    Network Working Group                                      U. Blumenthal
    Request for Comments: 2574                     IBM T. J. Watson Research
    Obsoletes: 2274                                                B. Wijnen
    Category: Standards Track                      IBM T. J. Watson Research
                                                                  April 1999

              User-based Security Model (USM) for version 3 of the
                  Simple Network Management Protocol (SNMPv3)

    Status of this Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (1999).  All Rights Reserved.

    Abstract

       This document describes the User-based Security Model (USM) for SNMP
       version 3 for use in the SNMP architecture [RFC2571].  It defines the
       Elements of Procedure for providing SNMP message level security.
       This document also includes a MIB for remotely monitoring/managing
       the configuration parameters for this Security Model.

    Table of Contents

       1.  Introduction                                                   3
       1.1.  Threats                                                      4
       1.2.  Goals and Constraints                                        5
       1.3.  Security Services                                            6
       1.4.  Module Organization                                          7
       1.4.1.  Timeliness Module                                          7
       1.4.2.  Authentication Protocol                                    8
       1.4.3.  Privacy Protocol                                           8
       1.5.  Protection against Message Replay, Delay and Redirection     8
       1.5.1.  Authoritative SNMP engine                                  8
       1.5.2.  Mechanisms                                                 9
       1.6.  Abstract Service Interfaces                                 10
       1.6.1.  User-based Security Model Primitives for Authentication   11
       1.6.2.  User-based Security Model Primitives for Privacy          11
       2.  Elements of the Model                                         12
       2.1.  User-based Security Model Users                             12

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       2.2.  Replay Protection                                           13
       2.2.1.  msgAuthoritativeEngineID                                  13
       2.2.2.  msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime14
       2.2.3.  Time Window                                               15
       2.3.  Time Synchronization                                        15
       2.4.  SNMP Messages Using this Security Model                     16
       2.5.  Services provided by the User-based Security Model          17
       2.5.1.  Services for Generating an Outgoing SNMP Message          17
       2.5.2.  Services for Processing an Incoming SNMP Message          19
       2.6.  Key Localization Algorithm.                                 21
       3.  Elements of Procedure                                         21
       3.1.  Generating an Outgoing SNMP Message                         22
       3.2.  Processing an Incoming SNMP Message                         25
       4.  Discovery                                                     30
       5.  Definitions                                                   31
       6.  HMAC-MD5-96 Authentication Protocol                           50
       6.1.  Mechanisms                                                  50
       6.1.1.  Digest Authentication Mechanism                           50
       6.2.  Elements of the Digest Authentication Protocol              51
       6.2.1.  Users                                                     51
       6.2.2.  msgAuthoritativeEngineID                                  51
       6.2.3.  SNMP Messages Using this Authentication Protocol          51
       6.2.4.  Services provided by the HMAC-MD5-96 Authentication Module52
       6.2.4.1.  Services for Generating an Outgoing SNMP Message        52
       6.2.4.2.  Services for Processing an Incoming SNMP Message        53
       6.3.  Elements of Procedure                                       53
       6.3.1.  Processing an Outgoing Message                            54
       6.3.2.  Processing an Incoming Message                            54
       7.  HMAC-SHA-96 Authentication Protocol                           55
       7.1.  Mechanisms                                                  55
       7.1.1.  Digest Authentication Mechanism                           56
       7.2.  Elements of the HMAC-SHA-96 Authentication Protocol         56
       7.2.1.  Users                                                     56
       7.2.2.  msgAuthoritativeEngineID                                  57
       7.2.3.  SNMP Messages Using this Authentication Protocol          57
       7.2.4.  Services provided by the HMAC-SHA-96 Authentication Module57
       7.2.4.1.  Services for Generating an Outgoing SNMP Message        57
       7.2.4.2.  Services for Processing an Incoming SNMP Message        58
       7.3.  Elements of Procedure                                       59
       7.3.1.  Processing an Outgoing Message                            59
       7.3.2.  Processing an Incoming Message                            60
       8.  CBC-DES Symmetric Encryption Protocol                         61
       8.1.  Mechanisms                                                  61
       8.1.1.  Symmetric Encryption Protocol                             61
       8.1.1.1.  DES key and Initialization Vector.                      62
       8.1.1.2.  Data Encryption.                                        63
       8.1.1.3.  Data Decryption                                         63
       8.2.  Elements of the DES Privacy Protocol                        63

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       8.2.1.  Users                                                     63
       8.2.2.  msgAuthoritativeEngineID                                  64
       8.2.3.  SNMP Messages Using this Privacy Protocol                 64
       8.2.4.  Services provided by the DES Privacy Module               64
       8.2.4.1.  Services for Encrypting Outgoing Data                   64
       8.2.4.2.  Services for Decrypting Incoming Data                   65
       8.3.  Elements of Procedure.                                      66
       8.3.1.  Processing an Outgoing Message                            66
       8.3.2.  Processing an Incoming Message                            66
       9.  Intellectual Property                                         67
       10. Acknowledgements                                              67
       11. Security Considerations                                       69
       11.1. Recommended Practices                                       69
       11.2. Defining Users                                              71
       11.3. Conformance                                                 72
       11.4. Use of Reports                                              72
       11.5. Access to the SNMP-USER-BASED-SM-MIB                        72
       12. References                                                    73
       13. Editors' Addresses                                            75
       A.1.  SNMP engine Installation Parameters                         76
       A.2.  Password to Key Algorithm                                   78
       A.2.1.  Password to Key Sample Code for MD5                       79
       A.2.2.  Password to Key Sample Code for SHA                       80
       A.3.  Password to Key Sample Results                              81
       A.3.1.  Password to Key Sample Results using MD5                  81
       A.3.2.  Password to Key Sample Results using SHA                  81
       A.4.  Sample encoding of msgSecurityParameters                    82
       A.5.  Sample keyChange Results                                    83
       A.5.1.  Sample keyChange Results using MD5                        83
       A.5.2.  Sample keyChange Results using SHA                        84
       B.  Change Log                                                    85
       C.  Full Copyright Statement                                      86

    1.  Introduction

       The Architecture for describing Internet Management Frameworks
       [RFC2571] describes that an SNMP engine is composed of:

         1) a Dispatcher
         2) a Message Processing Subsystem,
         3) a Security Subsystem, and
         4) an Access Control Subsystem.

       Applications make use of the services of these subsystems.

       It is important to understand the SNMP architecture and the
       terminology of the architecture to understand where the Security
       Model described in this document fits into the architecture and

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       interacts with other subsystems within the architecture.  The reader
       is expected to have read and understood the description of the SNMP
       architecture, as defined in [RFC2571].

       This memo [RFC2274] describes the User-based Security Model as it is
       used within the SNMP Architecture.  The main idea is that we use the
       traditional concept of a user (identified by a userName) with which
       to associate security information.

       This memo describes the use of HMAC-MD5-96 and HMAC-SHA-96 as the
       authentication protocols and the use of CBC-DES as the privacy
       protocol. The User-based Security Model however allows for other such
       protocols to be used instead of or concurrent with these protocols.
       Therefore, the description of HMAC-MD5-96, HMAC-SHA-96 and CBC-DES
       are in separate sections to reflect their self-contained nature and
       to indicate that they can be replaced or supplemented in the future.

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
       document are to be interpreted as described in [RFC2119].

    1.1.  Threats

       Several of the classical threats to network protocols are applicable
       to the network management problem and therefore would be applicable
       to any SNMP Security Model.  Other threats are not applicable to the
       network management problem.  This section discusses principal
       threats, secondary threats, and threats which are of lesser
       importance.

       The principal threats against which this SNMP Security Model should
       provide protection are:

       - Modification of Information
         The modification threat is the danger that some unauthorized entity
         may alter in-transit SNMP messages generated on behalf of an
         authorized principal in such a way as to effect unauthorized
         management operations, including falsifying the value of an object.

       - Masquerade
         The masquerade threat is the danger that management operations not
         authorized for some user may be attempted by assuming the identity
         of another user that has the appropriate authorizations.

       Two secondary threats are also identified.  The Security Model
       defined in this memo provides limited protection against:

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       - Disclosure
         The disclosure threat is the danger of eavesdropping on the
         exchanges between managed agents and a management station.
         Protecting against this threat may be required as a matter of local
         policy.

       - Message Stream Modification
         The SNMP protocol is typically based upon a connection-less
         transport service which may operate over any sub-network service.
         The re-ordering, delay or replay of messages can and does occur
         through the natural operation of many such sub-network services.
         The message stream modification threat is the danger that messages
         may be maliciously re-ordered, delayed or replayed to an extent
         which is greater than can occur through the natural operation of a
         sub-network service, in order to effect unauthorized management
         operations.

       There are at least two threats that an SNMP Security Model need not
       protect against.  The security protocols defined in this memo do not
       provide protection against:

       - Denial of Service
         This SNMP Security Model does not attempt to address the broad
         range of attacks by which service on behalf of authorized users is
         denied.  Indeed, such denial-of-service attacks are in many cases
         indistinguishable from the type of network failures with which any
         viable network management protocol must cope as a matter of course.
       - Traffic Analysis
         This SNMP Security Model does not attempt to address traffic
         analysis attacks.  Indeed, many traffic patterns are predictable -
         devices may be managed on a regular basis by a relatively small
         number of management applications - and therefore there is no
         significant advantage afforded by protecting against traffic
         analysis.

    1.2.  Goals and Constraints

       Based on the foregoing account of threats in the SNMP network
       management environment, the goals of this SNMP Security Model are as
       follows.

       1) Provide for verification that each received SNMP message has
          not been modified during its transmission through the network.

       2) Provide for verification of the identity of the user on whose
          behalf a received SNMP message claims to have been generated.

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       3) Provide for detection of received SNMP messages, which request
          or contain management information, whose time of generation was
          not recent.

       4) Provide, when necessary, that the contents of each received
          SNMP message are protected from disclosure.

       In addition to the principal goal of supporting secure network
       management, the design of this SNMP Security Model is also influenced
       by the following constraints:

       1) When the requirements of effective management in times of
          network stress are inconsistent with those of security, the design
          should prefer the former.

       2) Neither the security protocol nor its underlying security
          mechanisms should depend upon the ready availability of other
          network services (e.g., Network Time Protocol (NTP) or key
          management protocols).

       3) A security mechanism should entail no changes to the basic
          SNMP network management philosophy.

    1.3.  Security Services

       The security services necessary to support the goals of this SNMP
       Security Model are as follows:

       - Data Integrity
         is the provision of the property that data has not been altered or
         destroyed in an unauthorized manner, nor have data sequences been
         altered to an extent greater than can occur non-maliciously.

       - Data Origin Authentication
         is the provision of the property that the claimed identity of the
         user on whose behalf received data was originated is corroborated.

       - Data Confidentiality
         is the provision of the property that information is not made
         available or disclosed to unauthorized individuals, entities, or
         processes.

       - Message timeliness and limited replay protection
         is the provision of the property that a message whose generation
         time is outside of a specified time window is not accepted.  Note
         that message reordering is not dealt with and can occur in normal
         conditions too.

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       For the protocols specified in this memo, it is not possible to
       assure the specific originator of a received SNMP message; rather, it
       is the user on whose behalf the message was originated that is
       authenticated.

       For these protocols, it not possible to obtain data integrity without
       data origin authentication, nor is it possible to obtain data origin
       authentication without data integrity.  Further, there is no
       provision for data confidentiality without both data integrity and
       data origin authentication.

       The security protocols used in this memo are considered acceptably
       secure at the time of writing.  However, the procedures allow for new
       authentication and privacy methods to be specified at a future time
       if the need arises.

    1.4.  Module Organization

       The security protocols defined in this memo are split in three
       different modules and each has its specific responsibilities such
       that together they realize the goals and security services described
       above:

       - The authentication module MUST provide for:

         - Data Integrity,

         - Data Origin Authentication

       - The timeliness module MUST provide for:

         - Protection against message delay or replay (to an extent
           greater than can occur through normal operation)

       - The privacy module MUST provide for

         - Protection against disclosure of the message payload.

       The timeliness module is fixed for the User-based Security Model
       while there is provision for multiple authentication and/or privacy
       modules, each of which implements a specific authentication or
       privacy protocol respectively.

    1.4.1.  Timeliness Module

       Section 3 (Elements of Procedure) uses the timeliness values in an
       SNMP message to do timeliness checking.  The timeliness check is only
       performed if authentication is applied to the message.  Since the

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       complete message is checked for integrity, we can assume that the
       timeliness values in a message that passes the authentication module
       are trustworthy.

    1.4.2.  Authentication Protocol

       Section 6 describes the HMAC-MD5-96 authentication protocol which is
       the first authentication protocol that MUST be supported with the
       User-based Security Model.  Section 7 describes the HMAC-SHA-96
       authentication protocol which is another authentication protocol that
       SHOULD be supported with the User-based Security Model.  In the
       future additional or replacement authentication protocols may be
       defined as new needs arise.

       The User-based Security Model prescribes that, if authentication is
       used, then the complete message is checked for integrity in the
       authentication module.

       For a message to be authenticated, it needs to pass authentication
       check by the authentication module and the timeliness check which is
       a fixed part of this User-based Security model.

    1.4.3.  Privacy Protocol

       Section 8 describes the CBC-DES Symmetric Encryption Protocol which
       is the first privacy protocol to be used with the User-based Security
       Model.  In the future additional or replacement privacy protocols may
       be defined as new needs arise.

       The User-based Security Model prescribes that the scopedPDU is
       protected from disclosure when a message is sent with privacy.

       The User-based Security Model also prescribes that a message needs to
       be authenticated if privacy is in use.

    1.5.  Protection against Message Replay, Delay and Redirection

    1.5.1.  Authoritative SNMP engine

       In order to protect against message replay, delay and redirection,
       one of the SNMP engines involved in each communication is designated
       to be the authoritative SNMP engine.  When an SNMP message contains a
       payload which expects a response (those messages that contain a
       Confirmed Class PDU [RFC2571]), then the receiver of such messages is
       authoritative.  When an SNMP message contains a payload which does
       not expect a response (those messages that contain an Unconfirmed
       Class PDU [RFC2571]), then the sender of such a message is
       authoritative.

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    1.5.2.  Mechanisms

       The following mechanisms are used:

       1) To protect against the threat of message delay or replay (to an
          extent greater than can occur through normal operation), a set of
          timeliness indicators (for the authoritative SNMP engine) are
          included in each message generated.  An SNMP engine evaluates the
          timeliness indicators to determine if a received message is
          recent.  An SNMP engine may evaluate the timeliness indicators to
          ensure that a received message is at least as recent as the last
          message it received from the same source.  A non-authoritative
          SNMP engine uses received authentic messages to advance its notion
          of the timeliness indicators at the remote authoritative source.

          An SNMP engine MUST also use a mechanism to match incoming
          Responses to outstanding Requests and it MUST drop any Responses
          that do not match an outstanding request. For example, a msgID can
          be inserted in every message to cater for this functionality.

          These mechanisms provide for the detection of authenticated
          messages whose time of generation was not recent.

          This protection against the threat of message delay or replay does
          not imply nor provide any protection against unauthorized deletion
          or suppression of messages.  Also, an SNMP engine may not be able
          to detect message reordering if all the messages involved are sent
          within the Time Window interval.  Other mechanisms defined
          independently of the security protocol can also be used to detect
          the re-ordering replay, deletion, or suppression of messages
          containing Set operations (e.g., the MIB variable snmpSetSerialNo
          [RFC1907]).

       2) Verification that a message sent to/from one authoritative SNMP
          engine cannot be replayed to/as-if-from another authoritative SNMP
          engine.

          Included in each message is an identifier unique to the
          authoritative SNMP engine associated with the sender or intended
          recipient of the message.

          A message containing an Unconfirmed Class PDU sent by an
          authoritative SNMP engine to one non-authoritative SNMP engine can
          potentially be replayed to another non-authoritative SNMP engine.
          The latter non-authoritative SNMP engine might (if it knows about
          the same userName with the same secrets at the authoritative SNMP
          engine) as a result update its notion of timeliness indicators of
          the authoritative SNMP engine, but that is not considered a

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          threat.  In this case, A Report or Response message will be
          discarded by the Message Processing Model, because there should
          not be an outstanding Request message. A Trap will possibly be
          accepted.  Again, that is not considered a threat, because the
          communication was authenticated and timely. It is as if the
          authoritative SNMP engine was configured to start sending Traps to
          the second SNMP engine, which theoretically can happen without the
          knowledge of the second SNMP engine anyway. Anyway, the second
          SNMP engine may not expect to receive this Trap, but is allowed to
          see the management information contained in it.

       3) Detection of messages which were not recently generated.

          A set of time indicators are included in the message, indicating
          the time of generation.  Messages without recent time indicators
          are not considered authentic.  In addition, an SNMP engine MUST
          drop any Responses that do not match an outstanding request. This
          however is the responsibility of the Message Processing Model.

       This memo allows the same user to be defined on multiple SNMP
       engines.  Each SNMP engine maintains a value, snmpEngineID, which
       uniquely identifies the SNMP engine.  This value is included in each
       message sent to/from the SNMP engine that is authoritative (see
       section 1.5.1).  On receipt of a message, an authoritative SNMP
       engine checks the value to ensure that it is the intended recipient,
       and a non-authoritative SNMP engine uses the value to ensure that the
       message is processed using the correct state information.

       Each SNMP engine maintains two values, snmpEngineBoots and
       snmpEngineTime, which taken together provide an indication of time at
       that SNMP engine.  Both of these values are included in an
       authenticated message sent to/received from that SNMP engine.  On
       receipt, the values are checked to ensure that the indicated
       timeliness value is within a Time Window of the current time.  The
       Time Window represents an administrative upper bound on acceptable
       delivery delay for protocol messages.

       For an SNMP engine to generate a message which an authoritative SNMP
       engine will accept as authentic, and to verify that a message
       received from that authoritative SNMP engine is authentic, such an
       SNMP engine must first achieve timeliness synchronization with the
       authoritative SNMP engine. See section 2.3.

    1.6.  Abstract Service Interfaces

       Abstract service interfaces have been defined to describe the
       conceptual interfaces between the various subsystems within an SNMP
       entity. Similarly a set of abstract service interfaces have been

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       defined within the User-based Security Model (USM) to describe the
       conceptual interfaces between the generic USM services and the self-
       contained authentication and privacy services.

       These abstract service interfaces are defined by a set of primitives
       that define the services provided and the abstract data elements that
       must be passed when the services are invoked. This section lists the
       primitives that have been defined for the User-based Security Model.

    1.6.1.  User-based Security Model Primitives for Authentication

       The User-based Security Model provides the following internal
       primitives to pass data back and forth between the Security Model
       itself and the authentication service:

       statusInformation =
         authenticateOutgoingMsg(
         IN   authKey                   -- secret key for authentication
         IN   wholeMsg                  -- unauthenticated complete message
         OUT  authenticatedWholeMsg     -- complete authenticated message
              )

       statusInformation =
         authenticateIncomingMsg(
         IN   authKey                   -- secret key for authentication
         IN   authParameters            -- as received on the wire
         IN   wholeMsg                  -- as received on the wire
         OUT  authenticatedWholeMsg     -- complete authenticated message
              )

    1.6.2.  User-based Security Model Primitives for Privacy

       The User-based Security Model provides the following internal
       primitives to pass data back and forth between the Security Model
       itself and the privacy service:

       statusInformation =
         encryptData(
         IN    encryptKey               -- secret key for encryption
         IN    dataToEncrypt            -- data to encrypt (scopedPDU)
         OUT   encryptedData            -- encrypted data (encryptedPDU)
         OUT   privParameters           -- filled in by service provider
               )

       statusInformation =
         decryptData(
         IN    decryptKey               -- secret key for decrypting
         IN    privParameters           -- as received on the wire

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         IN    encryptedData            -- encrypted data (encryptedPDU)
         OUT   decryptedData            -- decrypted data (scopedPDU)
                  )

    2.  Elements of the Model

       This section contains definitions required to realize the security
       model defined by this memo.

    2.1.  User-based Security Model Users

       Management operations using this Security Model make use of a defined
       set of user identities.  For any user on whose behalf management
       operations are authorized at a particular SNMP engine, that SNMP
       engine must have knowledge of that user.  An SNMP engine that wishes
       to communicate with another SNMP engine must also have knowledge of a
       user known to that engine, including knowledge of the applicable
       attributes of that user.

       A user and its attributes are defined as follows:

       userName
         A string representing the name of the user.

       securityName
         A human-readable string representing the user in a format that is
         Security Model independent.

       authProtocol
         An indication of whether messages sent on behalf of this user can
         be authenticated, and if so, the type of authentication protocol
         which is used.  Two such protocols are defined in this memo:

           - the HMAC-MD5-96 authentication protocol.
           - the HMAC-SHA-96 authentication protocol.

       authKey
         If messages sent on behalf of this user can be authenticated,
         the (private) authentication key for use with the authentication
         protocol.  Note that a user's authentication key will normally
         be different at different authoritative SNMP engines. The authKey
         is not accessible via SNMP. The length requirements of the authKey
         are defined by the authProtocol in use.

       authKeyChange and authOwnKeyChange
         The only way to remotely update the authentication key.  Does
         that in a secure manner, so that the update can be completed
         without the need to employ privacy protection.

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       privProtocol
         An indication of whether messages sent on behalf of this user
         can be protected from disclosure, and if so, the type of privacy
         protocol which is used.  One such protocol is defined in this
         memo: the CBC-DES Symmetric Encryption Protocol.

       privKey
         If messages sent on behalf of this user can be en/decrypted,
         the (private) privacy key for use with the privacy protocol.
         Note that a user's privacy key will normally be different at
         different authoritative SNMP engines. The privKey is not
         accessible via SNMP. The length requirements of the privKey are
         defined by the privProtocol in use.

       privKeyChange and privOwnKeyChange
         The only way to remotely update the encryption key. Does that
         in a secure manner, so that the update can be completed without
         the need to employ privacy protection.

    2.2.  Replay Protection

       Each SNMP engine maintains three objects:

       - snmpEngineID, which (at least within an administrative domain)
         uniquely and unambiguously identifies an SNMP engine.

       - snmpEngineBoots, which is a count of the number of times the
         SNMP engine has re-booted/re-initialized since snmpEngineID
         was last configured; and,

       - snmpEngineTime, which is the number of seconds since the
         snmpEngineBoots counter was last incremented.

       Each SNMP engine is always authoritative with respect to these
       objects in its own SNMP entity.  It is the responsibility of a
       non-authoritative SNMP engine to synchronize with the
       authoritative SNMP engine, as appropriate.

       An authoritative SNMP engine is required to maintain the values of
       its snmpEngineID and snmpEngineBoots in non-volatile storage.

    2.2.1.  msgAuthoritativeEngineID

       The msgAuthoritativeEngineID value contained in an authenticated
       message is used to defeat attacks in which messages from one SNMP
       engine to another SNMP engine are replayed to a different SNMP
       engine. It represents the snmpEngineID at the authoritative SNMP
       engine involved in the exchange of the message.

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       When an authoritative SNMP engine is first installed, it sets its
       local value of snmpEngineID according to a enterprise-specific
       algorithm (see the definition of the Textual Convention for
       SnmpEngineID in the SNMP Architecture document [RFC2571]).

    2.2.2.  msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime

       The msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime
       values contained in an authenticated message are used to defeat
       attacks in which messages are replayed when they are no longer
       valid.  They represent the snmpEngineBoots and snmpEngineTime
       values at the authoritative SNMP engine involved in the exchange
       of the message.

       Through use of snmpEngineBoots and snmpEngineTime, there is no
       requirement for an SNMP engine to have a non-volatile clock which
       ticks (i.e., increases with the passage of time) even when the
       SNMP engine is powered off.  Rather, each time an SNMP engine
       re-boots, it retrieves, increments, and then stores snmpEngineBoots
       in non-volatile storage, and resets snmpEngineTime to zero.

       When an SNMP engine is first installed, it sets its local values
       of snmpEngineBoots and snmpEngineTime to zero.  If snmpEngineTime
       ever reaches its maximum value (2147483647), then snmpEngineBoots
       is incremented as if the SNMP engine has re-booted and
       snmpEngineTime is reset to zero and starts incrementing again.

       Each time an authoritative SNMP engine re-boots, any SNMP engines
       holding that authoritative SNMP engine's values of snmpEngineBoots
       and snmpEngineTime need to re-synchronize prior to sending
       correctly authenticated messages to that authoritative SNMP engine
       (see Section 2.3 for (re-)synchronization procedures).  Note,
       however, that the procedures do provide for a notification to be
       accepted as authentic by a receiving SNMP engine, when sent by an
       authoritative SNMP engine which has re-booted since the receiving
       SNMP engine last (re-)synchronized.

       If an authoritative SNMP engine is ever unable to determine its
       latest snmpEngineBoots value, then it must set its snmpEngineBoots
       value to 2147483647.

       Whenever the local value of snmpEngineBoots has the value 2147483647
       it latches at that value and an authenticated message always causes
       an notInTimeWindow authentication failure.

       In order to reset an SNMP engine whose snmpEngineBoots value has
       reached the value 2147483647, manual intervention is required.
       The engine must be physically visited and re-configured, either

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       with a new snmpEngineID value, or with new secret values for the
       authentication and privacy protocols of all users known to that
       SNMP engine. Note that even if an SNMP engine re-boots once a second
       that it would still take approximately 68 years before the max value
       of 2147483647 would be reached.

    2.2.3.  Time Window

       The Time Window is a value that specifies the window of time in
       which a message generated on behalf of any user is valid.  This
       memo specifies that the same value of the Time Window, 150 seconds,
       is used for all users.

    2.3.  Time Synchronization

       Time synchronization, required by a non-authoritative SNMP engine
       in order to proceed with authentic communications, has occurred
       when the non-authoritative SNMP engine has obtained a local notion
       of the authoritative SNMP engine's values of snmpEngineBoots and
       snmpEngineTime from the authoritative SNMP engine.  These values
       must be (and remain) within the authoritative SNMP engine's Time
       Window.  So the local notion of the authoritative SNMP engine's
       values must be kept loosely synchronized with the values stored
       at the authoritative SNMP engine.  In addition to keeping a local
       copy of snmpEngineBoots and snmpEngineTime from the authoritative
       SNMP engine, a non-authoritative SNMP engine must also keep one
       local variable, latestReceivedEngineTime.  This value records the
       highest value of snmpEngineTime that was received by the
       non-authoritative SNMP engine from the authoritative SNMP engine
       and is used to eliminate the possibility of replaying messages
       that would prevent the non-authoritative SNMP engine's notion of
       the snmpEngineTime from advancing.

       A non-authoritative SNMP engine must keep local notions of these
       values
       (snmpEngineBoots, snmpEngineTime and latestReceivedEngineTime)
       for each authoritative SNMP engine with which it wishes to
       communicate.  Since each authoritative SNMP engine is uniquely
       and unambiguously identified by its value of snmpEngineID, the
       non-authoritative SNMP engine may use this value as a key in
       order to cache its local notions of these values.

       Time synchronization occurs as part of the procedures of receiving
       an SNMP message (Section 3.2, step 7b). As such, no explicit time
       synchronization procedure is required by a non-authoritative SNMP
       engine.  Note, that whenever the local value of snmpEngineID is
       changed (e.g., through discovery) or when secure communications
       are first established with an authoritative SNMP engine, the local

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       values of snmpEngineBoots and latestReceivedEngineTime should be
       set to zero.  This will cause the time synchronization to occur
       when the next authentic message is received.

    2.4.  SNMP Messages Using this Security Model

       The syntax of an SNMP message using this Security Model adheres
       to the message format defined in the version-specific Message
       Processing Model document (for example [RFC2572]).

       The field msgSecurityParameters in SNMPv3 messages has a data type
       of OCTET STRING.  Its value is the BER serialization of the
       following ASN.1 sequence:

       USMSecurityParametersSyntax DEFINITIONS IMPLICIT TAGS ::= BEGIN

          UsmSecurityParameters ::=
              SEQUENCE {
               -- global User-based security parameters
                  msgAuthoritativeEngineID     OCTET STRING,
                  msgAuthoritativeEngineBoots  INTEGER (0..2147483647),
                  msgAuthoritativeEngineTime   INTEGER (0..2147483647),
                  msgUserName                  OCTET STRING (SIZE(0..32)),
               -- authentication protocol specific parameters
                  msgAuthenticationParameters  OCTET STRING,
               -- privacy protocol specific parameters
                  msgPrivacyParameters         OCTET STRING
              }
       END

       The fields of this sequence are:

       - The msgAuthoritativeEngineID specifies the snmpEngineID of the
         authoritative SNMP engine involved in the exchange of the message.

       - The msgAuthoritativeEngineBoots specifies the snmpEngineBoots value
         at the authoritative SNMP engine involved in the exchange of the
         message.

       - The msgAuthoritativeEngineTime specifies the snmpEngineTime value
         at the authoritative SNMP engine involved in the exchange of the
         message.

       - The msgUserName specifies the user (principal) on whose behalf the
         message is being exchanged.  Note that a zero-length userName will
         not match any user, but it can be used for snmpEngineID discovery.

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       - The msgAuthenticationParameters are defined by the authentication
         protocol in use for the message, as defined by the
         usmUserAuthProtocol column in the user's entry in the usmUserTable.

       - The msgPrivacyParameters are defined by the privacy protocol in use
         for the message, as defined by the usmUserPrivProtocol column in
         the user's entry in the usmUserTable).

       See appendix A.4 for an example of the BER encoding of field
       msgSecurityParameters.

    2.5.  Services provided by the User-based Security Model

       This section describes the services provided by the User-based
       Security Model with their inputs and outputs.

       The services are described as primitives of an abstract service
       interface and the inputs and outputs are described as abstract data
       elements as they are passed in these abstract service primitives.

    2.5.1.  Services for Generating an Outgoing SNMP Message

       When the Message Processing (MP) Subsystem invokes the User-based
       Security module to secure an outgoing SNMP message, it must use the
       appropriate service as provided by the Security module.  These two
       services are provided:

       1) A service to generate a Request message. The abstract service
          primitive is:

          statusInformation =            -- success or errorIndication
            generateRequestMsg(
            IN   messageProcessingModel  -- typically, SNMP version
            IN   globalData              -- message header, admin data
            IN   maxMessageSize          -- of the sending SNMP entity
            IN   securityModel           -- for the outgoing message
            IN   securityEngineID        -- authoritative SNMP entity
            IN   securityName            -- on behalf of this principal
            IN   securityLevel           -- Level of Security requested
            IN   scopedPDU               -- message (plaintext) payload
            OUT  securityParameters      -- filled in by Security Module
            OUT  wholeMsg                -- complete generated message
            OUT  wholeMsgLength          -- length of generated message
                 )

       2) A service to generate a Response message. The abstract service
          primitive is:

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          statusInformation =            -- success or errorIndication
            generateResponseMsg(
            IN   messageProcessingModel  -- typically, SNMP version
            IN   globalData              -- message header, admin data
            IN   maxMessageSize          -- of the sending SNMP entity
            IN   securityModel           -- for the outgoing message
            IN   securityEngineID        -- authoritative SNMP entity
            IN   securityName            -- on behalf of this principal
            IN   securityLevel           -- Level of Security requested
            IN   scopedPDU               -- message (plaintext) payload
            IN   securityStateReference  -- reference to security state
                                         -- information from original
                                         -- request
            OUT  securityParameters      -- filled in by Security Module
            OUT  wholeMsg                -- complete generated message
            OUT  wholeMsgLength          -- length of generated message
                 )

       The abstract data elements passed as parameters in the abstract
       service primitives are as follows:

        statusInformation
          An indication of whether the encoding and securing of the message
          was successful.  If not it is an indication of the problem.
        messageProcessingModel
          The SNMP version number for the message to be generated.  This
          data is not used by the User-based Security module.
        globalData
          The message header (i.e., its administrative information). This
          data is not used by the User-based Security module.
        maxMessageSize
          The maximum message size as included in the message.  This data is
          not used by the User-based Security module.
        securityParameters
          These are the security parameters. They will be filled in by the
          User-based Security module.

        securityModel
          The securityModel in use. Should be User-based Security Model.
          This data is not used by the User-based Security module.
        securityName
          Together with the snmpEngineID it identifies a row in the
          usmUserTable that is to be used for securing the message.  The
          securityName has a format that is independent of the Security
          Model. In case of a response this parameter is ignored and the
          value from the cache is used.

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        securityLevel
          The Level of Security from which the User-based Security module
          determines if the message needs to be protected from disclosure
          and if the message needs to be authenticated.
        securityEngineID
          The snmpEngineID of the authoritative SNMP engine to which a
          Request message is to be sent. In case of a response it is implied
          to be the processing SNMP engine's snmpEngineID and so if it is
          specified, then it is ignored.
        scopedPDU
          The message payload.  The data is opaque as far as the User-based
          Security Model is concerned.
        securityStateReference
          A handle/reference to cachedSecurityData to be used when securing
          an outgoing Response message.  This is the exact same
          handle/reference as it was generated by the User-based Security
          module when processing the incoming Request message to which this
          is the Response message.
        wholeMsg
          The fully encoded and secured message ready for sending on the
          wire.
        wholeMsgLength
          The length of the encoded and secured message (wholeMsg).

       Upon completion of the process, the User-based Security module
       returns statusInformation. If the process was successful, the
       completed message with privacy and authentication applied if such was
       requested by the specified securityLevel is returned. If the process
       was not successful, then an errorIndication is returned.

    2.5.2.  Services for Processing an Incoming SNMP Message

       When the Message Processing (MP) Subsystem invokes the User-based
       Security module to verify proper security of an incoming message, it
       must use the service provided for an incoming message. The abstract
       service primitive is:

       statusInformation =             -- errorIndication or success
                                       -- error counter OID/value if error
         processIncomingMsg(
         IN   messageProcessingModel   -- typically, SNMP version
         IN   maxMessageSize           -- of the sending SNMP entity
         IN   securityParameters       -- for the received message
         IN   securityModel            -- for the received message
         IN   securityLevel            -- Level of Security
         IN   wholeMsg                 -- as received on the wire
         IN   wholeMsgLength           -- length as received on the wire
         OUT  securityEngineID         -- authoritative SNMP entity

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         OUT  securityName             -- identification of the principal
         OUT  scopedPDU,               -- message (plaintext) payload
         OUT  maxSizeResponseScopedPDU -- maximum size of the Response PDU
         OUT  securityStateReference   -- reference to security state
              )                        -- information, needed for response

       The abstract data elements passed as parameters in the abstract
       service primitives are as follows:

        statusInformation
          An indication of whether the process was successful or not.  If
          not, then the statusInformation includes the OID and the value of
          the error counter that was incremented.
        messageProcessingModel
          The SNMP version number as received in the message.  This data is
          not used by the User-based Security module.
        maxMessageSize
          The maximum message size as included in the message.  The User-
          based Security module uses this value to calculate the
          maxSizeResponseScopedPDU.
        securityParameters
          These are the security parameters as received in the message.
        securityModel
          The securityModel in use.  Should be the User-based Security
          Model.  This data is not used by the User-based Security module.
        securityLevel
          The Level of Security from which the User-based Security module
          determines if the message needs to be protected from disclosure
          and if the message needs to be authenticated.
        wholeMsg
          The whole message as it was received.
        wholeMsgLength
          The length of the message as it was received (wholeMsg).
        securityEngineID
          The snmpEngineID that was extracted from the field
          msgAuthoritativeEngineID and that was used to lookup the secrets
          in the usmUserTable.
        securityName
          The security name representing the user on whose behalf the
          message was received.  The securityName has a format that is
          independent of the Security Model.
        scopedPDU
          The message payload.  The data is opaque as far as the User-based
          Security Model is concerned.

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        maxSizeResponseScopedPDU
          The maximum size of a scopedPDU to be included in a possible
          Response message.  The User-based Security module calculates this
          size based on the msgMaxSize (as received in the message) and the
          space required for the message header (including the
          securityParameters) for such a Response message.
        securityStateReference
          A handle/reference to cachedSecurityData to be used when securing
          an outgoing Response message.  When the Message Processing
          Subsystem calls the User-based Security module to generate a
          response to this incoming message it must pass this
          handle/reference.

       Upon completion of the process, the User-based Security module
       returns statusInformation and, if the process was successful, the
       additional data elements for further processing of the message.  If
       the process was not successful, then an errorIndication, possibly
       with a OID and value pair of an error counter that was incremented.

    2.6.  Key Localization Algorithm.

       A localized key is a secret key shared between a user U and one
       authoritative SNMP engine E.  Even though a user may have only one
       password and therefore one key for the whole network, the actual
       secrets shared between the user and each authoritative SNMP engine
       will be different. This is achieved by key localization [Localized-
       key].

       First, if a user uses a password, then the user's password is
       converted into a key Ku using one of the two algorithms described in
       Appendices A.2.1 and A.2.2.

       To convert key Ku into a localized key Kul of user U at the
       authoritative SNMP engine E, one appends the snmpEngineID of the
       authoritative SNMP engine to the key Ku and then appends the key Ku
       to the result, thus enveloping the snmpEngineID within the two copies
       of user's key Ku. Then one runs a secure hash function (which one
       depends on the authentication protocol defined for this user U at
       authoritative SNMP engine E; this document defines two authentication
       protocols with their associated algorithms based on MD5 and SHA). The
       output of the hash-function is the localized key Kul for user U at
       the authoritative SNMP engine E.

    3.  Elements of Procedure

       This section describes the security related procedures followed by an
       SNMP engine when processing SNMP messages according to the User-based
       Security Model.

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    3.1.  Generating an Outgoing SNMP Message

       This section describes the procedure followed by an SNMP engine
       whenever it generates a message containing a management operation
       (like a request, a response, a notification, or a report) on behalf
       of a user, with a particular securityLevel.

       1)  a) If any securityStateReference is passed (Response or Report
              message), then information concerning the user is extracted
              from the cachedSecurityData.  The cachedSecurityData can now
              be discarded.  The securityEngineID is set to the local
              snmpEngineID.  The securityLevel is set to the value specified
              by the calling module.

              Otherwise,

           b) based on the securityName, information concerning the user at
              the destination snmpEngineID, specified by the
              securityEngineID, is extracted from the Local Configuration
              Datastore (LCD, usmUserTable). If information about the user
              is absent from the LCD, then an error indication
              (unknownSecurityName) is returned to the calling module.

       2)  If the securityLevel specifies that the message is to be
           protected from disclosure, but the user does not support both an
           authentication and a privacy protocol then the message cannot be
           sent.  An error indication (unsupportedSecurityLevel) is returned
           to the calling module.

       3)  If the securityLevel specifies that the message is to be
           authenticated, but the user does not support an authentication
           protocol, then the message cannot be sent. An error indication
           (unsupportedSecurityLevel) is returned to the calling module.

       4)  a) If the securityLevel specifies that the message is to be
              protected from disclosure, then the octet sequence
              representing the serialized scopedPDU is encrypted according
              to the user's privacy protocol. To do so a call is made to the
              privacy module that implements the user's privacy protocol
              according to the abstract primitive:

              statusInformation =       -- success or failure
                encryptData(
                IN    encryptKey        -- user's localized privKey
                IN    dataToEncrypt     -- serialized scopedPDU
                OUT   encryptedData     -- serialized encryptedPDU
                OUT   privParameters    -- serialized privacy parameters
                      )

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              statusInformation
                indicates if the encryption process was successful or not.
              encryptKey
                the user's localized private privKey is the secret key that
                can be used by the encryption algorithm.
              dataToEncrypt
                the serialized scopedPDU is the data to be encrypted.
              encryptedData
                the encryptedPDU represents the encrypted scopedPDU,
                encoded as an OCTET STRING.
              privParameters
                the privacy parameters, encoded as an OCTET STRING.

              If the privacy module returns failure, then the message cannot
              be sent and an error indication (encryptionError) is returned
              to the calling module.

              If the privacy module returns success, then the returned
              privParameters are put into the msgPrivacyParameters field of
              the securityParameters and the encryptedPDU serves as the
              payload of the message being prepared.

              Otherwise,

           b) If the securityLevel specifies that the message is not to be
              be protected from disclosure, then a zero-length OCTET STRING
              is encoded into the msgPrivacyParameters field of the
              securityParameters and the plaintext scopedPDU serves as the
              payload of the message being prepared.

       5)  The securityEngineID is encoded as an OCTET STRING into the
           msgAuthoritativeEngineID field of the securityParameters.  Note
           that an empty (zero length) securityEngineID is OK for a Request
           message, because that will cause the remote (authoritative) SNMP
           engine to return a Report PDU with the proper securityEngineID
           included in the msgAuthoritativeEngineID in the
           securityParameters of that returned Report PDU.

       6)  a) If the securityLevel specifies that the message is to be
              authenticated, then the current values of snmpEngineBoots and
              snmpEngineTime corresponding to the securityEngineID from the
              LCD are used.

              Otherwise,

           b) If this is a Response or Report message, then the current
              value of snmpEngineBoots and snmpEngineTime corresponding to
              the local snmpEngineID from the LCD are used.

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              Otherwise,

           c) If this is a Request message, then a zero value is used for
              both snmpEngineBoots and snmpEngineTime. This zero value gets
              used if snmpEngineID is empty.

           The values are encoded as INTEGER respectively into the
           msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime fields
           of the securityParameters.

       7)  The userName is encoded as an OCTET STRING into the msgUserName
           field of the securityParameters.

       8)  a) If the securityLevel specifies that the message is to be
              authenticated, the message is authenticated according to the
              user's authentication protocol. To do so a call is made to the
              authentication module that implements the user's
              authentication protocol according to the abstract service
              primitive:

              statusInformation =
                authenticateOutgoingMsg(
                IN  authKey               -- the user's localized authKey
                IN  wholeMsg              -- unauthenticated message
                OUT authenticatedWholeMsg -- authenticated complete message
                    )

              statusInformation
                indicates if authentication was successful or not.
              authKey
                the user's localized private authKey is the secret key that
                can be used by the authentication algorithm.
              wholeMsg
                the complete serialized message to be authenticated.
              authenticatedWholeMsg
                the same as the input given to the authenticateOutgoingMsg
                service, but with msgAuthenticationParameters properly
                filled in.

              If the authentication module returns failure, then the message
              cannot be sent and an error indication (authenticationFailure)
              is returned to the calling module.

              If the authentication module returns success, then the
              msgAuthenticationParameters field is put into the
              securityParameters and the authenticatedWholeMsg represents
              the serialization of the authenticated message being prepared.

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              Otherwise,

           b) If the securityLevel specifies that the message is not to be
              authenticated then a zero-length OCTET STRING is encoded into
              the msgAuthenticationParameters field of the
              securityParameters.  The wholeMsg is now serialized and then
              represents the unauthenticated message being prepared.

       9)  The completed message with its length is returned to the calling
           module with the statusInformation set to success.

    3.2.  Processing an Incoming SNMP Message

       This section describes the procedure followed by an SNMP engine
       whenever it receives a message containing a management operation on
       behalf of a user, with a particular securityLevel.

       To simplify the elements of procedure, the release of state
       information is not always explicitly specified. As a general rule, if
       state information is available when a message gets discarded, the
       state information should also be released.  Also, an error indication
       can return an OID and value for an incremented counter and optionally
       a value for securityLevel, and values for contextEngineID or
       contextName for the counter.  In addition, the securityStateReference
       data is returned if any such information is available at the point
       where the error is detected.

       1)  If the received securityParameters is not the serialization
           (according to the conventions of [RFC1906]) of an OCTET STRING
           formatted according to the UsmSecurityParameters defined in
           section 2.4, then the snmpInASNParseErrs counter [RFC1907] is
           incremented, and an error indication (parseError) is returned to
           the calling module.  Note that we return without the OID and
           value of the incremented counter, because in this case there is
           not enough information to generate a Report PDU.

       2)  The values of the security parameter fields are extracted from
           the securityParameters. The securityEngineID to be returned to
           the caller is the value of the msgAuthoritativeEngineID field.
           The cachedSecurityData is prepared and a securityStateReference
           is prepared to reference this data. Values to be cached are:

               msgUserName

       3)  If the value of the msgAuthoritativeEngineID field in the
           securityParameters is unknown then:

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           a) a non-authoritative SNMP engine that performs discovery may
              optionally create a new entry in its Local Configuration
              Datastore (LCD) and continue processing;

              or

           b) the usmStatsUnknownEngineIDs counter is incremented, and
              an error indication (unknownEngineID) together with the
              OID and value of the incremented counter is returned to
              the calling module.

           Note in the event that a zero-length, or other illegally
           sized msgAuthoritativeEngineID is received, b) should be
           chosen to facilitate engineID discovery.
           Otherwise the choice between a) and b) is an implementation
           issue.

       4)  Information about the value of the msgUserName and
           msgAuthoritativeEngineID fields is extracted from the Local
           Configuration Datastore (LCD, usmUserTable).  If no information
           is available for the user, then the usmStatsUnknownUserNames
           counter is incremented and an error indication
           (unknownSecurityName) together with the OID and value of the
           incremented counter is returned to the calling module.

       5)  If the information about the user indicates that it does not
           support the securityLevel requested by the caller, then the
           usmStatsUnsupportedSecLevels counter is incremented and an
           error indication (unsupportedSecurityLevel) together with the
           OID and value of the incremented counter is returned to the
           calling module.

       6)  If the securityLevel specifies that the message is to be
           authenticated, then the message is authenticated according to
           the user's authentication protocol. To do so a call is made
           to the authentication module that implements the user's
           authentication protocol according to the abstract service
           primitive:

           statusInformation =          -- success or failure
             authenticateIncomingMsg(
             IN   authKey               -- the user's localized authKey
             IN   authParameters        -- as received on the wire
             IN   wholeMsg              -- as received on the wire
             OUT  authenticatedWholeMsg -- checked for authentication
                     )

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           statusInformation
             indicates if authentication was successful or not.
           authKey
             the user's localized private authKey is the secret key that
             can be used by the authentication algorithm.
           wholeMsg
             the complete serialized message to be authenticated.
           authenticatedWholeMsg
             the same as the input given to the authenticateIncomingMsg
             service, but after authentication has been checked.

           If the authentication module returns failure, then the message
           cannot be trusted, so the usmStatsWrongDigests counter is
           incremented and an error indication (authenticationFailure)
           together with the OID and value of the incremented counter is
           returned to the calling module.

           If the authentication module returns success, then the message
           is authentic and can be trusted so processing continues.

       7)  If the securityLevel indicates an authenticated message, then
           the local values of snmpEngineBoots, snmpEngineTime
           and latestReceivedEngineTime
           corresponding to the value of the msgAuthoritativeEngineID
           field are extracted from the Local Configuration Datastore.

           a) If the extracted value of msgAuthoritativeEngineID is the
              same as the value of snmpEngineID of the processing SNMP
              engine (meaning this is the authoritative SNMP engine),
              then if any of the following conditions is true, then the
              message is considered to be outside of the Time Window:

               - the local value of snmpEngineBoots is 2147483647;

               - the value of the msgAuthoritativeEngineBoots field differs
                 from the local value of snmpEngineBoots; or,

               - the value of the msgAuthoritativeEngineTime field differs
                 from the local notion of snmpEngineTime by more than
                 +/- 150 seconds.

              If the message is considered to be outside of the Time Window
              then the usmStatsNotInTimeWindows counter is incremented and
              an error indication (notInTimeWindow) together with the OID,
              the value of the incremented counter, and an indication that
              the error must be reported with a securityLevel of authNoPriv,
              is returned to the calling module

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           b) If the extracted value of msgAuthoritativeEngineID is not the
              same as the value snmpEngineID of the processing SNMP engine
              (meaning this is not the authoritative SNMP engine), then:

              1) if at least one of the following conditions is true:

                 - the extracted value of the msgAuthoritativeEngineBoots
                   field is greater than the local notion of the value of
                   snmpEngineBoots; or,

                 - the extracted value of the msgAuthoritativeEngineBoots
                   field is equal to the local notion of the value of
                   snmpEngineBoots, and the extracted value of
                   msgAuthoritativeEngineTime field is greater than the
                   value of latestReceivedEngineTime,

                 then the LCD entry corresponding to the extracted value
                 of the msgAuthoritativeEngineID field is updated, by
                 setting:

                    - the local notion of the value of snmpEngineBoots to
                      the value of the msgAuthoritativeEngineBoots field,
                    - the local notion of the value of snmpEngineTime to
                      the value of the msgAuthoritativeEngineTime field,
                      and
                    - the latestReceivedEngineTime to the value of the
                      value of the msgAuthoritativeEngineTime field.

              2) if any of the following conditions is true, then the
                 message is considered to be outside of the Time Window:

                 - the local notion of the value of snmpEngineBoots is
                   2147483647;

                 - the value of the msgAuthoritativeEngineBoots field is
                   less than the local notion of the value of
                   snmpEngineBoots; or,

                 - the value of the msgAuthoritativeEngineBoots field is
                   equal to the local notion of the value of
                   snmpEngineBoots and the value of the
                   msgAuthoritativeEngineTime field is more than 150
                   seconds less than the local notion of the value of
                   snmpEngineTime.

                 If the message is considered to be outside of the Time
                 Window then an error indication (notInTimeWindow) is
                 returned to the calling module.

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    RFC 2574                     USM for SNMPv3                   April 1999

                 Note that this means that a too old (possibly replayed)
                 message has been detected and is deemed unauthentic.

                 Note that this procedure allows for the value of
                 msgAuthoritativeEngineBoots in the message to be greater
                 than the local notion of the value of snmpEngineBoots to
                 allow for received messages to be accepted as authentic
                 when received from an authoritative SNMP engine that has
                 re-booted since the receiving SNMP engine last
                 (re-)synchronized.

       8)  a) If the securityLevel indicates that the message was protected
              from disclosure, then the OCTET STRING representing the
              encryptedPDU is decrypted according to the user's privacy
              protocol to obtain an unencrypted serialized scopedPDU value.
              To do so a call is made to the privacy module that implements
              the user's privacy protocol according to the abstract
              primitive:

              statusInformation =       -- success or failure
                decryptData(
                IN    decryptKey        -- the user's localized privKey
                IN    privParameters    -- as received on the wire
                IN    encryptedData     -- encryptedPDU as received
                OUT   decryptedData     -- serialized decrypted scopedPDU
                      )

              statusInformation
                indicates if the decryption process was successful or not.
              decryptKey
                the user's localized private privKey is the secret key that
                can be used by the decryption algorithm.
              privParameters
                the msgPrivacyParameters, encoded as an OCTET STRING.
              encryptedData
                the encryptedPDU represents the encrypted scopedPDU, encoded
                as an OCTET STRING.
              decryptedData
                the serialized scopedPDU if decryption is successful.

              If the privacy module returns failure, then the message can
              not be processed, so the usmStatsDecryptionErrors counter is
              incremented and an error indication (decryptionError) together
              with the OID and value of the incremented counter is returned
              to the calling module.

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    RFC 2574                     USM for SNMPv3                   April 1999

              If the privacy module returns success, then the decrypted
              scopedPDU is the message payload to be returned to the calling
              module.

              Otherwise,

           b) The scopedPDU component is assumed to be in plain text
              and is the message payload to be returned to the calling
              module.

       9)  The maxSizeResponseScopedPDU is calculated.  This is the
           maximum size allowed for a scopedPDU for a possible Response
           message.  Provision is made for a message header that allows the
           same securityLevel as the received Request.

       10) The securityName for the user is retrieved from the
           usmUserTable.

       11) The security data is cached as cachedSecurityData, so that a
           possible response to this message can and will use the same
           authentication and privacy secrets.  Information to be
           saved/cached is as follows:

              msgUserName,
              usmUserAuthProtocol, usmUserAuthKey
              usmUserPrivProtocol, usmUserPrivKey

       12) The statusInformation is set to success and a return is made to
           the calling module passing back the OUT parameters as specified
           in the processIncomingMsg primitive.

    4.  Discovery

       The User-based Security Model requires that a discovery process
       obtains sufficient information about other SNMP engines in order to
       communicate with them.  Discovery requires an non-authoritative SNMP
       engine to learn the authoritative SNMP engine's snmpEngineID value
       before communication may proceed.  This may be accomplished by
       generating a Request message with a securityLevel of noAuthNoPriv, a
       msgUserName of zero-length, a msgAuthoritativeEngineID value of zero
       length, and the varBindList left empty.  The response to this message
       will be a Report message containing the snmpEngineID of the
       authoritative SNMP engine as the value of the
       msgAuthoritativeEngineID field within the msgSecurityParameters
       field.  It contains a Report PDU with the usmStatsUnknownEngineIDs
       counter in the varBindList.

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    RFC 2574                     USM for SNMPv3                   April 1999

       If authenticated communication is required, then the discovery
       process should also establish time synchronization with the
       authoritative SNMP engine.  This may be accomplished by sending an
       authenticated Request message with the value of
       msgAuthoritativeEngineID set to the newly learned snmpEngineID and
       with the values of msgAuthoritativeEngineBoots and
       msgAuthoritativeEngineTime set to zero.  For an authenticated Request
       message, a valid userName must be used in the msgUserName field.  The
       response to this authenticated message will be a Report message
       containing the up to date values of the authoritative SNMP engine's
       snmpEngineBoots and snmpEngineTime as the value of the
       msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime fields
       respectively.  It also contains the usmStatsNotInTimeWindows counter
       in the varBindList of the Report PDU.  The time synchronization then
       happens automatically as part of the procedures in section 3.2 step
       7b. See also section 2.3.

    5.  Definitions

    SNMP-USER-BASED-SM-MIB DEFINITIONS ::= BEGIN

    IMPORTS
        MODULE-IDENTITY, OBJECT-TYPE,
        OBJECT-IDENTITY,
        snmpModules, Counter32                FROM SNMPv2-SMI
        TEXTUAL-CONVENTION, TestAndIncr,
        RowStatus, RowPointer,
        StorageType, AutonomousType           FROM SNMPv2-TC
        MODULE-COMPLIANCE, OBJECT-GROUP       FROM SNMPv2-CONF
        SnmpAdminString, SnmpEngineID,
        snmpAuthProtocols, snmpPrivProtocols  FROM SNMP-FRAMEWORK-MIB;

    snmpUsmMIB MODULE-IDENTITY
        LAST-UPDATED "9901200000Z"            -- 20 Jan 1999, midnight
        ORGANIZATION "SNMPv3 Working Group"
        CONTACT-INFO "WG-email:   snmpv3@lists.tislabs.com
                      Subscribe:  majordomo@lists.tislabs.com
                                  In msg body:  subscribe snmpv3

                      Chair:      Russ Mundy
                                  Trusted Information Systems
                      postal:     3060 Washington Rd
                                  Glenwood MD 21738
                                  USA
                      email:      mundy@tislabs.com
                      phone:      +1-301-854-6889

                      Co-editor   Uri Blumenthal

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    RFC 2574                     USM for SNMPv3                   April 1999

                                  IBM T. J. Watson Research
                      postal:     30 Saw Mill River Pkwy,
                                  Hawthorne, NY 10532
                                  USA
                      email:      uri@watson.ibm.com
                      phone:      +1-914-784-7964

                      Co-editor:  Bert Wijnen
                                  IBM T. J. Watson Research
                      postal:     Schagen 33
                                  3461 GL Linschoten
                                  Netherlands
                      email:      wijnen@vnet.ibm.com
                      phone:      +31-348-432-794
                     "
        DESCRIPTION  "The management information definitions for the
                      SNMP User-based Security Model.
                     "
    --  Revision history

        REVISION     "9901200000Z"            -- 20 Jan 1999, midnight
        DESCRIPTION  "Clarifications, published as RFC2574"

        REVISION     "9711200000Z"            -- 20 Nov 1997, midnight
        DESCRIPTION  "Initial version, published as RFC2274"

        ::= { snmpModules 15 }

    -- Administrative assignments ****************************************

    usmMIBObjects     OBJECT IDENTIFIER ::= { snmpUsmMIB 1 }
    usmMIBConformance OBJECT IDENTIFIER ::= { snmpUsmMIB 2 }

    -- Identification of Authentication and Privacy Protocols ************

    usmNoAuthProtocol OBJECT-IDENTITY
        STATUS        current
        DESCRIPTION  "No Authentication Protocol."
        ::= { snmpAuthProtocols 1 }

    usmHMACMD5AuthProtocol OBJECT-IDENTITY
        STATUS        current
        DESCRIPTION  "The HMAC-MD5-96 Digest Authentication Protocol."
        REFERENCE    "- H. Krawczyk, M. Bellare, R. Canetti HMAC:
                        Keyed-Hashing for Message Authentication,
                        RFC2104, Feb 1997.
                      - Rivest, R., Message Digest Algorithm MD5, RFC1321.
                     "

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    RFC 2574                     USM for SNMPv3                   April 1999

        ::= { snmpAuthProtocols 2 }

    usmHMACSHAAuthProtocol OBJECT-IDENTITY
        STATUS        current
        DESCRIPTION  "The HMAC-SHA-96 Digest Authentication Protocol."
        REFERENCE    "- H. Krawczyk, M. Bellare, R. Canetti, HMAC:
                        Keyed-Hashing for Message Authentication,
                        RFC2104, Feb 1997.
                      - Secure Hash Algorithm. NIST FIPS 180-1.
                     "
        ::= { snmpAuthProtocols 3 }

    usmNoPrivProtocol OBJECT-IDENTITY
        STATUS        current
        DESCRIPTION  "No Privacy Protocol."
        ::= { snmpPrivProtocols 1 }

    usmDESPrivProtocol OBJECT-IDENTITY
        STATUS        current
        DESCRIPTION  "The CBC-DES Symmetric Encryption Protocol."
        REFERENCE    "- Data Encryption Standard, National Institute of
                        Standards and Technology.  Federal Information
                        Processing Standard (FIPS) Publication 46-1.
                        Supersedes FIPS Publication 46,
                        (January, 1977; reaffirmed January, 1988).

                      - Data Encryption Algorithm, American National
                        Standards Institute.  ANSI X3.92-1981,
                        (December, 1980).

                      - DES Modes of Operation, National Institute of
                        Standards and Technology.  Federal Information
                        Processing Standard (FIPS) Publication 81,
                        (December, 1980).

                      - Data Encryption Algorithm - Modes of Operation,
                        American National Standards Institute.
                        ANSI X3.106-1983, (May 1983).
                     "
        ::= { snmpPrivProtocols 2 }

    -- Textual Conventions ***********************************************

    KeyChange ::=     TEXTUAL-CONVENTION
       STATUS         current
       DESCRIPTION

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    RFC 2574                     USM for SNMPv3                   April 1999

             "Every definition of an object with this syntax must identify
              a protocol P, a secret key K, and a hash algorithm H
              that produces output of L octets.

              The object's value is a manager-generated, partially-random
              value which, when modified, causes the value of the secret
              key K, to be modified via a one-way function.

              The value of an instance of this object is the concatenation
              of two components: first a 'random' component and then a
              'delta' component.

              The lengths of the random and delta components
              are given by the corresponding value of the protocol P;
              if P requires K to be a fixed length, the length of both the
              random and delta components is that fixed length; if P
              allows the length of K to be variable up to a particular
              maximum length, the length of the random component is that
              maximum length and the length of the delta component is any
              length less than or equal to that maximum length.
              For example, usmHMACMD5AuthProtocol requires K to be a fixed
              length of 16 octets and L - of 16 octets.
              usmHMACSHAAuthProtocol requires K to be a fixed length of
              20 octets and L - of 20 octets. Other protocols may define
              other sizes, as deemed appropriate.

              When a requester wants to change the old key K to a new
              key keyNew on a remote entity, the 'random' component is
              obtained from either a true random generator, or from a
              pseudorandom generator, and the 'delta' component is
              computed as follows:

               - a temporary variable is initialized to the existing value
                 of K;
               - if the length of the keyNew is greater than L octets,
                 then:
                  - the random component is appended to the value of the
                    temporary variable, and the result is input to the
                    the hash algorithm H to produce a digest value, and
                    the temporary variable is set to this digest value;
                  - the value of the temporary variable is XOR-ed with
                    the first (next) L-octets (16 octets in case of MD5)
                    of the keyNew to produce the first (next) L-octets
                    (16 octets in case of MD5) of the 'delta' component.
                  - the above two steps are repeated until the unused
                    portion of the keyNew component is L octets or less,
               - the random component is appended to the value of the
                 temporary variable, and the result is input to the

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    RFC 2574                     USM for SNMPv3                   April 1999

                 hash algorithm H to produce a digest value;
               - this digest value, truncated if necessary to be the same
                 length as the unused portion of the keyNew, is XOR-ed
                 with the unused portion of the keyNew to produce the
                 (final portion of the) 'delta' component.

               For example, using MD5 as the hash algorithm H:

                  iterations = (lenOfDelta - 1)/16; /* integer division */
                  temp = keyOld;
                  for (i = 0; i < iterations; i++) {
                      temp = MD5 (temp || random);
                      delta[i*16 .. (i*16)+15] =
                             temp XOR keyNew[i*16 .. (i*16)+15];
                  }
                  temp = MD5 (temp || random);
                  delta[i*16 .. lenOfDelta-1] =
                         temp XOR keyNew[i*16 .. lenOfDelta-1];

              The 'random' and 'delta' components are then concatenated as
              described above, and the resulting octet string is sent to
              the recipient as the new value of an instance of this object.

              At the receiver side, when an instance of this object is set
              to a new value, then a new value of K is computed as follows:

               - a temporary variable is initialized to the existing value
                 of K;
               - if the length of the delta component is greater than L
                 octets, then:
                  - the random component is appended to the value of the
                    temporary variable, and the result is input to the
                    hash algorithm H to produce a digest value, and the
                    temporary variable is set to this digest value;
                  - the value of the temporary variable is XOR-ed with
                    the first (next) L-octets (16 octets in case of MD5)
                    of the delta component to produce the first (next)
                    L-octets (16 octets in case of MD5) of the new value
                    of K.
                  - the above two steps are repeated until the unused
                    portion of the delta component is L octets or less,
               - the random component is appended to the value of the
                 temporary variable, and the result is input to the
                 hash algorithm H to produce a digest value;
               - this digest value, truncated if necessary to be the same
                 length as the unused portion of the delta component, is
                 XOR-ed with the unused portion of the delta component to
                 produce the (final portion of the) new value of K.

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    RFC 2574                     USM for SNMPv3                   April 1999

               For example, using MD5 as the hash algorithm H:

                  iterations = (lenOfDelta - 1)/16; /* integer division */
                  temp = keyOld;
                  for (i = 0; i < iterations; i++) {
                      temp = MD5 (temp || random);
                      keyNew[i*16 .. (i*16)+15] =
                             temp XOR delta[i*16 .. (i*16)+15];
                  }
                  temp = MD5 (temp || random);
                  keyNew[i*16 .. lenOfDelta-1] =
                         temp XOR delta[i*16 .. lenOfDelta-1];

              The value of an object with this syntax, whenever it is
              retrieved by the management protocol, is always the zero
              length string.

              Note that the keyOld and keyNew are the localized keys.

              Note that it is probably wise that when an SNMP entity sends
              a SetRequest to change a key, that it keeps a copy of the old
              key until it has confirmed that the key change actually
              succeeded.
             "
        SYNTAX       OCTET STRING

    -- Statistics for the User-based Security Model **********************

    usmStats         OBJECT IDENTIFIER ::= { usmMIBObjects 1 }

    usmStatsUnsupportedSecLevels OBJECT-TYPE
        SYNTAX       Counter32
        MAX-ACCESS   read-only
        STATUS       current
        DESCRIPTION "The total number of packets received by the SNMP
                     engine which were dropped because they requested a
                     securityLevel that was unknown to the SNMP engine
                     or otherwise unavailable.
                    "
        ::= { usmStats 1 }

    usmStatsNotInTimeWindows OBJECT-TYPE
        SYNTAX       Counter32
        MAX-ACCESS   read-only
        STATUS       current

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    RFC 2574                     USM for SNMPv3                   April 1999

        DESCRIPTION "The total number of packets received by the SNMP
                     engine which were dropped because they appeared
                     outside of the authoritative SNMP engine's window.
                    "
        ::= { usmStats 2 }

    usmStatsUnknownUserNames OBJECT-TYPE
        SYNTAX       Counter32
        MAX-ACCESS   read-only
        STATUS       current
        DESCRIPTION "The total number of packets received by the SNMP
                     engine which were dropped because they referenced a
                     user that was not known to the SNMP engine.
                    "
        ::= { usmStats 3 }

    usmStatsUnknownEngineIDs OBJECT-TYPE
        SYNTAX       Counter32
        MAX-ACCESS   read-only
        STATUS       current
        DESCRIPTION "The total number of packets received by the SNMP
                     engine which were dropped because they referenced an
                     snmpEngineID that was not known to the SNMP engine.
                    "
        ::= { usmStats 4 }

    usmStatsWrongDigests OBJECT-TYPE
        SYNTAX       Counter32
        MAX-ACCESS   read-only
        STATUS       current
        DESCRIPTION "The total number of packets received by the SNMP
                     engine which were dropped because they didn't
                     contain the expected digest value.
                    "
        ::= { usmStats 5 }

    usmStatsDecryptionErrors OBJECT-TYPE
        SYNTAX       Counter32
        MAX-ACCESS   read-only
        STATUS       current
        DESCRIPTION "The total number of packets received by the SNMP
                     engine which were dropped because they could not be
                     decrypted.
                    "
        ::= { usmStats 6 }

    -- The usmUser Group ************************************************

    Blumenthal & Wijnen         Standards Track                    [Page 37]

    RFC 2574                     USM for SNMPv3                   April 1999

    usmUser          OBJECT IDENTIFIER ::= { usmMIBObjects 2 }

    usmUserSpinLock  OBJECT-TYPE
        SYNTAX       TestAndIncr
        MAX-ACCESS   read-write
        STATUS       current
        DESCRIPTION "An advisory lock used to allow several cooperating
                     Command Generator Applications to coordinate their
                     use of facilities to alter secrets in the
                     usmUserTable.
                    "
        ::= { usmUser 1 }

    -- The table of valid users for the User-based Security Model ********

    usmUserTable     OBJECT-TYPE
        SYNTAX       SEQUENCE OF UsmUserEntry
        MAX-ACCESS   not-accessible
        STATUS       current
        DESCRIPTION "The table of users configured in the SNMP engine's
                     Local Configuration Datastore (LCD).

                     To create a new user (i.e., to instantiate a new
                     conceptual row in this table), it is recommended to
                     follow this procedure:

                       1)  GET(usmUserSpinLock.0) and save in sValue.
                       2)  SET(usmUserSpinLock.0=sValue,
                               usmUserCloneFrom=templateUser,
                               usmUserStatus=createAndWait)
                           You should use a template user to clone from
                           which has the proper auth/priv protocol defined.

                     If the new user is to use privacy:

                       3)  generate the keyChange value based on the secret
                           privKey of the clone-from user and the secret key
                           to be used for the new user. Let us call this
                           pkcValue.
                       4)  GET(usmUserSpinLock.0) and save in sValue.
                       5)  SET(usmUserSpinLock.0=sValue,
                               usmUserPrivKeyChange=pkcValue
                               usmUserPublic=randomValue1)
                       6)  GET(usmUserPulic) and check it has randomValue1.
                           If not, repeat steps 4-6.

                     If the new user will never use privacy:

    Blumenthal & Wijnen         Standards Track                    [Page 38]

    RFC 2574                     USM for SNMPv3                   April 1999

                       7)  SET(usmUserPrivProtocol=usmNoPrivProtocol)

                     If the new user is to use authentication:

                       8)  generate the keyChange value based on the secret
                           authKey of the clone-from user and the secret key
                           to be used for the new user. Let us call this
                           akcValue.
                       9)  GET(usmUserSpinLock.0) and save in sValue.
                       10) SET(usmUserSpinLock.0=sValue,
                               usmUserAuthKeyChange=akcValue
                               usmUserPublic=randomValue2)
                       11) GET(usmUserPulic) and check it has randomValue2.
                           If not, repeat steps 9-11.

                     If the new user will never use authentication:

                       12) SET(usmUserAuthProtocol=usmNoAuthProtocol)

                     Finally, activate the new user:

                       13) SET(usmUserStatus=active)

                     The new user should now be available and ready to be
                     used for SNMPv3 communication. Note however that access
                     to MIB data must be provided via configuration of the
                     SNMP-VIEW-BASED-ACM-MIB.

                     The use of usmUserSpinlock is to avoid conflicts with
                     another SNMP command responder application which may
                     also be acting on the usmUserTable.
                    "
        ::= { usmUser 2 }

    usmUserEntry     OBJECT-TYPE
        SYNTAX       UsmUserEntry
        MAX-ACCESS   not-accessible
        STATUS       current
        DESCRIPTION "A user configured in the SNMP engine's Local
                     Configuration Datastore (LCD) for the User-based
                     Security Model.
                    "
        INDEX       { usmUserEngineID,
                      usmUserName
                    }
        ::= { usmUserTable 1 }

    UsmUserEntry ::= SEQUENCE

    Blumenthal & Wijnen         Standards Track                    [Page 39]

    RFC 2574                     USM for SNMPv3                   April 1999

        {
            usmUserEngineID         SnmpEngineID,
            usmUserName             SnmpAdminString,
            usmUserSecurityName     SnmpAdminString,
            usmUserCloneFrom        RowPointer,
            usmUserAuthProtocol     AutonomousType,
            usmUserAuthKeyChange    KeyChange,
            usmUserOwnAuthKeyChange KeyChange,
            usmUserPrivProtocol     AutonomousType,
            usmUserPrivKeyChange    KeyChange,
            usmUserOwnPrivKeyChange KeyChange,
            usmUserPublic           OCTET STRING,
            usmUserStorageType      StorageType,
            usmUserStatus           RowStatus
        }

    usmUserEngineID  OBJECT-TYPE
        SYNTAX       SnmpEngineID
        MAX-ACCESS   not-accessible
        STATUS       current
        DESCRIPTION "An SNMP engine's administratively-unique identifier.

                     In a simple agent, this value is always that agent's
                     own snmpEngineID value.

                     The value can also take the value of the snmpEngineID
                     of a remote SNMP engine with which this user can
                     communicate.
                    "
        ::= { usmUserEntry 1 }

    usmUserName      OBJECT-TYPE
        SYNTAX       SnmpAdminString (SIZE(1..32))
        MAX-ACCESS   not-accessible
        STATUS       current
        DESCRIPTION "A human readable string representing the name of
                     the user.

                     This is the (User-based Security) Model dependent
                     security ID.
                    "
        ::= { usmUserEntry 2 }

    usmUserSecurityName OBJECT-TYPE
        SYNTAX       SnmpAdminString
        MAX-ACCESS   read-only
        STATUS       current
        DESCRIPTION "A human readable string representing the user in

    Blumenthal & Wijnen         Standards Track                    [Page 40]

    RFC 2574                     USM for SNMPv3                   April 1999

                     Security Model independent format.

                     The default transformation of the User-based Security
                     Model dependent security ID to the securityName and
                     vice versa is the identity function so that the
                     securityName is the same as the userName.
                    "
        ::= { usmUserEntry 3 }

    usmUserCloneFrom OBJECT-TYPE
        SYNTAX       RowPointer
        MAX-ACCESS   read-create
        STATUS       current
        DESCRIPTION "A pointer to another conceptual row in this
                     usmUserTable.  The user in this other conceptual
                     row is called the clone-from user.

                     When a new user is created (i.e., a new conceptual
                     row is instantiated in this table), the privacy and
                     authentication parameters of the new user must be
                     cloned from its clone-from user. These parameters are:
                       - authentication protocol (usmUserAuthProtocol)
                       - privacy protocol (usmUserPrivProtocol)
                     They will be copied regardless of what the current
                     value is.

                     Cloning also causes the initial values of the secret
                     authentication key (authKey) and the secret encryption
                     key (privKey) of the new user to be set to the same
                     value as the corresponding secret of the clone-from
                     user.

                     The first time an instance of this object is set by
                     a management operation (either at or after its
                     instantiation), the cloning process is invoked.
                     Subsequent writes are successful but invoke no
                     action to be taken by the receiver.
                     The cloning process fails with an 'inconsistentName'
                     error if the conceptual row representing the
                     clone-from user does not exist or is not in an active
                     state when the cloning process is invoked.

                     When this object is read, the ZeroDotZero OID
                     is returned.
                    "
        ::= { usmUserEntry 4 }

    usmUserAuthProtocol OBJECT-TYPE

    Blumenthal & Wijnen         Standards Track                    [Page 41]

    RFC 2574                     USM for SNMPv3                   April 1999

        SYNTAX       AutonomousType
        MAX-ACCESS   read-create
        STATUS       current
        DESCRIPTION "An indication of whether messages sent on behalf of
                     this user to/from the SNMP engine identified by
                     usmUserEngineID, can be authenticated, and if so,
                     the type of authentication protocol which is used.

                     An instance of this object is created concurrently
                     with the creation of any other object instance for
                     the same user (i.e., as part of the processing of
                     the set operation which creates the first object
                     instance in the same conceptual row).

                     If an initial set operation (i.e. at row creation time)
                     tries to set a value for an unknown or unsupported
                     protocol, then a 'wrongValue' error must be returned.

                     The value will be overwritten/set when a set operation
                     is performed on the corresponding instance of
                     usmUserCloneFrom.

                     Once instantiated, the value of such an instance of
                     this object can only be changed via a set operation to
                     the value of the usmNoAuthProtocol.

                     If a set operation tries to change the value of an
                     existing instance of this object to any value other
                     than usmNoAuthProtocol, then an 'inconsistentValue'
                     error must be returned.

                     If a set operation tries to set the value to the
                     usmNoAuthProtocol while the usmUserPrivProtocol value
                     in the same row is not equal to usmNoPrivProtocol,
                     then an 'inconsistentValue' error must be returned.
                     That means that an SNMP command generator application
                     must first ensure that the usmUserPrivProtocol is set
                     to the usmNoPrivProtocol value before it can set
                     the usmUserAuthProtocol value to usmNoAuthProtocol.
                    "
        DEFVAL      { usmNoAuthProtocol }
        ::= { usmUserEntry 5 }

    usmUserAuthKeyChange OBJECT-TYPE
        SYNTAX       KeyChange   -- typically (SIZE (0 | 32)) for HMACMD5
                                 -- typically (SIZE (0 | 40)) for HMACSHA
        MAX-ACCESS   read-create
        STATUS       current

    Blumenthal & Wijnen         Standards Track                    [Page 42]

    RFC 2574                     USM for SNMPv3                   April 1999

        DESCRIPTION "An object, which when modified, causes the secret
                     authentication key used for messages sent on behalf
                     of this user to/from the SNMP engine identified by
                     usmUserEngineID, to be modified via a one-way
                     function.

                     The associated protocol is the usmUserAuthProtocol.
                     The associated secret key is the user's secret
                     authentication key (authKey). The associated hash
                     algorithm is the algorithm used by the user's
                     usmUserAuthProtocol.

                     When creating a new user, it is an 'inconsistentName'
                     error for a set operation to refer to this object
                     unless it is previously or concurrently initialized
                     through a set operation on the corresponding instance
                     of usmUserCloneFrom.

                     When the value of the corresponding usmUserAuthProtocol
                     is usmNoAuthProtocol, then a set is successful, but
                     effectively is a no-op.

                     When this object is read, the zero-length (empty)
                     string is returned.

                     The recommended way to do a key change is as follows:

                       1) GET(usmUserSpinLock.0) and save in sValue.
                       2) generate the keyChange value based on the old
                          (existing) secret key and the new secret key,
                          let us call this kcValue.

                     If you do the key change on behalf of another user:

                       3) SET(usmUserSpinLock.0=sValue,
                              usmUserAuthKeyChange=kcValue
                              usmUserPublic=randomValue)

                     If you do the key change for yourself:

                       4) SET(usmUserSpinLock.0=sValue,
                              usmUserOwnAuthKeyChange=kcValue
                              usmUserPublic=randomValue)

                     If you get a response with error-status of noError,
                     then the SET succeeded and the new key is active.
                     If you do not get a response, then you can issue a
                     GET(usmUserPublic) and check if the value is equal

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                     to the randomValue you did send in the SET. If so, then
                     the key change succeeded and the new key is active
                     (probably the response got lost). If not, then the SET
                     request probably never reached the target and so you
                     can start over with the procedure above.
                    "
        DEFVAL      { ''H }    -- the empty string
        ::= { usmUserEntry 6 }

    usmUserOwnAuthKeyChange OBJECT-TYPE
        SYNTAX       KeyChange   -- typically (SIZE (0 | 32)) for HMACMD5
                                 -- typically (SIZE (0 | 40)) for HMACSHA
        MAX-ACCESS   read-create
        STATUS       current
        DESCRIPTION "Behaves exactly as usmUserAuthKeyChange, with one
                     notable difference: in order for the set operation
                     to succeed, the usmUserName of the operation
                     requester must match the usmUserName that
                     indexes the row which is targeted by this
                     operation.
                     In addition, the USM security model must be
                     used for this operation.

                     The idea here is that access to this column can be
                     public, since it will only allow a user to change
                     his own secret authentication key (authKey).
                     Note that this can only be done once the row is active.

                     When a set is received and the usmUserName of the
                     requester is not the same as the umsUserName that
                     indexes the row which is targeted by this operation,
                     then a 'noAccess' error must be returned.

                     When a set is received and the security model in use
                     is not USM, then a 'noAccess' error must be returned.
                    "
        DEFVAL      { ''H }    -- the empty string
        ::= { usmUserEntry 7 }

    usmUserPrivProtocol OBJECT-TYPE
        SYNTAX       AutonomousType
        MAX-ACCESS   read-create
        STATUS       current
        DESCRIPTION "An indication of whether messages sent on behalf of
                     this user to/from the SNMP engine identified by
                     usmUserEngineID, can be protected from disclosure,
                     and if so, the type of privacy protocol which is used.

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                     An instance of this object is created concurrently
                     with the creation of any other object instance for
                     the same user (i.e., as part of the processing of
                     the set operation which creates the first object
                     instance in the same conceptual row).

                     If an initial set operation (i.e. at row creation time)
                     tries to set a value for an unknown or unsupported
                     protocol, then a 'wrongValue' error must be returned.

                     The value will be overwritten/set when a set operation
                     is performed on the corresponding instance of
                     usmUserCloneFrom.

                     Once instantiated, the value of such an instance of
                     this object can only be changed via a set operation to
                     the value of the usmNoPrivProtocol.

                     If a set operation tries to change the value of an
                     existing instance of this object to any value other
                     than usmNoPrivProtocol, then an 'inconsistentValue'
                     error must be returned.

                     Note that if any privacy protocol is used, then you
                     must also use an authentication protocol. In other
                     words, if usmUserPrivProtocol is set to anything else
                     than usmNoPrivProtocol, then the corresponding instance
                     of usmUserAuthProtocol cannot have a value of
                     usmNoAuthProtocol. If it does, then an
                     'inconsistentValue' error must be returned.
                    "
        DEFVAL      { usmNoPrivProtocol }
        ::= { usmUserEntry 8 }

    usmUserPrivKeyChange OBJECT-TYPE
        SYNTAX       KeyChange  -- typically (SIZE (0 | 32)) for DES
        MAX-ACCESS   read-create
        STATUS       current
        DESCRIPTION "An object, which when modified, causes the secret
                     encryption key used for messages sent on behalf
                     of this user to/from the SNMP engine identified by
                     usmUserEngineID, to be modified via a one-way
                     function.

                     The associated protocol is the usmUserPrivProtocol.
                     The associated secret key is the user's secret
                     privacy key (privKey). The associated hash
                     algorithm is the algorithm used by the user's

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    RFC 2574                     USM for SNMPv3                   April 1999

                     usmUserAuthProtocol.

                     When creating a new user, it is an 'inconsistentName'
                     error for a set operation to refer to this object
                     unless it is previously or concurrently initialized
                     through a set operation on the corresponding instance
                     of usmUserCloneFrom.

                     When the value of the corresponding usmUserPrivProtocol
                     is usmNoPrivProtocol, then a set is successful, but
                     effectively is a no-op.

                     When this object is read, the zero-length (empty)
                     string is returned.
                     See the description clause of usmUserAuthKeyChange for
                     a recommended procedure to do a key change.
                    "
        DEFVAL      { ''H }    -- the empty string
        ::= { usmUserEntry 9 }

    usmUserOwnPrivKeyChange OBJECT-TYPE
        SYNTAX       KeyChange  -- typically (SIZE (0 | 32)) for DES
        MAX-ACCESS   read-create
        STATUS       current
        DESCRIPTION "Behaves exactly as usmUserPrivKeyChange, with one
                     notable difference: in order for the Set operation
                     to succeed, the usmUserName of the operation
                     requester must match the usmUserName that indexes
                     the row which is targeted by this operation.
                     In addition, the USM security model must be
                     used for this operation.

                     The idea here is that access to this column can be
                     public, since it will only allow a user to change
                     his own secret privacy key (privKey).
                     Note that this can only be done once the row is active.

                     When a set is received and the usmUserName of the
                     requester is not the same as the umsUserName that
                     indexes the row which is targeted by this operation,
                     then a 'noAccess' error must be returned.

                     When a set is received and the security model in use
                     is not USM, then a 'noAccess' error must be returned.
                    "
        DEFVAL      { ''H }    -- the empty string
        ::= { usmUserEntry 10 }

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    usmUserPublic    OBJECT-TYPE
        SYNTAX       OCTET STRING (SIZE(0..32))
        MAX-ACCESS   read-create
        STATUS       current
        DESCRIPTION "A publicly-readable value which can be written as part
                     of the procedure for changing a user's secret
                     authentication and/or privacy key, and later read to
                     determine whether the change of the secret was
                     effected.
                    "
        DEFVAL      { ''H }  -- the empty string
        ::= { usmUserEntry 11 }

    usmUserStorageType OBJECT-TYPE
        SYNTAX       StorageType
        MAX-ACCESS   read-create
        STATUS       current
        DESCRIPTION "The storage type for this conceptual row.

                     Conceptual rows having the value 'permanent' must
                     allow write-access at a minimum to:

                     - usmUserAuthKeyChange, usmUserOwnAuthKeyChange
                       and usmUserPublic for a user who employs
                       authentication, and
                     - usmUserPrivKeyChange, usmUserOwnPrivKeyChange
                       and usmUserPublic for a user who employs
                       privacy.

                     Note that any user who employs authentication or
                     privacy must allow its secret(s) to be updated and
                     thus cannot be 'readOnly'.

                     If an initial set operation tries to set the value to
                     'readOnly' for a user who employs authentication or
                     privacy, then an 'inconsistentValue' error must be
                     returned.  Note that if the value has been previously
                     set (implicit or explicit) to any value, then the rules
                     as defined in the StorageType Textual Convention apply.

                     It is an implementation issue to decide if a SET for
                     a readOnly or permanent row is accepted at all. In some
                     contexts this may make sense, in others it may not. If
                     a SET for a readOnly or permanent row is not accepted
                     at all, then a 'wrongValue' error must be returned.
                    "
        DEFVAL      { nonVolatile }
        ::= { usmUserEntry 12 }

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    RFC 2574                     USM for SNMPv3                   April 1999

    usmUserStatus    OBJECT-TYPE
        SYNTAX       RowStatus
        MAX-ACCESS   read-create
        STATUS       current
        DESCRIPTION "The status of this conceptual row.

                     Until instances of all corresponding columns are
                     appropriately configured, the value of the
                     corresponding instance of the usmUserStatus column
                     is 'notReady'.

                     In particular, a newly created row for a user who
                     employs authentication, cannot be made active until the
                     corresponding usmUserCloneFrom and usmUserAuthKeyChange
                     have been set.

                     Further, a newly created row for a user who also
                     employs privacy, cannot be made active until the
                     usmUserPrivKeyChange has been set.

                     The RowStatus TC [RFC2579] requires that this
                     DESCRIPTION clause states under which circumstances
                     other objects in this row can be modified:

                     The value of this object has no effect on whether
                     other objects in this conceptual row can be modified,
                     except for usmUserOwnAuthKeyChange and
                     usmUserOwnPrivKeyChange. For these 2 objects, the
                     value of usmUserStatus MUST be active.
                    "
        ::= { usmUserEntry 13 }

    -- Conformance Information *******************************************

    usmMIBCompliances OBJECT IDENTIFIER ::= { usmMIBConformance 1 }
    usmMIBGroups      OBJECT IDENTIFIER ::= { usmMIBConformance 2 }

    -- Compliance statements

    usmMIBCompliance MODULE-COMPLIANCE
        STATUS       current
        DESCRIPTION "The compliance statement for SNMP engines which
                     implement the SNMP-USER-BASED-SM-MIB.
                    "

        MODULE       -- this module
            MANDATORY-GROUPS { usmMIBBasicGroup }

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            OBJECT           usmUserAuthProtocol
            MIN-ACCESS       read-only
            DESCRIPTION     "Write access is not required."

            OBJECT           usmUserPrivProtocol
            MIN-ACCESS       read-only
            DESCRIPTION     "Write access is not required."

        ::= { usmMIBCompliances 1 }

    -- Units of compliance
    usmMIBBasicGroup OBJECT-GROUP
        OBJECTS     {
                      usmStatsUnsupportedSecLevels,
                      usmStatsNotInTimeWindows,
                      usmStatsUnknownUserNames,
                      usmStatsUnknownEngineIDs,
                      usmStatsWrongDigests,
                      usmStatsDecryptionErrors,
                      usmUserSpinLock,
                      usmUserSecurityName,
                      usmUserCloneFrom,
                      usmUserAuthProtocol,
                      usmUserAuthKeyChange,
                      usmUserOwnAuthKeyChange,
                      usmUserPrivProtocol,
                      usmUserPrivKeyChange,
                      usmUserOwnPrivKeyChange,
                      usmUserPublic,
                      usmUserStorageType,
                      usmUserStatus
                    }
        STATUS       current
        DESCRIPTION "A collection of objects providing for configuration
                     of an SNMP engine which implements the SNMP
                     User-based Security Model.
                    "
        ::= { usmMIBGroups 1 }

    END

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    6.  HMAC-MD5-96 Authentication Protocol

       This section describes the HMAC-MD5-96 authentication protocol.  This
       authentication protocol is the first defined for the User-based
       Security Model. It uses MD5 hash-function which is described in
       [MD5], in HMAC mode described in [RFC2104], truncating the output to
       96 bits.

       This protocol is identified by usmHMACMD5AuthProtocol.

       Over time, other authentication protocols may be defined either as a
       replacement of this protocol or in addition to this protocol.

    6.1.  Mechanisms

       - In support of data integrity, a message digest algorithm is
         required.  A digest is calculated over an appropriate portion of an
         SNMP message and included as part of the message sent to the
         recipient.

       - In support of data origin authentication and data integrity,
         a secret value is prepended to SNMP message prior to computing the
         digest; the calculated digest is partially inserted into the SNMP
         message prior to transmission, and the prepended value is not
         transmitted.  The secret value is shared by all SNMP engines
         authorized to originate messages on behalf of the appropriate user.

    6.1.1.  Digest Authentication Mechanism

       The Digest Authentication Mechanism defined in this memo provides
       for:

       - verification of the integrity of a received message, i.e., the
         message received is the message sent.

         The integrity of the message is protected by computing a digest
         over an appropriate portion of the message.  The digest is computed
         by the originator of the message, transmitted with the message, and
         verified by the recipient of the message.

       - verification of the user on whose behalf the message was generated.

         A secret value known only to SNMP engines authorized to generate
         messages on behalf of a user is used in HMAC mode (see [RFC2104]).
         It also recommends the hash-function output used as Message
         Authentication Code, to be truncated.

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       This protocol uses the MD5 [MD5] message digest algorithm.  A 128-bit
       MD5 digest is calculated in a special (HMAC) way over the designated
       portion of an SNMP message and the first 96 bits of this digest is
       included as part of the message sent to the recipient. The size of
       the digest carried in a message is 12 octets. The size of the private
       authentication key (the secret) is 16 octets. For the details see
       section 6.3.

    6.2.  Elements of the Digest Authentication Protocol

       This section contains definitions required to realize the
       authentication module defined in this section of this memo.

    6.2.1.  Users

       Authentication using this authentication protocol makes use of a
       defined set of userNames. For any user on whose behalf a message must
       be authenticated at a particular SNMP engine, that SNMP engine must
       have knowledge of that user. An SNMP engine that wishes to
       communicate with another SNMP engine must also have knowledge of a
       user known to that engine, including knowledge of the applicable
       attributes of that user.

       A user and its attributes are defined as follows:

       <userName>
         A string representing the name of the user.
       <authKey>
         A user's secret key to be used when calculating a digest.
         It MUST be 16 octets long for MD5.

    6.2.2.  msgAuthoritativeEngineID

       The msgAuthoritativeEngineID value contained in an authenticated
       message specifies the authoritative SNMP engine for that particular
       message (see the definition of SnmpEngineID in the SNMP Architecture
       document [RFC2571]).

       The user's (private) authentication key is normally different at each
       authoritative SNMP engine and so the snmpEngineID is used to select
       the proper key for the authentication process.

    6.2.3.  SNMP Messages Using this Authentication Protocol

       Messages using this authentication protocol carry a
       msgAuthenticationParameters field as part of the
       msgSecurityParameters.  For this protocol, the

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       msgAuthenticationParameters field is the serialized OCTET STRING
       representing the first 12 octets of the HMAC-MD5-96 output done over
       the wholeMsg.

       The digest is calculated over the wholeMsg so if a message is
       authenticated, that also means that all the fields in the message are
       intact and have not been tampered with.

    6.2.4.  Services provided by the HMAC-MD5-96 Authentication Module

       This section describes the inputs and outputs that the HMAC-MD5-96
       Authentication module expects and produces when the User-based
       Security module calls the HMAC-MD5-96 Authentication module for
       services.

    6.2.4.1.  Services for Generating an Outgoing SNMP Message

       The HMAC-MD5-96 authentication protocol assumes that the selection of
       the authKey is done by the caller and that the caller passes the
       secret key to be used.

       Upon completion the authentication module returns statusInformation
       and, if the message digest was correctly calculated, the wholeMsg
       with the digest inserted at the proper place. The abstract service
       primitive is:

       statusInformation =              -- success or failure
         authenticateOutgoingMsg(
         IN   authKey                   -- secret key for authentication
         IN   wholeMsg                  -- unauthenticated complete message
         OUT  authenticatedWholeMsg     -- complete authenticated message
              )

       The abstract data elements are:

         statusInformation
           An indication of whether the authentication process was
           successful.  If not it is an indication of the problem.
         authKey
           The secret key to be used by the authentication algorithm.
           The length of this key MUST be 16 octets.
         wholeMsg
           The message to be authenticated.
         authenticatedWholeMsg
           The authenticated message (including inserted digest) on output.

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       Note, that authParameters field is filled by the authentication
       module and this module and this field should be already present in
       the wholeMsg before the Message Authentication Code (MAC) is
       generated.

    6.2.4.2.  Services for Processing an Incoming SNMP Message

       The HMAC-MD5-96 authentication protocol assumes that the selection of
       the authKey is done by the caller and that the caller passes the
       secret key to be used.

       Upon completion the authentication module returns statusInformation
       and, if the message digest was correctly calculated, the wholeMsg as
       it was processed. The abstract service primitive is:

       statusInformation =              -- success or failure
         authenticateIncomingMsg(
         IN   authKey                   -- secret key for authentication
         IN   authParameters            -- as received on the wire
         IN   wholeMsg                  -- as received on the wire
         OUT  authenticatedWholeMsg     -- complete authenticated message
           )

       The abstract data elements are:

         statusInformation
           An indication of whether the authentication process was
           successful.  If not it is an indication of the problem.
         authKey
           The secret key to be used by the authentication algorithm.
           The length of this key MUST be 16 octets.
         authParameters
           The authParameters from the incoming message.
         wholeMsg
           The message to be authenticated on input and the authenticated
           message on output.
         authenticatedWholeMsg
           The whole message after the authentication check is complete.

    6.3.  Elements of Procedure

       This section describes the procedures for the HMAC-MD5-96
       authentication protocol.

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    6.3.1.  Processing an Outgoing Message

       This section describes the procedure followed by an SNMP engine
       whenever it must authenticate an outgoing message using the
       usmHMACMD5AuthProtocol.

       1) The msgAuthenticationParameters field is set to the serialization,
          according to the rules in [RFC1906], of an OCTET STRING containing
          12 zero octets.

       2) From the secret authKey, two keys K1 and K2 are derived:

             a) extend the authKey to 64 octets by appending 48 zero
                octets; save it as extendedAuthKey
             b) obtain IPAD by replicating the octet 0x36 64 times;
             c) obtain K1 by XORing extendedAuthKey with IPAD;
             d) obtain OPAD by replicating the octet 0x5C 64 times;
             e) obtain K2 by XORing extendedAuthKey with OPAD.

       3) Prepend K1 to the wholeMsg and calculate MD5 digest over it
          according to [MD5].

       4) Prepend K2 to the result of the step 4 and calculate MD5 digest
          over it according to [MD5]. Take the first 12 octets of the final
          digest - this is Message Authentication Code (MAC).

       5) Replace the msgAuthenticationParameters field with MAC obtained
          in the step 4.

       6) The authenticatedWholeMsg is then returned to the caller
          together with statusInformation indicating success.

    6.3.2.  Processing an Incoming Message

       This section describes the procedure followed by an SNMP engine
       whenever it must authenticate an incoming message using the
       usmHMACMD5AuthProtocol.

       1)  If the digest received in the msgAuthenticationParameters field
           is not 12 octets long, then an failure and an errorIndication
           (authenticationError) is returned to the calling module.

       2)  The MAC received in the msgAuthenticationParameters field
           is saved.

       3)  The digest in the msgAuthenticationParameters field is replaced
           by the 12 zero octets.

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       4)  From the secret authKey, two keys K1 and K2 are derived:

             a) extend the authKey to 64 octets by appending 48 zero
                octets; save it as extendedAuthKey
             b) obtain IPAD by replicating the octet 0x36 64 times;
             c) obtain K1 by XORing extendedAuthKey with IPAD;
             d) obtain OPAD by replicating the octet 0x5C 64 times;
             e) obtain K2 by XORing extendedAuthKey with OPAD.

       5)  The MAC is calculated over the wholeMsg:

             a) prepend K1 to the wholeMsg and calculate the MD5 digest
                over it;
             b) prepend K2 to the result of step 5.a and calculate the
                MD5 digest over it;
             c) first 12 octets of the result of step 5.b is the MAC.

           The msgAuthenticationParameters field is replaced with the MAC
           value that was saved in step 2.

       6)  Then the newly calculated MAC is compared with the MAC
           saved in step 2. If they do not match, then an failure and an
           errorIndication (authenticationFailure) is returned to the
           calling module.

       7)  The authenticatedWholeMsg and statusInformation indicating
           success are then returned to the caller.

    7.  HMAC-SHA-96 Authentication Protocol

       This section describes the HMAC-SHA-96 authentication protocol.  This
       protocol uses the SHA hash-function which is described in [SHA-NIST],
       in HMAC mode described in [RFC2104], truncating the output to 96
       bits.

       This protocol is identified by usmHMACSHAAuthProtocol.

       Over time, other authentication protocols may be defined either as a
       replacement of this protocol or in addition to this protocol.

    7.1.  Mechanisms

       - In support of data integrity, a message digest algorithm is
         required.  A digest is calculated over an appropriate portion of an
         SNMP message and included as part of the message sent to the
         recipient.

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       - In support of data origin authentication and data integrity,
         a secret value is prepended to the SNMP message prior to computing
         the digest; the calculated digest is then partially inserted into
         the message prior to transmission. The prepended secret is not
         transmitted.  The secret value is shared by all SNMP engines
         authorized to originate messages on behalf of the appropriate user.

    7.1.1.  Digest Authentication Mechanism

       The Digest Authentication Mechanism defined in this memo provides
       for:

       - verification of the integrity of a received message, i.e., the
         the message received is the message sent.

         The integrity of the message is protected by computing a digest
         over an appropriate portion of the message.  The digest is computed
         by the originator of the message, transmitted with the message, and
         verified by the recipient of the message.

       - verification of the user on whose behalf the message was generated.

         A secret value known only to SNMP engines authorized to generate
         messages on behalf of a user is used in HMAC mode (see [RFC2104]).
         It also recommends the hash-function output used as Message
         Authentication Code, to be truncated.

       This mechanism uses the SHA [SHA-NIST] message digest algorithm.  A
       160-bit SHA digest is calculated in a special (HMAC) way over the
       designated portion of an SNMP message and the first 96 bits of this
       digest is included as part of the message sent to the recipient. The
       size of the digest carried in a message is 12 octets. The size of the
       private authentication key (the secret) is 20 octets. For the details
       see section 7.3.

    7.2.  Elements of the HMAC-SHA-96 Authentication Protocol

       This section contains definitions required to realize the
       authentication module defined in this section of this memo.

    7.2.1.  Users

       Authentication using this authentication protocol makes use of a
       defined set of userNames.  For any user on whose behalf a message
       must be authenticated at a particular SNMP engine, that SNMP engine
       must have knowledge of that user.  An SNMP engine that wishes to

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    RFC 2574                     USM for SNMPv3                   April 1999

       communicate with another SNMP engine must also have knowledge of a
       user known to that engine, including knowledge of the applicable
       attributes of that user.

       A user and its attributes are defined as follows:

       <userName>
         A string representing the name of the user.
       <authKey>
         A user's secret key to be used when calculating a digest.
         It MUST be 20 octets long for SHA.

    7.2.2.  msgAuthoritativeEngineID

       The msgAuthoritativeEngineID value contained in an authenticated
       message specifies the authoritative SNMP engine for that particular
       message (see the definition of SnmpEngineID in the SNMP Architecture
       document [RFC2571]).

       The user's (private) authentication key is normally different at each
       authoritative SNMP engine and so the snmpEngineID is used to select
       the proper key for the authentication process.

    7.2.3.  SNMP Messages Using this Authentication Protocol

       Messages using this authentication protocol carry a
       msgAuthenticationParameters field as part of the
       msgSecurityParameters. For this protocol, the
       msgAuthenticationParameters field is the serialized OCTET STRING
       representing the first 12 octets of HMAC-SHA-96 output done over the
       wholeMsg.

       The digest is calculated over the wholeMsg so if a message is
       authenticated, that also means that all the fields in the message are
       intact and have not been tampered with.

    7.2.4.  Services provided by the HMAC-SHA-96 Authentication Module

       This section describes the inputs and outputs that the HMAC-SHA-96
       Authentication module expects and produces when the User-based
       Security module calls the HMAC-SHA-96 Authentication module for
       services.

    7.2.4.1.  Services for Generating an Outgoing SNMP Message

       HMAC-SHA-96 authentication protocol assumes that the selection of the
       authKey is done by the caller and that the caller passes the secret
       key to be used.

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       Upon completion the authentication module returns statusInformation
       and, if the message digest was correctly calculated, the wholeMsg
       with the digest inserted at the proper place. The abstract service
       primitive is:

       statusInformation =              -- success or failure
         authenticateOutgoingMsg(
         IN   authKey                   -- secret key for authentication
         IN   wholeMsg                  -- unauthenticated complete message
         OUT  authenticatedWholeMsg     -- complete authenticated message
              )

       The abstract data elements are:

         statusInformation
           An indication of whether the authentication process was
           successful.  If not it is an indication of the problem.
         authKey
           The secret key to be used by the authentication algorithm.
           The length of this key MUST be 20 octets.
         wholeMsg
           The message to be authenticated.
         authenticatedWholeMsg
           The authenticated message (including inserted digest) on output.

       Note, that authParameters field is filled by the authentication
       module and this field should be already present in the wholeMsg
       before the Message Authentication Code (MAC) is generated.

    7.2.4.2.  Services for Processing an Incoming SNMP Message

       HMAC-SHA-96 authentication protocol assumes that the selection of the
       authKey is done by the caller and that the caller passes the secret
       key to be used.

       Upon completion the authentication module returns statusInformation
       and, if the message digest was correctly calculated, the wholeMsg as
       it was processed. The abstract service primitive is:

       statusInformation =              -- success or failure
         authenticateIncomingMsg(
         IN   authKey                   -- secret key for authentication
         IN   authParameters            -- as received on the wire
         IN   wholeMsg                  -- as received on the wire
         OUT  authenticatedWholeMsg     -- complete authenticated message
           )

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       The abstract data elements are:

         statusInformation
           An indication of whether the authentication process was
           successful.  If not it is an indication of the problem.
         authKey
           The secret key to be used by the authentication algorithm.
           The length of this key MUST be 20 octets.
         authParameters
           The authParameters from the incoming message.
         wholeMsg
           The message to be authenticated on input and the authenticated
           message on output.
         authenticatedWholeMsg
           The whole message after the authentication check is complete.

    7.3.  Elements of Procedure

       This section describes the procedures for the HMAC-SHA-96
       authentication protocol.

    7.3.1.  Processing an Outgoing Message

       This section describes the procedure followed by an SNMP engine
       whenever it must authenticate an outgoing message using the
       usmHMACSHAAuthProtocol.

       1) The msgAuthenticationParameters field is set to the
          serialization, according to the rules in [RFC1906], of an OCTET
          STRING containing 12 zero octets.

       2) From the secret authKey, two keys K1 and K2 are derived:

             a) extend the authKey to 64 octets by appending 44 zero
                octets; save it as extendedAuthKey
             b) obtain IPAD by replicating the octet 0x36 64 times;
             c) obtain K1 by XORing extendedAuthKey with IPAD;
             d) obtain OPAD by replicating the octet 0x5C 64 times;
             e) obtain K2 by XORing extendedAuthKey with OPAD.

       3) Prepend K1 to the wholeMsg and calculate the SHA digest over it
          according to [SHA-NIST].

       4) Prepend K2 to the result of the step 4 and calculate SHA digest
          over it according to [SHA-NIST]. Take the first 12 octets of the
          final digest - this is Message Authentication Code (MAC).

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       5) Replace the msgAuthenticationParameters field with MAC obtained
          in the step 5.

       6) The authenticatedWholeMsg is then returned to the caller
          together with statusInformation indicating success.

    7.3.2.  Processing an Incoming Message

       This section describes the procedure followed by an SNMP engine
       whenever it must authenticate an incoming message using the
       usmHMACSHAAuthProtocol.

       1)  If the digest received in the msgAuthenticationParameters field
           is not 12 octets long, then an failure and an errorIndication
           (authenticationError) is returned to the calling module.

       2)  The MAC received in the msgAuthenticationParameters field
           is saved.

       3)  The digest in the msgAuthenticationParameters field is
           replaced by the 12 zero octets.

       4)  From the secret authKey, two keys K1 and K2 are derived:

             a) extend the authKey to 64 octets by appending 44 zero
                octets; save it as extendedAuthKey
             b) obtain IPAD by replicating the octet 0x36 64 times;
             c) obtain K1 by XORing extendedAuthKey with IPAD;
             d) obtain OPAD by replicating the octet 0x5C 64 times;
             e) obtain K2 by XORing extendedAuthKey with OPAD.

       5)  The MAC is calculated over the wholeMsg:

             a) prepend K1 to the wholeMsg and calculate the SHA digest
                over it;
             b) prepend K2 to the result of step 5.a and calculate the
                SHA digest over it;
             c) first 12 octets of the result of step 5.b is the MAC.

           The msgAuthenticationParameters field is replaced with the MAC
           value that was saved in step 2.

       6)  The the newly calculated MAC is compared with the MAC saved in
           step 2. If they do not match, then a failure and an
           errorIndication (authenticationFailure) are returned to the
           calling module.

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       7)  The authenticatedWholeMsg and statusInformation indicating
           success are then returned to the caller.

    8.  CBC-DES Symmetric Encryption Protocol

       This section describes the CBC-DES Symmetric Encryption Protocol.
       This protocol is the first privacy protocol defined for the User-
       based Security Model.

       This protocol is identified by usmDESPrivProtocol.

       Over time, other privacy protocols may be defined either as a
       replacement of this protocol or in addition to this protocol.

    8.1.  Mechanisms

       - In support of data confidentiality, an encryption algorithm is
         required.  An appropriate portion of the message is encrypted prior
         to being transmitted. The User-based Security Model specifies that
         the scopedPDU is the portion of the message that needs to be
         encrypted.

       - A secret value in combination with a timeliness value is used
         to create the en/decryption key and the initialization vector.  The
         secret value is shared by all SNMP engines authorized to originate
         messages on behalf of the appropriate user.

    8.1.1.  Symmetric Encryption Protocol

       The Symmetric Encryption Protocol defined in this memo provides
       support for data confidentiality.  The designated portion of an SNMP
       message is encrypted and included as part of the message sent to the
       recipient.

       Two organizations have published specifications defining the DES:
       the National Institute of Standards and Technology (NIST) [DES-NIST]
       and the American National Standards Institute [DES-ANSI].  There is a
       companion Modes of Operation specification for each definition
       ([DESO-NIST] and [DESO-ANSI], respectively).

       The NIST has published three additional documents that implementors
       may find useful.

       - There is a document with guidelines for implementing and using
         the DES, including functional specifications for the DES and its
         modes of operation [DESG-NIST].

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       - There is a specification of a validation test suite for the DES
         [DEST-NIST].  The suite is designed to test all aspects of the DES
         and is useful for pinpointing specific problems.

       - There is a specification of a maintenance test for the DES
         [DESM-NIST].  The test utilizes a minimal amount of data and
         processing to test all components of the DES.  It provides a simple
         yes-or-no indication of correct operation and is useful to run as
         part of an initialization step, e.g., when a computer re-boots.

    8.1.1.1.  DES key and Initialization Vector.

       The first 8 octets of the 16-octet secret (private privacy key) are
       used as a DES key.  Since DES uses only 56 bits, the Least
       Significant Bit in each octet is disregarded.

       The Initialization Vector for encryption is obtained using the
       following procedure.

       The last 8 octets of the 16-octet secret (private privacy key) are
       used as pre-IV.

       In order to ensure that the IV for two different packets encrypted by
       the same key, are not the same (i.e., the IV does not repeat) we need
       to "salt" the pre-IV with something unique per packet.  An 8-octet
       string is used as the "salt".  The concatenation of the generating
       SNMP engine's 32-bit snmpEngineBoots and a local 32-bit integer, that
       the encryption engine maintains, is input to the "salt".  The 32-bit
       integer is initialized to an arbitrary value at boot time.

       The 32-bit snmpEngineBoots is converted to the first 4 octets (Most
       Significant Byte first) of our "salt".  The 32-bit integer is then
       converted to the last 4 octet (Most Significant Byte first) of our
       "salt".  The resulting "salt" is then XOR-ed with the pre-IV to
       obtain the IV.  The 8-octet "salt" is then put into the
       privParameters field encoded as an OCTET STRING.  The "salt" integer
       is then modified.  We recommend that it be incremented by one and
       wrap when it reaches the maximum value.

       How exactly the value of the "salt" (and thus of the IV) varies, is
       an implementation issue, as long as the measures are taken to avoid
       producing a duplicate IV.

       The "salt" must be placed in the privParameters field to enable the
       receiving entity to compute the correct IV and to decrypt the
       message.

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    8.1.1.2.  Data Encryption.

       The data to be encrypted is treated as sequence of octets. Its length
       should be an integral multiple of 8 - and if it is not, the data is
       padded at the end as necessary.  The actual pad value is irrelevant.

       The data is encrypted in Cipher Block Chaining mode.

       The plaintext is divided into 64-bit blocks.

       The plaintext for each block is XOR-ed with the ciphertext of the
       previous block, the result is encrypted and the output of the
       encryption is the ciphertext for the block.  This procedure is
       repeated until there are no more plaintext blocks.

       For the very first block, the Initialization Vector is used instead
       of the ciphertext of the previous block.

    8.1.1.3.  Data Decryption

       Before decryption, the encrypted data length is verified.  If the
       length of the OCTET STRING to be decrypted is not an integral
       multiple of 8 octets, the decryption process is halted and an
       appropriate exception noted.  When decrypting, the padding is
       ignored.

       The first ciphertext block is decrypted, the decryption output is
       XOR-ed with the Initialization Vector, and the result is the first
       plaintext block.

       For each subsequent block, the ciphertext block is decrypted, the
       decryption output is XOR-ed with the previous ciphertext block and
       the result is the plaintext block.

    8.2.  Elements of the DES Privacy Protocol

       This section contains definitions required to realize the privacy
       module defined by this memo.

    8.2.1.  Users

       Data en/decryption using this Symmetric Encryption Protocol makes use
       of a defined set of userNames.  For any user on whose behalf a
       message must be en/decrypted at a particular SNMP engine, that SNMP
       engine must have knowledge of that user.  An SNMP engine that wishes
       to communicate with another SNMP engine must also have knowledge of a
       user known to that SNMP engine, including knowledge of the applicable
       attributes of that user.

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       A user and its attributes are defined as follows:

       <userName>
         An octet string representing the name of the user.

       <privKey>
         A user's secret key to be used as input for the DES key and IV.
         The length of this key MUST be 16 octets.

    8.2.2.  msgAuthoritativeEngineID

       The msgAuthoritativeEngineID value contained in an authenticated
       message specifies the authoritative SNMP engine for that particular
       message (see the definition of SnmpEngineID in the SNMP Architecture
       document [RFC2571]).

       The user's (private) privacy key is normally different at each
       authoritative SNMP engine and so the snmpEngineID is used to select
       the proper key for the en/decryption process.

    8.2.3.  SNMP Messages Using this Privacy Protocol

       Messages using this privacy protocol carry a msgPrivacyParameters
       field as part of the msgSecurityParameters. For this protocol, the
       msgPrivacyParameters field is the serialized OCTET STRING
       representing the "salt" that was used to create the IV.

    8.2.4.  Services provided by the DES Privacy Module

       This section describes the inputs and outputs that the DES Privacy
       module expects and produces when the User-based Security module
       invokes the DES Privacy module for services.

    8.2.4.1.  Services for Encrypting Outgoing Data

       This DES privacy protocol assumes that the selection of the privKey
       is done by the caller and that the caller passes the secret key to be
       used.

       Upon completion the privacy module returns statusInformation and, if
       the encryption process was successful, the encryptedPDU and the
       msgPrivacyParameters encoded as an OCTET STRING.  The abstract
       service primitive is:

       statusInformation =              -- success of failure
         encryptData(
         IN    encryptKey               -- secret key for encryption
         IN    dataToEncrypt            -- data to encrypt (scopedPDU)

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         OUT   encryptedData            -- encrypted data (encryptedPDU)
         OUT   privParameters           -- filled in by service provider
               )

       The abstract data elements are:

         statusInformation
           An indication of the success or failure of the encryption
           process.  In case of failure, it is an indication of the error.
         encryptKey
           The secret key to be used by the encryption algorithm.
           The length of this key MUST be 16 octets.
         dataToEncrypt
           The data that must be encrypted.
         encryptedData
           The encrypted data upon successful completion.
         privParameters
           The privParameters encoded as an OCTET STRING.

    8.2.4.2.  Services for Decrypting Incoming Data

       This DES privacy protocol assumes that the selection of the privKey
       is done by the caller and that the caller passes the secret key to be
       used.

       Upon completion the privacy module returns statusInformation and, if
       the decryption process was successful, the scopedPDU in plain text.
       The abstract service primitive is:

       statusInformation =
         decryptData(
         IN    decryptKey               -- secret key for decryption
         IN    privParameters           -- as received on the wire
         IN    encryptedData            -- encrypted data (encryptedPDU)
         OUT   decryptedData            -- decrypted data (scopedPDU)
               )

       The abstract data elements are:

         statusInformation
           An indication whether the data was successfully decrypted
           and if not an indication of the error.
         decryptKey
           The secret key to be used by the decryption algorithm.
           The length of this key MUST be 16 octets.
         privParameters
           The "salt" to be used to calculate the IV.

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         encryptedData
           The data to be decrypted.
         decryptedData
           The decrypted data.

    8.3.  Elements of Procedure.

       This section describes the procedures for the DES privacy protocol.

    8.3.1.  Processing an Outgoing Message

       This section describes the procedure followed by an SNMP engine
       whenever it must encrypt part of an outgoing message using the
       usmDESPrivProtocol.

       1)  The secret cryptKey is used to construct the DES encryption key,
           the "salt" and the DES pre-IV (from which the IV is computed as
           described in section 8.1.1.1).

       2)  The privParameters field is set to the serialization according
           to the rules in [RFC1906] of an OCTET STRING representing the the
           "salt" string.

       3)  The scopedPDU is encrypted (as described in section 8.1.1.2)
           and the encrypted data is serialized according to the rules in
           [RFC1906] as an OCTET STRING.

       4)  The serialized OCTET STRING representing the encrypted
           scopedPDU together with the privParameters and statusInformation
           indicating success is returned to the calling module.

    8.3.2.  Processing an Incoming Message

       This section describes the procedure followed by an SNMP engine
       whenever it must decrypt part of an incoming message using the
       usmDESPrivProtocol.

       1)  If the privParameters field is not an 8-octet OCTET STRING,
           then an error indication (decryptionError) is returned to the
           calling module.

       2)  The "salt" is extracted from the privParameters field.

       3)  The secret cryptKey and the "salt" are then used to construct the
           DES decryption key and pre-IV (from which the IV is computed as
           described in section 8.1.1.1).

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       4)  The encryptedPDU is then decrypted (as described in
           section 8.1.1.3).

       5)  If the encryptedPDU cannot be decrypted, then an error
           indication (decryptionError) is returned to the calling module.

       6)  The decrypted scopedPDU and statusInformation indicating
           success are returned to the calling module.

    9.  Intellectual Property

       The IETF takes no position regarding the validity or scope of any
       intellectual property or other rights that might be claimed to
       pertain to the implementation or use of the technology described in
       this document or the extent to which any license under such rights
       might or might not be available; neither does it represent that it
       has made any effort to identify any such rights.  Information on the
       IETF's procedures with respect to rights in standards-track and
       standards-related documentation can be found in BCP-11.  Copies of
       claims of rights made available for publication and any assurances of
       licenses to be made available, or the result of an attempt made to
       obtain a general license or permission for the use of such
       proprietary rights by implementors or users of this specification can
       be obtained from the IETF Secretariat.

       The IETF invites any interested party to bring to its attention any
       copyrights, patents or patent applications, or other proprietary
       rights which may cover technology that may be required to practice
       this standard.  Please address the information to the IETF Executive
       Director.

    10.  Acknowledgements

       This document is the result of the efforts of the SNMPv3 Working
       Group.  Some special thanks are in order to the following SNMPv3 WG
       members:

          Harald Tveit Alvestrand (Maxware)
          Dave Battle (SNMP Research, Inc.)
          Alan Beard (Disney Worldwide Services)
          Paul Berrevoets (SWI Systemware/Halcyon Inc.)
          Martin Bjorklund (Ericsson)
          Uri Blumenthal (IBM T.J. Watson Research Center)
          Jeff Case (SNMP Research, Inc.)
          John Curran (BBN)
          Mike Daniele (Compaq Computer Corporation))
          T. Max Devlin (Eltrax Systems)
          John Flick (Hewlett Packard)

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          Rob Frye (MCI)
          Wes Hardaker (U.C.Davis, Information Technology - D.C.A.S.)
          David Harrington (Cabletron Systems Inc.)
          Lauren Heintz (BMC Software, Inc.)
          N.C. Hien (IBM T.J. Watson Research Center)
          Michael Kirkham (InterWorking Labs, Inc.)
          Dave Levi (SNMP Research, Inc.)
          Louis A Mamakos (UUNET Technologies Inc.)
          Joe Marzot (Nortel Networks)
          Paul Meyer (Secure Computing Corporation)
          Keith McCloghrie (Cisco Systems)
          Bob Moore (IBM)
          Russ Mundy (TIS Labs at Network Associates)
          Bob Natale (ACE*COMM Corporation)
          Mike O'Dell (UUNET Technologies Inc.)
          Dave Perkins (DeskTalk)
          Peter Polkinghorne (Brunel University)
          Randy Presuhn (BMC Software, Inc.)
          David Reeder (TIS Labs at Network Associates)
          David Reid (SNMP Research, Inc.)
          Aleksey Romanov (Quality Quorum)
          Shawn Routhier (Epilogue)
          Juergen Schoenwaelder (TU Braunschweig)
          Bob Stewart (Cisco Systems)
          Mike Thatcher (Independent Consultant)
          Bert Wijnen (IBM T.J. Watson Research Center)

       The document is based on recommendations of the IETF Security and
       Administrative Framework Evolution for SNMP Advisory Team.  Members
       of that Advisory Team were:

          David Harrington (Cabletron Systems Inc.)
          Jeff Johnson (Cisco Systems)
          David Levi (SNMP Research Inc.)
          John Linn (Openvision)
          Russ Mundy (Trusted Information Systems) chair
          Shawn Routhier (Epilogue)
          Glenn Waters (Nortel)
          Bert Wijnen (IBM T. J. Watson Research Center)

       As recommended by the Advisory Team and the SNMPv3 Working Group
       Charter, the design incorporates as much as practical from previous
       RFCs and drafts. As a result, special thanks are due to the authors
       of previous designs known as SNMPv2u and SNMPv2*:

          Jeff Case (SNMP Research, Inc.)
          David Harrington (Cabletron Systems Inc.)
          David Levi (SNMP Research, Inc.)

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          Keith McCloghrie (Cisco Systems)
          Brian O'Keefe (Hewlett Packard)
          Marshall T. Rose (Dover Beach Consulting)
          Jon Saperia (BGS Systems Inc.)
          Steve Waldbusser (International Network Services)
          Glenn W. Waters (Bell-Northern Research Ltd.)

    11.  Security Considerations

    11.1.  Recommended Practices

       This section describes practices that contribute to the secure,
       effective operation of the mechanisms defined in this memo.

       - An SNMP engine must discard SNMP Response messages that do not
         correspond to any currently outstanding Request message. It is the
         responsibility of the Message Processing module to take care of
         this. For example it can use a msgID for that.

         An SNMP Command Generator Application must discard any Response
         Class PDU for which there is no currently outstanding Confirmed
         Class PDU; for example for SNMPv2 [RFC1905] PDUs, the request-id
         component in the PDU can be used to correlate Responses to
         outstanding Requests.

         Although it would be typical for an SNMP engine and an SNMP Command
         Generator Application to do this as a matter of course, when using
         these security protocols it is significant due to the possibility
         of message duplication (malicious or otherwise).

       - If an SNMP engine uses a msgID for correlating Response messages
         to outstanding Request messages, then it MUST use different msgIDs
         in all such Request messages that it sends out during a Time Window
         (150 seconds) period.

         A Command Generator or Notification Originator Application MUST use
         different request-ids in all Request PDUs that it sends out during
         a TimeWindow (150 seconds) period.

         This must be done to protect against the possibility of message
         duplication (malicious or otherwise).

         For example, starting operations with a msgID and/or request-id
         value of zero is not a good idea.  Initializing them with an
         unpredictable number (so they do not start out the same after each
         reboot) and then incrementing by one would be acceptable.

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       - An SNMP engine should perform time synchronization using
         authenticated messages in order to protect against the possibility
         of message duplication (malicious or otherwise).

       - When sending state altering messages to a managed authoritative
         SNMP engine, a Command Generator Application should delay sending
         successive messages to that managed SNMP engine until a positive
         acknowledgement is received for the previous message or until the
         previous message expires.

         No message ordering is imposed by the SNMP. Messages may be
         received in any order relative to their time of generation and each
         will be processed in the ordered received.  Note that when an
         authenticated message is sent to a managed SNMP engine, it will be
         valid for a period of time of approximately 150 seconds under
         normal circumstances, and is subject to replay during this period.
         Indeed, an SNMP engine and SNMP Command Generator Applications must
         cope with the loss and re-ordering of messages resulting from
         anomalies in the network as a matter of course.

         However, a managed object, snmpSetSerialNo [RFC1907], is
         specifically defined for use with SNMP Set operations in order to
         provide a mechanism to ensure that the processing of SNMP messages
         occurs in a specific order.

       - The frequency with which the secrets of a User-based Security
         Model user should be changed is indirectly related to the frequency
         of their use.

         Protecting the secrets from disclosure is critical to the overall
         security of the protocols.  Frequent use of a secret provides a
         continued source of data that may be useful to a cryptanalyst in
         exploiting known or perceived weaknesses in an algorithm.  Frequent
         changes to the secret avoid this vulnerability.

         Changing a secret after each use is generally regarded as the most
         secure practice, but a significant amount of overhead may be
         associated with that approach.

         Note, too, in a local environment the threat of disclosure may be
         less significant, and as such the changing of secrets may be less
         frequent.  However, when public data networks are used as the
         communication paths, more caution is prudent.

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    11.2  Defining Users

       The mechanisms defined in this document employ the notion of users on
       whose behalf messages are sent.  How "users" are defined is subject
       to the security policy of the network administration.  For example,
       users could be individuals (e.g., "joe" or "jane"), or a particular
       role (e.g., "operator" or "administrator"), or a combination (e.g.,
       "joe-operator", "jane-operator" or "joe-admin").  Furthermore, a user
       may be a logical entity, such as an SNMP Application or a set of SNMP
       Applications, acting on behalf of an individual or role, or set of
       individuals, or set of roles, including combinations.

       Appendix A describes an algorithm for mapping a user "password" to a
       16/20 octet value for use as either a user's authentication key or
       privacy key (or both).  Note however, that using the same password
       (and therefore the same key) for both authentication and privacy is
       very poor security practice and should be strongly discouraged.
       Passwords are often generated, remembered, and input by a human.
       Human-generated passwords may be less than the 16/20 octets required
       by the authentication and privacy protocols, and brute force attacks
       can be quite easy on a relatively short ASCII character set.
       Therefore, the algorithm is Appendix A performs a transformation on
       the password.  If the Appendix A algorithm is used, SNMP
       implementations (and SNMP configuration applications) must ensure
       that passwords are at least 8 characters in length.  Please note that
       longer passwords with repetitive strings may result in exactly the
       same key. For example, a password 'bertbert' will result in exactly
       the same key as password 'bertbertbert'.

       Because the Appendix A algorithm uses such passwords (nearly)
       directly, it is very important that they not be easily guessed.  It
       is suggested that they be composed of mixed-case alphanumeric and
       punctuation characters that don't form words or phrases that might be
       found in a dictionary.  Longer passwords improve the security of the
       system.  Users may wish to input multiword phrases to make their
       password string longer while ensuring that it is memorable.

       Since it is infeasible for human users to maintain different
       passwords for every SNMP engine, but security requirements strongly
       discourage having the same key for more than one SNMP engine, the
       User-based Security Model employs a compromise proposed in
       [Localized-key].  It derives the user keys for the SNMP engines from
       user's password in such a way that it is practically impossible to
       either determine the user's password, or user's key for another SNMP
       engine from any combination of user's keys on SNMP engines.

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       Note however, that if user's password is disclosed, then key
       localization will not help and network security may be compromised in
       this case. Therefore a user's password or non-localized key MUST NOT
       be stored on a managed device/node. Instead the localized key SHALL
       be stored (if at all) , so that, in case a device does get
       compromised, no other managed or managing devices get compromised.

    11.3.  Conformance

       To be termed a "Secure SNMP implementation" based on the User-based
       Security Model, an SNMP implementation MUST:

       - implement one or more Authentication Protocol(s). The HMAC-MD5-96
         and HMAC-SHA-96 Authentication Protocols defined in this memo are
         examples of such protocols.

       - to the maximum extent possible, prohibit access to the secret(s)
         of each user about which it maintains information in a Local
         Configuration Datastore (LCD) under all circumstances except as
         required to generate and/or validate SNMP messages with respect to
         that user.

       - implement the key-localization mechanism.

       - implement the SNMP-USER-BASED-SM-MIB.

       In addition, an authoritative SNMP engine SHOULD provide initial
       configuration in accordance with Appendix A.1.

       Implementation of a Privacy Protocol (the DES Symmetric Encryption
       Protocol defined in this memo is one such protocol) is optional.

    11.4.  Use of Reports

       The use of unsecure reports (i.e. sending them with a securityLevel
       of noAuthNoPriv) potentially exposes a non-authoritative SNMP engine
       to some form of attacks.  Some people consider these denial of
       service attacks, others don't.  An installation should evaluate the
       risk involved before deploying unsecure Report PDUs.

    11.5  Access to the SNMP-USER-BASED-SM-MIB

       The objects in this MIB may be considered sensitive in many
       environments. Specifically the objects in the usmUserTable contain
       information about users and their authentication and privacy
       protocols.  It is important to closely control (both read and write)

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       access to these MIB objects by using appropriately configured Access
       Control models (for example the View-based Access Control Model as
       specified in [RFC2575]).

    12.  References

       [RFC1321]       Rivest, R.,  "Message Digest Algorithm MD5", RFC
                       1321, April 1992.

       [RFC2579]       McCloghrie, K., Perkins, D. and J. Schoenwaelder,
                       "Textual Conventions for SMIv2", STD 58, RFC 2579,
                       April 1999.

       [RFC1905]       Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
                       "Protocol Operations for Version 2 of the Simple
                       Network Management Protocol (SNMPv2)", RFC 1905,
                       January 1996.

       [RFC1906]       Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
                       "Transport Mappings for Version 2 of the Simple
                       Network Management Protocol (SNMPv2)", RFC 1906,
                       January 1996.

       [RFC1907]       Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
                       "Management Information Base for Version 2 of the
                       Simple Network Management Protocol (SNMPv2)", RFC
                       1907 January 1996.

       [RFC2104]       Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
                       Keyed-Hashing  for Message Authentication", RFC 2104,
                       February 1997.

       [RFC2119]       Bradner, S., "Key words for use in RFCs to Indicate
                       Requirement Levels", BCP 14, RFC 2119, March 1997.

       [RFC2571]       Harrington, D., Presuhn, R. and B. Wijnen, "An
                       Architecture for describing SNMP Management
                       Frameworks", RFC 2571, April 1999.

       [RFC2572]       Case, J., Harrington, D., Presuhn, R. and B. Wijnen,
                       "Message Processing and Dispatching for the Simple
                       Network Management Protocol (SNMP)", RFC 2572, April
                       1999.

       [RF2575]        Wijnen, B., Presuhn, R. and K. McCloghrie, "View-
                       based Access Control Model for the Simple Network
                       Management Protocol (SNMP)", RFC 2575, April 1999.

    Blumenthal & Wijnen         Standards Track                    [Page 73]

    RFC 2574                     USM for SNMPv3                   April 1999

       [Localized-Key] U. Blumenthal, N. C. Hien, B. Wijnen "Key Derivation
                       for Network Management Applications" IEEE Network
                       Magazine, April/May issue, 1997.

       [DES-NIST]      Data Encryption Standard, National Institute of
                       Standards and Technology.  Federal Information
                       Processing Standard (FIPS) Publication 46-1.
                       Supersedes FIPS Publication 46, (January, 1977;
                       reaffirmed January, 1988).

       [DES-ANSI]      Data Encryption Algorithm, American National
                       Standards Institute.  ANSI X3.92-1981, (December,
                       1980).

       [DESO-NIST]     DES Modes of Operation, National Institute of
                       Standards and Technology.  Federal Information
                       Processing Standard (FIPS) Publication 81, (December,
                       1980).

       [DESO-ANSI]     Data Encryption Algorithm - Modes of Operation,
                       American National Standards Institute.  ANSI X3.106-
                       1983, (May 1983).

       [DESG-NIST]     Guidelines for Implementing and Using the NBS Data
                       Encryption Standard, National Institute of Standards
                       and Technology.  Federal Information Processing
                       Standard (FIPS) Publication 74, (April, 1981).

       [DEST-NIST]     Validating the Correctness of Hardware
                       Implementations of the NBS Data Encryption Standard,
                       National Institute of Standards and Technology.
                       Special Publication 500-20.

       [DESM-NIST]     Maintenance Testing for the Data Encryption Standard,
                       National Institute of Standards and Technology.
                       Special Publication 500-61, (August, 1980).

       [SHA-NIST]      Secure Hash Algorithm. NIST FIPS 180-1, (April, 1995)
                       http://csrc.nist.gov/fips/fip180-1.txt (ASCII)
                       http://csrc.nist.gov/fips/fip180-1.ps  (Postscript)

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    RFC 2574                     USM for SNMPv3                   April 1999

    13.  Editors' Addresses

       Uri Blumenthal
       IBM T. J. Watson Research
       30 Saw Mill River Pkwy,
       Hawthorne, NY 10532
       USA

       Phone:      +1-914-784-7064
       EMail:      uri@watson.ibm.com

       Bert Wijnen
       IBM T. J. Watson Research
       Schagen 33
       3461 GL Linschoten
       Netherlands

       Phone:      +31-348-432-794
       EMail:      wijnen@vnet.ibm.com

    Blumenthal & Wijnen         Standards Track                    [Page 75]

    RFC 2574                     USM for SNMPv3                   April 1999

    APPENDIX A - Installation

    A.1.  SNMP engine Installation Parameters

       During installation, an authoritative SNMP engine SHOULD (in the
       meaning as defined in [RFC2119]) be configured with several initial
       parameters.  These include:

       1) A security posture

          The choice of security posture determines if initial configuration
          is implemented and if so how.  One of three possible choices is
          selected:

                minimum-secure,
                semi-secure,
                very-secure (i.e., no-initial-configuration)

          In the case of a very-secure posture, there is no initial
          configuration, and so the following steps are irrelevant.

    2) one or more secrets

       These are the authentication/privacy secrets for the first user to be
       configured.

       One way to accomplish this is to have the installer enter a
       "password" for each required secret. The password is then
       algorithmically converted into the required secret by:

       - forming a string of length 1,048,576 octets by repeating the
         value of the password as often as necessary, truncating
         accordingly, and using the resulting string as the input to the MD5
         algorithm [MD5].  The resulting digest, termed "digest1", is used
         in the next step.

       - a second string is formed by concatenating digest1, the SNMP
         engine's snmpEngineID value, and digest1.  This string is used as
         input to the MD5 algorithm [MD5].

         The resulting digest is the required secret (see Appendix A.2).

       With these configured parameters, the SNMP engine instantiates the
       following usmUserEntry in the usmUserTable:

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    RFC 2574                     USM for SNMPv3                   April 1999

                               no privacy support     privacy support
                               ------------------     ---------------
       usmUserEngineID         localEngineID          localEngineID
       usmUserName             "initial"              "initial"
       usmUserSecurityName     "initial"              "initial"
       usmUserCloneFrom        ZeroDotZero            ZeroDotZero
       usmUserAuthProtocol     usmHMACMD5AuthProtocol usmHMACMD5AuthProtocol
       usmUserAuthKeyChange    ""                     ""
       usmUserOwnAuthKeyChange ""                     ""
       usmUserPrivProtocol     none                   usmDESPrivProtocol
       usmUserPrivKeyChange    ""                     ""
       usmUserOwnPrivKeyChange ""                     ""
       usmUserPublic           ""                     ""
       usmUserStorageType      anyValidStorageType    anyValidStorageType
       usmUserStatus           active                 active

       It is recommended to also instantiate a set of template
       usmUserEntries which can be used as clone-from users for newly
       created usmUserEntries.  These are the two suggested entries:

                               no privacy support     privacy support
                               ------------------     ---------------
       usmUserEngineID         localEngineID          localEngineID
       usmUserName             "templateMD5"          "templateMD5"
       usmUserSecurityName     "templateMD5"          "templateMD5"
       usmUserCloneFrom        ZeroDotZero            ZeroDotZero
       usmUserAuthProtocol     usmHMACMD5AuthProtocol usmHMACMD5AuthProtocol
       usmUserAuthKeyChange    ""                     ""
       usmUserOwnAuthKeyChange ""                     ""
       usmUserPrivProtocol     none                   usmDESPrivProtocol
       usmUserPrivKeyChange    ""                     ""
       usmUserOwnPrivKeyChange ""                     ""
       usmUserPublic           ""                     ""
       usmUserStorageType      permanent              permanent
       usmUserStatus           active                 active

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    RFC 2574                     USM for SNMPv3                   April 1999

                               no privacy support     privacy support
                               ------------------     ---------------
       usmUserEngineID         localEngineID          localEngineID
       usmUserName             "templateSHA"          "templateSHA"
       usmUserSecurityName     "templateSHA"          "templateSHA"
       usmUserCloneFrom        ZeroDotZero            ZeroDotZero
       usmUserAuthProtocol     usmHMACSHAAuthProtocol usmHMACSHAAuthProtocol
       usmUserAuthKeyChange    ""                     ""
       usmUserOwnAuthKeyChange ""                     ""
       usmUserPrivProtocol     none                   usmDESPrivProtocol
       usmUserPrivKeyChange    ""                     ""
       usmUserOwnPrivKeyChange ""                     ""
       usmUserPublic           ""                     ""
       usmUserStorageType      permanent              permanent
       usmUserStatus           active                 active

    A.2.  Password to Key Algorithm

       A sample code fragment (section A.2.1) demonstrates the password to
       key algorithm which can be used when mapping a password to an
       authentication or privacy key using MD5. The reference source code
       of MD5 is available in [RFC1321].

       Another sample code fragment (section A.2.2) demonstrates the
       password to key algorithm which can be used when mapping a password
       to an authentication or privacy key using SHA (documented in
       SHA-NIST).

       An example of the results of a correct implementation is provided
       (section A.3) which an implementor can use to check if his
       implementation produces the same result.

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    RFC 2574                     USM for SNMPv3                   April 1999

    A.2.1.  Password to Key Sample Code for MD5

    void password_to_key_md5(
       u_char *password,    /* IN */
       u_int   passwordlen, /* IN */
       u_char *engineID,    /* IN  - pointer to snmpEngineID  */
       u_int   engineLength,/* IN  - length of snmpEngineID */
       u_char *key)         /* OUT - pointer to caller 16-octet buffer */
    {
       MD5_CTX     MD;
       u_char     *cp, password_buf[64];
       u_long      password_index = 0;
       u_long      count = 0, i;

       MD5Init (&MD);   /* initialize MD5 */

       /**********************************************/
       /* Use while loop until we've done 1 Megabyte */
       /**********************************************/
       while (count < 1048576) {
          cp = password_buf;
          for (i = 0; i < 64; i++) {
              /*************************************************/
              /* Take the next octet of the password, wrapping */
              /* to the beginning of the password as necessary.*/
              /*************************************************/
              *cp++ = password[password_index++ % passwordlen];
          }
          MD5Update (&MD, password_buf, 64);
          count += 64;
       }
       MD5Final (key, &MD);          /* tell MD5 we're done */

       /*****************************************************/
       /* Now localize the key with the engineID and pass   */
       /* through MD5 to produce final key                  */
       /* May want to ensure that engineLength <= 32,       */
       /* otherwise need to use a buffer larger than 64     */
       /*****************************************************/
       memcpy(password_buf, key, 16);
       memcpy(password_buf+16, engineID, engineLength);
       memcpy(password_buf+16+engineLength, key, 16);

       MD5Init(&MD);
       MD5Update(&MD, password_buf, 32+engineLength);
       MD5Final(key, &MD);
       return;
    }

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    A.2.2.  Password to Key Sample Code for SHA

    void password_to_key_sha(
       u_char *password,    /* IN */
       u_int   passwordlen, /* IN */
       u_char *engineID,    /* IN  - pointer to snmpEngineID  */
       u_int   engineLength,/* IN  - length of snmpEngineID */
       u_char *key)         /* OUT - pointer to caller 20-octet buffer */
    {
       SHA_CTX     SH;
       u_char     *cp, password_buf[72];
       u_long      password_index = 0;
       u_long      count = 0, i;

       SHAInit (&SH);   /* initialize SHA */

       /**********************************************/
       /* Use while loop until we've done 1 Megabyte */
       /**********************************************/
       while (count < 1048576) {
          cp = password_buf;
          for (i = 0; i < 64; i++) {
              /*************************************************/
              /* Take the next octet of the password, wrapping */
              /* to the beginning of the password as necessary.*/
              /*************************************************/
              *cp++ = password[password_index++ % passwordlen];
          }
          SHAUpdate (&SH, password_buf, 64);
          count += 64;
       }
       SHAFinal (key, &SH);          /* tell SHA we're done */

       /*****************************************************/
       /* Now localize the key with the engineID and pass   */
       /* through SHA to produce final key                  */
       /* May want to ensure that engineLength <= 32,       */
       /* otherwise need to use a buffer larger than 72     */
       /*****************************************************/
       memcpy(password_buf, key, 20);
       memcpy(password_buf+20, engineID, engineLength);
       memcpy(password_buf+20+engineLength, key, 20);

       SHAInit(&SH);
       SHAUpdate(&SH, password_buf, 40+engineLength);
       SHAFinal(key, &SH);
       return;
    }

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    A.3.  Password to Key Sample Results

    A.3.1.  Password to Key Sample Results using MD5

       The following shows a sample output of the password to key algorithm
       for a 16-octet key using MD5.

       With a password of "maplesyrup" the output of the password to key
       algorithm before the key is localized with the SNMP engine's
       snmpEngineID is:

          '9f af 32 83 88 4e 92 83 4e bc 98 47 d8 ed d9 63'H

       After the intermediate key (shown above) is localized with the
       snmpEngineID value of:

          '00 00 00 00 00 00 00 00 00 00 00 02'H

       the final output of the password to key algorithm is:

          '52 6f 5e ed 9f cc e2 6f 89 64 c2 93 07 87 d8 2b'H

    A.3.2.  Password to Key Sample Results using SHA

          The following shows a sample output of the password to key
          algorithm for a 20-octet key using SHA.

          With a password of "maplesyrup" the output of the password to key
          algorithm before the key is localized with the SNMP engine's
          snmpEngineID is:

          '9f b5 cc 03 81 49 7b 37 93 52 89 39 ff 78 8d 5d 79 14 52 11'H

       After the intermediate key (shown above) is localized with the
       snmpEngineID value of:

          '00 00 00 00 00 00 00 00 00 00 00 02'H

       the final output of the password to key algorithm is:

          '66 95 fe bc 92 88 e3 62 82 23 5f c7 15 1f 12 84 97 b3 8f 3f'H

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    RFC 2574                     USM for SNMPv3                   April 1999

    A.4.  Sample encoding of msgSecurityParameters

       The msgSecurityParameters in an SNMP message are represented as an
       OCTET STRING. This OCTET STRING should be considered opaque outside a
       specific Security Model.

       The User-based Security Model defines the contents of the OCTET
       STRING as a SEQUENCE (see section 2.4).

       Given these two properties, the following is an example of the
       msgSecurityParameters for the User-based Security Model, encoded as
       an OCTET STRING:

         04 <length>
         30 <length>
         04 <length> <msgAuthoritativeEngineID>
         02 <length> <msgAuthoritativeEngineBoots>
         02 <length> <msgAuthoritativeEngineTime>
         04 <length> <msgUserName>
         04 0c       <HMAC-MD5-96-digest>
         04 08       <salt>

       Here is the example once more, but now with real values (except for
       the digest in msgAuthenticationParameters and the salt in
       msgPrivacyParameters, which depend on variable data that we have not
       defined here):

         Hex Data                         Description
         --------------  -----------------------------------------------
         04 39           OCTET STRING,                  length 57
         30 37           SEQUENCE,                      length 55
         04 0c 80000002  msgAuthoritativeEngineID:      IBM
               01                                       IPv4 address
               09840301                                 9.132.3.1
         02 01 01        msgAuthoritativeEngineBoots:   1
         02 02 0101      msgAuthoritativeEngineTime:    257
         04 04 62657274  msgUserName:                   bert
         04 0c 01234567  msgAuthenticationParameters:   sample value
               89abcdef
               fedcba98
         04 08 01234567  msgPrivacyParameters:          sample value
               89abcdef

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    RFC 2574                     USM for SNMPv3                   April 1999

    A.5.  Sample keyChange Results

    A.5.1.  Sample keyChange Results using MD5

       Let us assume that a user has a current password of "maplesyrup" as
       in section A.3.1. and let us also assume the snmpEngineID of 12
       octets:

          '00 00 00 00 00 00 00 00 00 00 00 02'H

       If we now want to change the password to "newsyrup", then we first
       calculate the key for the new password. It is as follows:

          '01 ad d2 73 10 7c 4e 59 6b 4b 00 f8 2b 1d 42 a7'H

       If we localize it for the above snmpEngineID, then the localized new
       key becomes:

          '87 02 1d 7b d9 d1 01 ba 05 ea 6e 3b f9 d9 bd 4a'H

       If we then use a (not so good, but easy to test) random value of:

          '00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00'H

       Then the value we must send for keyChange is:

          '00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
           88 05 61 51 41 67 6c c9 19 61 74 e7 42 a3 25 51'H

       If this were for the privacy key, then it would be exactly the same.

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    RFC 2574                     USM for SNMPv3                   April 1999

    A.5.2.  Sample keyChange Results using SHA

       Let us assume that a user has a current password of "maplesyrup" as
       in section A.3.2. and let us also assume the snmpEngineID of 12
       octets:

          '00 00 00 00 00 00 00 00 00 00 00 02'H

       If we now want to change the password to "newsyrup", then we first
       calculate the key for the new password. It is as follows:

          '3a 51 a6 d7 36 aa 34 7b 83 dc 4a 87 e3 e5 5e e4 d6 98 ac 71'H

       If we localize it for the above snmpEngineID, then the localized new
       key becomes:

          '78 e2 dc ce 79 d5 94 03 b5 8c 1b ba a5 bf f4 63 91 f1 cd 25'H

       If we then use a (not so good, but easy to test) random value of:

          '00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00'H

       Then the value we must send for keyChange is:

          '00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
           9c 10 17 f4 fd 48 3d 2d e8 d5 fa db f8 43 92 cb 06 45 70 51'

       For the key used for privacy, the new nonlocalized key would be:

          '3a 51 a6 d7 36 aa 34 7b 83 dc 4a 87 e3 e5 5e e4 d6 98 ac 71'H

       For the key used for privacy, the new localized key would be (note
       that they localized key gets truncated to 16 octets for DES):

          '78 e2 dc ce 79 d5 94 03 b5 8c 1b ba a5 bf f4 63'H

       If we then use a (not so good, but easy to test) random value of:

          '00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00'H

       Then the value we must send for keyChange for the privacy key is:

          '00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
          '7e f8 d8 a4 c9 cd b2 6b 47 59 1c d8 52 ff 88 b5'H

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    RFC 2574                     USM for SNMPv3                   April 1999

    B.  Change Log

       Changes made since RFC2274:
       - Fixed msgUserName to allow size of zero and explain that this can
       be used for snmpEngineID discovery.
       - Clarified section 3.1 steps 4.b, 5, 6 and 8.b.
       - Clarified section 3.2 paragraph 2.
       - Clarified section 3.2 step 7.a last paragraph, step 7.b.1 second
       bullet and step 7.b.2 third bullet.
       - Clarified section 4 to indicate that discovery can use a userName
       of zero length in unAuthenticated messages, whereas a valid userName
       must be used in authenticated messages.
       - Added REVISION clauses to MODULE-IDENTITY
       - Clarified KeyChange TC by adding a note that localized keys must be
       used when calculating a KeyChange value.
       - Added clarifying text to the DESCRIPTION clause of usmUserTable.
       Added text describes a recommended procedure for adding a new user.
       - Clarified the use of usmUserCloneFrom object.
       - Clarified how and under which conditions the usmUserAuthProtocol
       and usmUserPrivProtocol can be initialized and/or changed.
       - Added comment on typical sizes for usmUserAuthKeyChange and
       usmUserPrivKeyChange. Also for usmUserOwnAuthKeyChange and
       usmUserOwnPrivKeyChange.
       - Added clarifications to the DESCRIPTION clauses of
       usmUserAuthKeyChange, usmUserOwnAuthKeychange, usmUserPrivKeyChange
       and usmUserOwnPrivKeychange.  - Added clarification to DESCRIPTION
       clause of usmUserStorageType.  - Added clarification to DESCRIPTION
       clause of usmUserStatus.
       - Clarified IV generation procedure in section 8.1.1.1 and in
       addition clarified section 8.3.1 step 1 and section 8.3.2. step 3.
       - Clarified section 11.2 and added a warning that different size
       passwords with repetitive strings may result in same key.
       - Added template users to appendix A for cloning process.
       - Fixed C-code examples in Appendix A.
       - Fixed examples of generated keys in Appendix A.
       - Added examples of KeyChange values to Appendix A.
       - Used PDU Classes instead of RFC1905 PDU types.
       - Added text in the security section about Reports and Access Control
       to the MIB
       - Removed a incorrect note at the end of section 3.2 step 7.
       - Added a note in section 3.2 step 3.
       - Corrected various spelling errors and typos.
       - Corrected procedure for 3.2 step 2.a)
       - various clarifications.
       - Fixed references to new/revised documents
       - Change to no longer cache data that is not used

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    RFC 2574                     USM for SNMPv3                   April 1999

    C.  Full Copyright Statement

       Copyright (C) The Internet Society (1999).  All Rights Reserved.

       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       included on all such copies and derivative works.  However, this
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed, or as required to translate it into languages other than
       English.

       The limited permissions granted above are perpetual and will not be
       revoked by the Internet Society or its successors or assigns.

       This document and the information contained herein is provided on an
       "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
       TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
       BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
       HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
       MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

    Acknowledgement

       Funding for the RFC Editor function is currently provided by
       the Internet Society.

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