    Internet Engineering Task Force (IETF)                         J. Schaad
    Request for Comments: 8551                                August Cellars
    Obsoletes: 5751                                              B. Ramsdell
    Category: Standards Track                         Brute Squad Labs, Inc.
    ISSN: 2070-1721                                                S. Turner
                                                                       sn3rd
                                                                  April 2019

       Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
                             Message Specification

    Abstract

       This document defines Secure/Multipurpose Internet Mail Extensions
       (S/MIME) version 4.0.  S/MIME provides a consistent way to send and
       receive secure MIME data.  Digital signatures provide authentication,
       message integrity, and non-repudiation with proof of origin.
       Encryption provides data confidentiality.  Compression can be used to
       reduce data size.  This document obsoletes RFC 5751.

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 7841.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       https://www.rfc-editor.org/info/rfc8551.

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    Copyright Notice

       Copyright (c) 2019 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

       This document is subject to BCP 78 and the IETF Trust's Legal
       Provisions Relating to IETF Documents
       (https://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

       This document may contain material from IETF Documents or IETF
       Contributions published or made publicly available before November
       10, 2008.  The person(s) controlling the copyright in some of this
       material may not have granted the IETF Trust the right to allow
       modifications of such material outside the IETF Standards Process.
       Without obtaining an adequate license from the person(s) controlling
       the copyright in such materials, this document may not be modified
       outside the IETF Standards Process, and derivative works of it may
       not be created outside the IETF Standards Process, except to format
       it for publication as an RFC or to translate it into languages other
       than English.

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    RFC 8551            S/MIME 4.0 Message Specification          April 2019

    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
         1.1.  Specification Overview  . . . . . . . . . . . . . . . . .   5
         1.2.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   6
         1.3.  Conventions Used in This Document . . . . . . . . . . . .   7
         1.4.  Compatibility with Prior Practice of S/MIME . . . . . . .   8
         1.5.  Changes from S/MIME v3 to S/MIME v3.1 . . . . . . . . . .   9
         1.6.  Changes from S/MIME v3.1 to S/MIME v3.2 . . . . . . . . .   9
         1.7.  Changes for S/MIME v4.0 . . . . . . . . . . . . . . . . .  11
       2.  CMS Options . . . . . . . . . . . . . . . . . . . . . . . . .  12
         2.1.  DigestAlgorithmIdentifier . . . . . . . . . . . . . . . .  12
         2.2.  SignatureAlgorithmIdentifier  . . . . . . . . . . . . . .  12
         2.3.  KeyEncryptionAlgorithmIdentifier  . . . . . . . . . . . .  13
         2.4.  General Syntax  . . . . . . . . . . . . . . . . . . . . .  13
           2.4.1.  Data Content Type . . . . . . . . . . . . . . . . . .  14
           2.4.2.  SignedData Content Type . . . . . . . . . . . . . . .  14
           2.4.3.  EnvelopedData Content Type  . . . . . . . . . . . . .  14
           2.4.4.  AuthEnvelopedData Content Type  . . . . . . . . . . .  14
           2.4.5.  CompressedData Content Type . . . . . . . . . . . . .  14
         2.5.  Attributes and the SignerInfo Type  . . . . . . . . . . .  15
           2.5.1.  Signing Time Attribute  . . . . . . . . . . . . . . .  15
           2.5.2.  SMIMECapabilities Attribute . . . . . . . . . . . . .  16
           2.5.3.  Encryption Key Preference Attribute . . . . . . . . .  17
         2.6.  SignerIdentifier SignerInfo Type  . . . . . . . . . . . .  19
         2.7.  ContentEncryptionAlgorithmIdentifier  . . . . . . . . . .  19
           2.7.1.  Deciding Which Encryption Method to Use . . . . . . .  19
           2.7.2.  Choosing Weak Encryption  . . . . . . . . . . . . . .  21
           2.7.3.  Multiple Recipients . . . . . . . . . . . . . . . . .  21
       3.  Creating S/MIME Messages  . . . . . . . . . . . . . . . . . .  21
         3.1.  Preparing the MIME Entity for Signing, Enveloping, or
               Compressing . . . . . . . . . . . . . . . . . . . . . . .  22
           3.1.1.  Canonicalization  . . . . . . . . . . . . . . . . . .  23
           3.1.2.  Transfer Encoding . . . . . . . . . . . . . . . . . .  24
           3.1.3.  Transfer Encoding for Signing Using multipart/signed   25
           3.1.4.  Sample Canonical MIME Entity  . . . . . . . . . . . .  25
         3.2.  The application/pkcs7-mime Media Type . . . . . . . . . .  26
           3.2.1.  The name and filename Parameters  . . . . . . . . . .  27
           3.2.2.  The smime-type Parameter  . . . . . . . . . . . . . .  28
         3.3.  Creating an Enveloped-Only Message  . . . . . . . . . . .  29
         3.4.  Creating an Authenticated Enveloped-Only Message  . . . .  30
         3.5.  Creating a Signed-Only Message  . . . . . . . . . . . . .  31
           3.5.1.  Choosing a Format for Signed-Only Messages  . . . . .  32
           3.5.2.  Signing Using application/pkcs7-mime with SignedData   32
           3.5.3.  Signing Using the multipart/signed Format . . . . . .  33
         3.6.  Creating a Compressed-Only Message  . . . . . . . . . . .  36
         3.7.  Multiple Operations . . . . . . . . . . . . . . . . . . .  37
         3.8.  Creating a Certificate Management Message . . . . . . . .  38

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         3.9.  Registration Requests . . . . . . . . . . . . . . . . . .  38
         3.10. Identifying an S/MIME Message . . . . . . . . . . . . . .  39
       4.  Certificate Processing  . . . . . . . . . . . . . . . . . . .  39
         4.1.  Key Pair Generation . . . . . . . . . . . . . . . . . . .  40
         4.2.  Signature Generation  . . . . . . . . . . . . . . . . . .  40
         4.3.  Signature Verification  . . . . . . . . . . . . . . . . .  40
         4.4.  Encryption  . . . . . . . . . . . . . . . . . . . . . . .  41
         4.5.  Decryption  . . . . . . . . . . . . . . . . . . . . . . .  41
       5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  41
         5.1.  Media Type for application/pkcs7-mime . . . . . . . . . .  42
         5.2.  Media Type for application/pkcs7-signature  . . . . . . .  43
         5.3.  authEnveloped-data smime-type . . . . . . . . . . . . . .  44
         5.4.  Reference Updates . . . . . . . . . . . . . . . . . . . .  44
       6.  Security Considerations . . . . . . . . . . . . . . . . . . .  44
       7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  48
         7.1.  Reference Conventions . . . . . . . . . . . . . . . . . .  48
         7.2.  Normative References  . . . . . . . . . . . . . . . . . .  49
         7.3.  Informative References  . . . . . . . . . . . . . . . . .  52
       Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  57
       Appendix B.  Historic Mail Considerations . . . . . . . . . . . .  59
         B.1.  DigestAlgorithmIdentifier . . . . . . . . . . . . . . . .  59
         B.2.  Signature Algorithms  . . . . . . . . . . . . . . . . . .  59
         B.3.  ContentEncryptionAlgorithmIdentifier  . . . . . . . . . .  61
         B.4.  KeyEncryptionAlgorithmIdentifier  . . . . . . . . . . . .  62
       Appendix C.  Moving S/MIME v2 Message Specification to Historic
                    Status . . . . . . . . . . . . . . . . . . . . . . .  62
       Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  62
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  63

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    1.  Introduction

       S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a
       consistent way to send and receive secure MIME data.  Based on the
       popular Internet MIME standard, S/MIME provides the following
       cryptographic security services for electronic messaging
       applications: authentication, message integrity, and non-repudiation
       of origin (using digital signatures), and data confidentiality (using
       encryption).  As a supplementary service, S/MIME provides message
       compression.

       S/MIME can be used by traditional mail user agents (MUAs) to add
       cryptographic security services to mail that is sent, and to
       interpret cryptographic security services in mail that is received.
       However, S/MIME is not restricted to mail; it can be used with any
       transport mechanism that transports MIME data, such as HTTP or SIP.
       As such, S/MIME takes advantage of the object-based features of MIME
       and allows secure messages to be exchanged in mixed-transport
       systems.

       Further, S/MIME can be used in automated message transfer agents that
       use cryptographic security services that do not require any human
       intervention, such as the signing of software-generated documents and
       the encryption of FAX messages sent over the Internet.

       This document defines version 4.0 of the S/MIME Message
       Specification.  As such, this document obsoletes version 3.2 of the
       S/MIME Message Specification [RFC5751].

       This specification contains a number of references to documents that
       have been obsoleted or replaced.  This is intentional, as the updated
       documents often do not have the same information or protocol
       requirements in them.

    1.1.  Specification Overview

       This document describes a protocol for adding cryptographic signature
       and encryption services to MIME data.  The MIME standard [MIME-SPEC]
       provides a general structure for the content of Internet messages and
       allows extensions for new applications based on content-type.

       This specification defines how to create a MIME body part that has
       been cryptographically enhanced according to the Cryptographic
       Message Syntax (CMS) [CMS], which is derived from PKCS #7 [RFC2315].
       This specification also defines the application/pkcs7-mime media
       type, which can be used to transport those body parts.

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       This document also discusses how to use the multipart/signed media
       type defined in [RFC1847] to transport S/MIME signed messages.
       multipart/signed is used in conjunction with the
       application/pkcs7-signature media type, which is used to transport a
       detached S/MIME signature.

       In order to create S/MIME messages, an S/MIME agent MUST follow the
       specifications in this document, as well as the specifications listed
       in [CMS], [RFC3370], [RFC4056], [RFC3560], and [RFC5754].

       Throughout this specification, there are requirements and
       recommendations made for how receiving agents handle incoming
       messages.  There are separate requirements and recommendations for
       how sending agents create outgoing messages.  In general, the best
       strategy is to follow the Robustness Principle (be liberal in what
       you receive and conservative in what you send).  Most of the
       requirements are placed on the handling of incoming messages, while
       the recommendations are mostly on the creation of outgoing messages.

       The separation for requirements on receiving agents and sending
       agents also derives from the likelihood that there will be S/MIME
       systems that involve software other than traditional Internet mail
       clients.  S/MIME can be used with any system that transports MIME
       data.  An automated process that sends an encrypted message might not
       be able to receive an encrypted message at all, for example.  Thus,
       the requirements and recommendations for the two types of agents are
       listed separately when appropriate.

    1.2.  Definitions

       For the purposes of this specification, the following definitions
       apply.

       ASN.1:
          Abstract Syntax Notation One, as defined in ITU-T Recommendations
          X.680, X.681, X.682, and X.683 [ASN.1].

       BER:
          Basic Encoding Rules for ASN.1, as defined in ITU-T Recommendation
          X.690 [X.690].

       Certificate:
          A type that binds an entity's name to a public key with a digital
          signature.

       DER:
          Distinguished Encoding Rules for ASN.1, as defined in ITU-T
          Recommendation X.690 [X.690].

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       7-bit data:
          Text data with lines less than 998 characters long, where none of
          the characters have the 8th bit set, and there are no NULL
          characters.  <CR> and <LF> occur only as part of a <CR><LF>
          end-of-line delimiter.

       8-bit data:
          Text data with lines less than 998 characters, and where none of
          the characters are NULL characters.  <CR> and <LF> occur only as
          part of a <CR><LF> end-of-line delimiter.

       Binary data:
          Arbitrary data.

       Transfer encoding:
          A reversible transformation made on data so 8-bit or binary data
          can be sent via a channel that only transmits 7-bit data.

       Receiving agent:
          Software that interprets and processes S/MIME CMS objects, MIME
          body parts that contain CMS content types, or both.

       Sending agent:
          Software that creates S/MIME CMS content types, MIME body parts
          that contain CMS content types, or both.

       S/MIME agent:
          User software that is a receiving agent, a sending agent, or both.

       Data integrity service:
          A security service that protects against unauthorized changes to
          data by ensuring that changes to the data are detectable
          [RFC4949].

       Data confidentiality:
          The property that data is not disclosed to system entities unless
          they have been authorized to know the data [RFC4949].

    1.3.  Conventions Used in This Document

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
       "OPTIONAL" in this document are to be interpreted as described in
       BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
       capitals, as shown here.

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       We define the additional requirement levels:

       SHOULD+   This term means the same as SHOULD.  However, the authors
                 expect that a requirement marked as SHOULD+ will be
                 promoted at some future time to be a MUST.

       SHOULD-   This term means the same as SHOULD.  However, the authors
                 expect that a requirement marked as SHOULD- will be demoted
                 to a MAY in a future version of this document.

       MUST-     This term means the same as MUST.  However, the authors
                 expect that this requirement will no longer be a MUST in a
                 future document.  Although its status will be determined at
                 a later time, it is reasonable to expect that if a future
                 revision of a document alters the status of a MUST-
                 requirement, it will remain at least a SHOULD or a SHOULD-.

       The term "RSA" in this document almost always refers to the
       PKCS #1 v1.5 RSA [RFC2313] signature or encryption algorithms even
       when not qualified as such.  There are a couple of places where it
       refers to the general RSA cryptographic operation; these can be
       determined from the context where it is used.

    1.4.  Compatibility with Prior Practice of S/MIME

       S/MIME version 4.0 agents ought to attempt to have the greatest
       interoperability possible with agents for prior versions of S/MIME.

       -  S/MIME version 2 is described in RFC 2311 through RFC 2315
          inclusive [SMIMEv2].

       -  S/MIME version 3 is described in RFC 2630 through RFC 2634
          inclusive and RFC 5035 [SMIMEv3].

       -  S/MIME version 3.1 is described in RFC 2634, RFC 3850, RFC 3851,
          RFC 3852, and RFC 5035 [SMIMEv3.1].

       -  S/MIME version 3.2 is described in RFC 2634, RFC 5035, RFC 5652,
          RFC 5750, and RFC 5751 [SMIMEv3.2].

       -  [RFC2311] also has historical information about the development of
          S/MIME.

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    1.5.  Changes from S/MIME v3 to S/MIME v3.1

       This section describes the changes made between S/MIME v3 and
       S/MIME v3.1.  Note that the requirement levels indicated by the
       capitalized key words ("MUST", "SHOULD", etc.) may have changed in
       later versions of S/MIME.

       -  The RSA public key algorithm was changed to a MUST implement.  The
          key wrap algorithm and the Diffie-Hellman (DH) algorithm [RFC2631]
          were changed to a SHOULD implement.

       -  The AES symmetric encryption algorithm has been included as a
          SHOULD implement.

       -  The RSA public key algorithm was changed to a MUST implement
          signature algorithm.

       -  Ambiguous language about the use of "empty" SignedData messages to
          transmit certificates was clarified to reflect that transmission
          of Certificate Revocation Lists is also allowed.

       -  The use of binary encoding for some MIME entities is now
          explicitly discussed.

       -  Header protection through the use of the message/rfc822 media type
          has been added.

       -  Use of the CompressedData CMS type is allowed, along with required
          media type and file extension additions.

    1.6.  Changes from S/MIME v3.1 to S/MIME v3.2

       This section describes the changes made between S/MIME v3.1 and
       S/MIME v3.2.  Note that the requirement levels indicated by the
       capitalized key words ("MUST", "SHOULD", etc.) may have changed in
       later versions of S/MIME.  Note that the section numbers listed here
       (e.g., 3.4.3.2) are from [RFC5751].

       -  Made editorial changes, e.g., replaced "MIME type" with "media
          type", "content-type" with "Content-Type".

       -  Moved "Conventions Used in This Document" to Section 1.3.  Added
          definitions for SHOULD+, SHOULD-, and MUST-.

       -  Section 1.1 and Appendix A: Added references to RFCs for
          RSASSA-PSS, RSAES-OAEP, and SHA2 CMS algorithms.  Added CMS
          Multiple Signers Clarification to CMS reference.

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       -  Section 1.2: Updated references to ASN.1 to X.680, and BER and DER
          to X.690.

       -  Section 1.4: Added references to S/MIME v3.1 RFCs.

       -  Section 2.1 (digest algorithm): SHA-256 added as MUST, SHA-1 and
          MD5 made SHOULD-.

       -  Section 2.2 (signature algorithms): RSA with SHA-256 added as
          MUST; DSA with SHA-256 added as SHOULD+; RSA with SHA-1, DSA with
          SHA-1, and RSA with MD5 changed to SHOULD-; and RSASSA-PSS with
          SHA-256 added as SHOULD+.  Also added note about what S/MIME v3.1
          clients support.

       -  Section 2.3 (key encryption): DH changed to SHOULD-, and RSAES-
          OAEP added as SHOULD+.  Elaborated on requirements for key wrap
          algorithm.

       -  Section 2.5.1: Added requirement that receiving agents MUST
          support both GeneralizedTime and UTCTime.

       -  Section 2.5.2: Replaced reference "sha1WithRSAEncryption" with
          "sha256WithRSAEncryption", replaced "DES-3EDE-CBC" with "AES-128
          CBC", and deleted the RC5 example.

       -  Section 2.5.2.1: Deleted entire section (discussed
          deprecated RC2).

       -  Section 2.7, Section 2.7.1, and Appendix A: References to RC2/40
          removed.

       -  Section 2.7 (content encryption): AES-128 CBC added as MUST,
          AES-192 and AES-256 CBC SHOULD+, and tripleDES now SHOULD-.

       -  Section 2.7.1: Updated pointers from 2.7.2.1 through 2.7.2.4 to
          2.7.1.1 and 2.7.1.2.

       -  Section 3.1.1: Removed text about MIME character sets.

       -  Sections 3.2.2 and 3.6: Replaced "encrypted" with "enveloped".
          Updated OID example to use AES-128 CBC OID.

       -  Section 3.4.3.2: Replaced "micalg" parameter for "SHA-1" with
          "sha-1".

       -  Section 4: Updated reference to CERT v3.2.

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       -  Section 4.1: Updated RSA and DSA key size discussion.  Moved last
          four sentences to security considerations.  Updated reference to
          randomness requirements for security.

       -  Section 5: Added IANA registration templates to update media type
          registry to point to this document as opposed to RFC 2311.

       -  Section 6: Updated security considerations.

       -  Section 7: Moved references from Appendix B to this section.
          Updated references.  Added informative references to SMIMEv2,
          SMIMEv3, and SMIMEv3.1.

       -  Appendix B: Added Appendix B to move S/MIME v2 to Historic status.

    1.7.  Changes for S/MIME v4.0

       This section describes the changes made between S/MIME v3.2 and
       S/MIME v4.0.

       -  Added the use of AuthEnvelopedData, including defining and
          registering an smime-type value (Sections 2.4.4 and 3.4).

       -  Updated the content-encryption algorithms (Sections 2.7 and
          2.7.1.2): added AES-256 Galois/Counter Mode (GCM), added
          ChaCha20-Poly1305, removed mention of AES-192 Cipher Block
          Chaining (CBC), and marked tripleDES as historic.

       -  Updated the set of signature algorithms (Section 2.2): added the
          Edwards-curve Digital Signature Algorithm (EdDSA), added the
          Elliptic Curve Digital Signature Algorithm (ECDSA), and marked DSA
          as historic.

       -  Updated the set of digest algorithms (Section 2.1): added SHA-512,
          and marked SHA-1 as historic.

       -  Updated the size of keys to be used for RSA encryption and RSA
          signing (Section 4).

       -  Created Appendix B, which discusses considerations for dealing
          with historic email messages.

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    2.  CMS Options

       CMS allows for a wide variety of options in content, attributes, and
       algorithm support.  This section puts forth a number of support
       requirements and recommendations in order to achieve a base level of
       interoperability among all S/MIME implementations.  [RFC3370] and
       [RFC5754] provide additional details regarding the use of the
       cryptographic algorithms.  [ESS] provides additional details
       regarding the use of additional attributes.

    2.1.  DigestAlgorithmIdentifier

       The algorithms here are used for digesting the body of the message
       and are not the same as the digest algorithms used as part of the
       signature algorithms.  The result of this is placed in the
       message-digest attribute of the signed attributes.  It is RECOMMENDED
       that the algorithm used for digesting the body of the message be of
       similar strength to, or greater strength than, the signature
       algorithm.

       Sending and receiving agents:

       -  MUST support SHA-256.

       -  MUST support SHA-512.

       [RFC5754] provides the details for using these algorithms with
       S/MIME.

    2.2.  SignatureAlgorithmIdentifier

       There are different sets of requirements placed on receiving and
       sending agents.  By having the different requirements, the maximum
       amount of interoperability is achieved, as it allows for specialized
       protection of private key material but maximum signature validation.

       Receiving agents:

       -  MUST support ECDSA with curve P-256 and SHA-256.

       -  MUST support EdDSA with curve25519 using PureEdDSA mode [RFC8419].

       -  MUST- support RSA PKCS #1 v1.5 with SHA-256.

       -  SHOULD support the RSA Probabilistic Signature Scheme (RSASSA-PSS)
          with SHA-256.

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       Sending agents:

       -  MUST support at least one of the following algorithms: ECDSA with
          curve P-256 and SHA-256, or EdDSA with curve25519 using PureEdDSA
          mode.

       -  MUST- support RSA PKCS #1 v1.5 with SHA-256.

       -  SHOULD support RSASSA-PSS with SHA-256.

       See Section 4.1 for information on key size and algorithm references.

    2.3.  KeyEncryptionAlgorithmIdentifier

       Receiving and sending agents:

       -  MUST support Elliptic Curve Diffie-Hellman (ECDH) ephemeral-static
          mode for P-256, as specified in [RFC5753].

       -  MUST support ECDH ephemeral-static mode for X25519 using HKDF-256
          ("HKDF" stands for "HMAC-based Key Derivation Function") for the
          KDF, as specified in [RFC8418].

       -  MUST- support RSA encryption, as specified in [RFC3370].

       -  SHOULD+ support RSA Encryption Scheme - Optimal Asymmetric
          Encryption Padding (RSAES-OAEP), as specified in [RFC3560].

       When ECDH ephemeral-static is used, a key wrap algorithm is also
       specified in the KeyEncryptionAlgorithmIdentifier [RFC5652].  The
       underlying encryption functions for the key wrap and content-
       encryption algorithms [RFC3370] [RFC3565] and the key sizes for the
       two algorithms MUST be the same (e.g., AES-128 key wrap algorithm
       with AES-128 content-encryption algorithm).  As both 128-bit and
       256-bit AES modes are mandatory to implement as content-encryption
       algorithms (Section 2.7), both the AES-128 and AES-256 key wrap
       algorithms MUST be supported when ECDH ephemeral-static is used.
       Recipients MAY enforce this but MUST use the weaker of the two as
       part of any cryptographic strength computations they might do.

       Appendix B provides information on algorithm support in older
       versions of S/MIME.

    2.4.  General Syntax

       There are several CMS content types.  Of these, only the Data,
       SignedData, EnvelopedData, AuthEnvelopedData, and CompressedData
       content types are currently used for S/MIME.

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    2.4.1.  Data Content Type

       Sending agents MUST use the id-data content type identifier to
       identify the "inner" MIME message content.  For example, when
       applying a digital signature to MIME data, the CMS SignedData
       encapContentInfo eContentType MUST include the id-data object
       identifier (OID), and the media type MUST be stored in the SignedData
       encapContentInfo eContent OCTET STRING (unless the sending agent is
       using multipart/signed, in which case the eContent is absent, per
       Section 3.5.3 of this document).  As another example, when applying
       encryption to MIME data, the CMS EnvelopedData encryptedContentInfo
       contentType MUST include the id-data OID and the encrypted MIME
       content MUST be stored in the EnvelopedData encryptedContentInfo
       encryptedContent OCTET STRING.

    2.4.2.  SignedData Content Type

       Sending agents MUST use the SignedData content type to apply a
       digital signature to a message or, in a degenerate case where there
       is no signature information, to convey certificates.  Applying a
       signature to a message provides authentication, message integrity,
       and non-repudiation of origin.

    2.4.3.  EnvelopedData Content Type

       This content type is used to apply data confidentiality to a message.
       In order to distribute the symmetric key, a sender needs to have
       access to a public key for each intended message recipient to use
       this service.

    2.4.4.  AuthEnvelopedData Content Type

       This content type is used to apply data confidentiality and message
       integrity to a message.  This content type does not provide
       authentication or non-repudiation.  In order to distribute the
       symmetric key, a sender needs to have access to a public key for each
       intended message recipient to use this service.

    2.4.5.  CompressedData Content Type

       This content type is used to apply data compression to a message.
       This content type does not provide authentication, message integrity,
       non-repudiation, or data confidentiality; it is only used to reduce
       the message's size.

       See Section 3.7 for further guidance on the use of this type in
       conjunction with other CMS types.

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    2.5.  Attributes and the SignerInfo Type

       The SignerInfo type allows the inclusion of unsigned and signed
       attributes along with a signature.  These attributes can be required
       for the processing of messages (e.g., message digest), information
       the signer supplied (e.g., SMIME capabilities) that should be
       processed, or attributes that are not relevant to the current
       situation (e.g., mlExpansionHistory [RFC2634] for mail viewers).

       Receiving agents MUST be able to handle zero or one instance of each
       of the signed attributes listed here.  Sending agents SHOULD generate
       one instance of each of the following signed attributes in each
       S/MIME message:

       -  Signing time (Section 2.5.1 in this document)

       -  SMIME capabilities (Section 2.5.2 in this document)

       -  Encryption key Preference (Section 2.5.3 in this document)

       -  Message digest (Section 11.2 in [RFC5652])

       -  Content type (Section 11.1 in [RFC5652])

       Further, receiving agents SHOULD be able to handle zero or one
       instance of the signingCertificate and signingCertificateV2 signed
       attributes, as defined in Section 5 of RFC 2634 [ESS] and Section 3
       of RFC 5035 [ESS], respectively.

       Sending agents SHOULD generate one instance of the signingCertificate
       or signingCertificateV2 signed attribute in each SignerInfo
       structure.

       Additional attributes and values for these attributes might be
       defined in the future.  Receiving agents SHOULD handle attributes or
       values that they do not recognize in a graceful manner.

       Interactive sending agents that include signed attributes that are
       not listed here SHOULD display those attributes to the user, so that
       the user is aware of all of the data being signed.

    2.5.1.  Signing Time Attribute

       The signingTime attribute is used to convey the time that a message
       was signed.  The time of signing will most likely be created by a
       signer and therefore is only as trustworthy as that signer.

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       Sending agents MUST encode signing time through the year 2049 as
       UTCTime; signing times in 2050 or later MUST be encoded as
       GeneralizedTime.  When the UTCTime CHOICE is used, S/MIME agents MUST
       interpret the year field (YY) as follows:

          If YY is greater than or equal to 50, the year is interpreted as
          19YY; if YY is less than 50, the year is interpreted as 20YY.

       Receiving agents MUST be able to process signingTime attributes that
       are encoded in either UTCTime or GeneralizedTime.

    2.5.2.  SMIMECapabilities Attribute

       The SMIMECapabilities attribute includes signature algorithms (such
       as "sha256WithRSAEncryption"), symmetric algorithms (such as "AES-128
       CBC"), authenticated symmetric algorithms (such as "AES-128 GCM"),
       and key encipherment algorithms (such as "rsaEncryption").  The
       presence of an SMIMECapability attribute containing an algorithm
       implies that the sender can deal with the algorithm as well as
       understand the ASN.1 structures associated with that algorithm.
       There are also several identifiers that indicate support for other
       optional features such as binary encoding and compression.  The
       SMIMECapabilities attribute was designed to be flexible and
       extensible so that, in the future, a means of identifying other
       capabilities and preferences such as certificates can be added in a
       way that will not cause current clients to break.

       If present, the SMIMECapabilities attribute MUST be a
       SignedAttribute.  CMS defines SignedAttributes as a SET OF Attribute.
       The SignedAttributes in a signerInfo MUST include a single instance
       of the SMIMECapabilities attribute.  CMS defines the ASN.1 syntax for
       Attribute to include attrValues SET OF AttributeValue.  An
       SMIMECapabilities attribute MUST only include a single instance of
       AttributeValue.  If a signature is detected as violating these
       requirements, the signature SHOULD be treated as failing.

       The semantics of the SMIMECapabilities attribute specify a partial
       list as to what the client announcing the SMIMECapabilities can
       support.  A client does not have to list every capability it
       supports, and it need not list all its capabilities so that the
       capabilities list doesn't get too long.  In an SMIMECapabilities
       attribute, the OIDs are listed in order of their preference but
       SHOULD be separated logically along the lines of their categories
       (signature algorithms, symmetric algorithms, key encipherment
       algorithms, etc.).

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       The structure of the SMIMECapabilities attribute is to facilitate
       simple table lookups and binary comparisons in order to determine
       matches.  For instance, the encoding for the SMIMECapability for
       sha256WithRSAEncryption includes rather than omits the NULL
       parameter.  Because of the requirement for identical encoding,
       individuals documenting algorithms to be used in the
       SMIMECapabilities attribute SHOULD explicitly document the correct
       byte sequence for the common cases.

       For any capability, the associated parameters for the OID MUST
       specify all of the parameters necessary to differentiate between two
       instances of the same algorithm.

       The same OID that is used to identify an algorithm SHOULD also be
       used in the SMIMECapability for that algorithm.  There are cases
       where a single OID can correspond to multiple algorithms.  In these
       cases, a single algorithm MUST be assigned to the SMIMECapability
       using that OID.  Additional OIDs from the smimeCapabilities OID tree
       are then allocated for the other algorithms usages.  For instance, in
       an earlier specification, rsaEncryption was ambiguous because it
       could refer to either a signature algorithm or a key encipherment
       algorithm.  In the event that an OID is ambiguous, it needs to be
       arbitrated by the maintainer of the registered SMIMECapabilities list
       as to which type of algorithm will use the OID, and a new OID MUST be
       allocated under the smimeCapabilities OID to satisfy the other use of
       the OID.

       The registered SMIMECapabilities list specifies the parameters for
       OIDs that need them, most notably key lengths in the case of
       variable-length symmetric ciphers.  In the event that there are no
       differentiating parameters for a particular OID, the parameters MUST
       be omitted and MUST NOT be encoded as NULL.  Additional values for
       the SMIMECapabilities attribute might be defined in the future.
       Receiving agents MUST handle an SMIMECapabilities object that has
       values that it does not recognize in a graceful manner.

       Section 2.7.1 explains a strategy for caching capabilities.

    2.5.3.  Encryption Key Preference Attribute

       The encryption key preference attribute allows the signer to
       unambiguously describe which of the signer's certificates has the
       signer's preferred encryption key.  This attribute is designed to
       enhance behavior for interoperating with those clients that use
       separate keys for encryption and signing.  This attribute is used to
       convey to anyone viewing the attribute which of the listed
       certificates is appropriate for encrypting a session key for future
       encrypted messages.

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       If present, the SMIMEEncryptionKeyPreference attribute MUST be a
       SignedAttribute.  CMS defines SignedAttributes as a SET OF Attribute.
       The SignedAttributes in a signerInfo MUST include a single instance
       of the SMIMEEncryptionKeyPreference attribute.  CMS defines the ASN.1
       syntax for Attribute to include attrValues SET OF AttributeValue.  An
       SMIMEEncryptionKeyPreference attribute MUST only include a single
       instance of AttributeValue.  If a signature is detected as violating
       these requirements, the signature SHOULD be treated as failing.

       The sending agent SHOULD include the referenced certificate in the
       set of certificates included in the signed message if this attribute
       is used.  The certificate MAY be omitted if it has been previously
       made available to the receiving agent.  Sending agents SHOULD use
       this attribute if the commonly used or preferred encryption
       certificate is not the same as the certificate used to sign the
       message.

       Receiving agents SHOULD store the preference data if the signature on
       the message is valid and the signing time is greater than the
       currently stored value.  (As with the SMIMECapabilities, the clock
       skew SHOULD be checked and the data not used if the skew is too
       great.)  Receiving agents SHOULD respect the sender's encryption key
       preference attribute if possible.  This, however, represents only a
       preference, and the receiving agent can use any certificate in
       replying to the sender that is valid.

       Section 2.7.1 explains a strategy for caching preference data.

    2.5.3.1.  Selection of Recipient Key Management Certificate

       In order to determine the key management certificate to be used when
       sending a future CMS EnvelopedData message for a particular
       recipient, the following steps SHOULD be followed:

       -  If an SMIMEEncryptionKeyPreference attribute is found in a
          SignedData object received from the desired recipient, this
          identifies the X.509 certificate that SHOULD be used as the X.509
          key management certificate for the recipient.

       -  If an SMIMEEncryptionKeyPreference attribute is not found in a
          SignedData object received from the desired recipient, the set of
          X.509 certificates SHOULD be searched for an X.509 certificate
          with the same subject name as the signer of an X.509 certificate
          that can be used for key management.

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       -  Or, use some other method of determining the user's key management
          key.  If an X.509 key management certificate is not found, then
          encryption cannot be done with the signer of the message.  If
          multiple X.509 key management certificates are found, the S/MIME
          agent can make an arbitrary choice between them.

    2.6.  SignerIdentifier SignerInfo Type

       S/MIME v4.0 implementations MUST support both issuerAndSerialNumber
       and subjectKeyIdentifier.  Messages that use the subjectKeyIdentifier
       choice cannot be read by S/MIME v2 clients.

       It is important to understand that some certificates use a value for
       subjectKeyIdentifier that is not suitable for uniquely identifying a
       certificate.  Implementations MUST be prepared for multiple
       certificates for potentially different entities to have the same
       value for subjectKeyIdentifier and MUST be prepared to try each
       matching certificate during signature verification before indicating
       an error condition.

    2.7.  ContentEncryptionAlgorithmIdentifier

       Sending and receiving agents:

       -  MUST support encryption and decryption with AES-128 GCM and
          AES-256 GCM [RFC5084].

       -  MUST- support encryption and decryption with AES-128 CBC
          [RFC3565].

       -  SHOULD+ support encryption and decryption with ChaCha20-Poly1305
          [RFC7905].

    2.7.1.  Deciding Which Encryption Method to Use

       When a sending agent creates an encrypted message, it has to decide
       which type of encryption to use.  The decision process involves using
       information garnered from the capabilities lists included in messages
       received from the recipient, as well as out-of-band information such
       as private agreements, user preferences, legal restrictions, and
       so on.

       Section 2.5.2 defines a method by which a sending agent can
       optionally announce, among other things, its decrypting capabilities
       in its order of preference.  The following method for processing and
       remembering the encryption capabilities attribute in incoming signed
       messages SHOULD be used.

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       -  If the receiving agent has not yet created a list of capabilities
          for the sender's public key, then, after verifying the signature
          on the incoming message and checking the timestamp, the receiving
          agent SHOULD create a new list containing at least the signing
          time and the symmetric capabilities.

       -  If such a list already exists, the receiving agent SHOULD verify
          that the signing time in the incoming message is greater than the
          signing time stored in the list and that the signature is valid.
          If so, the receiving agent SHOULD update both the signing time and
          capabilities in the list.  Values of the signing time that lie far
          in the future (that is, a greater discrepancy than any reasonable
          clock skew), or a capabilities list in messages whose signature
          could not be verified, MUST NOT be accepted.

       The list of capabilities SHOULD be stored for future use in creating
       messages.

       Before sending a message, the sending agent MUST decide whether it is
       willing to use weak encryption for the particular data in the
       message.  If the sending agent decides that weak encryption is
       unacceptable for this data, then the sending agent MUST NOT use a
       weak algorithm.  The decision to use or not use weak encryption
       overrides any other decision in this section about which encryption
       algorithm to use.

       Sections 2.7.1.1 and 2.7.1.2 describe the decisions a sending agent
       SHOULD use when choosing which type of encryption will be applied to
       a message.  These rules are ordered, so the sending agent SHOULD make
       its decision in the order given.

    2.7.1.1.  Rule 1: Known Capabilities

       If the sending agent has received a set of capabilities from the
       recipient for the message the agent is about to encrypt, then the
       sending agent SHOULD use that information by selecting the first
       capability in the list (that is, the capability most preferred by the
       intended recipient) that the sending agent knows how to encrypt.  The
       sending agent SHOULD use one of the capabilities in the list if the
       agent reasonably expects the recipient to be able to decrypt the
       message.

    2.7.1.2.  Rule 2: Unknown Capabilities, Unknown Version of S/MIME

       If the following two conditions are met, the sending agent SHOULD use
       AES-256 GCM, as AES-256 GCM is a stronger algorithm and is required
       by S/MIME v4.0:

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       -  The sending agent has no knowledge of the encryption capabilities
          of the recipient.

       -  The sending agent has no knowledge of the version of S/MIME used
          or supported by the recipient.

       If the sending agent chooses not to use AES-256 GCM in this step,
       given the presumption is that a client implementing AES-GCM would do
       both AES-256 and AES-128, it SHOULD use AES-128 CBC.

    2.7.2.  Choosing Weak Encryption

       Algorithms such as RC2 are considered to be weak encryption
       algorithms.  Algorithms such as TripleDES are not state of the art
       and are considered to be weaker algorithms than AES.  A sending agent
       that is controlled by a human SHOULD allow a human sender to
       determine the risks of sending data using a weaker encryption
       algorithm before sending the data, and possibly allow the human to
       use a stronger encryption algorithm such as AES GCM or AES CBC even
       if there is a possibility that the recipient will not be able to
       process that algorithm.

    2.7.3.  Multiple Recipients

       If a sending agent is composing an encrypted message to a group of
       recipients where the encryption capabilities of some of the
       recipients do not overlap, the sending agent is forced to send more
       than one message.  Please note that if the sending agent chooses to
       send a message encrypted with a strong algorithm and then send the
       same message encrypted with a weak algorithm, someone watching the
       communications channel could learn the contents of the strongly
       encrypted message simply by decrypting the weakly encrypted message.

    3.  Creating S/MIME Messages

       This section describes the S/MIME message formats and how they are
       created.  S/MIME messages are a combination of MIME bodies and CMS
       content types.  Several media types as well as several CMS content
       types are used.  The data to be secured is always a canonical MIME
       entity.  The MIME entity and other data, such as certificates and
       algorithm identifiers, are given to CMS processing facilities that
       produce a CMS object.  Finally, the CMS object is wrapped in MIME.
       The "Enhanced Security Services for S/MIME" documents [ESS] provide
       descriptions of how nested, secured S/MIME messages are formatted.
       ESS provides a description of how a triple-wrapped S/MIME message is
       formatted using multipart/signed and application/pkcs7-mime for the
       signatures.

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       S/MIME provides one format for enveloped-only data, several formats
       for signed-only data, and several formats for signed and enveloped
       data.  Several formats are required to accommodate several
       environments -- in particular, for signed messages.  The criteria for
       choosing among these formats are also described.

       Anyone reading this section is expected to understand MIME as
       described in [MIME-SPEC] and [RFC1847].

    3.1.  Preparing the MIME Entity for Signing, Enveloping, or Compressing

       S/MIME is used to secure MIME entities.  A MIME message is composed
       of a MIME header and a MIME body.  A body can consist of a single
       MIME entity or a tree of MIME entities (rooted with a multipart).
       S/MIME can be used to secure either a single MIME entity or a tree of
       MIME entities.  These entities can be in locations other than the
       root.  S/MIME can be applied multiple times to different entities in
       a single message.  A MIME entity that is the whole message includes
       only the MIME message headers and MIME body and does not include the
       rfc822 header.  Note that S/MIME can also be used to secure MIME
       entities used in applications other than Internet mail.  For cases
       where protection of the rfc822 header is required, the use of the
       message/rfc822 media type is explained later in this section.

       The MIME entity that is secured and described in this section can be
       thought of as the "inside" MIME entity.  That is, it is the
       "innermost" object in what is possibly a larger MIME message.
       Processing "outside" MIME entities into CMS EnvelopedData,
       CompressedData, and AuthEnvelopedData content types is described in
       Sections 3.2 and 3.5.  Other documents define additional CMS content
       types; those documents should be consulted for processing those CMS
       content types.

       The procedure for preparing a MIME entity is given in [MIME-SPEC].
       The same procedure is used here with some additional restrictions
       when signing.  The description of the procedures from [MIME-SPEC] is
       repeated here, but it is suggested that the reader refer to those
       documents for the exact procedures.  This section also describes
       additional requirements.

       A single procedure is used for creating MIME entities that are to
       have any combination of signing, enveloping, and compressing applied.
       Some additional steps are recommended to defend against known
       corruptions that can occur during mail transport that are of
       particular importance for clear-signing using the multipart/signed
       format.  It is recommended that these additional steps be performed
       on enveloped messages, or signed and enveloped messages, so that the
       messages can be forwarded to any environment without modification.

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       These steps are descriptive rather than prescriptive.  The
       implementer is free to use any procedure as long as the result is
       the same.

       Step 1.  The MIME entity is prepared according to local conventions.

       Step 2.  The leaf parts of the MIME entity are converted to
                canonical form.

       Step 3.  Appropriate transfer encoding is applied to the leaves
                of the MIME entity.

       When an S/MIME message is received, the security services on the
       message are processed, and the result is the MIME entity.  That MIME
       entity is typically passed to a MIME-capable user agent where it is
       further decoded and presented to the user or receiving application.

       In order to protect outer, non-content-related message header fields
       (for instance, the "Subject", "To", "From", and "Cc" fields), the
       sending client MAY wrap a full MIME message in a message/rfc822
       wrapper in order to apply S/MIME security services to these header
       fields.  It is up to the receiving client to decide how to present
       this "inner" header along with the unprotected "outer" header.  Given
       the security difference between headers, it is RECOMMENDED that the
       receiving client provide a distinction between header fields,
       depending on where they are located.

       When an S/MIME message is received, if the top-level protected MIME
       entity has a Content-Type of message/rfc822, it can be assumed that
       the intent was to provide header protection.  This entity SHOULD be
       presented as the top-level message, taking into account
       header-merging issues as previously discussed.

    3.1.1.  Canonicalization

       Each MIME entity MUST be converted to a canonical form that is
       uniquely and unambiguously representable in the environment where the
       signature is created and the environment where the signature will be
       verified.  MIME entities MUST be canonicalized for enveloping and
       compressing as well as signing.

       The exact details of canonicalization depend on the actual media type
       and subtype of an entity and are not described here.  Instead, the
       standard for the particular media type SHOULD be consulted.  For
       example, canonicalization of type text/plain is different from
       canonicalization of audio/basic.  Other than text types, most types
       have only one representation, regardless of computing platform or
       environment, that can be considered their canonical representation.

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       In general, canonicalization will be performed by the non-security
       part of the sending agent rather than the S/MIME implementation.

       The most common and important canonicalization is for text, which is
       often represented differently in different environments.  MIME
       entities of major type "text" MUST have both their line endings and
       character set canonicalized.  The line ending MUST be the pair of
       characters <CR><LF>, and the charset SHOULD be a registered charset
       [CHARSETS].  The details of the canonicalization are specified in
       [MIME-SPEC].

       Note that some charsets such as ISO-2022 have multiple
       representations for the same characters.  When preparing such text
       for signing, the canonical representation specified for the charset
       MUST be used.

    3.1.2.  Transfer Encoding

       When generating any of the secured MIME entities below, except the
       signing using the multipart/signed format, no transfer encoding is
       required at all.  S/MIME implementations MUST be able to deal with
       binary MIME objects.  If no Content-Transfer-Encoding header field is
       present, the transfer encoding is presumed to be 7BIT.

       As a rule, S/MIME implementations SHOULD use transfer encoding as
       described in Section 3.1.3 for all MIME entities they secure.  The
       reason for securing only 7-bit MIME entities, even for enveloped data
       that is not exposed to the transport, is that it allows the MIME
       entity to be handled in any environment without changing it.  For
       example, a trusted gateway might remove the envelope, but not the
       signature, of a message, and then forward the signed message on to
       the end recipient so that they can verify the signatures directly.
       If the transport internal to the site is not 8-bit clean, such as on
       a wide-area network with a single mail gateway, verifying the
       signature will not be possible unless the original MIME entity was
       only 7-bit data.

       In the case where S/MIME implementations can determine that all
       intended recipients are capable of handling inner (all but the
       outermost) binary MIME objects, implementations SHOULD use binary
       encoding as opposed to a 7-bit-safe transfer encoding for the inner
       entities.  The use of a 7-bit-safe encoding (such as base64)
       unnecessarily expands the message size.  Implementations MAY
       determine that recipient implementations are capable of
       handling inner binary MIME entities by (1) interpreting the
       id-cap-preferBinaryInside SMIMECapabilities attribute, (2) prior
       agreement, or (3) other means.

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       If one or more intended recipients are unable to handle inner binary
       MIME objects or if this capability is unknown for any of the intended
       recipients, S/MIME implementations SHOULD use transfer encoding as
       described in Section 3.1.3 for all MIME entities they secure.

    3.1.3.  Transfer Encoding for Signing Using multipart/signed

       If a multipart/signed entity is ever to be transmitted over the
       standard Internet SMTP infrastructure or other transport that is
       constrained to 7-bit text, it MUST have transfer encoding applied so
       that it is represented as 7-bit text.  MIME entities that are already
       7-bit data need no transfer encoding.  Entities such as 8-bit text
       and binary data can be encoded with quoted-printable or base64
       transfer encoding.

       The primary reason for the 7-bit requirement is that the Internet
       mail transport infrastructure cannot guarantee transport of 8-bit or
       binary data.  Even though many segments of the transport
       infrastructure now handle 8-bit and even binary data, it is sometimes
       not possible to know whether the transport path is 8-bit clean.  If a
       mail message with 8-bit data were to encounter a message transfer
       agent that cannot transmit 8-bit or binary data, the agent has three
       options, none of which are acceptable for a clear-signed message:

       -  The agent could change the transfer encoding; this would
          invalidate the signature.

       -  The agent could transmit the data anyway, which would most likely
          result in the 8th bit being corrupted; this too would invalidate
          the signature.

       -  The agent could return the message to the sender.

       [RFC1847] prohibits an agent from changing the transfer encoding of
       the first part of a multipart/signed message.  If a compliant agent
       that cannot transmit 8-bit or binary data encountered a
       multipart/signed message with 8-bit or binary data in the first part,
       it would have to return the message to the sender as undeliverable.

    3.1.4.  Sample Canonical MIME Entity

       This example shows a multipart/mixed message with full transfer
       encoding.  This message contains a text part and an attachment.  The
       sample message text includes characters that are not ASCII and thus
       need to be transfer encoded.  Though not shown here, the end of each
       line is <CR><LF>.  The line ending of the MIME headers, the text, and
       the transfer-encoded parts all MUST be <CR><LF>.

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       Note that this example is not an example of an S/MIME message.

       Content-Type: multipart/mixed; boundary=bar

       --bar
       Content-Type: text/plain; charset=iso-8859-1
       Content-Transfer-Encoding: quoted-printable

       =A1Hola Michael!

       How do you like the new S/MIME specification?

       It's generally a good idea to encode lines that begin with
       From=20because some mail transport agents will insert a
       greater-than (>) sign, thus invalidating the signature.

       Also, in some cases it might be desirable to encode any =20
       trailing whitespace that occurs on lines in order to ensure =20
       that the message signature is not invalidated when passing =20
       a gateway that modifies such whitespace (like BITNET). =20

       --bar
       Content-Type: image/jpeg
       Content-Transfer-Encoding: base64

       iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
       jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
       uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
       HOxEa44b+EI=

       --bar--

    3.2.  The application/pkcs7-mime Media Type

       The application/pkcs7-mime media type is used to carry CMS content
       types, including EnvelopedData, SignedData, and CompressedData.  The
       details of constructing these entities are described in subsequent
       sections.  This section describes the general characteristics of the
       application/pkcs7-mime media type.

       The carried CMS object always contains a MIME entity that is prepared
       as described in Section 3.1 if the eContentType is id-data.  Other
       contents MAY be carried when the eContentType contains different
       values.  See [ESS] for an example of this with signed receipts.

       Since CMS content types are binary data, in most cases base64
       transfer encoding is appropriate -- in particular, when used with
       SMTP transport.  The transfer encoding used depends on the transport

    Schaad, et al.               Standards Track                   [Page 26]

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       through which the object is to be sent and is not a characteristic of
       the media type.

       Note that this discussion refers to the transfer encoding of the CMS
       object or "outside" MIME entity.  It is completely distinct from, and
       unrelated to, the transfer encoding of the MIME entity secured by the
       CMS object -- the "inside" object, which is described in Section 3.1.

       Because there are several types of application/pkcs7-mime objects, a
       sending agent SHOULD do as much as possible to help a receiving agent
       know about the contents of the object without forcing the receiving
       agent to decode the ASN.1 for the object.  The Content-Type header
       field of all application/pkcs7-mime objects SHOULD include the
       optional "smime-type" parameter, as described in the following
       sections.

    3.2.1.  The name and filename Parameters

       For application/pkcs7-mime, sending agents SHOULD emit the
       optional "name" parameter to the Content-Type field for compatibility
       with older systems.  Sending agents SHOULD also emit the optional
       Content-Disposition field [RFC2183] with the "filename" parameter.
       If a sending agent emits the above parameters, the value of the
       parameters SHOULD be a filename with the appropriate extension:

                                                                    File
       Media Type                                                Extension
       -------------------------------------------------------------------
       application/pkcs7-mime (SignedData, EnvelopedData,           .p7m
          AuthEnvelopedData)
       application/pkcs7-mime (degenerate SignedData certificate    .p7c
          management message)
       application/pkcs7-mime (CompressedData)                      .p7z
       application/pkcs7-signature (SignedData)                     .p7s

       In addition, the filename SHOULD be limited to eight characters
       followed by a three-letter extension.  The eight-character filename
       base can be any distinct name; the use of the filename base "smime"
       SHOULD be used to indicate that the MIME entity is associated with
       S/MIME.

       Including a filename serves two purposes.  It facilitates easier use
       of S/MIME objects as files on disk.  It also can convey type
       information across gateways.  When a MIME entity of type
       application/pkcs7-mime (for example) arrives at a gateway that has no
       special knowledge of S/MIME, it will default the entity's media type
       to application/octet-stream and treat it as a generic attachment,
       thus losing the type information.  However, the suggested filename

    Schaad, et al.               Standards Track                   [Page 27]

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       for an attachment is often carried across a gateway.  This often
       allows the receiving systems to determine the appropriate application
       to hand the attachment off to -- in this case, a standalone S/MIME
       processing application.  Note that this mechanism is provided as a
       convenience for implementations in certain environments.  A proper
       S/MIME implementation MUST use the media types and MUST NOT rely on
       the file extensions.

    3.2.2.  The smime-type Parameter

       The application/pkcs7-mime content type defines the optional
       "smime-type" parameter.  The intent of this parameter is to convey
       details about the security applied (signed or enveloped) along with
       information about the contained content.  This specification defines
       the following smime-types.

           Name                   CMS Type              Inner Content
           ----------------------------------------------------------
           enveloped-data         EnvelopedData         id-data
           signed-data            SignedData            id-data
           certs-only             SignedData            id-data
           compressed-data        CompressedData        id-data
           authEnveloped-data     AuthEnvelopedData     id-data

       In order for consistency to be obtained with future specifications,
       the following guidelines SHOULD be followed when assigning a new
       smime-type parameter.

       1.  If both signing and encryption can be applied to the content,
           then three values for smime-type SHOULD be assigned: "signed-*",
           "authEnv-*", and "enveloped-*".  If one operation can be
           assigned, then this can be omitted.  Thus, since "certs-only" can
           only be signed, "signed-" is omitted.

       2.  A common string for a content OID SHOULD be assigned.  We use
           "data" for the id-data content OID when MIME is the inner
           content.

       3.  If no common string is assigned, then the common string of
           "OID.<oid>" is recommended (for example,
           "OID.2.16.840.1.101.3.4.1.2" would be AES-128 CBC).

       It is explicitly intended that this field be a suitable hint for mail
       client applications to indicate whether a message is "signed",
       "authEnveloped", or "enveloped" without having to tunnel into the CMS
       payload.

    Schaad, et al.               Standards Track                   [Page 28]

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       A registry for additional smime-type parameter values has been
       defined in [RFC7114].

    3.3.  Creating an Enveloped-Only Message

       This section describes the format for enveloping a MIME entity
       without signing it.  It is important to note that sending enveloped
       but not signed messages does not provide for data integrity.  The
       "enveloped-only" structure does not support authenticated symmetric
       algorithms.  Use the "authenticated enveloped" structure for these
       algorithms.  Thus, it is possible to replace ciphertext in such a way
       that the processed message will still be valid, but the meaning can
       be altered.

       Step 1.  The MIME entity to be enveloped is prepared according to
                Section 3.1.

       Step 2.  The MIME entity and other required data are processed into a
                CMS object of type EnvelopedData.  In addition to encrypting
                a copy of the content-encryption key (CEK) for each
                recipient, a copy of the CEK SHOULD be encrypted for the
                originator and included in the EnvelopedData (see [RFC5652],
                Section 6).

       Step 3.  The EnvelopedData object is wrapped in a CMS ContentInfo
                object.

       Step 4.  The ContentInfo object is inserted into an
                application/pkcs7-mime MIME entity.

       The smime-type parameter for enveloped-only messages is
       "enveloped-data".  The file extension for this type of message
       is ".p7m".

       A sample message would be:

       Content-Type: application/pkcs7-mime; name=smime.p7m;
          smime-type=enveloped-data
       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7m

       MIIBHgYJKoZIhvcNAQcDoIIBDzCCAQsCAQAxgcAwgb0CAQAwJjASMRAwDgYDVQQDEw
       dDYXJsUlNBAhBGNGvHgABWvBHTbi7NXXHQMA0GCSqGSIb3DQEBAQUABIGAC3EN5nGI
       iJi2lsGPcP2iJ97a4e8kbKQz36zg6Z2i0yx6zYC4mZ7mX7FBs3IWg+f6KgCLx3M1eC
       bWx8+MDFbbpXadCDgO8/nUkUNYeNxJtuzubGgzoyEd8Ch4H/dd9gdzTd+taTEgS0ip
       dSJuNnkVY4/M652jKKHRLFf02hosdR8wQwYJKoZIhvcNAQcBMBQGCCqGSIb3DQMHBA
       gtaMXpRwZRNYAgDsiSf8Z9P43LrY4OxUk660cu1lXeCSFOSOpOJ7FuVyU=

    Schaad, et al.               Standards Track                   [Page 29]

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    3.4.  Creating an Authenticated Enveloped-Only Message

       This section describes the format for enveloping a MIME entity
       without signing it.  Authenticated enveloped messages provide
       confidentiality and data integrity.  It is important to note that
       sending authenticated enveloped messages does not provide for proof
       of origination when using S/MIME.  It is possible for a third party
       to replace ciphertext in such a way that the processed message will
       still be valid, but the meaning can be altered.  However, this is
       substantially more difficult than it is for an enveloped-only
       message, as the algorithm does provide a level of authentication.
       Any recipient for whom the message is encrypted can replace it
       without detection.

       Step 1.  The MIME entity to be enveloped is prepared according to
                Section 3.1.

       Step 2.  The MIME entity and other required data are processed into a
                CMS object of type AuthEnvelopedData.  In addition to
                encrypting a copy of the CEK for each recipient, a copy of
                the CEK SHOULD be encrypted for the originator and included
                in the AuthEnvelopedData (see [RFC5083]).

       Step 3.  The AuthEnvelopedData object is wrapped in a CMS ContentInfo
                object.

       Step 4.  The ContentInfo object is inserted into an
                application/pkcs7-mime MIME entity.

       The smime-type parameter for authenticated enveloped-only messages is
       "authEnveloped-data".  The file extension for this type of message
       is ".p7m".

    Schaad, et al.               Standards Track                   [Page 30]

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       A sample message would be:

       Content-Type: application/pkcs7-mime; smime-type=authEnveloped-data;
          name=smime.p7m
       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7m

       MIIDWQYLKoZIhvcNAQkQARegggNIMIIDRAIBADGBvjCBuwIBADAmMBIxEDAO
       BgNVBAMTB0NhcmxSU0ECEEY0a8eAAFa8EdNuLs1dcdAwCwYJKoZIhvcNAQEB
       BIGAgyZJo0ERTxA4xdTri5P5tVMyh0RARepTUCORZvlUbcUlaI8IpJZH3/J1
       Fv6MxTRS4O/K+ZcTlQmYeWLQvwdltQdOIP3mhpqXzTnOYhTK1IDtF2zx75Lg
       vE+ilpcLIzXfJB4RCBPtBWaHAof4Wb+VMQvLkk9OolX4mRSH1LPktgAwggJq
       BgkqhkiG9w0BBwEwGwYJYIZIAWUDBAEGMA4EDGPizioC9OHSsnNx4oCCAj7Y
       Cb8rOy8+55106newEJohC/aDgWbJhrMKzSOwa7JraXOV3HXD3NvKbl665dRx
       vmDwSCNaLCRU5q8/AxQx2SvnAbM+JKcEfC/VFdd4SiHNiUECAApLku2rMi5B
       WrhW/FXmx9d+cjum2BRwB3wj0q1wajdB0/kVRbQwg697dnlYyUog4vpJERjr
       7KAkawZx1RMHaM18wgZjUNpCBXFS3chQi9mTBp2i2Hf5iZ8OOtTx+rCQUmI6
       Jhy03vdcPCCARBjn3v0d3upZYDZddMA41CB9fKnnWFjadV1KpYwv80tqsEfx
       Vo0lJQ5VtJ8MHJiBpLVKadRIZ4iH2ULC0JtN5mXE1SrFKh7cqbJ4+7nqSRL3
       oBTud3rX41DGshOjpqcYHT4sqYlgZkc6dp0g1+hF1p3cGmjHdpysV2NVSUev
       ghHbvSqhIsXFzRSWKiZOigmlkv3R5LnjpYyP4brM62Jl7y0qborvV4dNMz7m
       D+5YxSlH0KAe8z6TT3LHuQdN7QCkFoiUSCaNhpAFaakkGIpqcqLhpOK4lXxt
       kptCG93eUwNCcTxtx6bXufPR5TUHohvZvfeqMp42kL37FJC/A8ZHoOxXy8+X
       X5QYxCQNuofWlvnIWv0Nr8w65x6lgVjPYmd/cHwzQKBTBMXN6pBud/PZL5zF
       tw3QHlQkBR+UflMWZKeN9L0KdQ27mQlCo5gQS85aifxoiiA2v9+0hxZw91rP
       IW4D+GS7oMMoKj8ZNyCJJsyf5smRZ+WxeBoolb3+TiGcBBCsRnfe6noLZiFO
       6Zeu2ZwE

    3.5.  Creating a Signed-Only Message

       There are two formats for signed messages defined for S/MIME:

       -  application/pkcs7-mime with SignedData.

       -  multipart/signed.

       In general, the multipart/signed form is preferred for sending, and
       receiving agents MUST be able to handle both.

    Schaad, et al.               Standards Track                   [Page 31]

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    3.5.1.  Choosing a Format for Signed-Only Messages

       There are no hard-and-fast rules as to when a particular signed-only
       format is chosen.  It depends on the capabilities of all the
       receivers and the relative importance of receivers with S/MIME
       facilities being able to verify the signature versus the importance
       of receivers without S/MIME software being able to view the message.

       Messages signed using the multipart/signed format can always be
       viewed by the receiver whether or not they have S/MIME software.
       They can also be viewed whether they are using a MIME-native user
       agent or they have messages translated by a gateway.  In this
       context, "be viewed" means the ability to process the message
       essentially as if it were not a signed message, including any other
       MIME structure the message might have.

       Messages signed using the SignedData format cannot be viewed by a
       recipient unless they have S/MIME facilities.  However, the
       SignedData format protects the message content from being changed by
       benign intermediate agents.  Such agents might do line wrapping or
       content-transfer encoding changes that would break the signature.

    3.5.2.  Signing Using application/pkcs7-mime with SignedData

       This signing format uses the application/pkcs7-mime media type.  The
       steps to create this format are as follows:

       Step 1.  The MIME entity is prepared according to Section 3.1.

       Step 2.  The MIME entity and other required data are processed into a
                CMS object of type SignedData.

       Step 3.  The SignedData object is wrapped in a CMS ContentInfo
                object.

       Step 4.  The ContentInfo object is inserted into an
                application/pkcs7-mime MIME entity.

       The smime-type parameter for messages using application/pkcs7-mime
       with SignedData is "signed-data".  The file extension for this type
       of message is ".p7m".

    Schaad, et al.               Standards Track                   [Page 32]

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       A sample message would be:

       Content-Type: application/pkcs7-mime; smime-type=signed-data;
          name=smime.p7m
       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7m

       MIIDmQYJKoZIhvcNAQcCoIIDijCCA4YCAQExCTAHBgUrDgMCGjAtBgkqhkiG9w0BBw
       GgIAQeDQpUaGlzIGlzIHNvbWUgc2FtcGxlIGNvbnRlbnQuoIIC4DCCAtwwggKboAMC
       AQICAgDIMAkGByqGSM44BAMwEjEQMA4GA1UEAxMHQ2FybERTUzAeFw05OTA4MTcwMT
       EwNDlaFw0zOTEyMzEyMzU5NTlaMBMxETAPBgNVBAMTCEFsaWNlRFNTMIIBtjCCASsG
       ByqGSM44BAEwggEeAoGBAIGNze2D6gqeOT7CSCij5EeT3Q7XqA7sU8WrhAhP/5Thc0
       h+DNbzREjR/p+vpKGJL+HZMMg23j+bv7dM3F9piuR10DcMkQiVm96nXvn89J8v3UOo
       i1TxP7AHCEdNXYjDw7Wz41UIddU5dhDEeL3/nbCElzfy5FEbteQJllzzflvbAhUA4k
       emGkVmuBPG2o+4NyErYov3k80CgYAmONAUiTKqOfs+bdlLWWpMdiM5BAI1XPLLGjDD
       HlBd3ZtZ4s2qBT1YwHuiNrhuB699ikIlp/R1z0oIXks+kPht6pzJIYo7dhTpzi5dow
       fNI4W4LzABfG1JiRGJNkS9+MiVSlNWteL5c+waYTYfEX/Cve3RUP+YdMLRgUpgObo2
       OQOBhAACgYBc47ladRSWC6l63eM/qeysXty9txMRNKYWiSgRI9k0hmd1dRMSPUNbb+
       VRv/qJ8qIbPiR9PQeNW2PIu0WloErjhdbOBoA/6CN+GvIkq1MauCcNHu8Iv2YUgFxi
       rGX6FYvxuzTU0pY39mFHssQyhPB+QUD9RqdjTjPypeL08oPluKOBgTB/MAwGA1UdEw
       EB/wQCMAAwDgYDVR0PAQH/BAQDAgbAMB8GA1UdIwQYMBaAFHBEPoIub4feStN14z0g
       vEMrk/EfMB0GA1UdDgQWBBS+bKGz48H37UNwpM4TAeL945f+zTAfBgNVHREEGDAWgR
       RBbGljZURTU0BleGFtcGxlLmNvbTAJBgcqhkjOOAQDAzAAMC0CFFUMpBkfQiuJcSIz
       jYNqtT1na79FAhUAn2FTUlQLXLLd2ud2HeIQUltDXr0xYzBhAgEBMBgwEjEQMA4GA1
       UEAxMHQ2FybERTUwICAMgwBwYFKw4DAhowCQYHKoZIzjgEAwQuMCwCFD1cSW6LIUFz
       eXle3YI5SKSBer/sAhQmCq7s/CTFHOEjgASeUjbMpx5g6A==

    3.5.3.  Signing Using the multipart/signed Format

       This format is a clear-signing format.  Recipients without any S/MIME
       or CMS processing facilities are able to view the message.  It makes
       use of the multipart/signed media type described in [RFC1847].  The
       multipart/signed media type has two parts.  The first part contains
       the MIME entity that is signed; the second part contains the
       "detached signature" CMS SignedData object in which the
       encapContentInfo eContent field is absent.

    3.5.3.1.  The application/pkcs7-signature Media Type

       This media type always contains a CMS ContentInfo containing a single
       CMS object of type SignedData.  The SignedData encapContentInfo
       eContent field MUST be absent.  The signerInfos field contains the
       signatures for the MIME entity.

       The file extension for signed-only messages using
       application/pkcs7-signature is ".p7s".

    Schaad, et al.               Standards Track                   [Page 33]

    RFC 8551            S/MIME 4.0 Message Specification          April 2019

    3.5.3.2.  Creating a multipart/signed Message

       Step 1.  The MIME entity to be signed is prepared according to
                Section 3.1, taking special care for clear-signing.

       Step 2.  The MIME entity is presented to CMS processing in order to
                obtain an object of type SignedData in which the
                encapContentInfo eContent field is absent.

       Step 3.  The MIME entity is inserted into the first part of a
                multipart/signed message with no processing other than that
                described in Section 3.1.

       Step 4.  Transfer encoding is applied to the "detached signature" CMS
                SignedData object, and it is inserted into a MIME entity of
                type application/pkcs7-signature.

       Step 5.  The MIME entity of the application/pkcs7-signature is
                inserted into the second part of the multipart/signed
                entity.

       The multipart/signed Content-Type has two required parameters: the
       protocol parameter and the micalg parameter.

       The protocol parameter MUST be "application/pkcs7-signature".  Note
       that quotation marks are required around the protocol parameter
       because MIME requires that the "/" character in the parameter value
       MUST be quoted.

    Schaad, et al.               Standards Track                   [Page 34]

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       The micalg parameter allows for one-pass processing when the
       signature is being verified.  The value of the micalg parameter is
       dependent on the message digest algorithm(s) used in the calculation
       of the Message Integrity Check.  If multiple message digest
       algorithms are used, they MUST be separated by commas per [RFC1847].
       The values to be placed in the micalg parameter SHOULD be from the
       following:

            Algorithm      Value Used
            -----------------------------------------------------------
            MD5*           md5
            SHA-1*         sha-1
            SHA-224        sha-224
            SHA-256        sha-256
            SHA-384        sha-384
            SHA-512        sha-512
            Any other      (defined separately in the algorithm profile
                            or "unknown" if not defined)

       *Note: MD5 and SHA-1 are historical and no longer considered secure.
       See Appendix B for details.

       (Historical note: Some early implementations of S/MIME emitted and
       expected "rsa-md5", "rsa-sha1", and "sha1" for the micalg parameter.)
       Receiving agents SHOULD be able to recover gracefully from a micalg
       parameter value that they do not recognize.  Future values for this
       parameter will be taken from the IANA "Hash Function Textual Names"
       registry.

    Schaad, et al.               Standards Track                   [Page 35]

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    3.5.3.3.  Sample multipart/signed Message

       Content-Type: multipart/signed;
           micalg=sha-256;
           boundary="----=_NextBoundary____Fri,_06_Sep_2002_00:25:21";
           protocol="application/pkcs7-signature"

       This is a multipart message in MIME format.

       ------=_NextBoundary____Fri,_06_Sep_2002_00:25:21

       This is some sample content.
       ------=_NextBoundary____Fri,_06_Sep_2002_00:25:21
       Content-Type: application/pkcs7-signature; name=smime.p7s
       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7s

       MIIBJgYJKoZIhvcNAQcCoIIBFzCCARMCAQExADALBgkqhkiG9w0BBwExgf4w
       gfsCAQIwJjASMRAwDgYDVQQDEwdDYXJsUlNBAhBGNGvHgABWvBHTbi7EELOw
       MAsGCWCGSAFlAwQCAaAxMC8GCSqGSIb3DQEJBDEiBCCxwpZGNZzTSsugsn+f
       lEidzQK4mf/ozKqfmbxhcIkKqjALBgkqhkiG9w0BAQsEgYB0XJV7fjPa5Nuh
       oth5msDfP8A5urYUMjhNpWgXG8ae3XpppqVrPi2nVO41onHnkByjkeD/wc31
       A9WH8MzFQgSTsrJ65JvffTTXkOpRPxsSHn3wJFwP/atWHkh8YK/jR9bULhUl
       Mv5jQEDiwVX5DRasxu6Ld8zv9u5/TsdBNiufGw==

       ------=_NextBoundary____Fri,_06_Sep_2002_00:25:21--

       The content that is digested (the first part of the multipart/signed)
       consists of the bytes:

       54 68 69 73 20 69 73 20 73 6f 6d 65 20 73 61 6d 70 6c 65 20 63 6f 6e
       74 65 6e 74 2e 0d 0a

    3.6.  Creating a Compressed-Only Message

       This section describes the format for compressing a MIME entity.
       Please note that versions of S/MIME prior to version 3.1 did not
       specify any use of CompressedData and will not recognize it.  The use
       of a capability to indicate the ability to receive CompressedData is
       described in [RFC3274] and is the preferred method for compatibility.

       Step 1.  The MIME entity to be compressed is prepared according to
                Section 3.1.

       Step 2.  The MIME entity and other required data are processed into a
                CMS object of type CompressedData.

    Schaad, et al.               Standards Track                   [Page 36]

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       Step 3.  The CompressedData object is wrapped in a CMS ContentInfo
                object.

       Step 4.  The ContentInfo object is inserted into an
                application/pkcs7-mime MIME entity.

       The smime-type parameter for compressed-only messages is
       "compressed-data".  The file extension for this type of message
       is ".p7z".

       A sample message would be:

       Content-Type: application/pkcs7-mime; smime-type=compressed-data;
          name=smime.p7z
       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7z

       eNoLycgsVgCi4vzcVIXixNyCnFSF5Py8ktS8Ej0AlCkKVA==

    3.7.  Multiple Operations

       The signed-only, enveloped-only, and compressed-only MIME formats can
       be nested.  This works because these formats are all MIME entities
       that encapsulate other MIME entities.

       An S/MIME implementation MUST be able to receive and process
       arbitrarily nested S/MIME within reasonable resource limits of the
       recipient computer.

       It is possible to apply any of the signing, encrypting, and
       compressing operations in any order.  It is up to the implementer and
       the user to choose.  When signing first, the signatories are then
       securely obscured by the enveloping.  When enveloping first, the
       signatories are exposed, but it is possible to verify signatures
       without removing the enveloping.  This can be useful in an
       environment where automatic signature verification is desired, as no
       private key material is required to verify a signature.

       There are security ramifications related to choosing whether to sign
       first or encrypt first.  A recipient of a message that is encrypted
       and then signed can validate that the encrypted block was unaltered
       but cannot determine any relationship between the signer and the
       unencrypted contents of the message.  A recipient of a message that
       is signed and then encrypted can assume that the signed message
       itself has not been altered but that a careful attacker could have
       changed the unauthenticated portions of the encrypted message.

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       When using compression, keep the following guidelines in mind:

       -  Compression of encrypted data that is transferred as binary data
       is discouraged, since it will not yield significant compression.
       Encrypted data that is transferred as base64-encoded data could
       benefit as well.

       -  If a lossy compression algorithm is used with signing, you will
       need to compress first, then sign.

    3.8.  Creating a Certificate Management Message

       The certificate management message or MIME entity is used to
       transport certificates and/or Certificate Revocation Lists (CRLs),
       such as in response to a registration request.

       Step 1.  The certificates and/or CRLs are made available to the CMS
                generating process that creates a CMS object of type
                SignedData.  The SignedData encapContentInfo eContent field
                MUST be absent, and the signerInfos field MUST be empty.

       Step 2.  The SignedData object is wrapped in a CMS ContentInfo
                object.

       Step 3.  The ContentInfo object is enclosed in an
                application/pkcs7-mime MIME entity.

       The smime-type parameter for a certificate management message is
       "certs-only".  The file extension for this type of message is ".p7c".

    3.9.  Registration Requests

       A sending agent that signs messages MUST have a certificate for the
       signature so that a receiving agent can verify the signature.  There
       are many ways of getting certificates, such as through an exchange
       with a certification authority, through a hardware token or diskette,
       and so on.

       S/MIME v2 [SMIMEv2] specified a method for "registering" public keys
       with certificate authorities using an application/pkcs10 body part.
       Since that time, the IETF PKIX Working Group has developed other
       methods for requesting certificates.  However, S/MIME v4.0 does not
       require a particular certificate request mechanism.

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    3.10.  Identifying an S/MIME Message

       Because S/MIME takes into account interoperation in non-MIME
       environments, several different mechanisms are employed to carry the
       type information, and it becomes a bit difficult to identify S/MIME
       messages.  The following table lists criteria for determining whether
       or not a message is an S/MIME message.  A message is considered an
       S/MIME message if it matches any of the criteria listed below.

       The file suffix in the table below comes from the "name" parameter in
       the Content-Type header field or the "filename" parameter in the
       Content-Disposition header field.  The MIME parameters that carry the
       file suffix are not listed below.

       Media Type                 Parameters                     File Suffix
       ---------------------------------------------------------------------
       application/pkcs7-mime     N/A                            N/A

       multipart/signed           protocol=                      N/A
                                  "application/pkcs7-signature"

       application/octet-stream   N/A                            p7m, p7s,
                                                                 p7c, p7z

    4.  Certificate Processing

       A receiving agent MUST provide some certificate retrieval mechanism
       in order to gain access to certificates for recipients of digital
       envelopes.  This specification does not cover how S/MIME agents
       handle certificates -- only what they do after a certificate has been
       validated or rejected.  S/MIME certificate issues are covered in
       [RFC5750].

       At a minimum, for initial S/MIME deployment, a user agent could
       automatically generate a message to an intended recipient requesting
       that recipient's certificate in a signed return message.  Receiving
       and sending agents SHOULD also provide a mechanism to allow a user to
       "store and protect" certificates for correspondents in such a way as
       to guarantee their later retrieval.

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    4.1.  Key Pair Generation

       All key pairs MUST be generated from a good source of
       non-deterministic random input [RFC4086], and the private key MUST be
       protected in a secure fashion.

       An S/MIME user agent MUST NOT generate asymmetric keys less than
       2048 bits for use with an RSA signature algorithm.

       For 2048-bit through 4096-bit RSA with SHA-256, see [RFC5754] and
       [FIPS186-4].  The first reference provides the signature algorithm's
       OID, and the second provides the signature algorithm's definition.

       For RSASSA-PSS with SHA-256, see [RFC4056].  For RSAES-OAEP, see
       [RFC3560].

    4.2.  Signature Generation

       The following are the requirements for an S/MIME agent when
       generating RSA and RSASSA-PSS signatures:

               key size <= 2047 : SHOULD NOT (Note 2)
       2048 <= key size <= 4096 : SHOULD     (Note 1)
       4096 <  key size         : MAY        (Note 1)

       Note 1: See Security Considerations in Section 6.
       Note 2: See Historical Mail Considerations in Appendix B.

       Key sizes for ECDSA and EdDSA are fixed by the curve.

    4.3.  Signature Verification

       The following are the requirements for S/MIME receiving agents during
       verification of RSA and RSASSA-PSS signatures:

               key size <= 2047 : SHOULD NOT (Note 2)
       2048 <= key size <= 4096 : MUST       (Note 1)
       4096 <  key size         : MAY        (Note 1)

       Note 1: See Security Considerations in Section 6.
       Note 2: See Historical Mail Considerations in Appendix B.

       Key sizes for ECDSA and EdDSA are fixed by the curve.

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    4.4.  Encryption

       The following are the requirements for an S/MIME agent when
       establishing keys for content encryption using the RSA and RSA-OAEP
       algorithms:

               key size <= 2047 : SHOULD NOT (Note 2)
       2048 <= key size <= 4096 : SHOULD     (Note 1)
       4096 <  key size         : MAY        (Note 1)

       Note 1: See Security Considerations in Section 6.
       Note 2: See Historical Mail Considerations in Appendix B.

       Key sizes for ECDH are fixed by the curve.

    4.5.  Decryption

       The following are the requirements for an S/MIME agent when
       establishing keys for content decryption using the RSA and RSAES-OAEP
       algorithms:

               key size <= 2047 : MAY        (Note 2)
       2048 <= key size <= 4096 : MUST       (Note 1)
       4096 <  key size         : MAY        (Note 1)

       Note 1: See Security Considerations in Section 6.
       Note 2: See Historical Mail Considerations in Appendix B.

       Key sizes for ECDH are fixed by the curve.

    5.  IANA Considerations

       This section (1) updates the media type registrations for
       application/pkcs7-mime and application/pkcs7-signature to refer to
       this document as opposed to RFC 5751, (2) adds authEnveloped-data to
       the list of values for smime-type, and (3) updates references from
       RFC 5751 to this document in general.

       Note that other documents can define additional media types for
       S/MIME.

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    5.1.  Media Type for application/pkcs7-mime

       Type name: application

       Subtype Name: pkcs7-mime

       Required Parameters: NONE

       Optional Parameters: smime-type
                            name

       Encoding Considerations: See Section 3 of this document

       Security Considerations: See Section 6 of this document

       Interoperability Considerations: See Sections 1-6 of this document

       Published Specification: RFC 2311, RFC 2633, RFC 5751,
                                and this document

       Applications that use this media type: Security applications

       Fragment identifier considerations: N/A

       Additional information:
           Deprecated alias names for this type: N/A
           Magic number(s): N/A
           File extensions(s): See Section 3.2.1 of this document
           Macintosh file type code(s): N/A

       Person & email address to contact for further information:
          The IESG <iesg@ietf.org>

       Intended usage: COMMON

       Restrictions on usage: NONE

       Author: Sean Turner

       Change Controller: LAMPS working group delegated from the IESG

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    5.2.  Media Type for application/pkcs7-signature

       Type name: application

       Subtype Name: pkcs7-signature

       Required Parameters: N/A

       Optional Parameters: N/A

       Encoding Considerations: See Section 3 of this document

       Security Considerations: See Section 6 of this document

       Interoperability Considerations: See Sections 1-6 of this document

       Published Specification: RFC 2311, RFC 2633, RFC 5751,
                                and this document

       Applications that use this media type: Security applications

       Fragment identifier considerations: N/A

       Additional information:
           Deprecated alias names for this type: N/A
           Magic number(s): N/A
           File extensions(s): See Section 3.2.1 of this document
           Macintosh file type code(s): N/A

       Person & email address to contact for further information:
          The IESG <iesg@ietf.org>

       Intended usage: COMMON

       Restrictions on usage: N/A

       Author: Sean Turner

       Change Controller: LAMPS working group delegated from the IESG

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    5.3.  authEnveloped-data smime-type

       IANA has registered the following value in the "Parameter Values for
       the smime-type Parameter" registry.

          smime-type value: authEnveloped-data

          Reference: RFC 8551, Section 3.2.2

    5.4.  Reference Updates

       IANA is to update all references to RFC 5751 to this document.  Known
       registries to be updated are "CoAP Content-Formats" and "media-
       types".

    6.  Security Considerations

       Cryptographic algorithms will be broken or weakened over time.
       Implementers and users need to check that the cryptographic
       algorithms listed in this document continue to provide the expected
       level of security.  The IETF from time to time may issue documents
       dealing with the current state of the art.  For example:

       -  The Million Message Attack described in RFC 3218 [RFC3218].

       -  The Diffie-Hellman "small-subgroup" attacks described in RFC 2785
          [RFC2785].

       -  The attacks against hash algorithms described in RFC 4270
          [RFC4270].

       This specification uses Public-Key Cryptography technologies.  It is
       assumed that the private key is protected to ensure that it is not
       accessed or altered by unauthorized parties.

       It is impossible for most people or software to estimate the value of
       a message's content.  Further, it is impossible for most people or
       software to estimate the actual cost of recovering an encrypted
       message's content that is encrypted with a key of a particular size.
       Further, it is quite difficult to determine the cost of a failed
       decryption if a recipient cannot process a message's content.  Thus,
       choosing between different key sizes (or choosing whether to just use
       plaintext) is also impossible for most people or software.  However,
       decisions based on these criteria are made all the time, and
       therefore this specification gives a framework for using those
       estimates in choosing algorithms.

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       The choice of 2048 bits as an RSA asymmetric key size in this
       specification is based on the desire to provide at least 100 bits of
       security.  The key sizes that must be supported to conform to this
       specification seem appropriate for the Internet, based on [RFC3766].
       Of course, there are environments, such as financial and medical
       systems, that may select different key sizes.  For this reason, an
       implementation MAY support key sizes beyond those recommended in this
       specification.

       Receiving agents that validate signatures and sending agents that
       encrypt messages need to be cautious of cryptographic processing
       usage when validating signatures and encrypting messages using keys
       larger than those mandated in this specification.  An attacker could
       send certificates with keys that would result in excessive
       cryptographic processing -- for example, keys larger than those
       mandated in this specification, as such keys could swamp the
       processing element.  Agents that use such keys without first
       validating the certificate to a trust anchor are advised to have some
       sort of cryptographic resource management system to prevent such
       attacks.

       Some cryptographic algorithms such as RC2 offer little actual
       security over sending plaintext.  Other algorithms such as TripleDES
       provide security but are no longer considered to be state of the art.
       S/MIME requires the use of current state-of-the-art algorithms such
       as AES and provides the ability to announce cryptographic
       capabilities to parties with whom you communicate.  This allows the
       sender to create messages that can use the strongest common
       encryption algorithm.  Using algorithms such as RC2 is never
       recommended unless the only alternative is no cryptography.

       RSA and DSA keys of less than 2048 bits are now considered by many
       experts to be cryptographically insecure (due to advances in
       computing power) and should no longer be used to protect messages.
       Such keys were previously considered secure, so processing previously
       received signed and encrypted mail will often result in the use of
       weak keys.  Implementations that wish to support previous versions of
       S/MIME or process old messages need to consider the security risks
       that result from smaller key sizes (e.g., spoofed messages) versus
       the costs of denial of service.  If an implementation supports
       verification of digital signatures generated with RSA and DSA keys of
       less than 1024 bits, it MUST warn the user.  Implementers should
       consider providing different warnings for newly received messages and
       previously stored messages.  Server implementations (e.g., secure
       mail list servers) where user warnings are not appropriate SHOULD
       reject messages with weak signatures.

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       Implementers SHOULD be aware that multiple active key pairs can be
       associated with a single individual.  For example, one key pair can
       be used to support confidentiality, while a different key pair can be
       used for digital signatures.

       If a sending agent is sending the same message using different
       strengths of cryptography, an attacker watching the communications
       channel might be able to determine the contents of the strongly
       encrypted message by decrypting the weakly encrypted version.  In
       other words, a sender SHOULD NOT send a copy of a message using
       weaker cryptography than they would use for the original of the
       message.

       Modification of the ciphertext in EnvelopedData can go undetected if
       authentication is not also used, which is the case when sending
       EnvelopedData without wrapping it in SignedData or enclosing
       SignedData within it.  This is one of the reasons for moving from
       EnvelopedData to AuthEnvelopedData, as the authenticated encryption
       algorithms provide the authentication without needing the SignedData
       layer.

       If an implementation is concerned about compliance with National
       Institute of Standards and Technology (NIST) key size
       recommendations, then see [SP800-57].

       If messaging environments make use of the fact that a message is
       signed to change the behavior of message processing (examples would
       be running rules or UI display hints), without first verifying that
       the message is actually signed and knowing the state of the
       signature, this can lead to incorrect handling of the message.
       Visual indicators on messages may need to have the signature
       validation code checked periodically if the indicator is supposed to
       give information on the current status of a message.

       Many people assume that the use of an authenticated encryption
       algorithm is all that is needed for the sender of the message to be
       authenticated.  In almost all cases, this is not a correct statement.
       There are a number of preconditions that need to hold for an
       authenticated encryption algorithm to provide this service:

       -  The starting key must be bound to a single entity.  The use of a
          group key only would allow for the statement that a message was
          sent by one of the entities that held the key but will not
          identify a specific entity.

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       -  The message must have exactly one sender and one recipient.
          Having more than one recipient would allow for the second
          recipient to create a message that the first recipient would
          believe is from the sender by stripping the second recipient from
          the message.

       -  A direct path needs to exist from the starting key to the key used
          as the CEK.  That path needs to guarantee that no third party
          could have seen the resulting CEK.  This means that one needs to
          be using an algorithm that is called a "Direct Encryption" or a
          "Direct Key Agreement" algorithm in other contexts.  This means
          that the starting key is (1) used directly as the CEK or (2) used
          to create a secret that is then transformed into the CEK via a
          KDF step.

       S/MIME implementations almost universally use ephemeral-static rather
       than static-static key agreement and do not use a shared secret for
       encryption.  This means that the first precondition is not met.
       [RFC6278] defines how to use static-static key agreement with CMS, so
       the first precondition can be met.  Currently, all S/MIME key
       agreement methods derive a key-encryption key (KEK) and wrap a CEK.
       This violates the third precondition above.  New key agreement
       algorithms that directly created the CEK without creating an
       intervening KEK would need to be defined.

       Even when all of the preconditions are met and origination of a
       message is established by the use of an authenticated encryption
       algorithm, users need to be aware that there is no way to prove this
       to a third party.  This is because either of the parties can
       successfully create the message (or just alter the content) based on
       the fact that the CEK is going to be known to both parties.  Thus,
       the origination is always built on a presumption that "I did not send
       this message to myself."

       All of the authenticated encryption algorithms in this document use
       counter mode for the encryption portion of the algorithm.  This means
       that the length of the plaintext will always be known, as the
       ciphertext length and the plaintext length are always the same.  This
       information can enable passive observers to infer information based
       solely on the length of the message.  Applications for which this is
       a concern need to provide some type of padding so that the length of
       the message does not provide this information.

       When compression is used with encryption, it has the potential to
       provide an additional layer of security.  However, care needs to be
       taken when designing a protocol that relies on using compression, so
       as not to create a compression oracle.  Compression oracle attacks
       require an adaptive input to the process and attack the unknown

    Schaad, et al.               Standards Track                   [Page 47]

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       content of a message based on the length of the compressed output.
       This means that no attack on the encryption key is necessarily
       required.

       A recent paper on S/MIME and OpenPGP email security [Efail] has
       pointed out a number of problems with the current S/MIME
       specifications and how people have implemented mail clients.  Due to
       the nature of how CBC mode operates, the modes allow for malleability
       of plaintexts.  This malleability allows for attackers to make
       changes in the ciphertext and, if parts of the plaintext are known,
       create arbitrary blocks of plaintext.  These changes can be made
       without the weak integrity check in CBC mode being triggered.  This
       type of attack can be prevented by the use of an Authenticated
       Encryption with Associated Data (AEAD) algorithm with a more robust
       integrity check on the decryption process.  It is therefore
       recommended that mail systems migrate to using AES-GCM as quickly as
       possible and that the decrypted content not be acted on prior to
       finishing the integrity check.

       The other attack that is highlighted in [Efail] is due to an error in
       how mail clients deal with HTML and multipart/mixed messages.
       Clients MUST require that a text/html content type be a complete HTML
       document (per [RFC1866]).  Clients SHOULD treat each of the different
       pieces of the multipart/mixed construct as being of different
       origins.  Clients MUST treat each encrypted or signed piece of a MIME
       message as being of different origins both from unprotected content
       and from each other.

    7.  References

    7.1.  Reference Conventions

       [ASN.1] refers to [X.680], [X.681], [X.682], and [X.683].

       [CMS] refers to [RFC5083] and [RFC5652].

       [ESS] refers to [RFC2634] and [RFC5035].

       [MIME-SPEC] refers to [RFC2045], [RFC2046], [RFC2047], [RFC2049],
       [RFC6838], and [RFC4289].

       [SMIMEv2] refers to [RFC2311], [RFC2312], [RFC2313], [RFC2314], and
       [RFC2315].

       [SMIMEv3] refers to [RFC2630], [RFC2631], [RFC2632], [RFC2633],
       [RFC2634], and [RFC5035].

    Schaad, et al.               Standards Track                   [Page 48]

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       [SMIMEv3.1] refers to [RFC2634], [RFC5035], [RFC5652], [RFC5750], and
       [RFC5751].

       [SMIMEv3.2] refers to [RFC2634], [RFC3850], [RFC3851], [RFC3852], and
       [RFC5035].

       [SMIMEv4] refers to [RFC2634], [RFC5035], [RFC5652], [RFC8550], and
       this document.

    7.2.  Normative References

       [CHARSETS] IANA, "Character sets assigned by IANA",
                  <http://www.iana.org/assignments/character-sets>.

       [FIPS186-4]
                  National Institute of Standards and Technology (NIST),
                  "Digital Signature Standard (DSS)", Federal Information
                  Processing Standards Publication 186-4,
                  DOI 10.6028/NIST.FIPS.186-4, July 2013,
                  <https://nvlpubs.nist.gov/nistpubs/fips/
                  nist.fips.186-4.pdf>.

       [RFC1847]  Galvin, J., Murphy, S., Crocker, S., and N. Freed,
                  "Security Multiparts for MIME: Multipart/Signed and
                  Multipart/Encrypted", RFC 1847, DOI 10.17487/RFC1847,
                  October 1995, <https://www.rfc-editor.org/info/rfc1847>.

       [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                  Extensions (MIME) Part One: Format of Internet Message
                  Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
                  <https://www.rfc-editor.org/info/rfc2045>.

       [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                  Extensions (MIME) Part Two: Media Types", RFC 2046,
                  DOI 10.17487/RFC2046, November 1996,
                  <https://www.rfc-editor.org/info/rfc2046>.

       [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
                  Part Three: Message Header Extensions for Non-ASCII Text",
                  RFC 2047, DOI 10.17487/RFC2047, November 1996,
                  <https://www.rfc-editor.org/info/rfc2047>.

       [RFC2049]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                  Extensions (MIME) Part Five: Conformance Criteria and
                  Examples", RFC 2049, DOI 10.17487/RFC2049, November 1996,
                  <https://www.rfc-editor.org/info/rfc2049>.

    Schaad, et al.               Standards Track                   [Page 49]

    RFC 8551            S/MIME 4.0 Message Specification          April 2019

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <https://www.rfc-editor.org/info/rfc2119>.

       [RFC2183]  Troost, R., Dorner, S., and K. Moore, Ed., "Communicating
                  Presentation Information in Internet Messages: The
                  Content-Disposition Header Field", RFC 2183,
                  DOI 10.17487/RFC2183, August 1997,
                  <https://www.rfc-editor.org/info/rfc2183>.

       [RFC2634]  Hoffman, P., Ed., "Enhanced Security Services for S/MIME",
                  RFC 2634, DOI 10.17487/RFC2634, June 1999,
                  <https://www.rfc-editor.org/info/rfc2634>.

       [RFC3274]  Gutmann, P., "Compressed Data Content Type for
                  Cryptographic Message Syntax (CMS)", RFC 3274,
                  DOI 10.17487/RFC3274, June 2002,
                  <https://www.rfc-editor.org/info/rfc3274>.

       [RFC3370]  Housley, R., "Cryptographic Message Syntax (CMS)
                  Algorithms", RFC 3370, DOI 10.17487/RFC3370, August 2002,
                  <https://www.rfc-editor.org/info/rfc3370>.

       [RFC3560]  Housley, R., "Use of the RSAES-OAEP Key Transport
                  Algorithm in Cryptographic Message Syntax (CMS)",
                  RFC 3560, DOI 10.17487/RFC3560, July 2003,
                  <https://www.rfc-editor.org/info/rfc3560>.

       [RFC3565]  Schaad, J., "Use of the Advanced Encryption Standard (AES)
                  Encryption Algorithm in Cryptographic Message Syntax
                  (CMS)", RFC 3565, DOI 10.17487/RFC3565, July 2003,
                  <https://www.rfc-editor.org/info/rfc3565>.

       [RFC4289]  Freed, N. and J. Klensin, "Multipurpose Internet Mail
                  Extensions (MIME) Part Four: Registration Procedures",
                  BCP 13, RFC 4289, DOI 10.17487/RFC4289, December 2005,
                  <https://www.rfc-editor.org/info/rfc4289>.

       [RFC4056]  Schaad, J., "Use of the RSASSA-PSS Signature Algorithm in
                  Cryptographic Message Syntax (CMS)", RFC 4056,
                  DOI 10.17487/RFC4056, June 2005,
                  <https://www.rfc-editor.org/info/rfc4056>.

       [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
                  "Randomness Requirements for Security", BCP 106, RFC 4086,
                  DOI 10.17487/RFC4086, June 2005,
                  <https://www.rfc-editor.org/info/rfc4086>.

    Schaad, et al.               Standards Track                   [Page 50]

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       [RFC5083]  Housley, R., "Cryptographic Message Syntax (CMS)
                  Authenticated-Enveloped-Data Content Type", RFC 5083,
                  DOI 10.17487/RFC5083, November 2007,
                  <https://www.rfc-editor.org/info/rfc5083>.

       [RFC5084]  Housley, R., "Using AES-CCM and AES-GCM Authenticated
                  Encryption in the Cryptographic Message Syntax (CMS)",
                  RFC 5084, DOI 10.17487/RFC5084, November 2007,
                  <https://www.rfc-editor.org/info/rfc5084>.

       [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
                  RFC 5652, DOI 10.17487/RFC5652, September 2009,
                  <https://www.rfc-editor.org/info/rfc5652>.

       [RFC5753]  Turner, S. and D. Brown, "Use of Elliptic Curve
                  Cryptography (ECC) Algorithms in Cryptographic Message
                  Syntax (CMS)", RFC 5753, DOI 10.17487/RFC5753,
                  January 2010, <https://www.rfc-editor.org/info/rfc5753>.

       [RFC5754]  Turner, S., "Using SHA2 Algorithms with Cryptographic
                  Message Syntax", RFC 5754, DOI 10.17487/RFC5754,
                  January 2010, <https://www.rfc-editor.org/info/rfc5754>.

       [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
                  Specifications and Registration Procedures", BCP 13,
                  RFC 6838, DOI 10.17487/RFC6838, January 2013,
                  <https://www.rfc-editor.org/info/rfc6838>.

       [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
                  2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
                  May 2017, <https://www.rfc-editor.org/info/rfc8174>.

       [RFC8418]  Housley, R., "Use of the Elliptic Curve Diffie-Hellman Key
                  Agreement Algorithm with X25519 and X448 in the
                  Cryptographic Message Syntax (CMS)", RFC 8418,
                  DOI 10.17487/RFC8418, August 2018,
                  <https://www.rfc-editor.org/info/rfc8418>.

       [RFC8419]  Housley, R., "Use of Edwards-Curve Digital Signature
                  Algorithm (EdDSA) Signatures in the Cryptographic Message
                  Syntax (CMS)", RFC 8419, DOI 10.17487/RFC8419,
                  August 2018, <https://www.rfc-editor.org/info/rfc8419>.

       [RFC8550]  Schaad, J., Ramsdell, B., and S. Turner,
                  "Secure/Multipurpose Internet Mail Extensions (S/MIME)
                  Version 4.0 Certificate Handling", RFC 8550,
                  DOI 10.17487/RFC8550, April 2019,
                  <https://www.rfc-editor.org/info/rfc8550>.

    Schaad, et al.               Standards Track                   [Page 51]

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       [X.680]    "Information Technology - Abstract Syntax Notation One
                  (ASN.1): Specification of basic notation", ITU-T
                  Recommendation X.680, ISO/IEC 8824-1:2015, August 2015,
                  <https://www.itu.int/rec/T-REC-X.680>.

       [X.681]    "Information Technology - Abstract Syntax Notation One
                  (ASN.1): Information object specification", ITU-T
                  Recommendation X.681, ISO/IEC 8824-2:2015, August 2015,
                  <https://www.itu.int/rec/T-REC-X.681>.

       [X.682]    "Information Technology - Abstract Syntax Notation One
                  (ASN.1): Constraint specification", ITU-T
                  Recommendation X.682, ISO/IEC 8824-3:2015, August 2015,
                  <https://www.itu.int/rec/T-REC-X.682>.

       [X.683]    "Information Technology - Abstract Syntax Notation One
                  (ASN.1): Parameterization of ASN.1 specifications", ITU-T
                  Recommendation X.683, ISO/IEC 8824-4:2015, August 2015,
                  <https://www.itu.int/rec/T-REC-X.683>.

       [X.690]    "Information Technology - ASN.1 encoding rules:
                  Specification of Basic Encoding Rules (BER), Canonical
                  Encoding Rules (CER) and Distinguished Encoding Rules
                  (DER)", ITU-T Recommendation X.690, ISO/IEC 8825-1:2015,
                  August 2015, <https://www.itu.int/rec/T-REC-X.690>.

    7.3.  Informative References

       [Efail]    Poddebniak, D., Dresen, C., Muller, J., Ising, F.,
                  Schinzel, S., Friedberger, S., Somorovsky, J., and J.
                  Schwenk, "Efail: Breaking S/MIME and OpenPGP Email
                  Encryption using Exfiltration Channels",
                  UsenixSecurity 2018, August 2018,
                  <https://www.usenix.org/system/files/conference/
                  usenixsecurity18/sec18-poddebniak.pdf>.

       [FIPS186-2]
                  National Institute of Standards and Technology (NIST),
                  "Digital Signature Standard (DSS) (also with Change
                  Notice 1)", Federal Information Processing Standards
                  Publication 186-2, January 2000,
                  <https://csrc.nist.gov/publications/detail/fips/186/2/
                  archive/2000-01-27>.

       [RFC1866]  Berners-Lee, T. and D. Connolly, "Hypertext Markup
                  Language - 2.0", RFC 1866, DOI 10.17487/RFC1866,
                  November 1995, <https://www.rfc-editor.org/info/rfc1866>.

    Schaad, et al.               Standards Track                   [Page 52]

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       [RFC2268]  Rivest, R., "A Description of the RC2(r) Encryption
                  Algorithm", RFC 2268, DOI 10.17487/RFC2268, March 1998,
                  <https://www.rfc-editor.org/info/rfc2268>.

       [RFC2311]  Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and
                  L. Repka, "S/MIME Version 2 Message Specification",
                  RFC 2311, DOI 10.17487/RFC2311, March 1998,
                  <https://www.rfc-editor.org/info/rfc2311>.

       [RFC2312]  Dusse, S., Hoffman, P., Ramsdell, B., and J. Weinstein,
                  "S/MIME Version 2 Certificate Handling", RFC 2312, DOI
                  10.17487/RFC2312, March 1998,
                  <https://www.rfc-editor.org/info/rfc2312>.

       [RFC2313]  Kaliski, B., "PKCS #1: RSA Encryption Version 1.5",
                  RFC 2313, DOI 10.17487/RFC2313, March 1998,
                  <https://www.rfc-editor.org/info/rfc2313>.

       [RFC2314]  Kaliski, B., "PKCS #10: Certification Request Syntax
                  Version 1.5", RFC 2314, DOI 10.17487/RFC2314, March 1998,
                  <https://www.rfc-editor.org/info/rfc2314>.

       [RFC2315]  Kaliski, B., "PKCS #7: Cryptographic Message Syntax
                  Version 1.5", RFC 2315, DOI 10.17487/RFC2315, March 1998,
                  <https://www.rfc-editor.org/info/rfc2315>.

       [RFC2630]  Housley, R., "Cryptographic Message Syntax", RFC 2630,
                  DOI 10.17487/RFC2630, June 1999,
                  <https://www.rfc-editor.org/info/rfc2630>.

       [RFC2631]  Rescorla, E., "Diffie-Hellman Key Agreement Method",
                  RFC 2631, DOI 10.17487/RFC2631, June 1999,
                  <https://www.rfc-editor.org/info/rfc2631>.

       [RFC2632]  Ramsdell, B., Ed., "S/MIME Version 3 Certificate
                  Handling", RFC 2632, DOI 10.17487/RFC2632, June 1999,
                  <https://www.rfc-editor.org/info/rfc2632>.

       [RFC2633]  Ramsdell, B., Ed., "S/MIME Version 3 Message
                  Specification", RFC 2633, DOI 10.17487/RFC2633, June 1999,
                  <https://www.rfc-editor.org/info/rfc2633>.

       [RFC2785]  Zuccherato, R., "Methods for Avoiding the "Small-Subgroup"
                  Attacks on the Diffie-Hellman Key Agreement Method for
                  S/MIME", RFC 2785, DOI 10.17487/RFC2785, March 2000,
                  <https://www.rfc-editor.org/info/rfc2785>.

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       [RFC3218]  Rescorla, E., "Preventing the Million Message Attack on
                  Cryptographic Message Syntax", RFC 3218,
                  DOI 10.17487/RFC3218, January 2002,
                  <https://www.rfc-editor.org/info/rfc3218>.

       [RFC3766]  Orman, H. and P. Hoffman, "Determining Strengths For
                  Public Keys Used For Exchanging Symmetric Keys", BCP 86,
                  RFC 3766, DOI 10.17487/RFC3766, April 2004,
                  <https://www.rfc-editor.org/info/rfc3766>.

       [RFC3850]  Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
                  Extensions (S/MIME) Version 3.1 Certificate Handling",
                  RFC 3850, DOI 10.17487/RFC3850, July 2004,
                  <https://www.rfc-editor.org/info/rfc3850>.

       [RFC3851]  Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
                  Extensions (S/MIME) Version 3.1 Message Specification",
                  RFC 3851, DOI 10.17487/RFC3851, July 2004,
                  <https://www.rfc-editor.org/info/rfc3851>.

       [RFC3852]  Housley, R., "Cryptographic Message Syntax (CMS)",
                  RFC 3852, DOI 10.17487/RFC3852, July 2004,
                  <https://www.rfc-editor.org/info/rfc3852>.

       [RFC4134]  Hoffman, P., Ed., "Examples of S/MIME Messages", RFC 4134,
                  DOI 10.17487/RFC4134, July 2005,
                  <https://www.rfc-editor.org/info/rfc4134>.

       [RFC4270]  Hoffman, P. and B. Schneier, "Attacks on Cryptographic
                  Hashes in Internet Protocols", RFC 4270,
                  DOI 10.17487/RFC4270, November 2005,
                  <https://www.rfc-editor.org/info/rfc4270>.

       [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
                  FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
                  <https://www.rfc-editor.org/info/rfc4949>.

       [RFC5035]  Schaad, J., "Enhanced Security Services (ESS) Update:
                  Adding CertID Algorithm Agility", RFC 5035, DOI
                  10.17487/RFC5035, August 2007,
                  <https://www.rfc-editor.org/info/rfc5035>.

       [RFC5750]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
                  Mail Extensions (S/MIME) Version 3.2 Certificate
                  Handling", RFC 5750, DOI 10.17487/RFC5750, January 2010,
                  <https://www.rfc-editor.org/info/rfc5750>.

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       [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
                  Mail Extensions (S/MIME) Version 3.2 Message
                  Specification", RFC 5751, DOI 10.17487/RFC5751,
                  January 2010, <https://www.rfc-editor.org/info/rfc5751>.

       [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
                  for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
                  RFC 6151, DOI 10.17487/RFC6151, March 2011,
                  <https://www.rfc-editor.org/info/rfc6151>.

       [RFC6194]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
                  Considerations for the SHA-0 and SHA-1 Message-Digest
                  Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
                  <https://www.rfc-editor.org/info/rfc6194>.

       [RFC6268]  Schaad, J. and S. Turner, "Additional New ASN.1 Modules
                  for the Cryptographic Message Syntax (CMS) and the Public
                  Key Infrastructure Using X.509 (PKIX)", RFC 6268,
                  DOI 10.17487/RFC6268, July 2011,
                  <https://www.rfc-editor.org/info/rfc6268>.

       [RFC6278]  Herzog, J. and R. Khazan, "Use of Static-Static Elliptic
                  Curve Diffie-Hellman Key Agreement in Cryptographic
                  Message Syntax", RFC 6278, DOI 10.17487/RFC6278,
                  June 2011, <https://www.rfc-editor.org/info/rfc6278>.

       [RFC7114]  Leiba, B., "Creation of a Registry for smime-type
                  Parameter Values", RFC 7114, DOI 10.17487/RFC7114,
                  January 2014, <https://www.rfc-editor.org/info/rfc7114>.

       [RFC7905]  Langley, A., Chang, W., Mavrogiannopoulos, N.,
                  Strombergson, J., and S. Josefsson, "ChaCha20-Poly1305
                  Cipher Suites for Transport Layer Security (TLS)",
                  RFC 7905, DOI 10.17487/RFC7905, June 2016,
                  <https://www.rfc-editor.org/info/rfc7905>.

       [SP800-56A]
                  National Institute of Standards and Technology (NIST),
                  "Recommendation for Pair-Wise Key Establishment Schemes
                  Using Discrete Logarithm Cryptography", NIST Special
                  Publication 800-56A Revision 2,
                  DOI 10.6028/NIST.SP.800-56Ar2, May 2013,
                  <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
                  NIST.SP.800-56Ar2.pdf>.

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       [SP800-57] National Institute of Standards and Technology (NIST),
                  "Recommendation for Key Management - Part 1: General",
                  NIST Special Publication 800-57 Revision 4,
                  DOI 10.6028/NIST.SP.800-57pt1r4, January 2016,
                  <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
                  NIST.SP.800-57pt1r4.pdf>.

       [TripleDES]
                  Tuchman, W., "Hellman Presents No Shortcut Solutions to
                  the DES", IEEE Spectrum v. 16, n. 7, pp. 40-41,
                  DOI 10.1109/MSPEC.1979.6368160, July 1979.

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    Appendix A.  ASN.1 Module

       Note: The ASN.1 module contained herein is unchanged from RFC 5751
       [SMIMEv2] and RFC 3851 [SMIMEv3.1], with the exception of a change to
       the preferBinaryInside ASN.1 comment in RFC 3851 [SMIMEv3.1].  If a
       module is needed that is compatible with current ASN.1 standards, one
       can be found in [RFC6268].  This module uses the 1988 version
       of ASN.1.

       SecureMimeMessageV3dot1

         { iso(1) member-body(2) us(840) rsadsi(113549)
                pkcs(1) pkcs-9(9) smime(16) modules(0) msg-v3dot1(21) }

       DEFINITIONS IMPLICIT TAGS ::=

       BEGIN

       IMPORTS

       -- Cryptographic Message Syntax [CMS]
          SubjectKeyIdentifier, IssuerAndSerialNumber,
          RecipientKeyIdentifier
              FROM  CryptographicMessageSyntax
                    { iso(1) member-body(2) us(840) rsadsi(113549)
                      pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2001(14) };

       -- id-aa is the arc with all new authenticated and unauthenticated
       -- attributes produced by the S/MIME Working Group.

       id-aa OBJECT IDENTIFIER ::= {iso(1) member-body(2) usa(840)
               rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) attributes(2)}

       -- S/MIME Capabilities provides a method of broadcasting the
       -- symmetric capabilities understood.  Algorithms SHOULD be ordered
       -- by preference and grouped by type.

       smimeCapabilities OBJECT IDENTIFIER ::= {iso(1) member-body(2)
               us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15}

       SMIMECapability ::= SEQUENCE {
          capabilityID OBJECT IDENTIFIER,
          parameters ANY DEFINED BY capabilityID OPTIONAL }

       SMIMECapabilities ::= SEQUENCE OF SMIMECapability

       -- Encryption Key Preference provides a method of broadcasting the
       -- preferred encryption certificate.

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       id-aa-encrypKeyPref OBJECT IDENTIFIER ::= {id-aa 11}

       SMIMEEncryptionKeyPreference ::= CHOICE {
          issuerAndSerialNumber   [0] IssuerAndSerialNumber,
          receipentKeyId          [1] RecipientKeyIdentifier,
          subjectAltKeyIdentifier [2] SubjectKeyIdentifier
       }

       -- "receipentKeyId" is spelled incorrectly but is kept for
       -- historical reasons.

       id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
               rsadsi(113549) pkcs(1) pkcs-9(9) 16 }

       id-cap  OBJECT IDENTIFIER ::= { id-smime 11 }

       -- The preferBinaryInside OID indicates an ability to receive
       -- messages with binary encoding inside the CMS wrapper.
       -- The preferBinaryInside attribute's value field is ABSENT.

       id-cap-preferBinaryInside  OBJECT IDENTIFIER ::= { id-cap 1 }

       -- The following is a list of OIDs to be used with S/MIME v3.

       -- Signature Algorithms Not Found in [RFC3370], [RFC5754], [RFC4056],
       -- and [RFC3560]

       --
       -- md2WithRSAEncryption OBJECT IDENTIFIER ::=
       --    {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
       --     2}

       --
       -- Other Signed Attributes
       --
       -- signingTime OBJECT IDENTIFIER ::=
       --    {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
       --     5}
       --    See [CMS] for a description of how to encode the attribute
       --    value.

       SMIMECapabilitiesParametersForRC2CBC ::= INTEGER
       --        (RC2 Key Length (number of bits))

       END

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    Appendix B.  Historic Mail Considerations

       Over the course of updating the S/MIME specifications, the set of
       recommended algorithms has been modified each time the documents have
       been updated.  This means that if a user has historic emails and
       their user agent has been updated to only support the current set of
       recommended algorithms, some of those old emails will no longer be
       accessible.  It is strongly suggested that user agents implement some
       of the following algorithms for dealing with historic emails.

       This appendix contains a number of references to documents that have
       been obsoleted or replaced.  This is intentional, as the updated
       documents often do not have the same information in them.

    B.1.  DigestAlgorithmIdentifier

       The following algorithms have been called out for some level of
       support by previous S/MIME specifications:

       -  SHA-1 was dropped in [SMIMEv4].  SHA-1 is no longer considered to
          be secure, as it is no longer collision resistant.  The IETF
          statement on SHA-1 can be found in [RFC6194], but it is out of
          date relative to the most recent advances.

       -  MD5 was dropped in [SMIMEv4].  MD5 is no longer considered to be
          secure, as it is no longer collision resistant.  Details can be
          found in [RFC6151].

    B.2.  Signature Algorithms

       There are a number of problems with validating signatures on
       sufficiently historic messages.  For this reason, it is strongly
       suggested that user agents treat these signatures differently from
       those on current messages.  These problems include the following:

       -  Certification authorities are not required to keep certificates on
          a CRL beyond one update after a certificate has expired.  This
          means that unless CRLs are cached as part of the message it is not
          always possible to check to see if a certificate has been revoked.
          The same problems exist with Online Certificate Status Protocol
          (OCSP) responses, as they may be based on a CRL rather than on the
          certificate database.

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       -  RSA and DSA keys of less than 2048 bits are now considered by many
          experts to be cryptographically insecure (due to advances in
          computing power).  Such keys were previously considered secure, so
          the processing of historic signed messages will often result in
          the use of weak keys.  Implementations that wish to support
          previous versions of S/MIME or process old messages need to
          consider the security risks that result from smaller key sizes
          (e.g., spoofed messages) versus the costs of denial of service.

          [SMIMEv3.1] set the lower limit on suggested key sizes for
          creating and validation at 1024 bits.  Prior to that, the lower
          bound on key sizes was 512 bits.

       -  Hash functions used to validate signatures on historic messages
          may no longer be considered to be secure (see below).  While there
          are not currently any known practical pre-image or second
          pre-image attacks against MD5 or SHA-1, the fact that they are no
          longer considered to be collision resistant implies that the
          security levels of the signatures are generally considered
          suspect.  If a message is known to be historic and it has been in
          the possession of the client for some time, then it might still be
          considered to be secure.

       -  The previous two issues apply to the certificates used to validate
          the binding of the public key to the identity that signed the
          message as well.

       The following algorithms have been called out for some level of
       support by previous S/MIME specifications:

       -  RSA with MD5 was dropped in [SMIMEv4].  MD5 is no longer
          considered to be secure, as it is no longer collision resistant.
          Details can be found in [RFC6151].

       -  RSA and DSA with SHA-1 were dropped in [SMIMEv4].  SHA-1 is no
          longer considered to be secure, as it is no longer collision
          resistant.  The IETF statement on SHA-1 can be found in [RFC6194],
          but it is out of date relative to the most recent advances.

       -  DSA with SHA-256 was dropped in [SMIMEv4].  DSA has been replaced
          by elliptic curve versions.

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       As requirements for "mandatory to implement" have changed over time,
       some issues have been created that can cause interoperability
       problems:

       -  S/MIME v2 clients are only required to verify digital signatures
          using the rsaEncryption algorithm with SHA-1 or MD5 and might not
          implement id-dsa-with-sha1 or id-dsa at all.

       -  S/MIME v3 clients might only implement signing or signature
          verification using id-dsa-with-sha1 and might also use id-dsa as
          an AlgorithmIdentifier in this field.

       -  Note that S/MIME v3.1 clients support verifying id-dsa-with-sha1
          and rsaEncryption and might not implement sha256WithRSAEncryption.

       NOTE: Receiving clients SHOULD recognize id-dsa as equivalent to
       id-dsa-with-sha1.

       For 512-bit RSA with SHA-1, see [RFC3370] and [FIPS186-2] without
       Change Notice 1; for 512-bit RSA with SHA-256, see [RFC5754] and
       [FIPS186-2] without Change Notice 1; and for 1024-bit through
       2048-bit RSA with SHA-256, see [RFC5754] and [FIPS186-2] with Change
       Notice 1.  The first reference provides the signature algorithm's
       OID, and the second provides the signature algorithm's definition.

       For 512-bit DSA with SHA-1, see [RFC3370] and [FIPS186-2] without
       Change Notice 1; for 512-bit DSA with SHA-256, see [RFC5754] and
       [FIPS186-2] without Change Notice 1; for 1024-bit DSA with SHA-1, see
       [RFC3370] and [FIPS186-2] with Change Notice 1; and for 1024-bit and
       above DSA with SHA-256, see [RFC5754] and [FIPS186-4].  The first
       reference provides the signature algorithm's OID, and the second
       provides the signature algorithm's definition.

    B.3.  ContentEncryptionAlgorithmIdentifier

       The following algorithms have been called out for some level of
       support by previous S/MIME specifications:

       -  RC2/40 [RFC2268] was dropped in [SMIMEv3.2].  The algorithm is
          known to be insecure and, if supported, should only be used to
          decrypt existing email.

       -  DES EDE3 CBC [TripleDES], also known as "tripleDES", was dropped
          in [SMIMEv4].  This algorithm is removed from the list of
          supported algorithms because (1) it has a 64-bit block size and
          (2) it offers less than 128 bits of security.  This algorithm
          should be supported only to decrypt existing email; it should not
          be used to encrypt new emails.

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    B.4.  KeyEncryptionAlgorithmIdentifier

       The following algorithms have been called out for some level of
       support by previous S/MIME specifications:

       -  DH ephemeral-static mode, as specified in [RFC3370] and
          [SP800-57], was dropped in [SMIMEv4].

       -  RSA key sizes have been increased over time.  Decrypting old mail
          with smaller key sizes is reasonable; however, new mail should use
          the updated key sizes.

       For 1024-bit DH, see [RFC3370].  For 1024-bit and larger DH, see
       [SP800-56A]; regardless, use the KDF, which is from X9.42, specified
       in [RFC3370].

    Appendix C.  Moving S/MIME v2 Message Specification to Historic Status

       The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], and v3.2 [SMIMEv3.2]
       specifications are backward compatible with the S/MIME v2 Message
       Specification [SMIMEv2], with the exception of the algorithms
       (dropped RC2/40 requirement and added DSA and RSASSA-PSS
       requirements).  Therefore, RFC 2311 [SMIMEv2] was moved to Historic
       status.

    Acknowledgements

       Many thanks go out to the other authors of the S/MIME version 2
       Message Specification RFC: Steve Dusse, Paul Hoffman, Laurence
       Lundblade, and Lisa Repka.  Without v2, there wouldn't be a v3, v3.1,
       v3.2, or v4.0.

       Some of the examples in this document were copied from [RFC4134].
       Thanks go to the people who wrote and verified the examples in that
       document.

       A number of the members of the S/MIME Working Group have also worked
       very hard and contributed to this document.  Any list of people is
       doomed to omission, and for that I apologize.  In alphabetical order,
       the following people stand out in my mind because they made direct
       contributions to this document:

       Tony Capel, Piers Chivers, Dave Crocker, Bill Flanigan, Peter
       Gutmann, Alfred Hoenes, Paul Hoffman, Russ Housley, William Ottaway,
       and John Pawling.

       The version 4 update to the S/MIME documents was done under the
       auspices of the LAMPS Working Group.

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    Authors' Addresses

       Jim Schaad
       August Cellars

       Email: ietf@augustcellars.com

       Blake Ramsdell
       Brute Squad Labs, Inc.

       Email: blaker@gmail.com

       Sean Turner
       sn3rd

       Email: sean@sn3rd.com

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