    ﻿

    Internet Engineering Task Force (IETF)                    P. Saint-Andre
    Request for Comments: 8265                                    Jabber.org
    Obsoletes: 7613                                              A. Melnikov
    Category: Standards Track                                      Isode Ltd
    ISSN: 2070-1721                                             October 2017

     Preparation, Enforcement, and Comparison of Internationalized Strings
                      Representing Usernames and Passwords

    Abstract

       This document describes updated methods for handling Unicode strings
       representing usernames and passwords.  The previous approach was
       known as SASLprep (RFC 4013) and was based on Stringprep (RFC 3454).
       The methods specified in this document provide a more sustainable
       approach to the handling of internationalized usernames and
       passwords.  This document obsoletes RFC 7613.

    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/rfc8265.

    Copyright Notice

       Copyright (c) 2017 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.

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    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
       2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
       3.  Usernames . . . . . . . . . . . . . . . . . . . . . . . . . .   5
         3.1.  Definition  . . . . . . . . . . . . . . . . . . . . . . .   5
         3.2.  Case Mapping vs. Case Preservation  . . . . . . . . . . .   6
         3.3.  UsernameCaseMapped Profile  . . . . . . . . . . . . . . .   7
           3.3.1.  Rules . . . . . . . . . . . . . . . . . . . . . . . .   7
           3.3.2.  Preparation . . . . . . . . . . . . . . . . . . . . .   8
           3.3.3.  Enforcement . . . . . . . . . . . . . . . . . . . . .   8
           3.3.4.  Comparison  . . . . . . . . . . . . . . . . . . . . .   9
         3.4.  UsernameCasePreserved Profile . . . . . . . . . . . . . .   9
           3.4.1.  Rules . . . . . . . . . . . . . . . . . . . . . . . .   9
           3.4.2.  Preparation . . . . . . . . . . . . . . . . . . . . .   9
           3.4.3.  Enforcement . . . . . . . . . . . . . . . . . . . . .  10
           3.4.4.  Comparison  . . . . . . . . . . . . . . . . . . . . .  10
         3.5.  Application-Layer Constructs  . . . . . . . . . . . . . .  11
         3.6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  11
       4.  Passwords . . . . . . . . . . . . . . . . . . . . . . . . . .  13
         4.1.  Definition  . . . . . . . . . . . . . . . . . . . . . . .  13
         4.2.  OpaqueString Profile  . . . . . . . . . . . . . . . . . .  14
           4.2.1.  Preparation . . . . . . . . . . . . . . . . . . . . .  14
           4.2.2.  Enforcement . . . . . . . . . . . . . . . . . . . . .  14
           4.2.3.  Comparison  . . . . . . . . . . . . . . . . . . . . .  15
         4.3.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  15
       5.  Use in Application Protocols  . . . . . . . . . . . . . . . .  16
       6.  Migration . . . . . . . . . . . . . . . . . . . . . . . . . .  17
         6.1.  Usernames . . . . . . . . . . . . . . . . . . . . . . . .  17
         6.2.  Passwords . . . . . . . . . . . . . . . . . . . . . . . .  19
       7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
         7.1.  UsernameCaseMapped Profile  . . . . . . . . . . . . . . .  20
         7.2.  UsernameCasePreserved Profile . . . . . . . . . . . . . .  20
         7.3.  OpaqueString Profile  . . . . . . . . . . . . . . . . . .  21
         7.4.  Stringprep Profile  . . . . . . . . . . . . . . . . . . .  22
       8.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
         8.1.  Password/Passphrase Strength  . . . . . . . . . . . . . .  22
         8.2.  Password/Passphrase Comparison  . . . . . . . . . . . . .  22
         8.3.  Identifier Comparison . . . . . . . . . . . . . . . . . .  22
         8.4.  Reuse of PRECIS . . . . . . . . . . . . . . . . . . . . .  22
         8.5.  Reuse of Unicode  . . . . . . . . . . . . . . . . . . . .  22
       9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
         9.1.  Normative References  . . . . . . . . . . . . . . . . . .  23
         9.2.  Informative References  . . . . . . . . . . . . . . . . .  24
       Appendix A.  Changes from RFC 7613  . . . . . . . . . . . . . . .  25
       Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  26
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

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    1.  Introduction

       Usernames and passwords are widely used for authentication and
       authorization on the Internet, either directly when provided in
       plaintext (as in the PLAIN Simple Authentication and Security Layer
       (SASL) mechanism [RFC4616] and the HTTP Basic scheme [RFC7617]) or
       indirectly when provided as the input to a cryptographic algorithm
       such as a hash function (as in the Salted Challenge Response
       Authentication Mechanism (SCRAM) SASL mechanism [RFC5802] and the
       HTTP Digest scheme [RFC7616]).

       To increase the likelihood that the input and comparison of usernames
       and passwords will work in ways that make sense for typical users
       throughout the world, this document defines rules for handling
       internationalized strings that represent usernames and passwords.
       Such strings consist of code points from the Unicode coded character
       set [Unicode], with special attention to code points outside the
       ASCII range [RFC20].  The rules for handling such strings are
       specified through profiles of the string classes defined in the
       preparation, enforcement, and comparison of internationalized strings
       (PRECIS) framework specification [RFC8264].

       Profiles of the PRECIS framework enable software to handle Unicode
       code points outside the ASCII range in an automated way, so that such
       code points are treated carefully and consistently in application
       protocols.  In large measure, these profiles are designed to protect
       application developers from the potentially negative consequences of
       supporting the full range of Unicode code points.  For instance, in
       almost all application protocols it would be dangerous to treat the
       Unicode code point "¹" (SUPERSCRIPT ONE, U+00B9) as equivalent to "1"
       (DIGIT ONE, U+0031), because that would result in false accepts
       during comparison, authentication, and authorization (e.g., an
       attacker could easily spoof an account "user1@example.com").

       Whereas a naive use of Unicode would make such attacks trivially
       easy, the PRECIS profile defined here for usernames generally
       protects applications from inadvertently causing such problems.
       (Similar considerations apply to passwords, although here it is
       desirable to support a wider range of characters so as to maximize
       entropy for purposes of authentication.)

       The methods defined here might be applicable wherever usernames or
       passwords are used.  However, the methods are not intended for use in
       preparing strings that are not usernames (e.g., Lightweight Directory
       Access Protocol (LDAP) distinguished names), nor in cases where
       identifiers or secrets are not strings (e.g., keys and certificates)
       or require specialized handling.

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       Although the historical predecessor of this document was the SASLprep
       profile of Stringprep [RFC3454]), the approach defined here can be
       used by technologies other than SASL [RFC4422], such as HTTP
       authentication as specified in [RFC7617] and [RFC7616].

       This document does not modify the handling of internationalized
       strings in usernames and passwords as prescribed by existing
       application protocols that use SASLprep.  If the community that uses
       such an application protocol wishes to modernize its handling of
       internationalized strings to use PRECIS instead of Stringprep, it
       needs to explicitly update the existing application protocol
       definition (one example is [RFC7622]).  Non-coordinated updates to
       protocol implementations are discouraged because they can have a
       negative impact on interoperability and security.

    2.  Terminology

       A "username" or "user identifier" is a string of characters
       designating an account on a computing device or system, often but not
       necessarily for use by a person.  Although some devices and systems
       might allow a username to be part or all of a person's name and a
       person might want their account designator to be part or all of their
       name, because of the complexities involved, that outcome is not
       guaranteed for all human names on all computing devices or systems
       that follow the rules defined in this specification.  Protocol
       designers and application developers who wish to allow a wider range
       of characters are encouraged to consider a separation between more
       restrictive account identifiers and more expressive display names or
       nicknames (see [RFC8266]).

       A "password" is a string of characters that allows access to a
       computing device or system, often associated with a particular
       username.  A password is not literally limited to a word, because a
       password could be a passphrase consisting of more than one word,
       perhaps separated by spaces, punctuation, or other non-alphanumeric
       characters.

       Some SASL mechanisms (e.g., CRAM-MD5, DIGEST-MD5, and SCRAM) specify
       that the authentication identity used in the context of such
       mechanisms is a "simple username" (see Section 2 of [RFC4422] as well
       as [RFC4013]).  Various application technologies also assume that the
       identity of a user or account takes the form of a username (e.g.,
       authentication for the Hypertext Transfer Protocol as specified in
       [RFC7617] and [RFC7616]), whether or not they use SASL.  Note well
       that the exact form of a username in any particular SASL mechanism or
       application technology is a matter for implementation and deployment;
       note also that a username does not necessarily map to any particular
       application identifier.

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       Many important terms used in this document are defined in [RFC5890],
       [RFC6365], [RFC8264], and [Unicode].  The term "non-ASCII space"
       refers to any Unicode code point having a Unicode general category of
       "Zs", naturally with the exception of SPACE (U+0020).

       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.

    3.  Usernames

    3.1.  Definition

       This document specifies that a username is a string of Unicode code
       points [Unicode] that is structured as an ordered sequence of
       "userparts" and expressed in a standard Unicode Encoding Form (such
       as UTF-8 [RFC3629]).  A userpart is allowed to contain only code
       points that are allowed by the PRECIS IdentifierClass defined in
       Section 4.2 of [RFC8264] and thus consists almost exclusively of
       letters and digits.  A username can consist of a single userpart or a
       space-separated sequence of userparts.

       The syntax for a username is defined as follows, using the Augmented
       Backus-Naur Form (ABNF) [RFC5234].

          username   = userpart *(1*SP userpart)
          userpart   = 1*(idpoint)
                       ;
                       ; an "idpoint" is a Unicode code point that
                       ; can be contained in a string conforming to
                       ; the PRECIS IdentifierClass
                       ;

       All code points and blocks not explicitly allowed in the PRECIS
       IdentifierClass are disallowed; this includes private-use code
       points, surrogate code points, and the other code points and blocks
       that were defined as "Prohibited Output" in Section 2.3 of [RFC4013]
       (when corrected per [Err1812]).  In addition, common constructions
       such as "user@example.com" (e.g., the Network Access Identifier from
       [RFC7542]) are allowed as usernames under this specification, as they
       were under [RFC4013].

          Implementation Note: The username construct defined in this
          document does not necessarily match what all deployed applications
          might refer to as a "username" or "userid" but instead provides a
          relatively safe subset of Unicode code points that can be used in

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          existing SASL mechanisms and in application protocols that use
          SASL, and even in most application protocols that do not currently
          use SASL.

       A username MUST NOT be zero bytes in length.  This rule is to be
       enforced after any normalization and mapping of code points.

       This specification defines two profiles for usernames: the
       UsernameCaseMapped profile performs case mapping, and the
       UsernameCasePreserved performs case preservation (see further
       discussion under Section 3.2).

       In protocols that provide usernames as input to a cryptographic
       algorithm such as a hash function, the client will need to perform
       enforcement of the rules for the UsernameCaseMapped or
       UsernameCasePreserved profile before applying the algorithm.

    3.2.  Case Mapping vs. Case Preservation

       In order to accommodate the widest range of username constructs in
       applications, this document defines two username profiles:
       UsernameCaseMapped and UsernameCasePreserved.  These two profiles
       differ only in their use (or not) of the Case Mapping Rule and are
       otherwise identical.

       Case mapping is a matter for the application protocol, protocol
       implementation, or end deployment.  In general, this document
       suggests that it is preferable to apply the UsernameCaseMapped
       profile and therefore perform case mapping, because not doing so can
       lead to false accepts during authentication and authorization (as
       described in [RFC6943]) and can result in confusion among end users,
       given the prevalence of case mapping in many existing protocols and
       applications.  However, there can be good reasons to apply the
       UsernameCasePreserved profile and thus not perform case mapping, such
       as backward compatibility with deployed infrastructure.

       In particular:

       o  SASL mechanisms that follow the recommendations in this document
          MUST specify whether and when case mapping is to be applied to
          authentication identifiers.  Because case mapping results in
          information loss, in order to retain that information for as long
          as possible during processing, implementations SHOULD delay any
          case mapping to the last possible moment, such as when doing a
          lookup by username, performing username comparisons, or generating
          a cryptographic salt from a username (if the last possible moment
          happens on a server, then decisions about case mapping can be a
          matter of service deployment policy).  In keeping with [RFC4422],

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          SASL mechanisms are not to apply this or any other profile to
          authorization identifiers, only to authentication identifiers.

       o  Application protocols that use SASL (such as IMAP [RFC3501] and
          the Extensible Messaging and Presence Protocol (XMPP) [RFC6120])
          and that directly reuse this profile MUST specify whether or not
          case mapping is to be applied to authorization identifiers.  Such
          "SASL application protocols" SHOULD delay any case mapping of
          authorization identifiers to the last possible moment, which
          happens to necessarily be on the server side (this enables
          decisions about case mapping to be a matter of service deployment
          policy).  In keeping with [RFC4422], SASL application protocols
          are not to apply this or any other profile to authentication
          identifiers, only to authorization identifiers.

       o  Application protocols that do not use SASL (such as HTTP
          authentication with the HTTP Basic and Digest schemes as specified
          in [RFC7617] and [RFC7616]) but that directly reuse this profile
          MUST specify whether and when case mapping is to be applied to
          authentication identifiers or authorization identifiers, or both.
          Such "non-SASL application protocols" SHOULD delay any case
          mapping to the last possible moment, such as when doing a lookup
          by username, performing username comparisons, or generating a
          cryptographic salt from a username (if the last possible moment
          happens on the server, then decisions about case mapping can be a
          matter of service deployment policy).

       If the specification for a SASL mechanism, SASL application protocol,
       or non-SASL application protocol uses the UsernameCaseMapped profile,
       it MUST clearly describe whether case mapping is to be applied at the
       level of the protocol itself, implementations thereof, or service
       deployments (each of these approaches can be legitimate, depending on
       the application in question).

    3.3.  UsernameCaseMapped Profile

    3.3.1.  Rules

       The following rules are defined for use within the UsernameCaseMapped
       profile of the PRECIS IdentifierClass.

       1.  Width Mapping Rule: Map fullwidth and halfwidth code points to
           their decomposition mappings (see Unicode Standard Annex #11
           [UAX11]).

       2.  Additional Mapping Rule: There is no additional mapping rule.

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       3.  Case Mapping Rule: Map uppercase and titlecase code points to
           their lowercase equivalents, preferably using the Unicode
           toLowerCase() operation as defined in the Unicode Standard
           [Unicode]; see further discussion in Section 3.2.

       4.  Normalization Rule: Apply Unicode Normalization Form C (NFC) to
           all strings.

       5.  Directionality Rule: Apply the "Bidi Rule" defined in [RFC5893]
           to strings that contain right-to-left code points (i.e., each of
           the six conditions of the Bidi Rule must be satisfied); for
           strings that do not contain right-to-left code points, there is
           no special processing for directionality.

    3.3.2.  Preparation

       An entity that prepares an input string for subsequent enforcement
       according to this profile MUST proceed as follows (applying the steps
       in the order shown).

       1.  Apply the width mapping rule specified in Section 3.3.1.  It is
           necessary to apply the rule at this point because otherwise the
           PRECIS "HasCompat" category specified in Section 9.17 of
           [RFC8264] would forbid fullwidth and halfwidth code points.

       2.  Ensure that the string consists only of Unicode code points that
           are explicitly allowed by the PRECIS IdentifierClass defined in
           Section 4.2 of [RFC8264].

    3.3.3.  Enforcement

       An entity that performs enforcement according to this profile MUST
       prepare an input string as described in Section 3.3.2 and MUST also
       apply the following rules specified in Section 3.3.1 in the order
       shown:

       1.  Case Mapping Rule

       2.  Normalization Rule

       3.  Directionality Rule

       After all of the foregoing rules have been enforced, the entity MUST
       ensure that the username is not zero bytes in length (this is done
       after enforcing the rules to prevent applications from mistakenly
       omitting a username entirely, because when internationalized strings
       are accepted, a non-empty sequence of characters can result in a
       zero-length username after canonicalization).

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       The result of the foregoing operations is an output string that
       conforms to the UsernameCaseMapped profile.  Until an implementation
       produces such an output string, it MUST NOT treat the string as
       conforming (in particular, it MUST NOT assume that an input string is
       conforming before the enforcement operation has been completed).

    3.3.4.  Comparison

       An entity that performs comparison of two strings according to this
       profile MUST prepare each string as specified in Section 3.3.2 and
       then MUST enforce the rules specified in Section 3.3.3.  The two
       strings are to be considered equivalent if and only if they are an
       exact octet-for-octet match (sometimes called "bit-string identity").

       Until an implementation determines whether two strings are to be
       considered equivalent, it MUST NOT treat them as equivalent (in
       particular, it MUST NOT assume that two input strings are equivalent
       before the comparison operation has been completed).

    3.4.  UsernameCasePreserved Profile

    3.4.1.  Rules

       The following rules are defined for use within the
       UsernameCasePreserved profile of the PRECIS IdentifierClass.

       1.  Width Mapping Rule: Map fullwidth and halfwidth code points to
           their decomposition mappings (see Unicode Standard Annex #11
           [UAX11]).

       2.  Additional Mapping Rule: There is no additional mapping rule.

       3.  Case Mapping Rule: There is no case mapping rule.

       4.  Normalization Rule: Apply Unicode Normalization Form C (NFC) to
           all strings.

       5.  Directionality Rule: Apply the "Bidi Rule" defined in [RFC5893]
           to strings that contain right-to-left code points (i.e., each of
           the six conditions of the Bidi Rule must be satisfied); for
           strings that do not contain right-to-left code points, there is
           no special processing for directionality.

    3.4.2.  Preparation

       An entity that prepares a string for subsequent enforcement according
       to this profile MUST proceed as follows (applying the steps in the
       order shown).

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       1.  Apply the width mapping rule specified in Section 3.4.1.  It is
           necessary to apply the rule at this point because otherwise the
           PRECIS "HasCompat" category specified in Section 9.17 of
           [RFC8264] would forbid fullwidth and halfwidth code points.

       2.  Ensure that the string consists only of Unicode code points that
           are explicitly allowed by the PRECIS IdentifierClass defined in
           Section 4.2 of [RFC8264].

    3.4.3.  Enforcement

       An entity that performs enforcement according to this profile MUST
       prepare a string as described in Section 3.4.2 and MUST also apply
       the following rules specified in Section 3.4.1 in the order shown:

       1.  Normalization Rule

       2.  Directionality Rule

       After all of the foregoing rules have been enforced, the entity MUST
       ensure that the username is not zero bytes in length (this is done
       after enforcing the rules to prevent applications from mistakenly
       omitting a username entirely, because when internationalized strings
       are accepted, a non-empty sequence of characters can result in a
       zero-length username after canonicalization).

       The result of the foregoing operations is an output string that
       conforms to the UsernameCasePreserved profile.  Until an
       implementation produces such an output string, it MUST NOT treat the
       string as conforming (in particular, it MUST NOT assume that an input
       string is conforming before the enforcement operation has been
       completed).

    3.4.4.  Comparison

       An entity that performs comparison of two strings according to this
       profile MUST prepare each string as specified in Section 3.4.2 and
       then MUST enforce the rules specified in Section 3.4.3.  The two
       strings are to be considered equivalent if and only if they are an
       exact octet-for-octet match (sometimes called "bit-string identity").

       Until an implementation determines whether two strings are to be
       considered equivalent, it MUST NOT treat them as equivalent (in
       particular, it MUST NOT assume that two input strings are equivalent
       before the comparison operation has been completed).

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    3.5.  Application-Layer Constructs

       Both the UsernameCaseMapped and UsernameCasePreserved profiles enable
       an application protocol, implementation, or deployment to create
       application-layer constructs such as a username that is a space-
       separated set of userparts like "Firstname Middlename Lastname".
       Such a construct is not a profile of the PRECIS IdentifierClass,
       because SPACE (U+0020) is not allowed in the IdentifierClass;
       however, it can be created at the application layer because SPACE
       (U+0020) can be used as a separator between instances of the PRECIS
       IdentifierClass (e.g., userparts as defined in this specification).

    3.6.  Examples

       The following examples illustrate a small number of userparts (not
       usernames) that are consistent with the format defined above (note
       that the characters "<" and ">" are used here to delineate the actual
       userparts and are not part of the userpart strings).

          +--------------------------+---------------------------------+
          | # | Userpart             | Notes                           |
          +--------------------------+---------------------------------+
          | 1 | <juliet@example.com> | The "at" sign ("@") is allowed  |
          |   |                      | in the PRECIS IdentifierClass   |
          +--------------------------+---------------------------------+
          | 2 | <fussball>           |                                 |
          +--------------------------+---------------------------------+
          | 3 | <fußball>            | The third character is LATIN    |
          |   |                      | SMALL LETTER SHARP S (U+00DF)   |
          +--------------------------+---------------------------------+
          | 4 | <π>                  | A userpart of GREEK SMALL       |
          |   |                      | LETTER PI (U+03C0)              |
          +--------------------------+---------------------------------+
          | 5 | <Σ>                  | A userpart of GREEK CAPITAL     |
          |   |                      | LETTER SIGMA (U+03A3)           |
          +--------------------------+---------------------------------+
          | 6 | <σ>                  | A userpart of GREEK SMALL       |
          |   |                      | LETTER SIGMA (U+03C3)           |
          +--------------------------+---------------------------------+
          | 7 | <ς>                  | A userpart of GREEK SMALL       |
          |   |                      | LETTER FINAL SIGMA (U+03C2)     |
          +--------------------------+---------------------------------+

                       Table 1: A Sample of Legal Userparts

       Regarding examples 2 and 3: although in German writing the character
       eszett "ß" (LATIN SMALL LETTER SHARP S, U+00DF) can mostly be used
       interchangeably with the two characters "ss", the userparts in these

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       examples are different and (if desired) a server would need to
       enforce a registration policy that disallows one of them if the other
       is registered.

       Regarding examples 5, 6, and 7: optional case mapping of "Σ" (GREEK
       CAPITAL LETTER SIGMA, U+03A3) to the lowercase character "σ" (GREEK
       SMALL LETTER SIGMA, U+03C3) during comparison would result in
       matching the userparts in examples 5 and 6; however, because the
       PRECIS mapping rules do not account for the special status of the
       character "ς" (GREEK SMALL LETTER FINAL SIGMA, U+03C2), the userparts
       in examples 5 and 7 or examples 6 and 7 would not be matched during
       comparison.

       The following examples illustrate strings that are not valid
       userparts (not usernames) because they violate the format defined
       above.

          +--------------------------+---------------------------------+
          | # | Non-Userpart String  | Notes                           |
          +--------------------------+---------------------------------+
          | 8 | <foo bar>            | SPACE (U+0020) is disallowed in |
          |   |                      | the userpart                    |
          +--------------------------+---------------------------------+
          | 9 | <>                   | Zero-length userpart            |
          +--------------------------+---------------------------------+
          | 10| <henryⅣ>            | The sixth character is ROMAN    |
          |   |                      | NUMERAL FOUR (U+2163)           |
          +--------------------------+---------------------------------+
          | 11| <∞>                  | A userpart of INFINITY (U+221E) |
          +--------------------------+---------------------------------+

           Table 2: A Sample of Strings That Violate the Userpart Rules

       Regarding example 8: although this is not a valid userpart, it is a
       valid username because it is a space-separated sequence of userparts.

       Regarding example 10: the character "Ⅳ" (ROMAN NUMERAL FOUR, U+2163)
       has a compatibility equivalent of the characters "I" (LATIN CAPITAL
       LETTER I, U+0049) and "V" (LATIN CAPITAL LETTER V, U+0056), but code
       points with compatibility equivalents are not allowed in the PRECIS
       IdentifierClass.

       Regarding example 11: symbol characters such as "∞" (INFINITY,
       U+221E) are not allowed in the PRECIS IdentifierClass.

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    4.  Passwords

    4.1.  Definition

       This document specifies that a password is a string of Unicode code
       points [Unicode] that is conformant to the OpaqueString profile
       (specified below) of the PRECIS FreeformClass defined in Section 4.3
       of [RFC8264] and expressed in a standard Unicode Encoding Form (such
       as UTF-8 [RFC3629]).

       The syntax for a password is defined as follows, using the Augmented
       Backus-Naur Form (ABNF) [RFC5234].

          password   = 1*(freepoint)
                       ;
                       ; a "freepoint" is a Unicode code point that
                       ; can be contained in a string conforming to
                       ; the PRECIS FreeformClass
                       ;

       All code points and blocks not explicitly allowed in the PRECIS
       FreeformClass are disallowed; this includes private-use code points,
       surrogate code points, and the other code points and blocks defined
       as "Prohibited Output" in Section 2.3 of [RFC4013] (when corrected
       per [Err1812]).

       A password MUST NOT be zero bytes in length.  This rule is to be
       enforced after any normalization and mapping of code points.

          Note: Some existing systems allow an empty string in places where
          a password would be expected (e.g., command-line tools that might
          be called from an automated script, or servers that might need to
          be restarted without human intervention).  From the perspective of
          this document (and RFC 4013 before it), these empty strings are
          not passwords but are workarounds for the practical difficulty of
          using passwords in certain scenarios.

          Note: The prohibition of zero-length passwords is not a
          recommendation regarding password strength (because a password of
          only one byte is highly insecure) but is meant to prevent
          applications from mistakenly omitting a password entirely; such an
          outcome is possible when internationalized strings are accepted,
          because a non-empty sequence of characters can result in a zero-
          length password after canonicalization.

       In protocols that provide passwords as input to a cryptographic
       algorithm such as a hash function, the client will need to perform
       enforcement of the rules for the OpaqueString profile before applying

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       the algorithm, because the password is not available to the server in
       plaintext form.

    4.2.  OpaqueString Profile

       The definition of the OpaqueString profile is provided in the
       following sections, including detailed information about preparation,
       enforcement, and comparison (for details on the distinction between
       these actions, refer to [RFC8264]).

    4.2.1.  Preparation

       An entity that prepares a string according to this profile MUST
       ensure that the string consists only of Unicode code points that are
       explicitly allowed by the FreeformClass string class defined in
       [RFC8264].

    4.2.2.  Enforcement

       An entity that performs enforcement according to this profile MUST
       prepare a string as described in Section 4.2.1 and MUST also apply
       the rules specified below for the OpaqueString profile (these rules
       MUST be applied in the order shown):

       1.  Width Mapping Rule: Fullwidth and halfwidth code points MUST NOT
           be mapped to their decomposition mappings (see Unicode Standard
           Annex #11 [UAX11]).

       2.  Additional Mapping Rule: Any instances of non-ASCII space MUST be
           mapped to SPACE (U+0020); a non-ASCII space is any Unicode code
           point having a Unicode general category of "Zs", with the
           exception of SPACE (U+0020).  As was the case in RFC 4013, the
           inclusion of only SPACE (U+0020) prevents confusion with various
           non-ASCII space code points, many of which are difficult to
           reproduce across different input methods.

       3.  Case Mapping Rule: There is no case mapping rule (because mapping
           uppercase and titlecase code points to their lowercase
           equivalents would lead to false accepts and thus to reduced
           security).

       4.  Normalization Rule: Unicode Normalization Form C (NFC) MUST be
           applied to all strings.

       5.  Directionality Rule: There is no directionality rule.  The "Bidi
           Rule" (defined in [RFC5893]) and similar rules are unnecessary
           and inapplicable to passwords, because they can reduce the
           repertoire of characters that are allowed in a string and

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           therefore reduce the amount of entropy that is possible in a
           password.  Such rules are intended to minimize the possibility
           that the same string will be displayed differently on a layout
           system set for right-to-left display and a layout system set for
           left-to-right display; however, passwords are typically not
           displayed at all and are rarely meant to be interoperable across
           different layout systems in the way that non-secret strings like
           domain names and usernames are.  Furthermore, it is perfectly
           acceptable for opaque strings other than passwords to be
           presented differently in different layout systems, as long as the
           presentation is consistent in any given layout system.

       The result of the foregoing operations is an output string that
       conforms to the OpaqueString profile.  Until an implementation
       produces such an output string, it MUST NOT treat the string as
       conforming (in particular, it MUST NOT assume that an input string is
       conforming before the enforcement operation has been completed).

    4.2.3.  Comparison

       An entity that performs comparison of two strings according to this
       profile MUST prepare each string as specified in Section 4.2.1 and
       then MUST enforce the rules specified in Section 4.2.2.  The two
       strings are to be considered equivalent if and only if they are an
       exact octet-for-octet match (sometimes called "bit-string identity").

       Until an implementation determines whether two strings are to be
       considered equivalent, it MUST NOT treat them as equivalent (in
       particular, it MUST NOT assume that two input strings are equivalent
       before the comparison operation has been completed).

       See Section 8.2 regarding comparison of passwords and passphrases.

    4.3.  Examples

       The following examples illustrate a small number of passwords that
       are consistent with the format defined above (note that the
       characters "<" and ">" are used here to delineate the actual
       passwords and are not part of the password strings).

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       +------------------------------------+------------------------------+
       | # | Password                       | Notes                        |
       +------------------------------------+------------------------------+
       | 12| <correct horse battery staple> | SPACE (U+0020) is allowed    |
       +------------------------------------+------------------------------+
       | 13| <Correct Horse Battery Staple> | Differs by case from         |
       |   |                                | example 12                   |
       +------------------------------------+------------------------------+
       | 14| <πßå>                          | Non-ASCII letters are OK     |
       |   |                                | (e.g., GREEK SMALL LETTER    |
       |   |                                | PI (U+03C0))                 |
       +------------------------------------+------------------------------+
       | 15| <Jack of ♦s>                   | Symbols are OK (e.g., BLACK  |
       |   |                                | DIAMOND SUIT (U+2666))       |
       +------------------------------------+------------------------------+
       | 16| <foo bar>                      | OGHAM SPACE MARK (U+1680) is |
       |   |                                | mapped to SPACE (U+0020);    |
       |   |                                | thus, the full string is     |
       |   |                                | mapped to <foo bar>          |
       +------------------------------------+------------------------------+

                       Table 3: A Sample of Legal Passwords

       The following examples illustrate strings that are not valid
       passwords because they violate the format defined above.

       +------------------------------------+------------------------------+
       | # | Password                       | Notes                        |
       +------------------------------------+------------------------------+
       | 17| <>                             | Zero-length passwords are    |
       |   |                                | disallowed                   |
       +------------------------------------+------------------------------+
       | 18| <my cat is a &#x9;by>          | Control characters like TAB  |
       |   |                                | (U+0009) are disallowed      |
       +------------------------------------+------------------------------+

           Table 4: A Sample of Strings That Violate the Password Rules

       Note: Following the "XML Notation" used in [RFC3987], the character
       TAB (U+0009) in example 18 is represented as &#x9 because otherwise
       it could not be shown in running text.

    5.  Use in Application Protocols

       This specification defines only the PRECIS-based rules for the
       handling of strings conforming to the UsernameCaseMapped and
       UsernameCasePreserved profiles of the PRECIS IdentifierClass, and
       strings conforming to the OpaqueString profile of the PRECIS

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       FreeformClass.  It is the responsibility of an application protocol
       to specify the protocol slots in which such strings can appear, the
       entities that are expected to enforce the rules governing such
       strings, and at what points during protocol processing or interface
       handling the rules need to be enforced.  See Section 6 of [RFC8264]
       for guidelines on using PRECIS profiles in applications.

       Above and beyond the PRECIS-based rules specified here, application
       protocols can also define application-specific rules governing such
       strings (rules regarding minimum or maximum length, further
       restrictions on allowable code points or character ranges, safeguards
       to mitigate the effects of visually similar characters, etc.),
       application-layer constructs (see Section 3.5), and related matters.

       Some PRECIS profile definitions encourage entities that enforce the
       rules to be liberal in what they accept.  However, for usernames and
       passwords such a policy can be problematic, because it can lead to
       false accepts.  An in-depth discussion can be found in [RFC6943].

       Applying the rules for any given PRECIS profile is not necessarily an
       idempotent procedure for all code points.  Therefore, an
       implementation SHOULD apply the rules repeatedly until the output
       string is stable; if the output string does not stabilize after
       reapplying the rules three (3) additional times after the first
       application, the implementation SHOULD terminate application of the
       rules and reject the input string as invalid.

    6.  Migration

       The rules defined in this specification differ slightly from those
       defined by the SASLprep specification [RFC4013] (but not from
       [RFC7613]).  In order to smooth the process of migrating from
       SASLprep to the approach defined herein, the following sections
       describe these differences, along with their implications for
       migration, in more detail.

    6.1.  Usernames

       Deployments that currently use SASLprep for handling usernames might
       need to scrub existing data when they migrate to the rules defined in
       this specification.  In particular:

       o  SASLprep specified the use of Unicode Normalization Form KC
          (NFKC), whereas the UsernameCaseMapped and UsernameCasePreserved
          profiles employ Unicode Normalization Form C (NFC).  In practice,
          this change is unlikely to cause significant problems, because
          NFKC provides methods for mapping Unicode code points with
          compatibility equivalents to those equivalents, whereas the PRECIS

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          IdentifierClass entirely disallows Unicode code points with
          compatibility equivalents (i.e., during comparison, NFKC is more
          "aggressive" about finding matches than NFC).  A few examples
          might suffice to indicate the nature of the problem:

          1.  "ſ" (LATIN SMALL LETTER LONG S, U+017F) is compatibility
              equivalent to "s" (LATIN SMALL LETTER S, U+0073).

          2.  "Ⅳ" (ROMAN NUMERAL FOUR, U+2163) is compatibility equivalent
              to "I" (LATIN CAPITAL LETTER I, U+0049) and "V" (LATIN CAPITAL
              LETTER V, U+0056).

          3.  "ﬁ" (LATIN SMALL LIGATURE FI, U+FB01) is compatibility
              equivalent to "f" (LATIN SMALL LETTER F, U+0066) and "i"
              (LATIN SMALL LETTER I, U+0069).

          Under SASLprep, the use of NFKC also handled the mapping of
          fullwidth and halfwidth code points to their decomposition
          mappings.

          For migration purposes, operators might want to search their
          database of usernames for names containing Unicode code points
          with compatibility equivalents and, where there is no conflict,
          map those code points to their equivalents.  Naturally, it is
          possible that during this process the operator will discover
          conflicting usernames; for instance, "HENRYIV" with the last two
          code points being LATIN CAPITAL LETTER I (U+0049) and LATIN
          CAPITAL LETTER V (U+0056) as opposed to "HENRYⅣ" with the last
          character being "Ⅳ" (ROMAN NUMERAL FOUR, U+2163), which is
          compatibility equivalent to U+0049 and U+0056).  In these cases,
          the operator will need to determine how to proceed, for instance,
          by disabling the account whose name contains a Unicode code point
          with a compatibility equivalent.  Such cases are probably rare,
          but it is important for operators to be aware of them.

       o  SASLprep mapped the "characters commonly mapped to nothing" (from
          Appendix B.1 of [RFC3454]) to nothing, whereas the PRECIS
          IdentifierClass entirely disallows most of these code points,
          which correspond to the code points from the PRECIS "M" category
          defined under Section 9.13 of [RFC8264].  For migration purposes,
          the operator might want to remove from usernames any code points
          contained in the PRECIS "M" category (e.g., SOFT HYPHEN (U+00AD)).
          Because these code points would have been "mapped to nothing" in
          Stringprep, in practice a user would not notice the difference if,
          upon migration to PRECIS, the code points are removed.

       o  SASLprep allowed uppercase and titlecase code points, whereas the
          UsernameCaseMapped profile maps uppercase and titlecase code

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          points to their lowercase equivalents (by contrast, the
          UsernameCasePreserved profile matches SASLprep in this regard).
          For migration purposes, the operator can use either the
          UsernameCaseMapped profile (thus losing the case information) or
          the UsernameCasePreserved profile (thus ignoring case difference
          when comparing usernames).

    6.2.  Passwords

       Depending on local service policy, migration from SASLprep to this
       specification might not involve any scrubbing of data (because
       passwords might not be stored in the clear anyway); however, service
       providers need to be aware of possible issues that might arise during
       migration.  In particular:

       o  SASLprep specified the use of Unicode Normalization Form KC
          (NFKC), whereas the OpaqueString profile employs Unicode
          Normalization Form C (NFC).  Because NFKC is more aggressive about
          finding matches than NFC, in practice this change is unlikely to
          cause significant problems and indeed has the security benefit of
          probably resulting in fewer false accepts when comparing
          passwords.  A few examples might suffice to indicate the nature of
          the problem:

          1.  "ſ" (LATIN SMALL LETTER LONG S, U+017F) is compatibility
              equivalent to "s" (LATIN SMALL LETTER S, U+0073).

          2.  "Ⅳ" (ROMAN NUMERAL FOUR, U+2163) is compatibility equivalent
              to "I" (LATIN CAPITAL LETTER I, U+0049) and "V" (LATIN CAPITAL
              LETTER V, U+0056).

          3.  "ﬁ" (LATIN SMALL LIGATURE FI, U+FB01) is compatibility
              equivalent to "f" (LATIN SMALL LETTER F, U+0066) and "i"
              (LATIN SMALL LETTER I, U+0069).

          Under SASLprep, the use of NFKC also handled the mapping of
          fullwidth and halfwidth code points to their decomposition
          mappings.  Although it is expected that code points with
          compatibility equivalents are rare in existing passwords, some
          passwords that matched when SASLprep was used might no longer work
          when the rules in this specification are applied.

       o  SASLprep mapped the "characters commonly mapped to nothing" (from
          Appendix B.1 of [RFC3454]) to nothing, whereas the PRECIS
          FreeformClass entirely disallows such code points, which
          correspond to the code points from the PRECIS "M" category defined
          under Section 9.13 of [RFC8264].  In practice, this change will
          probably have no effect on comparison, but user-oriented software

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          might reject such code points instead of ignoring them during
          password preparation.

    7.  IANA Considerations

       IANA has made the updates described below.

    7.1.  UsernameCaseMapped Profile

       IANA has added the following entry to the "PRECIS Profiles" registry.

       Name:  UsernameCaseMapped.

       Base Class:  IdentifierClass.

       Applicability:  Usernames in security and application protocols.

       Replaces:  The SASLprep profile of Stringprep.

       Width Mapping Rule:  Map fullwidth and halfwidth code points to their
          decomposition mappings.

       Additional Mapping Rule:  None.

       Case Mapping Rule:  Map uppercase and titlecase code points to
          lowercase.

       Normalization Rule:  NFC.

       Directionality Rule:  The "Bidi Rule" defined in RFC 5893 applies.

       Enforcement:  To be defined by security or application protocols that
          use this profile.

       Specification:  Section 3.3 of RFC 8265.

    7.2.  UsernameCasePreserved Profile

       IANA has added the following entry to the "PRECIS Profiles" registry.

       Name:  UsernameCasePreserved.

       Base Class:  IdentifierClass.

       Applicability:  Usernames in security and application protocols.

       Replaces:  The SASLprep profile of Stringprep.

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       Width Mapping Rule:  Map fullwidth and halfwidth code points to their
          decomposition mappings.

       Additional Mapping Rule:  None.

       Case Mapping Rule:  None.

       Normalization Rule:  NFC.

       Directionality Rule:  The "Bidi Rule" defined in RFC 5893 applies.

       Enforcement:  To be defined by security or application protocols that
          use this profile.

       Specification:  Section 3.4 of RFC 8265.

    7.3.  OpaqueString Profile

       IANA has added the following entry to the "PRECIS Profiles" registry.

       Name:  OpaqueString.

       Base Class:  FreeformClass.

       Applicability:  Passwords and other opaque strings in security and
          application protocols.

       Replaces:  The SASLprep profile of Stringprep.

       Width Mapping Rule:  None.

       Additional Mapping Rule:  Map non-ASCII space code points to SPACE
          (U+0020).

       Case Mapping Rule:  None.

       Normalization Rule:  NFC.

       Directionality Rule:  None.

       Enforcement:  To be defined by security or application protocols that
          use this profile.

       Specification:  Section 4.2 of RFC 8265.

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    7.4.  Stringprep Profile

       The Stringprep specification [RFC3454] did not provide for entries in
       the "Stringprep Profiles" registry to have any state except "Current"
       or "Not Current".  Because RFC 7613 obsoleted RFC 4013, which
       registered the SASLprep profile of Stringprep, IANA previously marked
       that profile as "Not Current" and cited RFC 7613 as an additional
       reference.  IANA has modified the profile so that the current
       document is now cited as the additional reference.

    8.  Security Considerations

    8.1.  Password/Passphrase Strength

       The ability to include a wide range of characters in passwords and
       passphrases can increase the potential for creating a strong password
       with high entropy.  However, in practice, the ability to include such
       characters ought to be weighed against the possible need to reproduce
       them on various devices using various input methods.

    8.2.  Password/Passphrase Comparison

       In systems that conform to modern best practices for security,
       verification of passwords during authentication will not use the
       comparison defined in Section 4.2.3.  Instead, because the system
       performs cryptographic calculations to verify the password, it will
       prepare the password as defined in Section 4.2.1 and enforce the
       rules as defined in Section 4.2.2 before performing the relevant
       calculations.

    8.3.  Identifier Comparison

       The process of comparing identifiers (such as SASL simple usernames,
       authentication identifiers, and authorization identifiers) can lead
       to either false rejects or false accepts, both of which have security
       implications.  A more detailed discussion can be found in [RFC6943].

    8.4.  Reuse of PRECIS

       The security considerations described in [RFC8264] apply to the
       IdentifierClass and FreeformClass string classes used in this
       document for usernames and passwords, respectively.

    8.5.  Reuse of Unicode

       The security considerations described in [UTS39] apply to the use of
       Unicode code points in usernames and passwords.

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    9.  References

    9.1.  Normative References

       [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>.

       [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
                  10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
                  2003, <https://www.rfc-editor.org/info/rfc3629>.

       [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
                  Specifications: ABNF", STD 68, RFC 5234,
                  DOI 10.17487/RFC5234, January 2008,
                  <https://www.rfc-editor.org/info/rfc5234>.

       [RFC5890]  Klensin, J., "Internationalized Domain Names for
                  Applications (IDNA): Definitions and Document Framework",
                  RFC 5890, DOI 10.17487/RFC5890, August 2010,
                  <https://www.rfc-editor.org/info/rfc5890>.

       [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in
                  Internationalization in the IETF", BCP 166, RFC 6365,
                  DOI 10.17487/RFC6365, September 2011,
                  <https://www.rfc-editor.org/info/rfc6365>.

       [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>.

       [RFC8264]  Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
                  Preparation, Enforcement, and Comparison of
                  Internationalized Strings in Application Protocols",
                  RFC 8264, DOI 10.17487/RFC8264, October 2017,
                  <https://www.rfc-editor.org/info/rfc8264>.

       [UAX11]    Unicode Standard Annex #11, "East Asian Width", edited by
                  Ken Lunde.  An integral part of The Unicode Standard,
                  <http://unicode.org/reports/tr11/>.

       [Unicode]  The Unicode Consortium, "The Unicode Standard",
                  <http://www.unicode.org/versions/latest/>.

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    9.2.  Informative References

       [Err1812]  RFC Errata, Erratum ID 1812, RFC 4013,
                  <https://www.rfc-editor.org/errata/eid1812>.

       [RFC20]    Cerf, V., "ASCII format for network interchange", STD 80,
                  RFC 20, DOI 10.17487/RFC0020, October 1969,
                  <https://www.rfc-editor.org/info/rfc20>.

       [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
                  Internationalized Strings ("stringprep")", RFC 3454,
                  DOI 10.17487/RFC3454, December 2002,
                  <https://www.rfc-editor.org/info/rfc3454>.

       [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
                  4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
                  <https://www.rfc-editor.org/info/rfc3501>.

       [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
                  Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
                  January 2005, <https://www.rfc-editor.org/info/rfc3987>.

       [RFC4013]  Zeilenga, K., "SASLprep: Stringprep Profile for User Names
                  and Passwords", RFC 4013, DOI 10.17487/RFC4013, February
                  2005, <https://www.rfc-editor.org/info/rfc4013>.

       [RFC4422]  Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
                  Authentication and Security Layer (SASL)", RFC 4422,
                  DOI 10.17487/RFC4422, June 2006,
                  <https://www.rfc-editor.org/info/rfc4422>.

       [RFC4616]  Zeilenga, K., Ed., "The PLAIN Simple Authentication and
                  Security Layer (SASL) Mechanism", RFC 4616,
                  DOI 10.17487/RFC4616, August 2006,
                  <https://www.rfc-editor.org/info/rfc4616>.

       [RFC5802]  Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
                  "Salted Challenge Response Authentication Mechanism
                  (SCRAM) SASL and GSS-API Mechanisms", RFC 5802,
                  DOI 10.17487/RFC5802, July 2010,
                  <https://www.rfc-editor.org/info/rfc5802>.

       [RFC5893]  Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts
                  for Internationalized Domain Names for Applications
                  (IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010,
                  <https://www.rfc-editor.org/info/rfc5893>.

    Saint-Andre & Melnikov       Standards Track                   [Page 24]

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       [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
                  Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
                  March 2011, <https://www.rfc-editor.org/info/rfc6120>.

       [RFC6943]  Thaler, D., Ed., "Issues in Identifier Comparison for
                  Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
                  2013, <https://www.rfc-editor.org/info/rfc6943>.

       [RFC7542]  DeKok, A., "The Network Access Identifier", RFC 7542,
                  DOI 10.17487/RFC7542, May 2015,
                  <https://www.rfc-editor.org/info/rfc7542>.

       [RFC7613]  Saint-Andre, P. and A. Melnikov, "Preparation,
                  Enforcement, and Comparison of Internationalized Strings
                  Representing Usernames and Passwords", RFC 7613,
                  DOI 10.17487/RFC7613, August 2015,
                  <https://www.rfc-editor.org/info/rfc7613>.

       [RFC7616]  Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
                  Digest Access Authentication", RFC 7616,
                  DOI 10.17487/RFC7616, September 2015,
                  <https://www.rfc-editor.org/info/rfc7616>.

       [RFC7617]  Reschke, J., "The 'Basic' HTTP Authentication Scheme",
                  RFC 7617, DOI 10.17487/RFC7617, September 2015,
                  <https://www.rfc-editor.org/info/rfc7617>.

       [RFC7622]  Saint-Andre, P., "Extensible Messaging and Presence
                  Protocol (XMPP): Address Format", RFC 7622,
                  DOI 10.17487/RFC7622, September 2015,
                  <https://www.rfc-editor.org/info/rfc7622>.

       [RFC8266]  Saint-Andre, P., "Preparation, Enforcement, and Comparison
                  of Internationalized Strings Representing Nicknames",
                  RFC 8266, DOI 10.17487/RFC8266, October 2017,
                  <https://www.rfc-editor.org/info/rfc8266>.

       [UTS39]    Unicode Technical Standard #39, "Unicode Security
                  Mechanisms", edited by Mark Davis and Michel Suignard,
                  <http://unicode.org/reports/tr39/>.

    Appendix A.  Changes from RFC 7613

       The following changes were made from [RFC7613].

       o  Corrected the order of operations for the UsernameCaseMapped
          profile to ensure consistency with [RFC8264].

    Saint-Andre & Melnikov       Standards Track                   [Page 25]

    RFC 8265             PRECIS: Usernames and Passwords        October 2017

       o  In accordance with working group discussions and updates to
          [RFC8264], removed the use of the Unicode toCaseFold() operation
          in favor of the Unicode toLowerCase() operation.

       o  Modified the presentation (but not the content) of the rules.

       o  Removed UTF-8 as a mandatory encoding, because that is a matter
          for the application.

       o  Clarified several editorial matters.

       o  Updated references.

       See [RFC7613] for a description of the differences from [RFC4013].

    Acknowledgements

       Thanks to Christian Schudt and Sam Whited for their bug reports and
       feedback.

       See [RFC7613] for acknowledgements related to the specification that
       this document supersedes.

    Authors' Addresses

       Peter Saint-Andre
       Jabber.org
       P.O. Box 787
       Parker, CO  80134
       United States of America

       Phone: +1 720 256 6756
       Email: stpeter@jabber.org
       URI:   https://www.jabber.org/

       Alexey Melnikov
       Isode Ltd
       5 Castle Business Village
       36 Station Road
       Hampton, Middlesex  TW12 2BX
       United Kingdom

       Email: Alexey.Melnikov@isode.com

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