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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.

Saint-Andre & Melnikov Standards Track [Page 1] RFC 8265 PRECIS: Usernames and Passwords October 2017

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

Saint-Andre & Melnikov Standards Track [Page 2] RFC 8265 PRECIS: Usernames and Passwords October 2017

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.

Saint-Andre & Melnikov Standards Track [Page 3] RFC 8265 PRECIS: Usernames and Passwords October 2017

 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.

Saint-Andre & Melnikov Standards Track [Page 4] RFC 8265 PRECIS: Usernames and Passwords October 2017

 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

Saint-Andre & Melnikov Standards Track [Page 5] RFC 8265 PRECIS: Usernames and Passwords October 2017

    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],

Saint-Andre & Melnikov Standards Track [Page 6] RFC 8265 PRECIS: Usernames and Passwords October 2017

    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.

Saint-Andre & Melnikov Standards Track [Page 7] RFC 8265 PRECIS: Usernames and Passwords October 2017

 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).

Saint-Andre & Melnikov Standards Track [Page 8] RFC 8265 PRECIS: Usernames and Passwords October 2017

 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).

Saint-Andre & Melnikov Standards Track [Page 9] RFC 8265 PRECIS: Usernames and Passwords October 2017

 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).

Saint-Andre & Melnikov Standards Track [Page 10] RFC 8265 PRECIS: Usernames and Passwords October 2017

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

Saint-Andre & Melnikov Standards Track [Page 11] RFC 8265 PRECIS: Usernames and Passwords October 2017

 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.

Saint-Andre & Melnikov Standards Track [Page 12] RFC 8265 PRECIS: Usernames and Passwords October 2017

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

Saint-Andre & Melnikov Standards Track [Page 13] RFC 8265 PRECIS: Usernames and Passwords October 2017

 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

Saint-Andre & Melnikov Standards Track [Page 14] RFC 8265 PRECIS: Usernames and Passwords October 2017

     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).

Saint-Andre & Melnikov Standards Track [Page 15] RFC 8265 PRECIS: Usernames and Passwords October 2017

 +------------------------------------+------------------------------+
 | # | 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

Saint-Andre & Melnikov Standards Track [Page 16] RFC 8265 PRECIS: Usernames and Passwords October 2017

 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

Saint-Andre & Melnikov Standards Track [Page 17] RFC 8265 PRECIS: Usernames and Passwords October 2017

    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

Saint-Andre & Melnikov Standards Track [Page 18] RFC 8265 PRECIS: Usernames and Passwords October 2017

    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

Saint-Andre & Melnikov Standards Track [Page 19] RFC 8265 PRECIS: Usernames and Passwords October 2017

    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.

Saint-Andre & Melnikov Standards Track [Page 20] RFC 8265 PRECIS: Usernames and Passwords October 2017

 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.

Saint-Andre & Melnikov Standards Track [Page 21] RFC 8265 PRECIS: Usernames and Passwords October 2017

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.

Saint-Andre & Melnikov Standards Track [Page 22] RFC 8265 PRECIS: Usernames and Passwords October 2017

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/>.

Saint-Andre & Melnikov Standards Track [Page 23] RFC 8265 PRECIS: Usernames and Passwords October 2017

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] RFC 8265 PRECIS: Usernames and Passwords October 2017

 [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

Saint-Andre & Melnikov Standards Track [Page 26]

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