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Internet Engineering Task Force (IETF) P. Saint-Andre Request for Comments: 7613 &yet Obsoletes: 4013 A. Melnikov Category: Standards Track Isode Ltd ISSN: 2070-1721 August 2015

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.  The preparation, enforcement, and comparison of
 internationalized strings (PRECIS) framework, RFC 7564, obsoletes RFC
 3454, and this document obsoletes RFC 4013.

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 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7613.

Saint-Andre & Melnikov Standards Track [Page 1] RFC 7613 PRECIS: Usernames and Passwords August 2015

Copyright Notice

 Copyright (c) 2015 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
 (http://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 2] RFC 7613 PRECIS: Usernames and Passwords August 2015

Table of Contents

 1. Introduction ....................................................4
 2. Terminology .....................................................5
 3. Usernames .......................................................6
    3.1. Definition .................................................6
    3.2. UsernameCaseMapped Profile .................................7
         3.2.1. Preparation .........................................7
         3.2.2. Enforcement .........................................7
         3.2.3. Comparison ..........................................8
    3.3. UsernameCasePreserved Profile ..............................8
         3.3.1. Preparation .........................................8
         3.3.2. Enforcement .........................................8
         3.3.3. Comparison ..........................................9
    3.4. Case Mapping vs. Case Preservation .........................9
    3.5. Application-Layer Constructs ..............................10
    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 ......................................................16
    6.1. Usernames .................................................17
    6.2. Passwords .................................................18
 7. IANA Considerations ............................................19
    7.1. UsernameCaseMapped Profile ................................19
    7.2. UsernameCasePreserved Profile .............................20
    7.3. OpaqueString Profile ......................................20
    7.4. Stringprep Profile ........................................21
 8. Security Considerations ........................................21
    8.1. Password/Passphrase Strength ..............................21
    8.2. Identifier Comparison .....................................21
    8.3. Reuse of PRECIS ...........................................21
    8.4. Reuse of Unicode ..........................................22
 9. References .....................................................22
    9.1. Normative References ......................................22
    9.2. Informative References ....................................23
 Appendix A. Differences from RFC 4013 .............................26
 Acknowledgements ..................................................27
 Authors' Addresses ................................................27

Saint-Andre & Melnikov Standards Track [Page 3] RFC 7613 PRECIS: Usernames and Passwords August 2015

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
 [HTTP-BASIC-AUTH]) 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 [HTTP-DIGEST-AUTH]).
 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 preparing,
 enforcing, and comparing internationalized strings that represent
 usernames and passwords.  Such strings consist of characters from the
 Unicode character set [Unicode], with special attention to characters
 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 [RFC7564].
 Profiles of the PRECIS framework enable software to handle Unicode
 characters outside the ASCII range in an automated way, so that such
 characters 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 characters.  For instance, in
 almost all application protocols it would be dangerous to treat the
 Unicode character SUPERSCRIPT ONE (U+00B9) as equivalent to DIGIT ONE
 (U+0031), because that would result in false positives during
 comparison, authentication, and authorization (e.g., an attacker
 could easy 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 4] RFC 7613 PRECIS: Usernames and Passwords August 2015

 This document obsoletes RFC 4013 (the SASLprep profile of stringprep
 [RFC3454]) but can be used by technologies other than SASL [RFC4422],
 such as HTTP authentication as specified in [HTTP-BASIC-AUTH] and
 [HTTP-DIGEST-AUTH].
 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 [XMPP-ADDR], which is intended to obsolete
 [RFC6122]).  Non-coordinated updates to protocol implementations are
 discouraged because they can have a negative impact on
 interoperability and security.

2. Terminology

 Many important terms used in this document are defined in [RFC5890],
 [RFC6365], [RFC7564], and [Unicode].  The term "non-ASCII space"
 refers to any Unicode code point having a Unicode general category of
 "Zs", with the exception of U+0020 (here called "ASCII space").
 As used here, the term "password" is not literally limited to a word;
 i.e., 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 user name" (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 [HTTP-BASIC-AUTH] and [HTTP-DIGEST-AUTH]), 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, and that a username does not
 necessarily map to any particular application identifier (such as the
 localpart of an email address).
 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
 [RFC2119].

Saint-Andre & Melnikov Standards Track [Page 5] RFC 7613 PRECIS: Usernames and Passwords August 2015

3. Usernames

3.1. Definition

 This document specifies that a username is a string of Unicode code
 points [Unicode], encoded using UTF-8 [RFC3629], and structured as an
 ordered sequence of "userparts".  A userpart is allowed to contain
 only code points that are in turn allowed by the PRECIS
 IdentifierClass defined in Section 4.2 of [RFC7564], 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*(idbyte)
                 ;
                 ; an "idbyte" is a byte used to represent a
                 ; UTF-8 encoded Unicode code point that can be
                 ; contained in a string that conforms to the
                 ; PRECIS IdentifierClass
                 ;
 All code points and blocks not explicitly allowed in the PRECIS
 IdentifierClass are disallowed; this includes private use characters,
 surrogate code points, and the other code points and blocks that were
 defined as "Prohibited Output" in [RFC4013].  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 characters that can be used in
    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.
 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.

Saint-Andre & Melnikov Standards Track [Page 6] RFC 7613 PRECIS: Usernames and Passwords August 2015

 This specification defines two profiles for usernames: one that
 performs case mapping and one that performs case preservation (see
 further discussion under Section 3.4).

3.2. UsernameCaseMapped Profile

 The definition of the UsernameCaseMapped profile of the
 IdentifierClass is provided in the following sections, including
 detailed information about preparation, enforcement, and comparison
 (for details on the distinction between these actions, refer to
 [RFC7564]).

3.2.1. Preparation

 An entity that prepares a string according to this profile MUST first
 map fullwidth and halfwidth characters to their decomposition
 mappings (see Unicode Standard Annex #11 [UAX11]).  This is necessary
 because the PRECIS "HasCompat" category specified in Section 9.17 of
 [RFC7564] would otherwise forbid fullwidth and halfwidth characters.
 After applying this width-mapping rule, the entity then MUST ensure
 that the string consists only of Unicode code points that conform to
 the PRECIS IdentifierClass defined in Section 4.2 of [RFC7564].  In
 addition, the entity then MUST encode the string as UTF-8 [RFC3629].

3.2.2. Enforcement

 An entity that performs enforcement according to this profile MUST
 prepare a string as described in Section 3.2.1 and MUST also apply
 the rules specified below for the UsernameCaseMapped profile (these
 rules MUST be applied in the order shown):
 1.  Width-Mapping Rule: Applied as part of preparation (see above).
 2.  Additional Mapping Rule: There is no additional mapping rule.
 3.  Case-Mapping Rule: Uppercase and titlecase characters MUST be
     mapped to their lowercase equivalents, preferably using Unicode
     Default Case Folding as defined in the Unicode Standard [Unicode]
     (at the time of this writing, the algorithm is specified in
     Chapter 3 of [Unicode7.0], but the chapter number might change in
     a future version of the Unicode Standard); see further discussion
     in Section 3.4.
 4.  Normalization Rule: Unicode Normalization Form C (NFC) MUST be
     applied to all characters.

Saint-Andre & Melnikov Standards Track [Page 7] RFC 7613 PRECIS: Usernames and Passwords August 2015

 5.  Directionality Rule: Applications MUST apply the "Bidi Rule"
     defined in [RFC5893] to strings that contain right-to-left
     characters (i.e., each of the six conditions of the Bidi Rule
     must be satisfied).

3.2.3. Comparison

 An entity that performs comparison of two strings according to this
 profile MUST prepare each string as specified in Section 3.2.1 and
 then enforce the rules specified in Section 3.2.2.  The two strings
 are to be considered equivalent if they are an exact octet-for-octet
 match (sometimes called "bit-string identity").

3.3. UsernameCasePreserved Profile

 The definition of the UsernameCasePreserved profile of the
 IdentifierClass is provided in the following sections, including
 detailed information about preparation, enforcement, and comparison
 (for details on the distinction between these actions, refer to
 [RFC7564]).

3.3.1. Preparation

 An entity that prepares a string according to this profile MUST first
 map fullwidth and halfwidth characters to their decomposition
 mappings (see Unicode Standard Annex #11 [UAX11]).  This is necessary
 because the PRECIS "HasCompat" category specified in Section 9.17 of
 [RFC7564] would otherwise forbid fullwidth and halfwidth characters.
 After applying this width-mapping rule, the entity then MUST ensure
 that the string consists only of Unicode code points that conform to
 the PRECIS IdentifierClass defined in Section 4.2 of [RFC7564].  In
 addition, the entity then MUST encode the string as UTF-8 [RFC3629].

3.3.2. Enforcement

 An entity that performs enforcement according to this profile MUST
 prepare a string as described in Section 3.3.1 and MUST also apply
 the rules specified below for the UsernameCasePreserved profile
 (these rules MUST be applied in the order shown):
 1.  Width-Mapping Rule: Applied as part of preparation (see above).
 2.  Additional Mapping Rule: There is no additional mapping rule.
 3.  Case-Mapping Rule: Uppercase and titlecase characters MUST NOT be
     mapped to their lowercase equivalents; see further discussion in
     Section 3.4.

Saint-Andre & Melnikov Standards Track [Page 8] RFC 7613 PRECIS: Usernames and Passwords August 2015

 4.  Normalization Rule: Unicode Normalization Form C (NFC) MUST be
     applied to all characters.
 5.  Directionality Rule: Applications MUST apply the "Bidi Rule"
     defined in [RFC5893] to strings that contain right-to-left
     characters (i.e., each of the six conditions of the Bidi Rule
     must be satisfied).

3.3.3. Comparison

 An entity that performs comparison of two strings according to this
 profile MUST prepare each string as specified in Section 3.3.1 and
 then enforce the rules specified in Section 3.3.2.  The two strings
 are to be considered equivalent if they are an exact octet-for-octet
 match (sometimes called "bit-string identity").

3.4. 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 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 positives 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.  SASL mechanisms 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 deployment policy).  In keeping with [RFC4422], SASL
    mechanisms are not to apply this or any other profile to
    authorization identifiers, only to authentication identifiers.

Saint-Andre & Melnikov Standards Track [Page 9] RFC 7613 PRECIS: Usernames and Passwords August 2015

 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 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 [HTTP-BASIC-AUTH] and [HTTP-DIGEST-AUTH]) 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 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).

3.5. 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".
 Although such a construct is not a profile of the PRECIS
 IdentifierClass (because U+0020 SPACE is not allowed in the
 IdentifierClass), it can be created at the application layer because
 U+0020 SPACE can be used as a separator between instances of the
 PRECIS IdentifierClass (e.g., userparts as defined in this
 specification).

Saint-Andre & Melnikov Standards Track [Page 10] RFC 7613 PRECIS: Usernames and Passwords August 2015

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 | <fu&#xDF;ball>       | The third character is LATIN    |
    |   |                      | SMALL LETTER SHARP S (U+00DF)   |
    +--------------------------+---------------------------------+
    | 4 | <&#x3C0;>            | A userpart of GREEK SMALL       |
    |   |                      | LETTER PI (U+03C0)              |
    +--------------------------+---------------------------------+
    | 5 | <&#x3A3;>            | A userpart of GREEK CAPITAL     |
    |   |                      | LETTER SIGMA (U+03A3)           |
    +--------------------------+---------------------------------+
    | 6 | <&#x3C3;>            | A userpart of GREEK SMALL       |
    |   |                      | LETTER SIGMA (U+03C3)           |
    +--------------------------+---------------------------------+
    | 7 | <&#x3C2;>            | A userpart of GREEK SMALL       |
    |   |                      | LETTER FINAL SIGMA (U+03C2)     |
    +--------------------------+---------------------------------+
                 Table 1: A Sample of Legal Userparts
 Several points are worth noting.  Regarding examples 2 and 3:
 although in German the character eszett (LATIN SMALL LETTER SHARP S
 (U+00DF)) can mostly be used interchangeably with the two characters
 "ss", the userparts in these 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 GREEK CAPITAL LETTER SIGMA (U+03A3) to
 lowercase (i.e., to 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 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.

Saint-Andre & Melnikov Standards Track [Page 11] RFC 7613 PRECIS: Usernames and Passwords August 2015

 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| <henry&#x2163;>      | The sixth character is ROMAN    |
    |   |                      | NUMERAL FOUR (U+2163)           |
    +--------------------------+---------------------------------+
    | 11| <&#x265A;>           | A localpart of BLACK CHESS KING |
    |   |                      | (U+265A)                        |
    +--------------------------+---------------------------------+
      Table 2: A Sample of Strings That Violate the Userpart Rule
 Here again, several points are worth noting.  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 Unicode character ROMAN NUMERAL FOUR (U+2163) has a compatibility
 equivalent of the string formed of LATIN CAPITAL LETTER I (U+0049)
 and LATIN CAPITAL LETTER V (U+0056), but characters with
 compatibility equivalents are not allowed in the PRECIS
 IdentifierClass.  Regarding example 11: symbol characters such as
 BLACK CHESS KING (U+265A) are not allowed in the PRECIS
 IdentifierClass.

Saint-Andre & Melnikov Standards Track [Page 12] RFC 7613 PRECIS: Usernames and Passwords August 2015

4. Passwords

4.1. Definition

 This document specifies that a password is a string of Unicode code
 points [Unicode], encoded using UTF-8 [RFC3629], and conformant to
 the OpaqueString profile (specified below) of the PRECIS
 FreeformClass defined in Section 4.3 of [RFC7564].
 The syntax for a password is defined as follows, using the Augmented
 Backus-Naur Form (ABNF) [RFC5234].
    password   = 1*(freebyte)
                 ;
                 ; a "freebyte" is a byte used to represent a
                 ; UTF-8 encoded Unicode code point that can be
                 ; contained in a string that conforms to the
                 ; PRECIS FreeformClass
                 ;
 All code points and blocks not explicitly allowed in the PRECIS
 FreeformClass are disallowed; this includes private use characters,
 surrogate code points, and the other code points and blocks defined
 as "Prohibited Output" in Section 2.3 of RFC 4013 (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.  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 characters are accepted, because a non-empty
    sequence of characters can result in a zero-length password after
    canonicalization.

Saint-Andre & Melnikov Standards Track [Page 13] RFC 7613 PRECIS: Usernames and Passwords August 2015

 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
 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 [RFC7564]).

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
 conform to the FreeformClass base string class defined in [RFC7564].
 In addition, the entity MUST encode the string as UTF-8 [RFC3629].

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 characters 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 ASCII space (U+0020); a non-ASCII space is any Unicode
     code point having a Unicode general category of "Zs" (with the
     exception of U+0020).
 3.  Case-Mapping Rule: Uppercase and titlecase characters MUST NOT be
     mapped to their lowercase equivalents.
 4.  Normalization Rule: Unicode Normalization Form C (NFC) MUST be
     applied to all characters.
 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 range
     of characters that are allowed in a string and therefore reduce
     the amount of entropy that is possible in a password.  Such rules
     are intended to minimize the possibility that the same string

Saint-Andre & Melnikov Standards Track [Page 14] RFC 7613 PRECIS: Usernames and Passwords August 2015

     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.

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 enforce the rules specified in Section 4.2.2.  The two strings
 are to be considered equivalent if they are an exact octet-for-octet
 match (sometimes called "bit-string identity").

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).
 +------------------------------------+------------------------------+
 | # | Password                       | Notes                        |
 +------------------------------------+------------------------------+
 | 12| <correct horse battery staple> | ASCII space is allowed       |
 +------------------------------------+------------------------------+
 | 13| <Correct Horse Battery Staple> | Differs by case from         |
 |   |                                | example 12                   |
 +------------------------------------+------------------------------+
 | 14| <&#x3C0;&#xDF;&#xE5;>          | Non-ASCII letters are OK     |
 |   |                                | (e.g., GREEK SMALL LETTER    |
 |   |                                | PI (U+03C0))                 |
 +------------------------------------+------------------------------+
 | 15| <Jack of &#x2666;s>            | Symbols are OK (e.g., BLACK  |
 |   |                                | DIAMOND SUIT (U+2666))       |
 +------------------------------------+------------------------------+
 | 16| <foo&#x1680;bar>               | OGHAM SPACE MARK (U+1680) is |
 |   |                                | mapped to U+0020, and thus   |
 |   |                                | the full string is mapped to |
 |   |                                | <foo bar>                    |
 +------------------------------------+------------------------------+
                 Table 3: A Sample of Legal Passwords

Saint-Andre & Melnikov Standards Track [Page 15] RFC 7613 PRECIS: Usernames and Passwords August 2015

 The following example illustrates a string that is not a valid
 password because it violates the format defined above.
 +------------------------------------+------------------------------+
 | # | Password                       | Notes                        |
 +------------------------------------+------------------------------+
 | 17| <my cat is a &#x9;by>          | Controls are disallowed      |
 +------------------------------------+------------------------------+
          Table 4: A String That Violates the Password Rules

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
 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 [RFC7564]
 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 characters 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 positives.  An in-depth discussion can be found in [RFC6943].

6. Migration

 The rules defined in this specification differ slightly from those
 defined by the SASLprep specification [RFC4013].  The following
 sections describe these differences, along with their implications
 for migration, in more detail.

Saint-Andre & Melnikov Standards Track [Page 16] RFC 7613 PRECIS: Usernames and Passwords August 2015

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
    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.  LATIN SMALL LETTER LONG S (U+017F) is compatibility equivalent
        to LATIN SMALL LETTER S (U+0073).
    2.  ROMAN NUMERAL FOUR (U+2163) is compatibility equivalent to
        LATIN CAPITAL LETTER I (U+0049) and LATIN CAPITAL LETTER V
        (U+0056).
    3.  LATIN SMALL LIGATURE FI (U+FB01) is compatibility equivalent
        to LATIN SMALL LETTER F (U+0066) and 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 (e.g., HENRYIV with the last two characters
    being LATIN CAPITAL LETTER I (U+0049) and LATIN CAPITAL LETTER V
    (U+0056) vs. "HENRYIV" with the last character being 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.

Saint-Andre & Melnikov Standards Track [Page 17] RFC 7613 PRECIS: Usernames and Passwords August 2015

 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 characters, which
    correspond to the code points from the PRECIS "M" category defined
    under Section 9.13 of [RFC7564] (with the exception of MONGOLIAN
    TODO SOFT HYPHEN (U+1806), which was "commonly mapped to nothing"
    in Unicode 3.2 but at the time of this writing does not have a
    derived property of Default_Ignorable_Code_Point in Unicode 7.0).
    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 characters, whereas the
    UsernameCaseMapped profile maps uppercase and titlecase characters
    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 RFC 4013 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 positives when comparing
    passwords.  A few examples might suffice to indicate the nature of
    the problem:
    1.  LATIN SMALL LETTER LONG S (U+017F) is compatibility equivalent
        to LATIN SMALL LETTER S (U+0073).
    2.  ROMAN NUMERAL FOUR (U+2163) is compatibility equivalent to
        LATIN CAPITAL LETTER I (U+0049) and LATIN CAPITAL LETTER V
        (U+0056).

Saint-Andre & Melnikov Standards Track [Page 18] RFC 7613 PRECIS: Usernames and Passwords August 2015

    3.  LATIN SMALL LIGATURE FI (U+FB01) is compatibility equivalent
        to LATIN SMALL LETTER F (U+0066) and 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 characters, which correspond
    to the code points from the PRECIS "M" category defined under
    Section 9.13 of [RFC7564] (with the exception of MONGOLIAN TODO
    SOFT HYPHEN (U+1806), which was commonly mapped to nothing in
    Unicode 3.2 but at the time of this writing is allowed by
    Unicode 7.0).  In practice, this change will probably have no
    effect on comparison, but user-oriented software 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 characters to their
    decomposition mappings.
 Additional Mapping Rule:  None.
 Case-Mapping Rule:  Map uppercase and titlecase characters to
    lowercase.
 Normalization Rule:  NFC.

Saint-Andre & Melnikov Standards Track [Page 19] RFC 7613 PRECIS: Usernames and Passwords August 2015

 Directionality Rule:  The "Bidi Rule" defined in RFC 5893 applies.
 Enforcement:  To be defined by security or application protocols that
    use this profile.
 Specification:  RFC 7613 (this document), Section 3.2.

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.
 Width-Mapping Rule:  Map fullwidth and halfwidth characters 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:  RFC 7613 (this document), Section 3.3.

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.

Saint-Andre & Melnikov Standards Track [Page 20] RFC 7613 PRECIS: Usernames and Passwords August 2015

 Width-Mapping Rule:  None.
 Additional Mapping Rule:  Map non-ASCII space characters to ASCII
    space.
 Case-Mapping Rule:  None.
 Normalization Rule:  NFC.
 Directionality Rule:  None.
 Enforcement:  To be defined by security or application protocols that
    use this profile.
 Specification:  RFC 7613 (this document), Section 4.2.

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 this document obsoletes RFC 4013, which
 registered the SASLprep profile of stringprep, IANA has marked that
 profile as "Not Current" and cited this document as an 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. Identifier Comparison

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

8.3. Reuse of PRECIS

 The security considerations described in [RFC7564] apply to the
 IdentifierClass and FreeformClass base string classes used in this
 document for usernames and passwords, respectively.

Saint-Andre & Melnikov Standards Track [Page 21] RFC 7613 PRECIS: Usernames and Passwords August 2015

8.4. Reuse of Unicode

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

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,
            <http://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, <http://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,
            <http://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,
            <http://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,
            <http://www.rfc-editor.org/info/rfc6365>.
 [RFC7564]  Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
            Preparation, Enforcement, and Comparison of
            Internationalized Strings in Application Protocols",
            RFC 7564, DOI 10.17487/RFC7564, May 2015,
            <http://www.rfc-editor.org/info/rfc7564>.
 [UAX11]    Unicode Standard Annex #11, "East Asian Width", edited by
            Ken Lunde.  An integral part of The Unicode Standard,
            <http://unicode.org/reports/tr11/>.

Saint-Andre & Melnikov Standards Track [Page 22] RFC 7613 PRECIS: Usernames and Passwords August 2015

 [Unicode]  The Unicode Consortium, "The Unicode Standard",
            <http://www.unicode.org/versions/latest/>.
 [Unicode7.0]
            The Unicode Consortium, "The Unicode Standard,
            Version 7.0.0", (Mountain View, CA: The Unicode
            Consortium, 2014 ISBN 978-1-936213-09-2),
            <http://www.unicode.org/versions/Unicode7.0.0/>.

9.2. Informative References

 [Err1812]  RFC Errata, Erratum ID 1812, RFC 4013,
            <http://www.rfc-editor.org>.
 [HTTP-BASIC-AUTH]
            Reschke, J., "The 'Basic' HTTP Authentication Scheme",
            Work in Progress, draft-ietf-httpauth-basicauth-update-07,
            February 2015.
 [HTTP-DIGEST-AUTH]
            Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
            Digest Access Authentication", Work in Progress,
            draft-ietf-httpauth-digest-19, April 2015.
 [RFC20]    Cerf, V., "ASCII format for network interchange", STD 80,
            RFC 20, DOI 10.17487/RFC0020, October 1969,
            <http://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,
            <http://www.rfc-editor.org/info/rfc3454>.
 [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
            VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501,
            March 2003, <http://www.rfc-editor.org/info/rfc3501>.
 [RFC4013]  Zeilenga, K., "SASLprep: Stringprep Profile for User Names
            and Passwords", RFC 4013, DOI 10.17487/RFC4013,
            February 2005, <http://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,
            <http://www.rfc-editor.org/info/rfc4422>.

Saint-Andre & Melnikov Standards Track [Page 23] RFC 7613 PRECIS: Usernames and Passwords August 2015

 [RFC4616]  Zeilenga, K., Ed., "The PLAIN Simple Authentication and
            Security Layer (SASL) Mechanism", RFC 4616,
            DOI 10.17487/RFC4616, August 2006,
            <http://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,
            <http://www.rfc-editor.org/info/rfc5802>.
 [RFC5891]  Klensin, J., "Internationalized Domain Names in
            Applications (IDNA): Protocol", RFC 5891,
            DOI 10.17487/RFC5891, August 2010,
            <http://www.rfc-editor.org/info/rfc5891>.
 [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,
            <http://www.rfc-editor.org/info/rfc5893>.
 [RFC5894]  Klensin, J., "Internationalized Domain Names for
            Applications (IDNA): Background, Explanation, and
            Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010,
            <http://www.rfc-editor.org/info/rfc5894>.
 [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
            Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
            March 2011, <http://www.rfc-editor.org/info/rfc6120>.
 [RFC6122]  Saint-Andre, P., "Extensible Messaging and Presence
            Protocol (XMPP): Address Format", RFC 6122,
            DOI 10.17487/RFC6122, March 2011,
            <http://www.rfc-editor.org/info/rfc6122>.
 [RFC6943]  Thaler, D., Ed., "Issues in Identifier Comparison for
            Security Purposes", RFC 6943, DOI 10.17487/RFC6943,
            May 2013, <http://www.rfc-editor.org/info/rfc6943>.
 [RFC7542]  DeKok, A., "The Network Access Identifier", RFC 7542,
            DOI 10.17487/RFC7542, May 2015,
            <http://www.rfc-editor.org/info/rfc7542>.

Saint-Andre & Melnikov Standards Track [Page 24] RFC 7613 PRECIS: Usernames and Passwords August 2015

 [UTS39]    Unicode Technical Standard #39, "Unicode Security
            Mechanisms", edited by Mark Davis and Michel Suignard,
            <http://unicode.org/reports/tr39/>.
 [XMPP-ADDR]
            Saint-Andre, P., "Extensible Messaging and Presence
            Protocol (XMPP): Address Format", Work in Progress,
            draft-ietf-xmpp-6122bis-24, June 2015.

Saint-Andre & Melnikov Standards Track [Page 25] RFC 7613 PRECIS: Usernames and Passwords August 2015

Appendix A. Differences from RFC 4013

 This document builds upon the PRECIS framework defined in [RFC7564],
 which differs fundamentally from the stringprep technology [RFC3454]
 used in SASLprep [RFC4013].  The primary difference is that
 stringprep profiles allowed all characters except those characters
 that were explicitly disallowed, whereas PRECIS profiles disallow all
 characters except those characters that are explicitly allowed (this
 "inclusion model" was originally used for internationalized domain
 names in [RFC5891]; see [RFC5894] for further discussion).  It is
 important to keep this distinction in mind when comparing the
 technology defined in this document to SASLprep [RFC4013].
 The following substantive modifications were made from RFC 4013.
 o  A single SASLprep algorithm was replaced by three separate
    algorithms: one for usernames with case mapping, one for usernames
    with case preservation, and one for passwords.
 o  The new preparation algorithms use PRECIS instead of a stringprep
    profile.  The new algorithms work independently of Unicode
    versions.
 o  As recommended in the PRECIS framework, changed the Unicode
    normalization form from NFKC to NFC.
 o  Some Unicode code points that were mapped to nothing in RFC 4013
    are simply disallowed by PRECIS.

Saint-Andre & Melnikov Standards Track [Page 26] RFC 7613 PRECIS: Usernames and Passwords August 2015

Acknowledgements

 This document borrows some text from [RFC4013] and [RFC6120].
 The following individuals provided helpful feedback on this document:
 Marc Blanchet, Ben Campbell, Alan DeKok, Joe Hildebrand, Jeffrey
 Hutzelman, Simon Josefsson, Jonathan Lennox, James Manger, Matt
 Miller, Chris Newman, Yutaka OIWA, Pete Resnick, Andrew Sullivan,
 Nico Williams, and Yoshiro YONEYA.  Nico Williams in particular
 deserves special recognition for providing text that was used in
 Section 3.4.  Thanks also to Takahiro NEMOTO and Yoshiro YONEYA for
 implementation feedback.
 Robert Sparks and Derek Atkins reviewed the document on behalf of the
 General Area Review Team and the Security Directorate, respectively.
 Benoit Claise and Stephen Farrell provided helpful input during IESG
 review.
 Thanks to Matt Miller as document shepherd, Marc Blanchet and Yoshiro
 YONEYA as working group chairs, and Pete Resnick and Barry Leiba as
 area directors.
 Peter Saint-Andre wishes to acknowledge Cisco Systems, Inc., for
 employing him during his work on earlier draft versions of this
 document.

Authors' Addresses

 Peter Saint-Andre
 &yet
 Email: peter@andyet.com
 URI:   https://andyet.com/
 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 27]

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