Preparation, Enforcement, and Comparison of Internationalized Strings Representing Usernames and Passwords
RFC 8265
| Document | Type |
RFC
- Proposed Standard
(October 2017)
Errata
Obsoletes RFC 7613
|
|
|---|---|---|---|
| Authors | Peter Saint-Andre , Alexey Melnikov | ||
| Last updated | 2018-08-27 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Last Call review
(of
-07)
by Linda Dunbar
Ready w/nits
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Marc Blanchet | ||
| Shepherd write-up | Show Last changed 2017-05-30 | ||
| IESG | IESG state | RFC 8265 (Proposed Standard) | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | Ben Campbell | ||
| Send notices to | Marc Blanchet <Marc.Blanchet@viagenie.ca> | ||
| IANA | IANA review state | Version Changed - Review Needed | |
| IANA action state | RFC-Ed-Ack |
RFC 8265
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
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1. Introduction
Usernames and passwords are widely used for authentication and
authorization on the Internet, either directly when provided in
plaintext (as in the PLAIN Simple Authentication and Security Layer
(SASL) mechanism [RFC4616] and the HTTP Basic scheme [RFC7617]) or
indirectly when provided as the input to a cryptographic algorithm
such as a hash function (as in the Salted Challenge Response
Authentication Mechanism (SCRAM) SASL mechanism [RFC5802] and the
HTTP Digest scheme [RFC7616]).
To increase the likelihood that the input and comparison of usernames
and passwords will work in ways that make sense for typical users
throughout the world, this document defines rules for handling
internationalized strings that represent usernames and passwords.
Such strings consist of code points from the Unicode coded character
set [Unicode], with special attention to code points outside the
ASCII range [RFC20]. The rules for handling such strings are
specified through profiles of the string classes defined in the
preparation, enforcement, and comparison of internationalized strings
(PRECIS) framework specification [RFC8264].
Profiles of the PRECIS framework enable software to handle Unicode
code points outside the ASCII range in an automated way, so that such
code points are treated carefully and consistently in application
protocols. In large measure, these profiles are designed to protect
application developers from the potentially negative consequences of
supporting the full range of Unicode code points. For instance, in
almost all application protocols it would be dangerous to treat the
Unicode code point "¹" (SUPERSCRIPT ONE, U+00B9) as equivalent to "1"
(DIGIT ONE, U+0031), because that would result in false accepts
during comparison, authentication, and authorization (e.g., an
attacker could easily spoof an account "user1@example.com").
Whereas a naive use of Unicode would make such attacks trivially
easy, the PRECIS profile defined here for usernames generally
protects applications from inadvertently causing such problems.
(Similar considerations apply to passwords, although here it is
desirable to support a wider range of characters so as to maximize
entropy for purposes of authentication.)
The methods defined here might be applicable wherever usernames or
passwords are used. However, the methods are not intended for use in
preparing strings that are not usernames (e.g., Lightweight Directory
Access Protocol (LDAP) distinguished names), nor in cases where
identifiers or secrets are not strings (e.g., keys and certificates)
or require specialized handling.
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Although the historical predecessor of this document was the SASLprep
profile of Stringprep [RFC3454]), the approach defined here can be
used by technologies other than SASL [RFC4422], such as HTTP
authentication as specified in [RFC7617] and [RFC7616].
This document does not modify the handling of internationalized
strings in usernames and passwords as prescribed by existing
application protocols that use SASLprep. If the community that uses
such an application protocol wishes to modernize its handling of
internationalized strings to use PRECIS instead of Stringprep, it
needs to explicitly update the existing application protocol
definition (one example is [RFC7622]). Non-coordinated updates to
protocol implementations are discouraged because they can have a
negative impact on interoperability and security.
2. Terminology
A "username" or "user identifier" is a string of characters
designating an account on a computing device or system, often but not
necessarily for use by a person. Although some devices and systems
might allow a username to be part or all of a person's name and a
person might want their account designator to be part or all of their
name, because of the complexities involved, that outcome is not
guaranteed for all human names on all computing devices or systems
that follow the rules defined in this specification. Protocol
designers and application developers who wish to allow a wider range
of characters are encouraged to consider a separation between more
restrictive account identifiers and more expressive display names or
nicknames (see [RFC8266]).
A "password" is a string of characters that allows access to a
computing device or system, often associated with a particular
username. A password is not literally limited to a word, because a
password could be a passphrase consisting of more than one word,
perhaps separated by spaces, punctuation, or other non-alphanumeric
characters.
Some SASL mechanisms (e.g., CRAM-MD5, DIGEST-MD5, and SCRAM) specify
that the authentication identity used in the context of such
mechanisms is a "simple username" (see Section 2 of [RFC4422] as well
as [RFC4013]). Various application technologies also assume that the
identity of a user or account takes the form of a username (e.g.,
authentication for the Hypertext Transfer Protocol as specified in
[RFC7617] and [RFC7616]), whether or not they use SASL. Note well
that the exact form of a username in any particular SASL mechanism or
application technology is a matter for implementation and deployment;
note also that a username does not necessarily map to any particular
application identifier.
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Many important terms used in this document are defined in [RFC5890],
[RFC6365], [RFC8264], and [Unicode]. The term "non-ASCII space"
refers to any Unicode code point having a Unicode general category of
"Zs", naturally with the exception of SPACE (U+0020).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Usernames
3.1. Definition
This document specifies that a username is a string of Unicode code
points [Unicode] that is structured as an ordered sequence of
"userparts" and expressed in a standard Unicode Encoding Form (such
as UTF-8 [RFC3629]). A userpart is allowed to contain only code
points that are allowed by the PRECIS IdentifierClass defined in
Section 4.2 of [RFC8264] and thus consists almost exclusively of
letters and digits. A username can consist of a single userpart or a
space-separated sequence of userparts.
The syntax for a username is defined as follows, using the Augmented
Backus-Naur Form (ABNF) [RFC5234].
username = userpart *(1*SP userpart)
userpart = 1*(idpoint)
;
; an "idpoint" is a Unicode code point that
; can be contained in a string conforming to
; the PRECIS IdentifierClass
;
All code points and blocks not explicitly allowed in the PRECIS
IdentifierClass are disallowed; this includes private-use code
points, surrogate code points, and the other code points and blocks
that were defined as "Prohibited Output" in Section 2.3 of [RFC4013]
(when corrected per [Err1812]). In addition, common constructions
such as "user@example.com" (e.g., the Network Access Identifier from
[RFC7542]) are allowed as usernames under this specification, as they
were under [RFC4013].
Implementation Note: The username construct defined in this
document does not necessarily match what all deployed applications
might refer to as a "username" or "userid" but instead provides a
relatively safe subset of Unicode code points that can be used in
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existing SASL mechanisms and in application protocols that use
SASL, and even in most application protocols that do not currently
use SASL.
A username MUST NOT be zero bytes in length. This rule is to be
enforced after any normalization and mapping of code points.
This specification defines two profiles for usernames: the
UsernameCaseMapped profile performs case mapping, and the
UsernameCasePreserved performs case preservation (see further
discussion under Section 3.2).
In protocols that provide usernames as input to a cryptographic
algorithm such as a hash function, the client will need to perform
enforcement of the rules for the UsernameCaseMapped or
UsernameCasePreserved profile before applying the algorithm.
3.2. Case Mapping vs. Case Preservation
In order to accommodate the widest range of username constructs in
applications, this document defines two username profiles:
UsernameCaseMapped and UsernameCasePreserved. These two profiles
differ only in their use (or not) of the Case Mapping Rule and are
otherwise identical.
Case mapping is a matter for the application protocol, protocol
implementation, or end deployment. In general, this document
suggests that it is preferable to apply the UsernameCaseMapped
profile and therefore perform case mapping, because not doing so can
lead to false accepts during authentication and authorization (as
described in [RFC6943]) and can result in confusion among end users,
given the prevalence of case mapping in many existing protocols and
applications. However, there can be good reasons to apply the
UsernameCasePreserved profile and thus not perform case mapping, such
as backward compatibility with deployed infrastructure.
In particular:
o SASL mechanisms that follow the recommendations in this document
MUST specify whether and when case mapping is to be applied to
authentication identifiers. Because case mapping results in
information loss, in order to retain that information for as long
as possible during processing, implementations SHOULD delay any
case mapping to the last possible moment, such as when doing a
lookup by username, performing username comparisons, or generating
a cryptographic salt from a username (if the last possible moment
happens on a server, then decisions about case mapping can be a
matter of service deployment policy). In keeping with [RFC4422],
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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.
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3. Case Mapping Rule: Map uppercase and titlecase code points to
their lowercase equivalents, preferably using the Unicode
toLowerCase() operation as defined in the Unicode Standard
[Unicode]; see further discussion in Section 3.2.
4. Normalization Rule: Apply Unicode Normalization Form C (NFC) to
all strings.
5. Directionality Rule: Apply the "Bidi Rule" defined in [RFC5893]
to strings that contain right-to-left code points (i.e., each of
the six conditions of the Bidi Rule must be satisfied); for
strings that do not contain right-to-left code points, there is
no special processing for directionality.
3.3.2. Preparation
An entity that prepares an input string for subsequent enforcement
according to this profile MUST proceed as follows (applying the steps
in the order shown).
1. Apply the width mapping rule specified in Section 3.3.1. It is
necessary to apply the rule at this point because otherwise the
PRECIS "HasCompat" category specified in Section 9.17 of
[RFC8264] would forbid fullwidth and halfwidth code points.
2. Ensure that the string consists only of Unicode code points that
are explicitly allowed by the PRECIS IdentifierClass defined in
Section 4.2 of [RFC8264].
3.3.3. Enforcement
An entity that performs enforcement according to this profile MUST
prepare an input string as described in Section 3.3.2 and MUST also
apply the following rules specified in Section 3.3.1 in the order
shown:
1. Case Mapping Rule
2. Normalization Rule
3. Directionality Rule
After all of the foregoing rules have been enforced, the entity MUST
ensure that the username is not zero bytes in length (this is done
after enforcing the rules to prevent applications from mistakenly
omitting a username entirely, because when internationalized strings
are accepted, a non-empty sequence of characters can result in a
zero-length username after canonicalization).
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The result of the foregoing operations is an output string that
conforms to the UsernameCaseMapped profile. Until an implementation
produces such an output string, it MUST NOT treat the string as
conforming (in particular, it MUST NOT assume that an input string is
conforming before the enforcement operation has been completed).
3.3.4. Comparison
An entity that performs comparison of two strings according to this
profile MUST prepare each string as specified in Section 3.3.2 and
then MUST enforce the rules specified in Section 3.3.3. The two
strings are to be considered equivalent if and only if they are an
exact octet-for-octet match (sometimes called "bit-string identity").
Until an implementation determines whether two strings are to be
considered equivalent, it MUST NOT treat them as equivalent (in
particular, it MUST NOT assume that two input strings are equivalent
before the comparison operation has been completed).
3.4. UsernameCasePreserved Profile
3.4.1. Rules
The following rules are defined for use within the
UsernameCasePreserved profile of the PRECIS IdentifierClass.
1. Width Mapping Rule: Map fullwidth and halfwidth code points to
their decomposition mappings (see Unicode Standard Annex #11
[UAX11]).
2. Additional Mapping Rule: There is no additional mapping rule.
3. Case Mapping Rule: There is no case mapping rule.
4. Normalization Rule: Apply Unicode Normalization Form C (NFC) to
all strings.
5. Directionality Rule: Apply the "Bidi Rule" defined in [RFC5893]
to strings that contain right-to-left code points (i.e., each of
the six conditions of the Bidi Rule must be satisfied); for
strings that do not contain right-to-left code points, there is
no special processing for directionality.
3.4.2. Preparation
An entity that prepares a string for subsequent enforcement according
to this profile MUST proceed as follows (applying the steps in the
order shown).
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1. Apply the width mapping rule specified in Section 3.4.1. It is
necessary to apply the rule at this point because otherwise the
PRECIS "HasCompat" category specified in Section 9.17 of
[RFC8264] would forbid fullwidth and halfwidth code points.
2. Ensure that the string consists only of Unicode code points that
are explicitly allowed by the PRECIS IdentifierClass defined in
Section 4.2 of [RFC8264].
3.4.3. Enforcement
An entity that performs enforcement according to this profile MUST
prepare a string as described in Section 3.4.2 and MUST also apply
the following rules specified in Section 3.4.1 in the order shown:
1. Normalization Rule
2. Directionality Rule
After all of the foregoing rules have been enforced, the entity MUST
ensure that the username is not zero bytes in length (this is done
after enforcing the rules to prevent applications from mistakenly
omitting a username entirely, because when internationalized strings
are accepted, a non-empty sequence of characters can result in a
zero-length username after canonicalization).
The result of the foregoing operations is an output string that
conforms to the UsernameCasePreserved profile. Until an
implementation produces such an output string, it MUST NOT treat the
string as conforming (in particular, it MUST NOT assume that an input
string is conforming before the enforcement operation has been
completed).
3.4.4. Comparison
An entity that performs comparison of two strings according to this
profile MUST prepare each string as specified in Section 3.4.2 and
then MUST enforce the rules specified in Section 3.4.3. The two
strings are to be considered equivalent if and only if they are an
exact octet-for-octet match (sometimes called "bit-string identity").
Until an implementation determines whether two strings are to be
considered equivalent, it MUST NOT treat them as equivalent (in
particular, it MUST NOT assume that two input strings are equivalent
before the comparison operation has been completed).
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3.5. Application-Layer Constructs
Both the UsernameCaseMapped and UsernameCasePreserved profiles enable
an application protocol, implementation, or deployment to create
application-layer constructs such as a username that is a space-
separated set of userparts like "Firstname Middlename Lastname".
Such a construct is not a profile of the PRECIS IdentifierClass,
because SPACE (U+0020) is not allowed in the IdentifierClass;
however, it can be created at the application layer because SPACE
(U+0020) can be used as a separator between instances of the PRECIS
IdentifierClass (e.g., userparts as defined in this specification).
3.6. Examples
The following examples illustrate a small number of userparts (not
usernames) that are consistent with the format defined above (note
that the characters "<" and ">" are used here to delineate the actual
userparts and are not part of the userpart strings).
+--------------------------+---------------------------------+
| # | Userpart | Notes |
+--------------------------+---------------------------------+
| 1 | <juliet@example.com> | The "at" sign ("@") is allowed |
| | | in the PRECIS IdentifierClass |
+--------------------------+---------------------------------+
| 2 | <fussball> | |
+--------------------------+---------------------------------+
| 3 | <fußball> | The third character is LATIN |
| | | SMALL LETTER SHARP S (U+00DF) |
+--------------------------+---------------------------------+
| 4 | <π> | A userpart of GREEK SMALL |
| | | LETTER PI (U+03C0) |
+--------------------------+---------------------------------+
| 5 | <Σ> | A userpart of GREEK CAPITAL |
| | | LETTER SIGMA (U+03A3) |
+--------------------------+---------------------------------+
| 6 | <σ> | A userpart of GREEK SMALL |
| | | LETTER SIGMA (U+03C3) |
+--------------------------+---------------------------------+
| 7 | <ς> | A userpart of GREEK SMALL |
| | | LETTER FINAL SIGMA (U+03C2) |
+--------------------------+---------------------------------+
Table 1: A Sample of Legal Userparts
Regarding examples 2 and 3: although in German writing the character
eszett "ß" (LATIN SMALL LETTER SHARP S, U+00DF) can mostly be used
interchangeably with the two characters "ss", the userparts in these
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examples are different and (if desired) a server would need to
enforce a registration policy that disallows one of them if the other
is registered.
Regarding examples 5, 6, and 7: optional case mapping of "Σ" (GREEK
CAPITAL LETTER SIGMA, U+03A3) to the lowercase character "σ" (GREEK
SMALL LETTER SIGMA, U+03C3) during comparison would result in
matching the userparts in examples 5 and 6; however, because the
PRECIS mapping rules do not account for the special status of the
character "ς" (GREEK SMALL LETTER FINAL SIGMA, U+03C2), the userparts
in examples 5 and 7 or examples 6 and 7 would not be matched during
comparison.
The following examples illustrate strings that are not valid
userparts (not usernames) because they violate the format defined
above.
+--------------------------+---------------------------------+
| # | Non-Userpart String | Notes |
+--------------------------+---------------------------------+
| 8 | <foo bar> | SPACE (U+0020) is disallowed in |
| | | the userpart |
+--------------------------+---------------------------------+
| 9 | <> | Zero-length userpart |
+--------------------------+---------------------------------+
| 10| <henryⅣ> | The sixth character is ROMAN |
| | | NUMERAL FOUR (U+2163) |
+--------------------------+---------------------------------+
| 11| <∞> | A userpart of INFINITY (U+221E) |
+--------------------------+---------------------------------+
Table 2: A Sample of Strings That Violate the Userpart Rules
Regarding example 8: although this is not a valid userpart, it is a
valid username because it is a space-separated sequence of userparts.
Regarding example 10: the character "Ⅳ" (ROMAN NUMERAL FOUR, U+2163)
has a compatibility equivalent of the characters "I" (LATIN CAPITAL
LETTER I, U+0049) and "V" (LATIN CAPITAL LETTER V, U+0056), but code
points with compatibility equivalents are not allowed in the PRECIS
IdentifierClass.
Regarding example 11: symbol characters such as "∞" (INFINITY,
U+221E) are not allowed in the PRECIS IdentifierClass.
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4. Passwords
4.1. Definition
This document specifies that a password is a string of Unicode code
points [Unicode] that is conformant to the OpaqueString profile
(specified below) of the PRECIS FreeformClass defined in Section 4.3
of [RFC8264] and expressed in a standard Unicode Encoding Form (such
as UTF-8 [RFC3629]).
The syntax for a password is defined as follows, using the Augmented
Backus-Naur Form (ABNF) [RFC5234].
password = 1*(freepoint)
;
; a "freepoint" is a Unicode code point that
; can be contained in a string conforming to
; the PRECIS FreeformClass
;
All code points and blocks not explicitly allowed in the PRECIS
FreeformClass are disallowed; this includes private-use code points,
surrogate code points, and the other code points and blocks defined
as "Prohibited Output" in Section 2.3 of [RFC4013] (when corrected
per [Err1812]).
A password MUST NOT be zero bytes in length. This rule is to be
enforced after any normalization and mapping of code points.
Note: Some existing systems allow an empty string in places where
a password would be expected (e.g., command-line tools that might
be called from an automated script, or servers that might need to
be restarted without human intervention). From the perspective of
this document (and RFC 4013 before it), these empty strings are
not passwords but are workarounds for the practical difficulty of
using passwords in certain scenarios.
Note: The prohibition of zero-length passwords is not a
recommendation regarding password strength (because a password of
only one byte is highly insecure) but is meant to prevent
applications from mistakenly omitting a password entirely; such an
outcome is possible when internationalized strings are accepted,
because a non-empty sequence of characters can result in a zero-
length password after canonicalization.
In protocols that provide passwords as input to a cryptographic
algorithm such as a hash function, the client will need to perform
enforcement of the rules for the OpaqueString profile before applying
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the algorithm, because the password is not available to the server in
plaintext form.
4.2. OpaqueString Profile
The definition of the OpaqueString profile is provided in the
following sections, including detailed information about preparation,
enforcement, and comparison (for details on the distinction between
these actions, refer to [RFC8264]).
4.2.1. Preparation
An entity that prepares a string according to this profile MUST
ensure that the string consists only of Unicode code points that are
explicitly allowed by the FreeformClass string class defined in
[RFC8264].
4.2.2. Enforcement
An entity that performs enforcement according to this profile MUST
prepare a string as described in Section 4.2.1 and MUST also apply
the rules specified below for the OpaqueString profile (these rules
MUST be applied in the order shown):
1. Width Mapping Rule: Fullwidth and halfwidth code points MUST NOT
be mapped to their decomposition mappings (see Unicode Standard
Annex #11 [UAX11]).
2. Additional Mapping Rule: Any instances of non-ASCII space MUST be
mapped to SPACE (U+0020); a non-ASCII space is any Unicode code
point having a Unicode general category of "Zs", with the
exception of SPACE (U+0020). As was the case in RFC 4013, the
inclusion of only SPACE (U+0020) prevents confusion with various
non-ASCII space code points, many of which are difficult to
reproduce across different input methods.
3. Case Mapping Rule: There is no case mapping rule (because mapping
uppercase and titlecase code points to their lowercase
equivalents would lead to false accepts and thus to reduced
security).
4. Normalization Rule: Unicode Normalization Form C (NFC) MUST be
applied to all strings.
5. Directionality Rule: There is no directionality rule. The "Bidi
Rule" (defined in [RFC5893]) and similar rules are unnecessary
and inapplicable to passwords, because they can reduce the
repertoire of characters that are allowed in a string and
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therefore reduce the amount of entropy that is possible in a
password. Such rules are intended to minimize the possibility
that the same string will be displayed differently on a layout
system set for right-to-left display and a layout system set for
left-to-right display; however, passwords are typically not
displayed at all and are rarely meant to be interoperable across
different layout systems in the way that non-secret strings like
domain names and usernames are. Furthermore, it is perfectly
acceptable for opaque strings other than passwords to be
presented differently in different layout systems, as long as the
presentation is consistent in any given layout system.
The result of the foregoing operations is an output string that
conforms to the OpaqueString profile. Until an implementation
produces such an output string, it MUST NOT treat the string as
conforming (in particular, it MUST NOT assume that an input string is
conforming before the enforcement operation has been completed).
4.2.3. Comparison
An entity that performs comparison of two strings according to this
profile MUST prepare each string as specified in Section 4.2.1 and
then MUST enforce the rules specified in Section 4.2.2. The two
strings are to be considered equivalent if and only if they are an
exact octet-for-octet match (sometimes called "bit-string identity").
Until an implementation determines whether two strings are to be
considered equivalent, it MUST NOT treat them as equivalent (in
particular, it MUST NOT assume that two input strings are equivalent
before the comparison operation has been completed).
See Section 8.2 regarding comparison of passwords and passphrases.
4.3. Examples
The following examples illustrate a small number of passwords that
are consistent with the format defined above (note that the
characters "<" and ">" are used here to delineate the actual
passwords and are not part of the password strings).
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+------------------------------------+------------------------------+
| # | Password | Notes |
+------------------------------------+------------------------------+
| 12| <correct horse battery staple> | SPACE (U+0020) is allowed |
+------------------------------------+------------------------------+
| 13| <Correct Horse Battery Staple> | Differs by case from |
| | | example 12 |
+------------------------------------+------------------------------+
| 14| <πßå> | Non-ASCII letters are OK |
| | | (e.g., GREEK SMALL LETTER |
| | | PI (U+03C0)) |
+------------------------------------+------------------------------+
| 15| <Jack of ♦s> | Symbols are OK (e.g., BLACK |
| | | DIAMOND SUIT (U+2666)) |
+------------------------------------+------------------------------+
| 16| <foo bar> | OGHAM SPACE MARK (U+1680) is |
| | | mapped to SPACE (U+0020); |
| | | thus, the full string is |
| | | mapped to <foo bar> |
+------------------------------------+------------------------------+
Table 3: A Sample of Legal Passwords
The following examples illustrate strings that are not valid
passwords because they violate the format defined above.
+------------------------------------+------------------------------+
| # | Password | Notes |
+------------------------------------+------------------------------+
| 17| <> | Zero-length passwords are |
| | | disallowed |
+------------------------------------+------------------------------+
| 18| <my cat is a 	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 	 because otherwise
it could not be shown in running text.
5. Use in Application Protocols
This specification defines only the PRECIS-based rules for the
handling of strings conforming to the UsernameCaseMapped and
UsernameCasePreserved profiles of the PRECIS IdentifierClass, and
strings conforming to the OpaqueString profile of the PRECIS
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FreeformClass. It is the responsibility of an application protocol
to specify the protocol slots in which such strings can appear, the
entities that are expected to enforce the rules governing such
strings, and at what points during protocol processing or interface
handling the rules need to be enforced. See Section 6 of [RFC8264]
for guidelines on using PRECIS profiles in applications.
Above and beyond the PRECIS-based rules specified here, application
protocols can also define application-specific rules governing such
strings (rules regarding minimum or maximum length, further
restrictions on allowable code points or character ranges, safeguards
to mitigate the effects of visually similar characters, etc.),
application-layer constructs (see Section 3.5), and related matters.
Some PRECIS profile definitions encourage entities that enforce the
rules to be liberal in what they accept. However, for usernames and
passwords such a policy can be problematic, because it can lead to
false accepts. An in-depth discussion can be found in [RFC6943].
Applying the rules for any given PRECIS profile is not necessarily an
idempotent procedure for all code points. Therefore, an
implementation SHOULD apply the rules repeatedly until the output
string is stable; if the output string does not stabilize after
reapplying the rules three (3) additional times after the first
application, the implementation SHOULD terminate application of the
rules and reject the input string as invalid.
6. Migration
The rules defined in this specification differ slightly from those
defined by the SASLprep specification [RFC4013] (but not from
[RFC7613]). In order to smooth the process of migrating from
SASLprep to the approach defined herein, the following sections
describe these differences, along with their implications for
migration, in more detail.
6.1. Usernames
Deployments that currently use SASLprep for handling usernames might
need to scrub existing data when they migrate to the rules defined in
this specification. In particular:
o SASLprep specified the use of Unicode Normalization Form KC
(NFKC), whereas the UsernameCaseMapped and UsernameCasePreserved
profiles employ Unicode Normalization Form C (NFC). In practice,
this change is unlikely to cause significant problems, because
NFKC provides methods for mapping Unicode code points with
compatibility equivalents to those equivalents, whereas the PRECIS
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IdentifierClass entirely disallows Unicode code points with
compatibility equivalents (i.e., during comparison, NFKC is more
"aggressive" about finding matches than NFC). A few examples
might suffice to indicate the nature of the problem:
1. "ſ" (LATIN SMALL LETTER LONG S, U+017F) is compatibility
equivalent to "s" (LATIN SMALL LETTER S, U+0073).
2. "Ⅳ" (ROMAN NUMERAL FOUR, U+2163) is compatibility equivalent
to "I" (LATIN CAPITAL LETTER I, U+0049) and "V" (LATIN CAPITAL
LETTER V, U+0056).
3. "fi" (LATIN SMALL LIGATURE FI, U+FB01) is compatibility
equivalent to "f" (LATIN SMALL LETTER F, U+0066) and "i"
(LATIN SMALL LETTER I, U+0069).
Under SASLprep, the use of NFKC also handled the mapping of
fullwidth and halfwidth code points to their decomposition
mappings.
For migration purposes, operators might want to search their
database of usernames for names containing Unicode code points
with compatibility equivalents and, where there is no conflict,
map those code points to their equivalents. Naturally, it is
possible that during this process the operator will discover
conflicting usernames; for instance, "HENRYIV" with the last two
code points being LATIN CAPITAL LETTER I (U+0049) and LATIN
CAPITAL LETTER V (U+0056) as opposed to "HENRYⅣ" with the last
character being "Ⅳ" (ROMAN NUMERAL FOUR, U+2163), which is
compatibility equivalent to U+0049 and U+0056). In these cases,
the operator will need to determine how to proceed, for instance,
by disabling the account whose name contains a Unicode code point
with a compatibility equivalent. Such cases are probably rare,
but it is important for operators to be aware of them.
o SASLprep mapped the "characters commonly mapped to nothing" (from
Appendix B.1 of [RFC3454]) to nothing, whereas the PRECIS
IdentifierClass entirely disallows most of these code points,
which correspond to the code points from the PRECIS "M" category
defined under Section 9.13 of [RFC8264]. For migration purposes,
the operator might want to remove from usernames any code points
contained in the PRECIS "M" category (e.g., SOFT HYPHEN (U+00AD)).
Because these code points would have been "mapped to nothing" in
Stringprep, in practice a user would not notice the difference if,
upon migration to PRECIS, the code points are removed.
o SASLprep allowed uppercase and titlecase code points, whereas the
UsernameCaseMapped profile maps uppercase and titlecase code
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points to their lowercase equivalents (by contrast, the
UsernameCasePreserved profile matches SASLprep in this regard).
For migration purposes, the operator can use either the
UsernameCaseMapped profile (thus losing the case information) or
the UsernameCasePreserved profile (thus ignoring case difference
when comparing usernames).
6.2. Passwords
Depending on local service policy, migration from SASLprep to this
specification might not involve any scrubbing of data (because
passwords might not be stored in the clear anyway); however, service
providers need to be aware of possible issues that might arise during
migration. In particular:
o SASLprep specified the use of Unicode Normalization Form KC
(NFKC), whereas the OpaqueString profile employs Unicode
Normalization Form C (NFC). Because NFKC is more aggressive about
finding matches than NFC, in practice this change is unlikely to
cause significant problems and indeed has the security benefit of
probably resulting in fewer false accepts when comparing
passwords. A few examples might suffice to indicate the nature of
the problem:
1. "ſ" (LATIN SMALL LETTER LONG S, U+017F) is compatibility
equivalent to "s" (LATIN SMALL LETTER S, U+0073).
2. "Ⅳ" (ROMAN NUMERAL FOUR, U+2163) is compatibility equivalent
to "I" (LATIN CAPITAL LETTER I, U+0049) and "V" (LATIN CAPITAL
LETTER V, U+0056).
3. "fi" (LATIN SMALL LIGATURE FI, U+FB01) is compatibility
equivalent to "f" (LATIN SMALL LETTER F, U+0066) and "i"
(LATIN SMALL LETTER I, U+0069).
Under SASLprep, the use of NFKC also handled the mapping of
fullwidth and halfwidth code points to their decomposition
mappings. Although it is expected that code points with
compatibility equivalents are rare in existing passwords, some
passwords that matched when SASLprep was used might no longer work
when the rules in this specification are applied.
o SASLprep mapped the "characters commonly mapped to nothing" (from
Appendix B.1 of [RFC3454]) to nothing, whereas the PRECIS
FreeformClass entirely disallows such code points, which
correspond to the code points from the PRECIS "M" category defined
under Section 9.13 of [RFC8264]. In practice, this change will
probably have no effect on comparison, but user-oriented software
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might reject such code points instead of ignoring them during
password preparation.
7. IANA Considerations
IANA has made the updates described below.
7.1. UsernameCaseMapped Profile
IANA has added the following entry to the "PRECIS Profiles" registry.
Name: UsernameCaseMapped.
Base Class: IdentifierClass.
Applicability: Usernames in security and application protocols.
Replaces: The SASLprep profile of Stringprep.
Width Mapping Rule: Map fullwidth and halfwidth code points to their
decomposition mappings.
Additional Mapping Rule: None.
Case Mapping Rule: Map uppercase and titlecase code points to
lowercase.
Normalization Rule: NFC.
Directionality Rule: The "Bidi Rule" defined in RFC 5893 applies.
Enforcement: To be defined by security or application protocols that
use this profile.
Specification: Section 3.3 of RFC 8265.
7.2. UsernameCasePreserved Profile
IANA has added the following entry to the "PRECIS Profiles" registry.
Name: UsernameCasePreserved.
Base Class: IdentifierClass.
Applicability: Usernames in security and application protocols.
Replaces: The SASLprep profile of Stringprep.
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Width Mapping Rule: Map fullwidth and halfwidth code points to their
decomposition mappings.
Additional Mapping Rule: None.
Case Mapping Rule: None.
Normalization Rule: NFC.
Directionality Rule: The "Bidi Rule" defined in RFC 5893 applies.
Enforcement: To be defined by security or application protocols that
use this profile.
Specification: Section 3.4 of RFC 8265.
7.3. OpaqueString Profile
IANA has added the following entry to the "PRECIS Profiles" registry.
Name: OpaqueString.
Base Class: FreeformClass.
Applicability: Passwords and other opaque strings in security and
application protocols.
Replaces: The SASLprep profile of Stringprep.
Width Mapping Rule: None.
Additional Mapping Rule: Map non-ASCII space code points to SPACE
(U+0020).
Case Mapping Rule: None.
Normalization Rule: NFC.
Directionality Rule: None.
Enforcement: To be defined by security or application protocols that
use this profile.
Specification: Section 4.2 of RFC 8265.
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7.4. Stringprep Profile
The Stringprep specification [RFC3454] did not provide for entries in
the "Stringprep Profiles" registry to have any state except "Current"
or "Not Current". Because RFC 7613 obsoleted RFC 4013, which
registered the SASLprep profile of Stringprep, IANA previously marked
that profile as "Not Current" and cited RFC 7613 as an additional
reference. IANA has modified the profile so that the current
document is now cited as the additional reference.
8. Security Considerations
8.1. Password/Passphrase Strength
The ability to include a wide range of characters in passwords and
passphrases can increase the potential for creating a strong password
with high entropy. However, in practice, the ability to include such
characters ought to be weighed against the possible need to reproduce
them on various devices using various input methods.
8.2. Password/Passphrase Comparison
In systems that conform to modern best practices for security,
verification of passwords during authentication will not use the
comparison defined in Section 4.2.3. Instead, because the system
performs cryptographic calculations to verify the password, it will
prepare the password as defined in Section 4.2.1 and enforce the
rules as defined in Section 4.2.2 before performing the relevant
calculations.
8.3. Identifier Comparison
The process of comparing identifiers (such as SASL simple usernames,
authentication identifiers, and authorization identifiers) can lead
to either false rejects or false accepts, both of which have security
implications. A more detailed discussion can be found in [RFC6943].
8.4. Reuse of PRECIS
The security considerations described in [RFC8264] apply to the
IdentifierClass and FreeformClass string classes used in this
document for usernames and passwords, respectively.
8.5. Reuse of Unicode
The security considerations described in [UTS39] apply to the use of
Unicode code points in usernames and passwords.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<https://www.rfc-editor.org/info/rfc5890>.
[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365,
DOI 10.17487/RFC6365, September 2011,
<https://www.rfc-editor.org/info/rfc6365>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8264] Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
Preparation, Enforcement, and Comparison of
Internationalized Strings in Application Protocols",
RFC 8264, DOI 10.17487/RFC8264, October 2017,
<https://www.rfc-editor.org/info/rfc8264>.
[UAX11] Unicode Standard Annex #11, "East Asian Width", edited by
Ken Lunde. An integral part of The Unicode Standard,
<http://unicode.org/reports/tr11/>.
[Unicode] The Unicode Consortium, "The Unicode Standard",
<http://www.unicode.org/versions/latest/>.
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9.2. Informative References
[Err1812] RFC Errata, Erratum ID 1812, RFC 4013,
<https://www.rfc-editor.org/errata/eid1812>.
[RFC20] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
DOI 10.17487/RFC3454, December 2002,
<https://www.rfc-editor.org/info/rfc3454>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<https://www.rfc-editor.org/info/rfc3501>.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
January 2005, <https://www.rfc-editor.org/info/rfc3987>.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names
and Passwords", RFC 4013, DOI 10.17487/RFC4013, February
2005, <https://www.rfc-editor.org/info/rfc4013>.
[RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422,
DOI 10.17487/RFC4422, June 2006,
<https://www.rfc-editor.org/info/rfc4422>.
[RFC4616] Zeilenga, K., Ed., "The PLAIN Simple Authentication and
Security Layer (SASL) Mechanism", RFC 4616,
DOI 10.17487/RFC4616, August 2006,
<https://www.rfc-editor.org/info/rfc4616>.
[RFC5802] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
"Salted Challenge Response Authentication Mechanism
(SCRAM) SASL and GSS-API Mechanisms", RFC 5802,
DOI 10.17487/RFC5802, July 2010,
<https://www.rfc-editor.org/info/rfc5802>.
[RFC5893] Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts
for Internationalized Domain Names for Applications
(IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010,
<https://www.rfc-editor.org/info/rfc5893>.
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[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <https://www.rfc-editor.org/info/rfc6120>.
[RFC6943] Thaler, D., Ed., "Issues in Identifier Comparison for
Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
2013, <https://www.rfc-editor.org/info/rfc6943>.
[RFC7542] DeKok, A., "The Network Access Identifier", RFC 7542,
DOI 10.17487/RFC7542, May 2015,
<https://www.rfc-editor.org/info/rfc7542>.
[RFC7613] Saint-Andre, P. and A. Melnikov, "Preparation,
Enforcement, and Comparison of Internationalized Strings
Representing Usernames and Passwords", RFC 7613,
DOI 10.17487/RFC7613, August 2015,
<https://www.rfc-editor.org/info/rfc7613>.
[RFC7616] Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
Digest Access Authentication", RFC 7616,
DOI 10.17487/RFC7616, September 2015,
<https://www.rfc-editor.org/info/rfc7616>.
[RFC7617] Reschke, J., "The 'Basic' HTTP Authentication Scheme",
RFC 7617, DOI 10.17487/RFC7617, September 2015,
<https://www.rfc-editor.org/info/rfc7617>.
[RFC7622] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Address Format", RFC 7622,
DOI 10.17487/RFC7622, September 2015,
<https://www.rfc-editor.org/info/rfc7622>.
[RFC8266] Saint-Andre, P., "Preparation, Enforcement, and Comparison
of Internationalized Strings Representing Nicknames",
RFC 8266, DOI 10.17487/RFC8266, October 2017,
<https://www.rfc-editor.org/info/rfc8266>.
[UTS39] Unicode Technical Standard #39, "Unicode Security
Mechanisms", edited by Mark Davis and Michel Suignard,
<http://unicode.org/reports/tr39/>.
Appendix A. Changes from RFC 7613
The following changes were made from [RFC7613].
o Corrected the order of operations for the UsernameCaseMapped
profile to ensure consistency with [RFC8264].
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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]