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.
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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Usernames . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Definition . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. UsernameCaseMapped Profile . . . . . . . . . . . . . . . 6
3.2.1. Preparation . . . . . . . . . . . . . . . . . . . . . 6
3.2.2. Enforcement . . . . . . . . . . . . . . . . . . . . . 6
3.2.3. Comparison . . . . . . . . . . . . . . . . . . . . . 7
3.3. UsernameCasePreserved Profile . . . . . . . . . . . . . . 7
3.3.1. Preparation . . . . . . . . . . . . . . . . . . . . . 7
3.3.2. Enforcement . . . . . . . . . . . . . . . . . . . . . 7
3.3.3. Comparison . . . . . . . . . . . . . . . . . . . . . 8
3.4. Case Mapping vs. Case Preservation . . . . . . . . . . . 8
3.5. Application-Layer Constructs . . . . . . . . . . . . . . 9
3.6. Examples . . . . . . . . . . . . . . . . . . . . . . . . 9
4. Passwords . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Definition . . . . . . . . . . . . . . . . . . . . . . . 11
4.2. OpaqueString Profile . . . . . . . . . . . . . . . . . . 12
4.2.1. Preparation . . . . . . . . . . . . . . . . . . . . . 12
4.2.2. Enforcement . . . . . . . . . . . . . . . . . . . . . 13
4.2.3. Comparison . . . . . . . . . . . . . . . . . . . . . 13
4.3. Examples . . . . . . . . . . . . . . . . . . . . . . . . 14
5. Use in Application Protocols . . . . . . . . . . . . . . . . 14
6. Migration . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. Usernames . . . . . . . . . . . . . . . . . . . . . . . . 15
6.2. Passwords . . . . . . . . . . . . . . . . . . . . . . . . 17
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7.1. UsernameCaseMapped Profile . . . . . . . . . . . . . . . 18
7.2. UsernameCasePreserved Profile . . . . . . . . . . . . . . 18
7.3. OpaqueString Profile . . . . . . . . . . . . . . . . . . 19
7.4. Stringprep Profile . . . . . . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
8.1. Password/Passphrase Strength . . . . . . . . . . . . . . 20
8.2. Identifier Comparison . . . . . . . . . . . . . . . . . . 20
8.3. Reuse of PRECIS . . . . . . . . . . . . . . . . . . . . . 20
8.4. Reuse of Unicode . . . . . . . . . . . . . . . . . . . . 20
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
9.1. Normative References . . . . . . . . . . . . . . . . . . 20
9.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Differences from RFC 4013 . . . . . . . . . . . . . 23
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
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.
This document obsoletes RFC 4013 (the "SASLprep" 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].
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.
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 the previous section 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.
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 the previous section 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.
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.
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).
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
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.
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 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.
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.
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 the previous section 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
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| <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 U+0020, and thus |
| | | the full string is mapped to |
| | | <foo bar> |
+------------------------------------+------------------------------+
Table 3: A Sample of Legal Passwords
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 	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.
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.
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).
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.
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.
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" 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.
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/>.
[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>.
[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>.
[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, June
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.
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