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PRECIS P. Saint-Andre
Internet-Draft &yet
Obsoletes: 4013 (if approved) A. Melnikov
Intended status: Standards Track Isode Ltd
Expires: May 25, 2015 November 21, 2014
Preparation, Enforcement, and Comparison of Internationalized Strings
Representing Usernames and Passwords
draft-ietf-precis-saslprepbis-10
Abstract
This document describes methods for handling Unicode strings
representing usernames and passwords. This document obsoletes RFC
4013.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 25, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Usernames . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Definition . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. UsernameIdentifierClass Profile . . . . . . . . . . . . . 5
3.2.1. Preparation . . . . . . . . . . . . . . . . . . . . . 5
3.2.2. Enforcement . . . . . . . . . . . . . . . . . . . . . 5
3.2.3. Comparison . . . . . . . . . . . . . . . . . . . . . 6
3.3. Case Mapping . . . . . . . . . . . . . . . . . . . . . . 6
3.4. Application-Layer Constructs . . . . . . . . . . . . . . 7
3.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Passwords . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Definition . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. PasswordFreeformClass Profile . . . . . . . . . . . . . . 10
4.2.1. Preparation . . . . . . . . . . . . . . . . . . . . . 10
4.2.2. Enforcement . . . . . . . . . . . . . . . . . . . . . 10
4.2.3. Comparison . . . . . . . . . . . . . . . . . . . . . 11
4.3. Examples . . . . . . . . . . . . . . . . . . . . . . . . 11
5. Migration . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Usernames . . . . . . . . . . . . . . . . . . . . . . . . 12
5.2. Passwords . . . . . . . . . . . . . . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
6.1. UsernameIdentifierClass Profile . . . . . . . . . . . . . 14
6.2. PasswordFreeformClass Profile . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7.1. Password/Passphrase Strength . . . . . . . . . . . . . . 15
7.2. Identifier Comparison . . . . . . . . . . . . . . . . . . 16
7.3. Reuse of PRECIS . . . . . . . . . . . . . . . . . . . . . 16
7.4. Reuse of Unicode . . . . . . . . . . . . . . . . . . . . 16
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
8.1. Normative References . . . . . . . . . . . . . . . . . . 16
8.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Differences from RFC 4013 . . . . . . . . . . . . . 18
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
Usernames and passwords are widely used for authentication and
authorization on the Internet, either directly when provided in
plaintext (as in the SASL PLAIN mechanism [RFC4616] or the HTTP Basic
scheme [RFC2617] / [I-D.ietf-httpauth-basicauth-update]) or
indirectly when provided as the input to a cryptographic algorithm
such as a hash function (as in the SASL SCRAM mechanism [RFC5802] or
the HTTP Digest scheme [RFC2617] / [I-D.ietf-httpauth-digest]).
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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
PRECIS framework specification [I-D.ietf-precis-framework].
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+0089) as equivalent to DIGIT ONE
(U+0031), since 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 during 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., email addresses and
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" profile of
stringprep [RFC3454]) but can be used by technologies other than the
Simple Authentication and Security Layer (SASL) [RFC4422], such as
HTTP authentication [RFC2617] / [I-D.ietf-httpauth-basicauth-update]
/ [I-D.ietf-httpauth-digest].
2. Terminology
Many important terms used in this document are defined in
[I-D.ietf-precis-framework], [RFC5890], [RFC6365], and [UNICODE].
The term "non-ASCII space" refers to any Unicode code point having a
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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 or other such 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 [RFC2617] /
[I-D.ietf-httpauth-basicauth-update] / [I-D.ietf-httpauth-digest]),
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" (where the complete 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"
;
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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" 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 SASL-using application protocols, 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
proper preparation of the username before applying the algorithm.
3.2. UsernameIdentifierClass Profile
The definition of the UsernameIdentifierClass profile is provided in
the following sections, including detailed information about
preparation, enforcement, and comparison (on the distinction between
these actions, refer to [I-D.ietf-precis-framework]).
3.2.1. Preparation
An entity that prepares a string for inclusion in a username slot
MUST ensure that the string consists only of Unicode code points that
conform to the "IdentifierClass" base string class defined in
[I-D.ietf-precis-framework]. In addition, the string MUST be encoded
as UTF-8 [RFC3629].
3.2.2. Enforcement
An entity that performs enforcement in username slots MUST prepare a
string as described in the previous section and MUST also apply the
rules specified below for the UsernameIdentifierClass profile (these
rules MUST be applied in the order shown).
1. Width Mapping Rule: Fullwidth and halfwidth characters MUST be
mapped to their decomposition mappings.
2. Additional Mapping Rule: There is no additional mapping rule.
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3. Case Mapping Rule: There is no case mapping rule (although see
Section 3.3 below).
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] (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 before or after
their inclusion in username slots MUST prepare each string and
enforce the rules specified in the previous two sections. The two
strings are to be considered equivalent if they are an exact octet-
for-octet match (sometimes called "bit-string identity").
3.3. Case Mapping
Case mapping is a matter for the application protocol, protocol
implementation, or end deployment. In general, this document
suggests that it is preferable to perform case mapping, since 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
not perform case mapping, such as backward compatibility with
deployed infrastructure.
In particular:
o SASL mechanisms that directly re-use this profile 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,
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.
o Application protocols that use SASL (such as IMAP [RFC3501] and
XMPP [RFC6120]) and that directly re-use this profile MUST specify
whether 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
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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.
o Application protocols that do not use SASL (such as HTTP
authentication with the Basic and Digest schemes [RFC2617] /
[I-D.ietf-httpauth-basicauth-update] / [I-D.ietf-httpauth-digest])
MUST specify whether and when case mapping is to be applied to
authentication identifiers and authorization identifiers. Such
"non-SASL application protocols" SHOULD delay any case mapping to
the last possible moment, such as when doing a lookup by username,
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 specifies the handling of case
mapping for strings that conform to the UsernameIdentifierClass, it
MUST clearly describe whether case mapping is required, recommended,
or optional at the level of the protocol itself, implementations
thereof, or service deployments.
Informational Note: The LocalpartIdentifierClass profile defined
in [I-D.ietf-xmpp-6122bis] is identical to the
UsernameIdentifierClass profile defined here, except that the
LocalpartIdentifierClass profile specifies case mapping.
3.4. Application-Layer Constructs
The username rule allows an application protocol, implementation, or
deployment to create application-layer constructs such as
"user@domain" or "Firstname Middlename Lastname" (e.g., because the
PRECIS IdentifierClass allows any ASCII7 character, because spaces
can be used to separate userpart instances, and because domain names
as specified in [RFC5890] and [RFC5892] are a subset of the PRECIS
IdentifierClass).
3.5. 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).
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Table 1: A sample of legal userparts
+--------------------------+---------------------------------+
| # | 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) |
+--------------------------+---------------------------------+
Several points are worth noting. Regarding examples 2 and 3:
although in German the character esszett (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.
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Table 2: A sample of strings that violate the userpart rule
+--------------------------+---------------------------------+
| # | 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) |
+--------------------------+---------------------------------+
Here again, several points are worth noting. 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
IdentiferClass. 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 PRECIS FreeformClass.
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 "FreefromClass"
;
All code points and blocks not explicitly allowed in the PRECIS
FreeformClass are disallowed; this includes private use characters,
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surrogate code points, and the other code points and blocks defined
as "Prohibited Output" in Section 2.3 of RFC 4013.
A password MUST NOT be zero bytes in length. This rule is to be
enforced after any normalization and mapping of code points.
Note: The prohibition on zero-length passwords is not a
recommendation regarding password strength (since a password of
only one byte is highly insecure), but is meant to prevent
applications from omitting a password entirely.
In protocols that provide passwords as input to a cryptographic
algorithm such as a hash function, the client will need to perform
proper preparation of the password before applying the algorithm,
since the password is not available to the server in plaintext form.
4.2. PasswordFreeformClass Profile
The definition of the PasswordFreeformClass profile is provided in
the following sections, including detailed information about
preparation, enforcement, and comparison (on the distinction between
these actions, refer to [I-D.ietf-precis-framework]).
4.2.1. Preparation
An entity that prepares a string for inclusion in a password slot
MUST ensure that the string consists only of Unicode code points that
conform to the "FreeformClass" base string class defined in
[I-D.ietf-precis-framework]. In addition, the string MUST be encoded
as UTF-8 [RFC3629].
4.2.2. Enforcement
An entity that performs enforcement in password slots MUST prepare a
string as described in the previous section and MUST also apply the
rules specified below for the PasswordFreeformClass (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.
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 general category of "Zs", naturally with the
exception of U+0020.
3. Case Mapping Rule: Uppercase and titlecase characters MUST NOT be
mapped to their lowercase equivalents.
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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, since 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. Furthermore,
such rules are intended to minimize the possibility that the same
string will be displayed differently on a system set for right-
to-left display and a system set for left-to-right display;
however, passwords are typically not displayed at all and are
rarely meant to be interoperable across different systems in the
way that non-secret strings like domain names and usernames are.
4.2.3. Comparison
An entity that performs comparison of two strings before or after
their inclusion in password slots MUST prepare each string and
enforce the rules specified in the previous two sections. 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).
Table 3: A sample of legal passwords
+------------------------------------+------------------------------+
| # | Password | Notes |
+------------------------------------+------------------------------+
| 12| <correct horse battery staple> | ASCII space is allowed |
+------------------------------------+------------------------------+
| 13| <Correct Horse Battery Staple> | Different 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) |
+------------------------------------+------------------------------+
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The following examples illustrate strings that are not valid
passwords because they violate the format defined above.
Table 4: A sample of strings that violate the password rules
+------------------------------------+------------------------------+
| # | Password | Notes |
+------------------------------------+------------------------------+
| 16| <foo bar> | Non-ASCII space (here, OGHAM |
| | | SPACE MARK, U+1680) is not |
| | | allowed |
+------------------------------------+------------------------------+
| 17| <my cat is a 	by> | Controls are disallowed |
+------------------------------------+------------------------------+
5. 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.
5.1. Usernames
Deployments that currently use SASLprep for handling usernames might
need to scrub existing data when migrating to use of the rules
defined in this specification. In particular:
o SASLprep specified the use of Unicode Normalization Form KC
(NFKC), whereas the UsernameIdentifierClass profile employs
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. U+017F LATIN SMALL LETTER LONG S is compatibility equivalent
to U+0073 LATIN SMALL LETTER S
2. U+2163 ROMAN NUMERAL FOUR is compatibility equivalent to
U+0049 LATIN CAPITAL LETTER I and U+0056 LATIN CAPITAL LETTER
V
3. U+FB01 LATIN SMALL LIGATURE FI is compatibility equivalent to
U+0066 LATIN SMALL LETTER F and U+0069 LATIN SMALL LETTER I
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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 usernames, for
migration purposes deployments might want to search their database
of usernames for Unicode code points with compatibility
equivalents and map those code points to their compatibility
equivalents.
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 "M" category defined under
Section 8.13 of [I-D.ietf-precis-framework] (with the exception of
U+1806 MONGOLIAN TODO SOFT HYPHEN, 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, deployments might want to remove
code points contained in the PRECIS "M" category from usernames.
o SASLprep allowed uppercase and titlecase characters, whereas the
UsernameIdentifierClass profile maps uppercase and titlecase
characters to their lowercase equivalents. For migration
purposes, deployments can either convert uppercase and titlecase
characters to their lowercase equivalents in usernames (thus
losing the case information) or preserve uppercase and titlecase
characters and ignore the case difference when comparing
usernames.
5.2. Passwords
Depending on local service policy, migration from RFC 4013 to this
specification might not involve any scrubbing of data (since
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 PasswordFreeformClass 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. U+017F LATIN SMALL LETTER LONG S is compatibility equivalent
to U+0073 LATIN SMALL LETTER S
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2. U+2163 ROMAN NUMERAL FOUR is compatibility equivalent to
U+0049 LATIN CAPITAL LETTER I and U+0056 LATIN CAPITAL LETTER
V
3. U+FB01 LATIN SMALL LIGATURE FI is compatibility equivalent to
U+0066 LATIN SMALL LETTER F and U+0069 LATIN SMALL LETTER I
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 "M" category defined under
Section 8.13 of [I-D.ietf-precis-framework] (with the exception of
U+1806 MONGOLIAN TODO SOFT HYPHEN, 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.
6. IANA Considerations
The IANA shall add the following entries to the PRECIS Profiles
Registry.
6.1. UsernameIdentifierClass Profile
Name: UsernameIdentifierClass.
Applicability: Usernames in security and application protocols.
Base Class: IdentifierClass.
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: To be defined by security or application
protocols that use this profile.
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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 XXXX. [Note to RFC Editor: please change XXXX to
the number issued for this specification.]
6.2. PasswordFreeformClass Profile
Name: PasswordFreeformClass.
Applicability: Passwords in security and application protocols.
Base Class: FreeformClass
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 XXXX. [Note to RFC Editor: please change XXXX to
the number issued for this specification.]
7. Security Considerations
7.1. 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.
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7.2. 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].
7.3. Reuse of PRECIS
The security considerations described in [I-D.ietf-precis-framework]
apply to the "IdentifierClass" and "FreeformClass" base string
classes used in this document for usernames and passwords,
respectively.
7.4. Reuse of Unicode
The security considerations described in [UTS39] apply to the use of
Unicode characters in usernames and passwords.
8. References
8.1. Normative References
[I-D.ietf-precis-framework]
Saint-Andre, P. and M. Blanchet, "Precis Framework:
Handling Internationalized Strings in Protocols", draft-
ietf-precis-framework-20 (work in progress), November
2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[UNICODE] The Unicode Consortium, "The Unicode Standard, Version
6.3", 2013,
<http://www.unicode.org/versions/Unicode6.3.0/>.
8.2. Informative References
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[I-D.ietf-httpauth-basicauth-update]
Reschke, J., "The 'Basic' HTTP Authentication Scheme",
draft-ietf-httpauth-basicauth-update-02 (work in
progress), October 2014.
[I-D.ietf-httpauth-digest]
Shekh-Yusef, R., Ahrens, D., and S. Bremer, "HTTP Digest
Access Authentication", draft-ietf-httpauth-digest-08
(work in progress), August 2014.
[I-D.ietf-xmpp-6122bis]
Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Address Format", draft-ietf-xmpp-
6122bis-16 (work in progress), November 2014.
[RFC20] Cerf, V., "ASCII format for network interchange", RFC 20,
October 1969.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names
and Passwords", RFC 4013, February 2005.
[RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422, June
2006.
[RFC4616] Zeilenga, K., "The PLAIN Simple Authentication and
Security Layer (SASL) Mechanism", RFC 4616, August 2006.
[RFC5802] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
"Salted Challenge Response Authentication Mechanism
(SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
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[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010.
[RFC5892] Faltstrom, P., "The Unicode Code Points and
Internationalized Domain Names for Applications (IDNA)",
RFC 5892, August 2010.
[RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
Internationalized Domain Names for Applications (IDNA)",
RFC 5893, August 2010.
[RFC5894] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Background, Explanation, and
Rationale", RFC 5894, August 2010.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011.
[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365,
September 2011.
[RFC6943] Thaler, D., "Issues in Identifier Comparison for Security
Purposes", RFC 6943, May 2013.
[UTS39] The Unicode Consortium, "Unicode Technical Standard #39:
Unicode Security Mechanisms", July 2012,
<http://unicode.org/reports/tr39/>.
Appendix A. Differences from RFC 4013
This document builds upon the PRECIS framework defined in
[I-D.ietf-precis-framework], which differs fundamentally from the
stringprep technology [RFC3454] used in SASLprep [RFC4013]. The
primary difference is that stringprep profiles allowed all characters
except those which were explicitly disallowed, whereas PRECIS
profiles disallow all characters except those which 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 two separate
algorithms: one for usernames and another for passwords.
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o The new preparation algorithms use PRECIS instead of a stringprep
profile. The new algorithms work independenctly 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.
Appendix B. Acknowledgements
The following individuals provided helpful feedback on this document:
Marc Blanchet, Alan DeKok, Joe Hildebrand, Jeffrey Hutzelman, Simon
Josefsson, Jonathan Lennox, James Manger, Matt Miller, Chris Newman,
Yutaka OIWA, Pete Resnick, Andrew Sullivan, and Nico Williams. Nico
in particular deserves special recognition for providing text that
was used in Section 3.3. Thanks also to Yoshiro YONEYA and Takahiro
NEMOTO for implementation feedback.
This document borrows some text from [RFC4013] and [RFC6120].
Peter Saint-Andre wishes to acknowledge Cisco Systems, Inc., for
employing him during his work on earlier 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
UK
Email: Alexey.Melnikov@isode.com
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