Precis P. Saint-Andre
Internet-Draft Cisco Systems, Inc.
Obsoletes: 4013 (if approved) A. Melnikov
Intended status: Standards Track Isode Ltd
Expires: March 17, 2013 September 13, 2012
Preparation and Comparison of Internationalized Strings Representing
Simple User Names and Passwords
draft-melnikov-precis-saslprepbis-02
Abstract
This document describes how to handle Unicode strings representing
simple user names and passwords, primarily for purposes of
comparison. This profile is intended to be used by Simple
Authentication and Security Layer (SASL) mechanisms (such as PLAIN
and SCRAM-SHA-1), as well as other protocols that exchange simple
user names or user 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
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This Internet-Draft will expire on March 17, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Simple User Names . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Definition . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Preparation . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Migration . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Passwords . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Definition . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Preparation . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Migration . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5.1. Password/Passphrase Strength . . . . . . . . . . . . . . . 8
5.2. Reuse of PRECIS . . . . . . . . . . . . . . . . . . . . . 8
5.3. Reuse of Unicode . . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6.1. Use of NameClass . . . . . . . . . . . . . . . . . . . . . 8
6.2. Use of FreeClass . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. Differences from RFC 4013 . . . . . . . . . . . . . . 10
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
1.1. Overview
User names and passwords are used pervasively in authentication and
authorization on the Internet. To increase the likelihood that the
input and comparison of user names and passwords will work in ways
that make sense for typical users throughout the world, this document
defines rules for preparing and comparing internationalized strings
that represent simple user names and passwords.
The algorithms defined in this document assume that all strings are
comprised of characters from the Unicode character set [UNICODE].
The algorithms are designed for use in Simple Authentication and
Security Layer (SASL) [RFC4422] mechanisms, such as PLAIN [RFC4616]
and SCRAM-SHA-1 [RFC5802]. However, they might be applicable
wherever simple user names or passwords are used. This profile is
not intended for use in preparing strings that are not simple user
names (e.g., email addresses, DNS domain names, LDAP distinguished
names), nor in cases where identifiers or secrets are not character
data or require different handling (e.g., case folding).
This document builds upon the PRECIS framework defined in
[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].
This document obsoletes RFC 4013.
1.2. Terminology
Many important terms used in this document are defined in
[FRAMEWORK], [RFC4422], [RFC5890], [RFC6365], and [UNICODE]. The
term "non-ASCII" space refers to any Unicode code point with a
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.
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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].
2. Simple User Names
2.1. Definition
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]). However, the exact form of a simple user name in
any particular mechanism or deployment thereof is a local matter, and
a simple user name does not necessarily map to an application
identifier such as the localpart of an email address.
For purposes of preparation and comparison of authentication
identities, this document specifies that a simple user name is a
string of [UNICODE] code points, encoded using UTF-8 [RFC3629], and
structured as an ordered sequence of "simpleparts" (where the
complete simple user name can consist of a single simplepart or a
space-separated sequence of simpleparts).
Therefore the syntax for a simple user name is defined as follows
using the Augmented Backus-Naur Form (ABNF) as specified in
[RFC5234].
simpleusername = simplepart [1*(1*SP simplepart)]
simplepart = 1*(namepoint)
;
; a "namepoint" is a UTF-8 encoded
; Unicode code point that conforms to
; the "NameClass" string class defined
; in draft-ietf-precis-framework
;
2.2. Preparation
A simple user name MUST NOT be zero bytes in length. This rule is to
be enforced after any normalization or mapping of code points.
Each simplepart of a simple user name MUST be treated as follows,
where the operations specified MUST be completed in the order shown:
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1. Apply Unicode Normalization Form C (NFC) to all characters.
2. Map uppercase and titlecase characters to their lowercase
equivalents.
3. Optionally apply additional mappings, such as those defined in
[MAPPINGS].
4. Ensure that the resulting string conforms to the definition of
the PRECIS NameClass.
With regard to directionality, the "Bidi Rule" provided in [RFC5893]
applies.
2.3. Migration
The rules defined in the previous section differ slightly from those
defined by the SASLprep specification [RFC4013]. Therefore,
deployments that currently use SASLprep for handling user names will
need to scrub existing data when migrating to use of the rules
defined here. In particular:
o SASLprep specified the use of Unicode Normalization Form KC
(NFKC), whereas this usage of the PRECIS NameClass 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 NameClass entirely disallows
Unicode code points with compatibility equivalents. For migration
purposes, deployments need to search their simple user names for
Unicode code points with compatibility equivalents and map those
code points to their compatibility equivalents.
o SASLprep mapped non-ASCII spaces to ASCII space (U+0020), whereas
the PRECIS NameClass entirely disallows non-ASCII spaces. For
migration purposes, deployments need to convert non-ASCII space
characters to ASCII space in simple user names.
o SASLprep mapped the "characters commonly mapped to nothing" from
Appendix B.1 of [RFC3454]) to nothing, whereas the PRECIS
NameClass entirely disallows such characters, which correspond to
the code points from the "M" category defined under Section 6.13
of [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 6.1). For
migration purposes, deployments need to remove code points from
the PRECIS "M" category in simple user names.
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o SASLprep allowed uppercase and titlecase characters, whereas this
usage of the PRECIS NameClass 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 simple user names
(thus losing the case information) or preserve uppercase and
titlecase characters and ignore the case difference when comparing
simple user names.
Note well that all code points and blocks not explicitly allowed in
the PRECIS NameClass 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.
3. Passwords
3.1. Definition
For purposes of preparation and comparison of passwords, this
document specifies that a password is a string of [UNICODE] code
points, encoded using UTF-8 [RFC3629], and conformant to the PRECIS
FreeClass.
Therefore the syntax for a password is defined as follows using the
Augmented Backus-Naur Form (ABNF) as specified in [RFC5234].
password = 1*(freepoint)
;
; a "freepoint" is a UTF-8 encoded
; Unicode code point that conforms to
; the "FreeClass" string class defined
; in draft-ietf-precis-framework
;
3.2. Preparation
A password MUST NOT be zero bytes in length. This rule is to be
enforced after any normalization or mapping of code points.
A password MUST be treated as follows, where the operations specified
MUST be completed in the order shown:
1. Apply Unicode Normalization Form C (NFC) to all characters.
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2. Map any instances of non-ASCII space to ASCII space (U+0020).
3. Ensure that the resulting string conforms to the definition of
the PRECIS FreeClass.
With regard to directionality, the "Bidi Rule" provided in [RFC5893]
applies.
3.3. Migration
The rules defined in the previous section differ slightly from those
defined by the SASLprep specification [RFC4013]. 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 this usage of the PRECIS FreeClass 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 will probably result in
fewer false positives when comparing passwords.
o SASLprep mapped the "characters commonly mapped to nothing" from
Appendix B.1 of [RFC3454]) to nothing, whereas the PRECIS
FreeClass entirely disallows such characters, which correspond to
the code points from the "M" category defined under Section 6.13
of [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 6.1).
Note well that all code points and blocks not explicitly allowed in
the PRECIS FreeClass 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.
4. Open Issues
We need to compare the output obtained when applying the new rules
with Unicode 3.2 and Unicode 6.1 data to the output obtained when
applying the SASLprep rules with Unicode 3.2 data, then make sure
that the PRECIS Working Group and KITTEN Working Group are
comfortable with any changes to the Unicode characters that are
allowed and disallowed. (See also the migration issues described in
the foregoing sections.)
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5. Security Considerations
5.1. 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.
5.2. Reuse of PRECIS
The security considerations described in [FRAMEWORK] apply to the
"NameClass" and "FreeClass" base string classes used in this document
for user names and passwords, respectively.
5.3. Reuse of Unicode
The security considerations described in [UTR39] apply to the use of
Unicode characters in user names and passwords.
6. IANA Considerations
6.1. Use of NameClass
The IANA shall add an entry to the PRECIS Usage Registry for reuse of
the PRECIS NameClass in SASL, as follows:
Application Protocol: SASL/Kerberos.
Base Class: NameClass.
Subclassing: No.
Directionality: The "Bidi Rule" defined in RFC 5893 applies.
Casemapping: Map uppercase and titlecase code points to their
lowercase equivalents.
Normalization: NFC.
Specification: RFC XXXX.
6.2. Use of FreeClass
The IANA shall add an entry to the PRECIS Usage Registry for reuse of
the PRECIS FreeClass in SASL, as follows:
Application Protocol: SASL/Kerberos.
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Base Class: FreeClass
Subclassing: No.
Directionality: The "Bidi Rule" defined in RFC 5893 applies.
Casemapping: None.
Normalization: NFC.
Specification: RFC XXXX.
7. References
7.1. Normative References
[FRAMEWORK]
Saint-Andre, P. and M. Blanchet, "Precis Framework:
Handling Internationalized Strings in Protocols",
draft-ietf-precis-framework-05 (work in progress),
August 2012.
[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
6.1", 2012,
.
7.2. Informative References
[MAPPINGS]
YONEYA, Y. and T. NEMOTO, "Mapping characters for PRECIS
classes", draft-yoneya-precis-mappings-02 (work in
progress), July 2012.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[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.
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[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.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, 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.
[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365,
September 2011.
[UTR39] The Unicode Consortium, "Unicode Technical Report #39:
Unicode Security Mechanisms", August 2010,
.
Appendix A. Differences from RFC 4013
The following substantive modifications were made from RFC 3920.
o A single SASLprep algorithm was replaced by two separate
algorithms: one for user names and another for passwords.
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 framwork, 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.
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Appendix B. Acknowledgements
Thanks to Yoshiro YONEYA and Takahiro NEMOTO for their implementation
feedback.
This document borrows some text from RFC 4013 and RFC 6120.
Authors' Addresses
Peter Saint-Andre
Cisco Systems, Inc.
1899 Wynkoop Street, Suite 600
Denver, CO 80202
USA
Phone: +1-303-308-3282
Email: psaintan@cisco.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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