PRECIS P. Saint-Andre
Internet-Draft Cisco Systems, Inc.
Obsoletes: 4013 (if approved) A. Melnikov
Intended status: Standards Track Isode Ltd
Expires: August 17, 2013 February 13, 2013
Preparation and Comparison of Internationalized Strings Representing
Simple User Names and Passwords
draft-ietf-precis-saslprepbis-00
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 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.
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This Internet-Draft will expire on August 17, 2013.
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document authors. All rights reserved.
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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Simple User Names . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Definition . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Preparation . . . . . . . . . . . . . . . . . . . . . . . 4
3. Passwords . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Definition . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Preparation . . . . . . . . . . . . . . . . . . . . . . . 5
4. Migration . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. User Names . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Passwords . . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . 9
6.2. Use of FreeClass . . . . . . . . . . . . . . . . . . . . 9
7. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Differences from RFC 4013 . . . . . . . . . . . . . 11
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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. (In many
authentication technologies passwords are not directly compared
because the actual password is used as input to an algorithm such as
a hash function; however, non-ASCII code points in the input string
still need to be handled correctly.)
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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 strings
(e.g., keys or certificates) 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.
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].
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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 [UNICODE], 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
;
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.
2.2. Preparation
A simple user name MUST NOT be zero bytes in length. This rule is to
be enforced after any normalization and mapping of code points.
Each simplepart of a simple user name MUST conform to the definition
of the PRECIS NameClass provided in [FRAMEWORK], where the
normalization, casemapping, and directionality rules are as described
below.
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1. Unicode Normalization Form C (NFC) MUST be applied to all
characters.
2. Uppercase and titlecase characters MUST be mapped to their
lowercase equivalents.
3. Additional mappings MAY be applied, such as those defined in
[I-D.ietf-precis-mappings].
With regard to directionality, the "Bidi Rule" provided in [RFC5893]
applies.
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
[UNICODE], 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
;
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.
3.2. Preparation
A password MUST NOT be zero bytes in length. This rule is to be
enforced after any normalization and 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" (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 user names are.
4. 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.
4.1. User Names
Deployments that currently use SASLprep for handling user names 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 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 (i.e., during
comparison NFKC is more "aggressive" about finding matches than is
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. Under SASLprep, the use of NFKC also
handled the mapping of fullwidth and halfwidth code points to
their decomposition equivalents (see [I-D.ietf-precis-mappings]).
Although it is expected that code points with compatibility
equivalents are rare in existing user names, for migration
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purposes deployments might want to search their database of 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. The
non-ASCII space characters are U+00A0 NO-BREAK SPACE, U+1680 OGHAM
SPACE MARK, U+180E MONGOLIAN VOWEL SEPARATOR, U+2000 EN QUAD
through U+200A HAIR SPACE, U+202F NARROW NO-BREAK SPACE, U+205F
MEDIUM MATHEMATICAL SPACE, and U+3000 IDEOGRAPHIC SPACE. For
migration purposes, deployments might want 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 most of these 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 does not have a
derived property of Default_Ignorable_Code_Point in Unicode 6.1).
For migration purposes, deployments might want to remove code
points contained in the PRECIS "M" category from simple user
names.
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.
4.2. 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 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 has the security benefit of
probably resulting in fewer false positives when comparing
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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 (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 equivalents (see [I-D.ietf-precis-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
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). 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.
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 simple 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
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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:
Applicability: Usernames in SASL and Kerberos.
Base Class: NameClass.
Subclass: No.
Replaces: The SASLprep profile of Stringprep.
Normalization: NFC.
Casemapping: Map uppercase and titlecase characters to lowercase.
Additional Mappings: None.
Directionality: The "Bidi Rule" defined in RFC 5893 applies.
Specification: RFC XXXX. [Note to RFC Editor: please change XXXX to
the number issued for this specification.]
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:
Applicability: Passwords in SASL and Kerberos.
Base Class: FreeClass
Subclass: No.
Replaces: The SASLprep profile of Stringprep.
Normalization: NFC.
Casemapping: None.
Additional Mappings: Map non-ASCII space characters to ASCII space.
Directionality: None.
Specification: RFC XXXX. [Note to RFC Editor: please change XXXX to
the number issued for this specification.]
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7. 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
under Section 4.)
8. References
8.1. Normative References
[FRAMEWORK]
Saint-Andre, P. and M. Blanchet, "Precis Framework:
Handling Internationalized Strings in Protocols", draft-
ietf-precis-framework-06 (work in progress), September
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,
.
8.2. Informative References
[I-D.ietf-precis-mappings]
YONEYA, Y. and T. NEMOTO, "Mapping characters for PRECIS
classes", draft-ietf-precis-mappings-01 (work in
progress), December 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.
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[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.
[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 4013.
o A single SASLprep algorithm was replaced by two separate
algorithms: one for simple 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.
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o Some Unicode code points that were mapped to nothing in RFC 4013
are simply disallowed by PRECIS.
Appendix B. Acknowledgements
Thanks to Yoshiro YONEYA and Takahiro NEMOTO for implementation
feedback. Thanks also to Marc Blanchet, Joe Hildebrand, Alan DeKok,
Simon Josefsson, Jonathan Lennox, Matt Miller, Pete Resnick, and
Andrew Sullivan for their input regarding the text.
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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