INTERNET-DRAFT Kurt D. Zeilenga
Intended Category: Standards Track OpenLDAP Foundation
October 2004 28 April January 2005 18 July 2004
SASLprep: Stringprep profile for user names and passwords
Status of Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.
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revision, submitted to the RFC Editor as a Standards Track document.
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This document describes how to prepare Unicode strings representing
user names and passwords for comparison. The document defines the
"SASLprep" profile of the "stringprep" algorithm to be used for both
user names and passwords. This profile is intended to be used by
Simple Authentication and Security Layer (SASL) mechanisms (such as
PLAIN, CRAM-MD5, and DIGEST-MD5) as well as other protocols exchanging
simple user names and/or passwords.
The use of simple user names and passwords in authentication and
authorization is pervasive on the Internet. To increase the
likelihood that user name and password input and comparison work in
ways that make sense for typical users throughout the world, this
document defines rules for preparing internationalized user names and
passwords for comparison. For simplicity and implementation ease, a
single algorithm is defined for both user names and passwords.
The algorithm assumes all strings are comprised of characters from the
Unicode [Unicode] character set.
This document defines the "SASLprep" profile of the "stringprep"
The profile is designed for use in Simple Authentication and Security
Layer ([SASL]) mechanisms such as [PLAIN]. It may be applicable
elsewhere simple user names and passwords are used. This profile is
not intended to be used to prepare identity strings which are not
simple user names (e.g., e-mail email addresses, domain names, distinguished
names), or where identity or password strings which are not character
data, or require different handling (e.g., case folding).
This document by itself does not alter the technical specification any
existing protocols. Any specification that wishes to use the
algorithm described in this document needs to explicitly incorporate
this document and provide precise details as to where and how this
algorithm is used by implementations of that specification.
2. The SASLprep profile
This section defines the "SASLprep" profile of the "stringprep"
algorithm [StringPrep]. This profile is intended to be used to
prepare strings representing simple user names and passwords.
This profile uses Unicode 3.2 [Unicode].
Character names in this document use the notation for code points and
names from the Unicode Standard [Unicode]. For example, the letter
"a" may be represented as either <U+0061> or <LATIN SMALL LETTER A>.
In the lists of mappings and the prohibited characters, the "U+" is
left off to make the lists easier to read. The comments for character
ranges are shown in square brackets (such as "[CONTROL CHARACTERS]")
and do not come from the standard.
Note: a glossary of terms used in Unicode can be found in [Glossary].
Information on the Unicode character encoding model can be found in
This profile specifies:
- non-ASCII space characters [StringPrep, C.1.2] be mapped to SPACE
- the "commonly mapped to nothing" characters [StringPrep, B.1] be
mapped to nothing.
This profile specifies using Unicode normalization form KC, as
described in Section 4 of [StringPrep].
2.3. Prohibited Output
This profile specifies the following characters:
- Non-ASCII space characters [StringPrep, C.1.2],
- ASCII control characters [StringPrep, C.2.1],
- Non-ASCII control characters [StringPrep, C.2.2],
- Private Use [StringPrep, C.3],
- Non-character code points [StringPrep, C.4],
- Surrogate code points [StringPrep, C.5],
- Inappropriate for plain text [StringPrep, C.6],
- Inappropriate for canonical representation [StringPrep, C.7],
- Change display properties or are deprecated [StringPrep, C.8], and
- Tagging characters [StringPrep, C.9].
are prohibited output.
2.4. Bidirectional characters
This profile specifies checking bidirectional strings as described in
[StringPrep, Section 6].
2.5. Unassigned Code Points
This profile specifies [StringPrep, A.1] table as its list of
unassigned code points.
The following table provides examples of how various character data is
transformed by SASLprep string preparation algorithm
# Input Output Comments
- ----- ------ --------
1 I<U+00AD>X IX SOFT HYPHEN mapped to nothing
2 user user no transformation
3 USER USER case preserved, will not match #3 #2
4 <U+00AA> a output is NFKC, input in ISO 8859-1
5 <U+2168> IX output is NFKC, will match #1
6 <U+0007> Error - prohibited character
7 <U+0627><U+0031> Error - bidirectional check
4. Security Considerations
This profile is intended to be used to prepare simple user names and
passwords strings for comparison or use in cryptographic functions
(e.g., message digests). The preparation algorithm is was specifically
designed such that its output is canonical. canonical, and it is well-formed.
However, due to an anomaly [PR29] in the specification of Unicode
normalization, canonical equivalence is not guaranteed for a select
few character sequences. These sequences, however, do not appear in
well-formed text. This specification was published despite this known
technical problem. It is expected that this specification will be
revised before further progression on the Standards Track (after
[Unicode] and/or [StringPrep] specifications have been updated to
address this problem).
It is not intended to be used for to prepare identity strings which
are not simple user names (e.g., distinguished names and names, domain
names). Nor names),
nor is the profile intended to be used for simple user names which
require different handling (such as case folding). Protocols (or
applications of those protocols) which have application-specific
identity forms and/or comparison algorithms should use mechanisms
specifically designed for these forms and algorithms.
Application of string preparation may have an impact upon the
feasibility of brute force and dictionary attacks. While the number
of possible prepared strings is less than the number of possible
Unicode strings, the number of usable names and passwords is greater
than if only ASCII was used. Though SASLprep eliminates some of
Unicode code point sequences as possible prepared strings, that
elimination generally makes the (canonical) output forms practicable
and prohibits nonsensical inputs.
User names and passwords should be protected from eavesdropping.
General "stringprep" and Unicode security considerations apply. Both
are discussed in [StringPrep].
5. IANA Considerations
This document details the "SASLprep" profile of [StringPrep] protocol.
Upon Standards Action the profile should be registered in the
stringprep profile registry.
Name of this profile: SASLprep
RFC in which the profile is defined: This RFC
Indicator whether or not this is the newest version of the
profile: This is the first version of the SASPprep profile.
This document borrows text from "Preparation of Internationalized
Strings ('stringprep')" and "Nameprep: A Stringprep Profile for
Internationalized Domain Names", both by Paul Hoffman and Marc
This document is a product of the IETF SASL WG.
7. Normative References
[StringPrep] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ('stringprep')", RFC 3454,
[Unicode] The Unicode Consortium, "The Unicode Standard, Version
3.2.0" is defined by "The Unicode Standard, Version 3.0"
(Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5),
as amended by the "Unicode Standard Annex #27: Unicode
3.1" (http://www.unicode.org/reports/tr27/) and by the
"Unicode Standard Annex #28: Unicode 3.2"
8. Informative References
[Glossary] The Unicode Consortium, "Unicode Glossary",
[CharModel] Whistler, K. and M. Davis, "Unicode Technical Report
#17, Character Encoding Model", UTR17,
[SASL] Melnikov, A. (Editor), "Simple Authentication and
Security Layer (SASL)",
draft-ietf-sasl-rfc2222bis-xx.txt, a work in progress.
[CRAM-MD5] Nerenberg, L., "The CRAM-MD5 SASL Mechanism",
draft-ietf-sasl-crammd5-xx.txt, a work in progress.
[DIGEST-MD5] Leach, P., C. Newman, and A. Melnikov, "Using Digest
Authentication as a SASL Mechanism",
draft-ietf-sasl-rfc2831bis-xx.txt, a work in progress.
[PLAIN] Zeilenga, K. (Editor), "The Plain SASL Mechanism",
draft-ietf-sasl-plain-xx.txt, a work in progress.
[PR29] "Public Review Issue #29: Normalization Issue",
9. Author's Address
Kurt D. Zeilenga
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