Network Working Group K. Zeilenga
Request for Comments: 4013 OpenLDAP Foundation
Category: Standards Track February 2005
 SASLprep: Stringprep Profile for User Names and Passwords
Status of This Memo
 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements. Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
 Copyright (C) The Internet Society (2005).
Abstract
 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.
1. Introduction
 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"
 algorithm [StringPrep].
 The profile is designed for use in Simple Authentication and Security
 Layer ([SASL]) mechanisms, such as [PLAIN], [CRAM-MD5], and
 [DIGEST-MD5]. It may be applicable where simple user names and
 passwords are used. This profile is not intended for use in
 preparing identity strings that are not simple user names (e.g.,
 email addresses, domain names, distinguished names), or where
 identity or password strings that are not character data, or require
 different handling (e.g., case folding).
 This document does not alter the technical specification of 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 for use in
 preparing 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
 [CharModel].
2.1. Mapping
 This profile specifies:
 - non-ASCII space characters [StringPrep, C.1.2] that can be
 mapped to SPACE (U+0020), and
 - the "commonly mapped to nothing" characters [StringPrep, B.1]
 that can be mapped to nothing.
2.2. Normalization
 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 as prohibited input:
 - 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 characters [StringPrep, C.3]
 - Non-character code points [StringPrep, C.4]
 - Surrogate code points [StringPrep, C.5]
 - Inappropriate for plain text characters [StringPrep, C.6]
 - Inappropriate for canonical representation characters
 [StringPrep, C.7]
 - Change display properties or deprecated characters
 [StringPrep, C.8]
 - Tagging characters [StringPrep, C.9]
2.4. Bidirectional Characters
 This profile specifies checking bidirectional strings as described in
 [StringPrep, Section 6].
2.5. Unassigned Code Points
 This profile specifies the [StringPrep, A.1] table as its list of
 unassigned code points.
3. Examples
 The following table provides examples of how various character data
 is transformed by the 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 #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 prepare simple user name and password
 strings for comparison or use in cryptographic functions (e.g.,
 message digests). The preparation algorithm was specifically
 designed such that its output is 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 for preparing identity strings that are not simple
 user names (e.g., distinguished names, domain names), nor is the
 profile intended for use of simple user names that require different
 handling (such as case folding). Protocols (or applications of those
 protocols) that 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 as if only ASCII was used. Though SASLprep eliminates some
 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 the [StringPrep]
 protocol. This profile has been registered in the stringprep profile
 registry.
 Name of this profile: SASLprep
 RFC in which the profile is defined: RFC 4013
 Indicator whether or not this is the newest version of the
 profile: This is the first version of the SASPprep profile.
6. Acknowledgement
 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
 Blanchet. 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,
 December 2002.
 [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"
 (http://www.unicode.org/reports/tr28/).
8. Informative References
 [Glossary] The Unicode Consortium, "Unicode Glossary",
 <http://www.unicode.org/glossary/>.
 [CharModel] Whistler, K. and M. Davis, "Unicode Technical Report
 #17, Character Encoding Model", UTR17,
 <http://www.unicode.org/unicode/reports/tr17/>, August
 2000.
 [SASL] Melnikov, A., Ed., "Simple Authentication and Security
 Layer (SASL)", Work in Progress.
 [CRAM-MD5] Nerenberg, L., "The CRAM-MD5 SASL Mechanism", Work in
 Progress.
 [DIGEST-MD5] Leach, P., Newman, C., and A. Melnikov, "Using Digest
 Authentication as a SASL Mechanism", Work in Progress.
 [PLAIN] Zeilenga, K., Ed., "The Plain SASL Mechanism", Work in
 Progress.
 [PR29] "Public Review Issue #29: Normalization Issue",
 <http://www.unicode.org/review/pr-29.html>, February
 2004.
Author's Address
 Kurt D. Zeilenga
 OpenLDAP Foundation
 EMail: Kurt@OpenLDAP.org
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