SASLprep: Stringprep Profile for User Names and Passwords

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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