Network Working Group K. Zeilenga

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

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RFC 4013 SASLprep February 2005

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].

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RFC 4013 SASLprep February 2005

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.

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RFC 4013 SASLprep February 2005

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.

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RFC 4013 SASLprep February 2005

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: [email protected]

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RFC 4013 SASLprep February 2005

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Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

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