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Network Working Group K. Zeilenga
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Request for Comments: 4013 OpenLDAP Foundation
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Category: Standards Track February 2005
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SASLprep: Stringprep Profile for User Names and Passwords
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Status of This Memo
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This document specifies an Internet standards track protocol for the
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Internet community, and requests discussion and suggestions for
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improvements. Please refer to the current edition of the "Internet
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Official Protocol Standards" (STD 1) for the standardization state
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and status of this protocol. Distribution of this memo is unlimited.
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Copyright Notice
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Copyright (C) The Internet Society (2005).
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Abstract
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This document describes how to prepare Unicode strings representing
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user names and passwords for comparison. The document defines the
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"SASLprep" profile of the "stringprep" algorithm to be used for both
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user names and passwords. This profile is intended to be used by
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Simple Authentication and Security Layer (SASL) mechanisms (such as
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PLAIN, CRAM-MD5, and DIGEST-MD5), as well as other protocols
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exchanging simple user names and/or passwords.
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1. Introduction
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The use of simple user names and passwords in authentication and
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authorization is pervasive on the Internet. To increase the
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likelihood that user name and password input and comparison work in
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ways that make sense for typical users throughout the world, this
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document defines rules for preparing internationalized user names and
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passwords for comparison. For simplicity and implementation ease, a
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single algorithm is defined for both user names and passwords.
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The algorithm assumes all strings are comprised of characters from
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the Unicode [Unicode] character set.
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This document defines the "SASLprep" profile of the "stringprep"
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algorithm [StringPrep].
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The profile is designed for use in Simple Authentication and Security
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Layer ([SASL]) mechanisms, such as [PLAIN], [CRAM-MD5], and
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[DIGEST-MD5]. It may be applicable where simple user names and
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Zeilenga Standards Track [Page 1]
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RFC 4013 SASLprep February 2005
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passwords are used. This profile is not intended for use in
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preparing identity strings that are not simple user names (e.g.,
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email addresses, domain names, distinguished names), or where
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identity or password strings that are not character data, or require
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different handling (e.g., case folding).
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This document does not alter the technical specification of any
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existing protocols. Any specification that wishes to use the
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algorithm described in this document needs to explicitly incorporate
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this document and provide precise details as to where and how this
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algorithm is used by implementations of that specification.
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2. The SASLprep Profile
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This section defines the "SASLprep" profile of the "stringprep"
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algorithm [StringPrep]. This profile is intended for use in
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preparing strings representing simple user names and passwords.
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This profile uses Unicode 3.2 [Unicode].
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Character names in this document use the notation for code points and
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names from the Unicode Standard [Unicode]. For example, the letter
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"a" may be represented as either <U+0061> or <LATIN SMALL LETTER A>.
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In the lists of mappings and the prohibited characters, the "U+" is
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left off to make the lists easier to read. The comments for
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character ranges are shown in square brackets (such as "[CONTROL
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CHARACTERS]") and do not come from the standard.
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Note: A glossary of terms used in Unicode can be found in [Glossary].
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Information on the Unicode character encoding model can be found in
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[CharModel].
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2.1. Mapping
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This profile specifies:
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- non-ASCII space characters [StringPrep, C.1.2] that can be
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mapped to SPACE (U+0020), and
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- the "commonly mapped to nothing" characters [StringPrep, B.1]
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that can be mapped to nothing.
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2.2. Normalization
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This profile specifies using Unicode normalization form KC, as
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described in Section 4 of [StringPrep].
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Zeilenga Standards Track [Page 2]
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RFC 4013 SASLprep February 2005
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2.3. Prohibited Output
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This profile specifies the following characters as prohibited input:
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- Non-ASCII space characters [StringPrep, C.1.2]
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- ASCII control characters [StringPrep, C.2.1]
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- Non-ASCII control characters [StringPrep, C.2.2]
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- Private Use characters [StringPrep, C.3]
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- Non-character code points [StringPrep, C.4]
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- Surrogate code points [StringPrep, C.5]
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- Inappropriate for plain text characters [StringPrep, C.6]
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- Inappropriate for canonical representation characters
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[StringPrep, C.7]
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- Change display properties or deprecated characters
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[StringPrep, C.8]
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- Tagging characters [StringPrep, C.9]
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2.4. Bidirectional Characters
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This profile specifies checking bidirectional strings as described in
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[StringPrep, Section 6].
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2.5. Unassigned Code Points
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This profile specifies the [StringPrep, A.1] table as its list of
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unassigned code points.
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3. Examples
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The following table provides examples of how various character data
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is transformed by the SASLprep string preparation algorithm
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# Input Output Comments
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- ----- ------ --------
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1 I<U+00AD>X IX SOFT HYPHEN mapped to nothing
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2 user user no transformation
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3 USER USER case preserved, will not match #2
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4 <U+00AA> a output is NFKC, input in ISO 8859-1
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5 <U+2168> IX output is NFKC, will match #1
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6 <U+0007> Error - prohibited character
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7 <U+0627><U+0031> Error - bidirectional check
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4. Security Considerations
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This profile is intended to prepare simple user name and password
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strings for comparison or use in cryptographic functions (e.g.,
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message digests). The preparation algorithm was specifically
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designed such that its output is canonical, and it is well-formed.
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Zeilenga Standards Track [Page 3]
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RFC 4013 SASLprep February 2005
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However, due to an anomaly [PR29] in the specification of Unicode
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normalization, canonical equivalence is not guaranteed for a select
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few character sequences. These sequences, however, do not appear in
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well-formed text. This specification was published despite this
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known technical problem. It is expected that this specification will
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be revised before further progression on the Standards Track (after
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[Unicode] and/or [StringPrep] specifications have been updated to
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address this problem).
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It is not intended for preparing identity strings that are not simple
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user names (e.g., distinguished names, domain names), nor is the
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profile intended for use of simple user names that require different
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handling (such as case folding). Protocols (or applications of those
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protocols) that have application-specific identity forms and/or
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comparison algorithms should use mechanisms specifically designed for
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these forms and algorithms.
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Application of string preparation may have an impact upon the
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feasibility of brute force and dictionary attacks. While the number
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of possible prepared strings is less than the number of possible
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Unicode strings, the number of usable names and passwords is greater
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than as if only ASCII was used. Though SASLprep eliminates some
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Unicode code point sequences as possible prepared strings, that
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elimination generally makes the (canonical) output forms practicable
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and prohibits nonsensical inputs.
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User names and passwords should be protected from eavesdropping.
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General "stringprep" and Unicode security considerations apply. Both
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are discussed in [StringPrep].
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5. IANA Considerations
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This document details the "SASLprep" profile of the [StringPrep]
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protocol. This profile has been registered in the stringprep profile
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registry.
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Name of this profile: SASLprep
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RFC in which the profile is defined: RFC 4013
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Indicator whether or not this is the newest version of the
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profile: This is the first version of the SASPprep profile.
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6. Acknowledgement
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This document borrows text from "Preparation of Internationalized
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Strings ('stringprep')" and "Nameprep: A Stringprep Profile for
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Internationalized Domain Names", both by Paul Hoffman and Marc
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Blanchet. This document is a product of the IETF SASL WG.
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Zeilenga Standards Track [Page 4]
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RFC 4013 SASLprep February 2005
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7. Normative References
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[StringPrep] Hoffman, P. and M. Blanchet, "Preparation of
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Internationalized Strings ("stringprep")", RFC 3454,
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December 2002.
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[Unicode] The Unicode Consortium, "The Unicode Standard, Version
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3.2.0" is defined by "The Unicode Standard, Version
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3.0" (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-
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61633-5), as amended by the "Unicode Standard Annex
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#27: Unicode 3.1"
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(http://www.unicode.org/reports/tr27/) and by the
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"Unicode Standard Annex #28: Unicode 3.2"
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(http://www.unicode.org/reports/tr28/).
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8. Informative References
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[Glossary] The Unicode Consortium, "Unicode Glossary",
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<http://www.unicode.org/glossary/>.
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[CharModel] Whistler, K. and M. Davis, "Unicode Technical Report
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#17, Character Encoding Model", UTR17,
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<http://www.unicode.org/unicode/reports/tr17/>, August
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2000.
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[SASL] Melnikov, A., Ed., "Simple Authentication and Security
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Layer (SASL)", Work in Progress.
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[CRAM-MD5] Nerenberg, L., "The CRAM-MD5 SASL Mechanism", Work in
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Progress.
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[DIGEST-MD5] Leach, P., Newman, C., and A. Melnikov, "Using Digest
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Authentication as a SASL Mechanism", Work in Progress.
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[PLAIN] Zeilenga, K., Ed., "The Plain SASL Mechanism", Work in
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Progress.
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[PR29] "Public Review Issue #29: Normalization Issue",
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<http://www.unicode.org/review/pr-29.html>, February
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2004.
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Author's Address
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Kurt D. Zeilenga
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OpenLDAP Foundation
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EMail: Kurt@OpenLDAP.org
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Zeilenga Standards Track [Page 5]
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RFC 4013 SASLprep February 2005
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Full Copyright Statement
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Copyright (C) The Internet Society (2005).
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This document is subject to the rights, licenses and restrictions
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contained in BCP 78, and except as set forth therein, the authors
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retain all their rights.
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This document and the information contained herein are provided on an
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"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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Intellectual Property
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The IETF takes no position regarding the validity or scope of any
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Intellectual Property Rights or other rights that might be claimed to
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pertain to the implementation or use of the technology described in
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this document or the extent to which any license under such rights
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might or might not be available; nor does it represent that it has
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made any independent effort to identify any such rights. Information
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on the IETF's procedures with respect to rights in IETF Documents can
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be found in BCP 78 and BCP 79.
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Copies of IPR disclosures made to the IETF Secretariat and any
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assurances of licenses to be made available, or the result of an
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attempt made to obtain a general license or permission for the use of
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such proprietary rights by implementers or users of this
|
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specification can be obtained from the IETF on-line IPR repository at
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http://www.ietf.org/ipr.
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The IETF invites any interested party to bring to its attention any
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copyrights, patents or patent applications, or other proprietary
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rights that may cover technology that may be required to implement
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this standard. Please address the information to the IETF at ietf-
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ipr@ietf.org.
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Acknowledgement
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Funding for the RFC Editor function is currently provided by the
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Internet Society.
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Zeilenga Standards Track [Page 6]
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