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Versions: (draft-melnikov-precis-saslprepbis) 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 RFC 7613

PRECIS                                                    P. Saint-Andre
Internet-Draft                                       Cisco Systems, Inc.
Obsoletes: 4013 (if approved)                                A. Melnikov
Intended status: Standards Track                               Isode Ltd
Expires: January 13, 2014                                  July 12, 2013


  Preparation and Comparison of Internationalized Strings Representing
                    Simple User Names and Passwords
                    draft-ietf-precis-saslprepbis-03

Abstract

   This document describes how to handle Unicode strings representing
   simple user names and passwords, primarily for purposes of
   comparison.  This profile is intended to be used by Simple
   Authentication and Security Layer (SASL) mechanisms (such as PLAIN
   and SCRAM-SHA-1), as well as other protocols that exchange simple
   user names or passwords.  This document obsoletes RFC 4013.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 13, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Simple User Names  . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Definition . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Preparation  . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Passwords  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Definition . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Preparation  . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Migration  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  User Names . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Passwords  . . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
     5.1.  Password/Passphrase Strength . . . . . . . . . . . . . . . 10
     5.2.  Identifier Comparison  . . . . . . . . . . . . . . . . . . 10
     5.3.  Reuse of PRECIS  . . . . . . . . . . . . . . . . . . . . . 10
     5.4.  Reuse of Unicode . . . . . . . . . . . . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
     6.1.  Use of IdentifierClass . . . . . . . . . . . . . . . . . . 10
     6.2.  Use of FreeformClass . . . . . . . . . . . . . . . . . . . 11
   7.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 11
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Differences from RFC 4013 . . . . . . . . . . . . . . 13
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13

















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1.  Introduction

1.1.  Overview

   User names and passwords are used pervasively in authentication and
   authorization on the Internet.  To increase the likelihood that the
   input and comparison of user names and passwords will work in ways
   that make sense for typical users throughout the world, this document
   defines rules for preparing and comparing internationalized strings
   that represent simple user names and passwords.  (In many
   authentication technologies passwords are not directly compared
   because the actual password is used as input to an algorithm such as
   a hash function; however, non-ASCII code points in the input string
   still need to be handled correctly.)

   The algorithms defined in this document assume that all strings are
   comprised of characters from the Unicode character set [UNICODE].

   The algorithms are designed for use in Simple Authentication and
   Security Layer (SASL) [RFC4422] mechanisms, such as PLAIN [RFC4616]
   and SCRAM-SHA-1 [RFC5802].  However, they might be applicable
   wherever simple user names or passwords are used.  This profile is
   not intended for use in preparing strings that are not simple user
   names (e.g., email addresses, DNS domain names, LDAP distinguished
   names), nor in cases where identifiers or secrets are not strings
   (e.g., keys or certificates) or require specialized handling (e.g.,
   case folding).

   This document builds upon the PRECIS framework defined in
   [I-D.ietf-precis-framework], which differs fundamentally from the
   stringprep technology [RFC3454] used in SASLprep [RFC4013].  The
   primary difference is that stringprep profiles allowed all characters
   except those which were explicitly disallowed, whereas PRECIS
   profiles disallow all characters except those which are explicitly
   allowed (this "inclusion model" was originally used for
   internationalized domain names in [RFC5891]; see [RFC5894] for
   further discussion).  It is important to keep this distinction in
   mind when comparing the technology defined in this document to
   SASLprep [RFC4013].

   This document obsoletes RFC 4013.

1.2.  Terminology

   Many important terms used in this document are defined in
   [I-D.ietf-precis-framework], [RFC4422], [RFC5890], [RFC6365], and
   [UNICODE].  The term "non-ASCII space" refers to any Unicode code
   point with a general category of "Zs", with the exception of U+0020



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   (here called "ASCII space").

   As used here, the term "password" is not literally limited to a word;
   i.e., a password could be a passphrase consisting of more than one
   word, perhaps separated by spaces or other such characters.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].


2.  Simple User Names

2.1.  Definition

   Some SASL mechanisms (e.g., CRAM-MD5, DIGEST-MD5, and SCRAM) specify
   that the authentication identity used in the context of such
   mechanisms is a "simple user name" (see Section 2 of [RFC4422] as
   well as [RFC4013]).  However, the exact form of a simple user name in
   any particular mechanism or deployment thereof is a local matter, and
   a simple user name does not necessarily map to an application
   identifier such as the localpart of an email address.

   For purposes of preparation and comparison of authentication
   identities, this document specifies that a simple user name is a
   string of Unicode code points [UNICODE], encoded using UTF-8
   [RFC3629], and structured either as an ordered sequence of
   "simpleparts" (where the complete simple user name can consist of a
   single simplepart or a space-separated sequence of simpleparts) or as
   a simplepart@domainpart (where the domainpart is an IP literal, an
   IPv4 address, or a fully-qualified domain name).

   Therefore the syntax for a simple user name is defined as follows
   using the Augmented Backus-Naur Form (ABNF) as specified in
   [RFC5234].















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      simpleusername = simplepart [1*(1*SP simplepart)]
                       / simplepart '@' domainpart
      simplepart     = 1*(idpoint)
                       ;
                       ; an "idpoint" is a UTF-8 encoded
                       ; Unicode code point that conforms to
                       ; the PRECIS "IdentifierClass"
                       ;
      domainpart     = IP-literal / IPv4address / ifqdn
                       ;
                       ; the "IPv4address" and "IP-literal"
                       ; rules are defined in RFC 3986, and
                       ; the first-match-wins (a.k.a. "greedy")
                       ; algorithm described in RFC 3986
                       ; applies to the matching process
                       ;
                       ; note well that reuse of the IP-literal
                       ; rule from RFC 3986 implies that IPv6
                       ; addresses are enclosed in square
                       ; brackets (i.e., beginning with '['
                       ; and ending with ']')
                       ;
      ifqdn          = 1*1023(domainpoint)
                       ;
                       ; a "domainpoint" is a UTF-8 encoded
                       ; Unicode code point that conforms to
                       ; RFC 5890
                       ;

   Note well that all code points and blocks not explicitly allowed in
   the PRECIS IdentifierClass are disallowed; this includes private use
   characters, surrogate code points, and the other code points and
   blocks defined as "Prohibited Output" in Section 2.3 of RFC 4013.

   Note also that common constructions such as "user@example.com" are
   allowed as simple user names when using software that conforms to
   this specification, as they were under [RFC4013].

2.2.  Preparation

   A simple user name MUST NOT be zero bytes in length.  This rule is to
   be enforced after any normalization and mapping of code points.

   Each simplepart of a simple user name MUST conform to the definition
   of the PRECIS IdentifierClass provided in
   [I-D.ietf-precis-framework], where the width mapping, additional
   mapping, case mapping, normalization, and directionality rules are as
   described below.



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   1.  Fullwidth and halfwidth characters MUST be mapped to their
       decomposition equivalents.
   2.  So-called additional mappings MAY be applied, such as those
       defined in [I-D.ietf-precis-mappings].
   3.  Uppercase and titlecase characters MAY be mapped to their
       lowercase equivalents.
   4.  Unicode Normalization Form C (NFC) MUST be applied to all
       characters.

   With regard to directionality, the "Bidi Rule" provided in [RFC5893]
   applies.

   SASL mechanisms that directly re-use this profile MUST specify
   whether case mapping is to be applied to authentication IDs, and
   when.  SASL mechanisms SHOULD delay any case mapping to the last
   possible moment, such as when doing a lookup by username, username
   comparisons, or generating a cryptographic salt from a username.

   Application protocols that use SASL (such as IMAP [RFC4616] and XMPP
   [RFC6120]) and that directly re-use this profile MUST specify whether
   case mapping is to be applied to authorization IDs.  Such "SASL
   application protocols" SHOULD delay any case mapping of authorization
   IDs to the last possible moment, which happens to necessarily be on
   the server side.

   In keeping with RFC4422, SASL application protocols are not to apply
   this or any other profile to authentication IDs, and SASL mechanisms
   are not to apply this or any other profile to authorization IDs.


3.  Passwords

3.1.  Definition

   For purposes of preparation and comparison of passwords, this
   document specifies that a password is a string of Unicode code points
   [UNICODE], encoded using UTF-8 [RFC3629], and conformant to the
   PRECIS FreeformClass.

   Therefore the syntax for a password is defined as follows using the
   Augmented Backus-Naur Form (ABNF) as specified in [RFC5234].

      password       = 1*(freepoint)
                       ;
                       ; a "freepoint" is a UTF-8 encoded
                       ; Unicode code point that conforms to
                       ; the PRECIS "FreeformClass"
                       ;



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   Note well that all code points and blocks not explicitly allowed in
   the PRECIS FreeformClass are disallowed; this includes private use
   characters, surrogate code points, and the other code points and
   blocks defined as "Prohibited Output" in Section 2.3 of RFC 4013.

3.2.  Preparation

   A password MUST NOT be zero bytes in length.  This rule is to be
   enforced after any normalization and mapping of code points.

   A password MUST be treated as follows, where the operations specified
   MUST be completed in the order shown:

   1.  Width mapping is not applied.
   2.  Map any instances of non-ASCII space to ASCII space (U+0020).
   3.  Case mapping is not applied.
   4.  Apply Unicode Normalization Form C (NFC) to all characters.
   5.  Ensure that the resulting string conforms to the definition of
       the PRECIS FreeformClass.

   With regard to directionality, the "Bidi Rule" (defined in [RFC5893])
   and similar rules are unnecessary and inapplicable to passwords,
   since they can reduce the range of characters that are allowed in a
   string and therefore reduce the amount of entropy that is possible in
   a password.  Furthermore, such rules are intended to minimize the
   possibility that the same string will be displayed differently on a
   system set for right-to-left display and a system set for left-to-
   right display; however, passwords are typically not displayed at all
   and are rarely meant to be interoperable across different systems in
   the way that non-secret strings like domain names and user names are.


4.  Migration

   The rules defined in this specification differ slightly from those
   defined by the SASLprep specification [RFC4013].  The following
   sections describe these differences, along with their implications
   for migration, in more detail.

4.1.  User Names

   Deployments that currently use SASLprep for handling user names might
   need to scrub existing data when migrating to use of the rules
   defined in this specification.  In particular:

   o  SASLprep specified the use of Unicode Normalization Form KC
      (NFKC), whereas this usage of the PRECIS IdentifierClass employs
      Unicode Normalization Form C (NFC).  In practice this change is



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      unlikely to cause significant problems, because NFKC provides
      methods for mapping Unicode code points with compatibility
      equivalents to those equivalents, whereas the PRECIS
      IdentifierClass entirely disallows Unicode code points with
      compatibility equivalents (i.e., during comparison NFKC is more
      "aggressive" about finding matches than is NFC).  A few examples
      might suffice to indicate the nature of the problem: (1) U+017F
      LATIN SMALL LETTER LONG S is compatibility equivalent to U+0073
      LATIN SMALL LETTER S (2) U+2163 ROMAN NUMERAL FOUR is
      compatibility equivalent to U+0049 LATIN CAPITAL LETTER I and
      U+0056 LATIN CAPITAL LETTER V (3) U+FB01 LATIN SMALL LIGATURE FI
      is compatibility equivalent to U+0066 LATIN SMALL LETTER F and
      U+0069 LATIN SMALL LETTER I. Under SASLprep, the use of NFKC also
      handled the mapping of fullwidth and halfwidth code points to
      their decomposition equivalents (see [I-D.ietf-precis-mappings]).
      Although it is expected that code points with compatibility
      equivalents are rare in existing user names, for migration
      purposes deployments might want to search their database of user
      names for Unicode code points with compatibility equivalents and
      map those code points to their compatibility equivalents.

   o  SASLprep mapped non-ASCII spaces to ASCII space (U+0020), whereas
      the PRECIS IdentifierClass entirely disallows non-ASCII spaces.
      The non-ASCII space characters are U+00A0 NO-BREAK SPACE, U+1680
      OGHAM SPACE MARK, U+180E MONGOLIAN VOWEL SEPARATOR, U+2000 EN QUAD
      through U+200A HAIR SPACE, U+202F NARROW NO-BREAK SPACE, U+205F
      MEDIUM MATHEMATICAL SPACE, and U+3000 IDEOGRAPHIC SPACE.  For
      migration purposes, deployments might want to convert non-ASCII
      space characters to ASCII space in simple user names.

   o  SASLprep mapped the "characters commonly mapped to nothing" from
      Appendix B.1 of [RFC3454]) to nothing, whereas the PRECIS
      IdentifierClass entirely disallows most of these characters, which
      correspond to the code points from the "M" category defined under
      Section 6.13 of [I-D.ietf-precis-framework] (with the exception of
      U+1806 MONGOLIAN TODO SOFT HYPHEN, which was "commonly mapped to
      nothing" in Unicode 3.2 but at the time of this writing does not
      have a derived property of Default_Ignorable_Code_Point in Unicode
      6.1).  For migration purposes, deployments might want to remove
      code points contained in the PRECIS "M" category from simple user
      names.

   o  SASLprep allowed uppercase and titlecase characters, whereas this
      usage of the PRECIS IdentifierClass maps uppercase and titlecase
      characters to their lowercase equivalents.  For migration
      purposes, deployments can either convert uppercase and titlecase
      characters to their lowercase equivalents in simple user names
      (thus losing the case information) or preserve uppercase and



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      titlecase characters and ignore the case difference when comparing
      simple user names.

4.2.  Passwords

   Depending on local service policy, migration from RFC 4013 to this
   specification might not involve any scrubbing of data (since
   passwords might not be stored in the clear anyway); however, service
   providers need to be aware of possible issues that might arise during
   migration.  In particular:

   o  SASLprep specified the use of Unicode Normalization Form KC
      (NFKC), whereas this usage of the PRECIS FreeformClass employs
      Unicode Normalization Form C (NFC).  Because NFKC is more
      aggressive about finding matches than NFC, in practice this change
      is unlikely to cause significant problems and indeed has the
      security benefit of probably resulting in fewer false positives
      when comparing passwords.  A few examples might suffice to
      indicate the nature of the problem: (1) U+017F LATIN SMALL LETTER
      LONG S is compatibility equivalent to U+0073 LATIN SMALL LETTER S
      (2) U+2163 ROMAN NUMERAL FOUR is compatibility equivalent to
      U+0049 LATIN CAPITAL LETTER I and U+0056 LATIN CAPITAL LETTER V
      (3) U+FB01 LATIN SMALL LIGATURE FI is compatibility equivalent to
      U+0066 LATIN SMALL LETTER F and U+0069 LATIN SMALL LETTER I. Under
      SASLprep, the use of NFKC also handled the mapping of fullwidth
      and halfwidth code points to their decomposition equivalents (see
      [I-D.ietf-precis-mappings]).  Although it is expected that code
      points with compatibility equivalents are rare in existing
      passwords, some passwords that matched when SASLprep was used
      might no longer work when the rules in this specification are
      applied.

   o  SASLprep mapped the "characters commonly mapped to nothing" from
      Appendix B.1 of [RFC3454]) to nothing, whereas the PRECIS
      FreeformClass entirely disallows such characters, which correspond
      to the code points from the "M" category defined under Section
      6.13 of [I-D.ietf-precis-framework] (with the exception of U+1806
      MONGOLIAN TODO SOFT HYPHEN, which was commonly mapped to nothing
      in Unicode 3.2 but at the time of this writing is allowed by
      Unicode 6.1).  In practice, this change will probably have no
      effect on comparison, but user-oriented software might reject such
      code points instead of ignoring them during password preparation.


5.  Security Considerations






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5.1.  Password/Passphrase Strength

   The ability to include a wide range of characters in passwords and
   passphrases can increase the potential for creating a strong password
   with high entropy.  However, in practice, the ability to include such
   characters ought to be weighed against the possible need to reproduce
   them on various devices using various input methods.

5.2.  Identifier Comparison

   The process of comparing identifiers (such as SASL simple user names,
   authentication identifiers, and authorization identifiers) can lead
   to either false negatives or false positives, both of which have
   security implications.  A more detailed discussion can be found in
   [RFC6943].

5.3.  Reuse of PRECIS

   The security considerations described in [I-D.ietf-precis-framework]
   apply to the "IdentifierClass" and "FreeformClass" base string
   classes used in this document for simple user names and passwords,
   respectively.

5.4.  Reuse of Unicode

   The security considerations described in [UTR39] apply to the use of
   Unicode characters in user names and passwords.


6.  IANA Considerations

6.1.  Use of IdentifierClass

   The IANA shall add an entry to the PRECIS Usage Registry for reuse of
   the PRECIS IdentifierClass in SASL, as follows:

   Applicability:  Usernames in SASL and Kerberos.
   Base Class:  IdentifierClass.
   Subclass:  No.
   Replaces:  The SASLprep profile of Stringprep.
   Width Mapping:  Map fullwidth and halfwidth characters to their
      decomposition equivalents.
   Additional Mappings:  None.
   Case Mapping:  To be defined by application protocols that use this
      profile.






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   Normalization:  NFC.
   Directionality:  The "Bidi Rule" defined in RFC 5893 applies.
   Specification:  RFC XXXX.  [Note to RFC Editor: please change XXXX to
      the number issued for this specification.]

6.2.  Use of FreeformClass

   The IANA shall add an entry to the PRECIS Usage Registry for reuse of
   the PRECIS FreeformClass in SASL, as follows:

   Applicability:  Passwords in SASL and Kerberos.
   Base Class:  FreeformClass
   Subclass:  No.
   Replaces:  The SASLprep profile of Stringprep.
   Width Mapping:  None.
   Additional Mappings:  Map non-ASCII space characters to ASCII space.
   Case Mapping:  None.
   Normalization:  NFC.
   Directionality:  None.
   Specification:  RFC XXXX.  [Note to RFC Editor: please change XXXX to
      the number issued for this specification.]


7.  Open Issues

   We need to compare the output obtained when applying the new rules
   with Unicode 3.2 and Unicode 6.1 data to the output obtained when
   applying the SASLprep rules with Unicode 3.2 data, then make sure
   that the PRECIS Working Group and KITTEN Working Group are
   comfortable with any changes to the Unicode characters that are
   allowed and disallowed.  (See also the migration issues described
   under Section 4.)


8.  References

8.1.  Normative References

   [I-D.ietf-precis-framework]
              Saint-Andre, P. and M. Blanchet, "Precis Framework:
              Handling Internationalized Strings in Protocols",
              draft-ietf-precis-framework-09 (work in progress),
              July 2013.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO



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              10646", STD 63, RFC 3629, November 2003.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [UNICODE]  The Unicode Consortium, "The Unicode Standard, Version
              6.1", 2012,
              <http://www.unicode.org/versions/Unicode6.1.0/>.

8.2.  Informative References

   [I-D.ietf-precis-mappings]
              YONEYA, Y. and T. NEMOTO, "Mapping characters for PRECIS
              classes", draft-ietf-precis-mappings-02 (work in
              progress), May 2013.

   [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.

   [RFC4013]  Zeilenga, K., "SASLprep: Stringprep Profile for User Names
              and Passwords", RFC 4013, February 2005.

   [RFC4422]  Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
              Authentication and Security Layer (SASL)", RFC 4422,
              June 2006.

   [RFC4616]  Zeilenga, K., "The PLAIN Simple Authentication and
              Security Layer (SASL) Mechanism", RFC 4616, August 2006.

   [RFC5802]  Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
              "Salted Challenge Response Authentication Mechanism
              (SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891, August 2010.

   [RFC5893]  Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5893, August 2010.

   [RFC5894]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Background, Explanation, and
              Rationale", RFC 5894, August 2010.



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   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, March 2011.

   [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in
              Internationalization in the IETF", BCP 166, RFC 6365,
              September 2011.

   [RFC6943]  Thaler, D., "Issues in Identifier Comparison for Security
              Purposes", RFC 6943, May 2013.

   [UTR39]    The Unicode Consortium, "Unicode Technical Report #39:
              Unicode Security Mechanisms", August 2010,
              <http://unicode.org/reports/tr39/>.


Appendix A.  Differences from RFC 4013

   The following substantive modifications were made from RFC 4013.

   o  A single SASLprep algorithm was replaced by two separate
      algorithms: one for simple user names and another for passwords.
   o  The new preparation algorithms use PRECIS instead of a stringprep
      profile.  The new algorithms work independenctly of Unicode
      versions.
   o  As recommended in the PRECIS framwork, changed the Unicode
      normalization form to NFC (from NFKC).
   o  Some Unicode code points that were mapped to nothing in RFC 4013
      are simply disallowed by PRECIS.


Appendix B.  Acknowledgements

   The following individuals provided helpful feedback on this document:
   Marc Blanchet, Alan DeKok, Joe Hildebrand, Jeffrey Hutzelman, Simon
   Josefsson, Jonathan Lennox, Matt Miller, Chris Newman, Pete Resnick,
   Andrew Sullivan, and Nico Williams (Nico in particular provided text
   that was used in Section 2.2).  Thanks also to Yoshiro YONEYA and
   Takahiro NEMOTO for implementation feedback.

   This document borrows some text from [RFC4013] and [RFC6120].











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Authors' Addresses

   Peter Saint-Andre
   Cisco Systems, Inc.
   1899 Wynkoop Street, Suite 600
   Denver, CO  80202
   USA

   Phone: +1-303-308-3282
   Email: psaintan@cisco.com


   Alexey Melnikov
   Isode Ltd
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX
   UK

   Email: Alexey.Melnikov@isode.com































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