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Versions: 00 01 02 03 04 draft-ietf-precis-saslprepbis

Precis                                                    P. Saint-Andre
Internet-Draft                                       Cisco Systems, Inc.
Obsoletes: 4013 (if approved)                                A. Melnikov
Intended status: Standards Track                               Isode Ltd
Expires: March 27, 2013                               September 23, 2012


  Preparation and Comparison of Internationalized Strings Representing
                    Simple User Names and Passwords
                  draft-melnikov-precis-saslprepbis-04

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 March 27, 2013.

Copyright Notice

   Copyright (c) 2012 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  . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Passwords  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Definition . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Preparation  . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Migration  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  User Names . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.2.  Passwords  . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
     5.1.  Password/Passphrase Strength . . . . . . . . . . . . . . .  8
     5.2.  Reuse of PRECIS  . . . . . . . . . . . . . . . . . . . . .  8
     5.3.  Reuse of Unicode . . . . . . . . . . . . . . . . . . . . .  9
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
     6.1.  Use of NameClass . . . . . . . . . . . . . . . . . . . . .  9
     6.2.  Use of FreeClass . . . . . . . . . . . . . . . . . . . . .  9
   7.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . .  9
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Differences from RFC 4013 . . . . . . . . . . . . . . 11
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12


















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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.

   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 different handling (e.g.,
   case folding).

   This document builds upon the PRECIS framework defined in
   [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
   [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 (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.



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   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 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).

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

      simpleusername = simplepart [1*(1*SP simplepart)]
      simplepart     = 1*(namepoint)
                       ;
                       ; a "namepoint" is a UTF-8 encoded
                       ; Unicode code point that conforms to
                       ; the "NameClass" string class defined
                       ; in draft-ietf-precis-framework
                       ;

   Note well that all code points and blocks not explicitly allowed in
   the PRECIS NameClass 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.

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.




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   Each simplepart of a simple user name MUST conform to the definition
   of the PRECIS NameClass provided in [FRAMEWORK], where the
   normalization, casemapping, and directionality rules are as described
   below.

   1.  Unicode Normalization Form C (NFC) MUST be applied to all
       characters.

   2.  Uppercase and titlecase characters MUST be mapped to their
       lowercase equivalents.

   3.  Additional mappings MAY be applied, such as those defined in
       [I-D.yoneya-precis-mappings].

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


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 FreeClass.

   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 "FreeClass" string class defined
                       ; in draft-ietf-precis-framework
                       ;

   Note well that all code points and blocks not explicitly allowed in
   the PRECIS FreeClass 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.




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

   1.  Apply Unicode Normalization Form C (NFC) to all characters.

   2.  Map any instances of non-ASCII space to ASCII space (U+0020).

   3.  Ensure that the resulting string conforms to the definition of
       the PRECIS FreeClass.

   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 NameClass employs Unicode
      Normalization Form C (NFC).  In practice this change is unlikely
      to cause significant problems, because NFKC provides methods for
      mapping Unicode code points with compatibility equivalents to
      those equivalents, whereas the PRECIS NameClass 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



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      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.yoneya-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 NameClass 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
      NameClass entirely disallows most of these characters, which
      correspond to the code points from the "M" category defined under
      Section 6.13 of [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 NameClass 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
      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:



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   o  SASLprep specified the use of Unicode Normalization Form KC
      (NFKC), whereas this usage of the PRECIS FreeClass 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.yoneya-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
      FreeClass entirely disallows such characters, which correspond to
      the code points from the "M" category defined under Section 6.13
      of [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

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.  Reuse of PRECIS

   The security considerations described in [FRAMEWORK] apply to the
   "NameClass" and "FreeClass" base string classes used in this document
   for simple user names and passwords, respectively.



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5.3.  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 NameClass

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

   Applicability:  Usernames in SASL and Kerberos.
   Base Class:  NameClass.
   Subclass:  No.
   Replaces:  The SASLprep profile of Stringprep.
   Normalization:  NFC.
   Casemapping:  Map uppercase and titlecase characters to lowercase.
   Additional Mappings:  None.
   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 FreeClass

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

   Applicability:  Passwords in SASL and Kerberos.
   Base Class:  FreeClass
   Subclass:  No.
   Replaces:  The SASLprep profile of Stringprep.
   Normalization:  NFC.
   Casemapping:  None.
   Additional Mappings:  Map non-ASCII space characters to ASCII space.
   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



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   allowed and disallowed.  (See also the migration issues described
   under Section 4.)


8.  References

8.1.  Normative References

   [FRAMEWORK]
              Saint-Andre, P. and M. Blanchet, "Precis Framework:
              Handling Internationalized Strings in Protocols",
              draft-ietf-precis-framework-05 (work in progress),
              August 2012.

   [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
              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.yoneya-precis-mappings]
              YONEYA, Y. and T. NEMOTO, "Mapping characters for PRECIS
              classes", draft-yoneya-precis-mappings-02 (work in
              progress), July 2012.

   [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.




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   [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.

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

   [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 from NFKC to NFC.
   o  Some Unicode code points that were mapped to nothing in RFC 4013
      are simply disallowed by PRECIS.


Appendix B.  Acknowledgements

   Thanks to Yoshiro YONEYA and Takahiro NEMOTO for implementation
   feedback.  Thanks also to Marc Blanchet, Joe Hildebrand, Alan DeKok,
   Simon Josefsson, Jonathan Lennox, Matt Miller, Pete Resnick, and



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   Andrew Sullivan for their input regarding the text.

   This document borrows some text from RFC 4013 and RFC 6120.


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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