draft-hoffman-rfc3491bis-01.txt                               P. Hoffman
April 14, 2004                                                IMC & VPNC
Expires in six months                                        M. Blanchet

                   Nameprep: A Stringprep Profile for
                  Internationalized Domain Names (IDN)


   This document describes how to prepare internationalized domain name
   (IDN) labels in order to increase the likelihood that name input and
   name comparison work in ways that make sense for typical users
   throughout the world.  This profile of the stringprep protocol is
   used as part of a suite of on-the-wire protocols for
   internationalizing the Domain Name System (DNS).

1. Introduction

   This document specifies processing rules that will allow users to
   enter internationalized domain names (IDNs) into applications and
   have the highest chance of getting the content of the strings
   correct.  It is a profile of stringprep [STRINGPREP].  These
   processing rules are only intended for internationalized domain
   names, not for arbitrary text.

   This profile defines the following, as required by [STRINGPREP].

   -  The intended applicability of the profile: internationalized
      domain names processed by IDNA.

   -  The character repertoire that is the input and output to
      stringprep:  Unicode 3.2, specified in section 2.

   -  The mappings used: specified in section 3.

   -  The Unicode normalization used: specified in section 4.

   -  The characters that are prohibited as output: specified in section

   -  Bidirectional character handling: specified in section 6.

1.1 Interaction of protocol parts

   Nameprep is used by the IDNA [IDNA] protocol for preparing domain
   names; it is not designed for any other purpose.  It is explicitly
   not designed for processing arbitrary free text and SHOULD NOT be
   used for that purpose.  Nameprep is a profile of Stringprep
   [STRINGPREP].  Implementations of Nameprep MUST fully implement
   Stringprep.  Specifically, Nameprep mandates a set of steps that
   must be executed in the same manner and the same order as they
   are described in Stringprep.
      1) Map
      2) Normalize
      3) Prohibit
      4) Check bidi

   Nameprep is used to process domain name labels, not domain names.
   IDNA calls nameprep for each label in a domain name, not for the
   whole domain name.

1.2 Terminology

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
   in this document are to be interpreted as described in BCP 14, RFC
   2119 [RFC2119].

2. Character Repertoire

   This profile uses Unicode 3.2, as defined in [STRINGPREP] Appendix A.

3. Mapping

   This profile specifies mapping using the following tables from

   Table B.1
   Table B.2

4. Normalization

   This profile specifies using Unicode normalization form KC, as
   described in [STRINGPREP].

5. Prohibited Output

   This profile specifies prohibiting using the following tables from

   Table C.1.2
   Table C.2.2
   Table C.3
   Table C.4
   Table C.5
   Table C.6
   Table C.7
   Table C.8
   Table C.9

   IMPORTANT NOTE: The IDNA protocol has additional prohibitions
   that are checked outside of this profile.

6. Bidirectional characters

   This profile specifies checking bidirectional strings as described in
   [STRINGPREP] section 6.

7. Unassigned Code Points in Internationalized Domain Names

   If the processing in [IDNA] specifies that a list of unassigned code
   points be used, the system uses table A.1 from [STRINGPREP] as its
   list of unassigned code points.

8. References

8.1 Normative References

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

   [STRINGPREP] Hoffman, P. and M. Blanchet, "Preparation of
                Internationalized Strings ("stringprep")",

   [IDNA]       Faltstrom, P., Hoffman, P. and A. Costello,
                "Internationalizing Domain Names in Applications
                (IDNA)", draft-hoffman-rfc3490bis.

8.2 Informative references

   [STD13]      Mockapetris, P., "Domain names - concepts and
                facilities", STD 13, RFC 1034, and "Domain names -
                implementation and specification", STD 13, RFC 1035,
                November 1987.

9. Security Considerations

   The Unicode and ISO/IEC 10646 repertoires have many characters that
   look similar.  In many cases, users of security protocols might do
   visual matching, such as when comparing the names of trusted third
   parties.  Because it is impossible to map similar-looking characters
   without a great deal of context such as knowing the fonts used,
   stringprep does nothing to map similar-looking characters together
   nor to prohibit some characters because they look like others.

   Security on the Internet partly relies on the DNS.  Thus, any change
   to the characteristics of the DNS can change the security of much of
   the Internet.

   Domain names are used by users to connect to Internet servers.  The
   security of the Internet would be compromised if a user entering a
   single internationalized name could be connected to different servers
   based on different interpretations of the internationalized domain

   Current applications might assume that the characters allowed in
   domain names will always be the same as they are in [STD13].  This
   document vastly increases the number of characters available in
   domain names.  Every program that uses "special" characters in
   conjunction with domain names may be vulnerable to attack based on
   the new characters allowed by this specification.

10. IANA Considerations

   This is a profile of stringprep.  It has been registered by the IANA
   in the stringprep profile registry

      Name of this profile:

      RFC in which the profile is defined:
         This document.

      Indicator whether or not this is the newest version of the
         This is the first version of Nameprep.

11. Acknowledgements

   Many people from the IETF IDN Working Group and the Unicode Technical
   Committee contributed ideas that went into this document.

   The IDN Nameprep design team made many useful changes to the
   document.  That team and its advisors include:

      Asmus Freytag
      Cathy Wissink
      Francois Yergeau
      James Seng
      Marc Blanchet
      Mark Davis
      Martin Duerst
      Patrik Faltstrom
      Paul Hoffman

   Additional significant improvements were proposed by:

      Jonathan Rosenne
      Kent Karlsson
      Scott Hollenbeck
      Dave Crocker
      Erik Nordmark
      Matitiahu Allouche

12. Authors' Addresses

   Paul Hoffman
   Internet Mail Consortium and VPN Consortium
   127 Segre Place
   Santa Cruz, CA  95060 USA

   EMail: paul.hoffman@imc.org and paul.hoffman@vpnc.org

   Marc Blanchet
   Viagenie inc.
   2875 boul. Laurier, bur. 300
   Ste-Foy, Quebec, Canada, G1V 2M2

   EMail: Marc.Blanchet@viagenie.qc.ca

A. Changes from RFC 3491

This document is a revision of RFC 3491. None of the changes affect the
protocol described in RFC 3491; that is, all implementations of RFC 3491
will be identical with implementations of the specification in this
document. The items that have changed RFC 3491 document are:

- In section 1.1, a sentence was added to the end of the first paragraph
  to make it clear that implementations must follow the same steps in
  Stringprep, not just use the tables from Stringprep. The numbered list
  added as well.

- In section 5, removed "This profile MUST be used with the IDNA
  protocol" because it is expected that other protocols might use this
  profile as well.