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Versions: (draft-blanchet-precis-problem-statement) 00 01 02 03 04 05 06 07 08 09 RFC 6885

Network Working Group                                        M. Blanchet
Internet-Draft                                                  Viagenie
Intended status: Informational                               A. Sullivan
Expires: April 21, 2011                                 October 18, 2010


                 Stringprep Revision Problem Statement
               draft-ietf-precis-problem-statement-00.txt

Abstract

   Using Unicode codepoints in protocol strings that expect comparison
   with other strings [[anchor1: The WG will need to decide whether
   "other strings" is too broad.  In particular, what about protocol
   slots that can take strings other than plain ASCII?
   --ajs@shinkuro.com]] requires preparation of the string that contains
   the Unicode codepoints.  Internationalizing Domain Names in
   Applications (IDNA2003) defined and used Stringprep and Nameprep.
   Other protocols subsequently defined Stringprep profiles.  A new
   approach different from Stringprep and Nameprep is used for a
   revision of IDNA2003 (called IDNA2008).  Other Stringprep profiles
   need to be similarly updated or a replacement of Stringprep need to
   be designed.  This document outlines the issues to be faced by those
   designing a Stringprep replacement.

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 April 21, 2011.

Copyright Notice

   Copyright (c) 2010 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



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   Provisions Relating to IETF Documents
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   it for publication as an RFC or to translate it into languages other
   than English.































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Usage and Issues of Stringprep . . . . . . . . . . . . . . . .  5
     2.1.  Issues raised during newprep BOF . . . . . . . . . . . . .  5
     2.2.  Specific issues with particular Stringprep profiles  . . .  6
     2.3.  Inclusion vs. exclusion of characters  . . . . . . . . . .  6
     2.4.  Stringprep and NFKC  . . . . . . . . . . . . . . . . . . .  7
     2.5.  Case mapping . . . . . . . . . . . . . . . . . . . . . . .  7
     2.6.  Whether to use ASCII-compatible encoding . . . . . . . . .  7
     2.7.  Issues with delimiters . . . . . . . . . . . . . . . . . .  8
   3.  Considerations for Stringprep replacement  . . . . . . . . . .  8
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   6.  Discussion home for this draft . . . . . . . . . . . . . . . .  9
   7.  Informative References . . . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12


































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1.  Introduction

   Internationalizing Domain Names in Applications (IDNA2003) [RFC3490],
   [RFC3491], [RFC3492], [RFC3454] described a mechanism for encoding
   UTF-8 labels making up Internationalized Domain Names (IDNs) as
   standard DNS labels.  The labels were processed using a method called
   Nameprep [RFC3491] and Punycode [RFC3492].  That method was specific
   to IDNA2003, but is generalized as Stringprep [RFC3454].  The general
   mechanism can be used to help other protocols with similar needs, but
   with different constraints than IDNA2003.

   Stringprep defines a framework within which protocols define their
   Stringprep profiles.  Known IETF specifications using Stringprep are
   listed below:
   o  The Nameprep profile [RFC3490] for use in Internationalized Domain
      Names (IDNs);
   o  NFSv4 [RFC3530] and NFSv4.1 [RFC5661];
   o  The iSCSI profile [RFC3722] for use in Internet Small Computer
      Systems Interface (iSCSI) Names;
   o  EAP [RFC3748];
   o  The Nodeprep and Resourceprep profiles [RFC3920] for use in the
      Extensible Messaging and Presence Protocol (XMPP), and the XMPP to
      CPIM mapping [RFC3922];
   o  The Policy MIB profile [RFC4011] for use in the Simple Network
      Management Protocol (SNMP);
   o  The SASLprep profile [RFC4013] for use in the Simple
      Authentication and Security Layer (SASL), and SASL itself
      [RFC4422];
   o  TLS [RFC4279];
   o  IMAP4 using SASLprep [RFC4314];
   o  The trace profile [RFC4505] for use with the SASL ANONYMOUS
      mechanism;
   o  The LDAP profile [RFC4518] for use with LDAP [RFC4511] and its
      authentication methods [RFC4513];
   o  Plain SASL using SASLprep [RFC4616];
   o  NNTP using SASLprep [RFC4643];
   o  PKIX subject identification using LDAPprep [RFC4683];
   o  Internet Application Protocol Collation Registry [RFC4790];
   o  SMTP Auth using SASLprep [RFC4954];
   o  POP3 Auth using SASLprep [RFC5034];
   o  TLS SRP using SASLprep [RFC5054];
   o  IRI and URI in XMPP [RFC5122];
   o  PKIX CRL using LDAPprep [RFC5280];
   o  IAX using Nameprep [RFC5456];
   o  SASL SCRAM using SASLprep [RFC5802];
   o  Remote management of Sieve using SASLprep [RFC5804];





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   o  The i;unicode-casemap Unicode Collation [RFC5051].

   There turned out to be some difficulties with IDNA2003, documented in
   [RFC4690].  These difficulties led to a new IDN specification, called
   IDNA2008 [RFC5890], [RFC5891], [RFC5892], [RFC5893].  Additional
   background and explanations of the decisions embodied in IDNA2008 is
   presented in [RFC5894].  One of the effects of IDNA2008 is that
   Nameprep and Stringprep are not used at all.  Instead, an algorithm
   based on Unicode properties of codepoints is defined.  That algorithm
   generates a stable and complete table of the supported Unicode
   codepoints.  This algorithm is based on an inclusion-based approach,
   instead of the exclusion-based approach of Stringprep/Nameprep.

   This document lists the shortcomings and issues found by protocols
   listed above that defined Stringprep profiles.  It also lists some
   early conclusions and requirements for a potential replacement of
   Stringprep.


2.  Usage and Issues of Stringprep

2.1.  Issues raised during newprep BOF

   During IETF 77, a BOF discussed the current state of the protocols
   that have defined Stringprep profiles [NEWPREP].  The main
   conclusions are :
   o  Stringprep is bound to a specific version of Unicode: 3.2.
      Stringprep has not been updated to new versions of Unicode.
      Therefore, the protocols using Stringprep are stuck to Unicode
      3.2.
   o  The protocols need to be updated to support new versions of
      Unicode.  The protocols would like to not be bound to a specific
      version of Unicode, but rather have better Unicode agility in the
      way of IDNA2008.  This is important partly because it is usually
      impossible for an application to require Unicode 3.2; the
      application gets whatever version of Unicode is available on the
      host.
   o  The protocols require better bidirectional support (bidi) than
      currently offered by Stringprep.
   o  If the protocols are updated to use a new version of Stringprep or
      another framework, then backward compatibility is an important
      requirement.  For example, Stringprep is based on and may use NFKC
      [UAX15], while IDNA2008 mostly uses NFC [UAX15].
   o  Protocols use each other; for example, a protocol can use user
      identifiers that are later passed to SASL, LDAP or another
      authentication mechanism.  Therefore, common set of rules or
      classes of strings are preferred over specific rules for each
      protocol.



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   Protocols that use Stringprep profiles use strings for different
   purposes:
   o  XMPP uses a different Stringprep profile for each part of the XMPP
      address (JID): a localpart which is similar to a username and used
      for authentication, a domainpart which is a domain name and a
      resource part which is less restrictive than the localpart.
   o  iSCSI uses a Stringprep profile for the IQN, which is very similar
      to (often is) a DNS domain name.
   o  SASL and LDAP uses a Stringprep profile for usernames.
   o  LDAP uses a set of Stringprep profiles.

   During the newprep BOF, it was the consensus of the attendees that it
   would be highly preferable to have a replacement of Stringprep, with
   similar characteristics to IDNA2008.  That replacement should be
   defined so that the protocols could use internationalized strings
   without a lot of specialized internationalization work, since
   internationalization expertise is not available in the respective
   protocols or working groups.

2.2.  Specific issues with particular Stringprep profiles

   [[anchor6: This section is where issues raised in the individual
   profile reviews goes.  A review of the WG trac state on 2010-10-06 of
   the tracker suggests those reviews haven't happened yet.
   --ajs@shinkuro.com]]

2.3.  Inclusion vs. exclusion of characters

   One of the primary changes of IDNA2008 is in the way it approaches
   Unicode characters.  IDNA2003 created an explicit list of excluded or
   mapped-away characters; anything in Unicode 3.2 that was not so
   listed could be assumed to be allowed under the protocol.  IDNA2008
   begins instead from the assumption that characters are disallowed,
   and then relies on Unicode properties to derive whether a given
   character actually is allowed in the protocol.

   Moreover, there is more than one class of "allowed in the protocol".
   While some characters are simply disallowed, some are allowed only in
   certain contexts.  The reasons for the context-dependent rules have
   to do with the way some characters are used.  For instance, the ZERO
   WIDTH JOINER and ZERO WIDTH NON-JOINER characters (ZWJ, U+200D and
   ZWNJ, U+200C) are allowed with contextual rules because they are
   required in some circumstances, yet are considered punctuation by
   Unicode and would therefore be DISALLOWED under the usual IDNA2008
   derivation rules.

   The working group needs to decide whether similar contextual cases
   need to be supported.



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2.4.  Stringprep and NFKC

   Stringprep profiles may use normalization.  If they do, they use NFKC
   [UAX15].  It is not clear that NFKC is the right normalization to use
   in all cases.  In [UAX15], there is the following observation
   regarding Normalization Forms KC and KD: "It is best to think of
   these Normalization Forms as being like uppercase or lowercase
   mappings: useful in certain contexts for identifying core meanings,
   but also performing modifications to the text that may not always be
   appropriate."  For things like the spelling of users' names, then,
   NKFC may not be the best form to use.  At the same time, one of the
   nice things about NFKC is that it deals with the width of characters
   that are otherwise similar, by canonicalizing half-width to full-
   width.  This mapping step can be crucial in practice.  The WG will
   need to analyze the different use profiles and consider whether NFKC
   or NFC is a better normalization for each profile.

2.5.  Case mapping

   In IDNA2003, labels are always mapped to lower case before the
   Punycode transformation.  In IDNA2003, there is no mapping at all:
   input is either a valid U-label or it is not.  At the same time,
   upper-case characters are by definition not valid U-labels, because
   they fall into the Unstable category (category B) of [RFC5892].

   If there are protocols that require upper and lower cases be
   preserved, then the analogy with IDNA2008 will break down.  The
   working group will need to decide whether there are any cases that
   require upper case, and what to do about it if so.

2.6.  Whether to use ASCII-compatible encoding

   The development of IDNA2008 depended on the notion that there was a
   narrow repertoire of reasonable traditional labels, and what was
   necessary was to internationalize that repertoire rather than to
   incorporate any characters into domain name labels.  More exactly,
   the idea was to internationalize the traditional hostname rules (the
   "LDH rule".  See [RFC4690], section 5.1.).  Efforts to
   internationalize email ([RFC5336]) have started from different
   assumptions.  The email example suggests that in some cases, the
   right answer might be to internationalize the target protocol rather
   than to depend on a technology to ensure protocol slots can use only
   ASCII.  The working group will need to determine which approach is
   correct for the different use-cases.







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2.7.  Issues with delimiters

   There are two kinds of issues to address with delimiters.  First,
   exactly where a delimiter will appear on the screen when dealing with
   bidirectional parts of a string can be extremely surprising.  In the
   case of IDNA2008, just what to do in these cases remains a display
   issue (there is no question about the wire format, because the wire
   format is an A-label and it is always left to right).

   Second, there is the question of whether to include different kinds
   of protocol separators.  For instance, FULL STOP, U+002E (.) may not
   be available on all keyboards.  In addition, in some languages there
   is more than one full stop which are variants of one another.  The
   working group will need to decide how to handle such cases: whether
   there will be a mapping, some restrictions, or something else.


3.  Considerations for Stringprep replacement

   The above suggests the following direction for the working group:
   o  A stringprep replacement should be defined.
   o  The replacement should take an approach similar to IDNA2008, in
      that it enables Unicode agility.
   o  Protocols share similar characteristics of strings.  Therefore,
      defining i18n preparation algorithms for a (small) set of string
      classes may be sufficient for most cases and provides the
      coherence among a set of protocol friends.
   o  The sets of string classes need to be evaluated for the following
      properties:
      *  the normalization needed (NFC vs NFKC);
      *  whether case-folding, case preservation, and case-insensitive
         matching is needed;
      *  what restrictions on input are reasonable for the class (i.e.
         whether there is something like an "LDH rule" for the class),
         or whether the ASCII-only input in the protocol slot is lightly
         constrained;
      *  the extent to which bidi considerations are important for the
         class.

   Existing deployments already depend on Stringprep profiles.
   Therefore, the working group will need to consider the effects of any
   new strategy on existing deployments.  By way of comparison, it is
   worth noting that some characters were acceptable in IDNA labels
   under IDNA2003, but are not protocol-valid under IDNA2008 (and
   conversely).  Different implementers may make different decisions
   about what to do in such cases; this could have interoperability
   effects.  The working group will need to trade better support for
   different linguistic environments against the potential side effects



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   of backward incompatibility.


4.  Security Considerations

   This document merely states what problems are to be solved, and does
   not define a protocol.  There are undoubtedly security implications
   of the particular results that will come from the work to be
   completed.


5.  IANA Considerations

   This document has no actions for IANA.


6.  Discussion home for this draft

   This document is intended to define the problem space discussed on
   the precis@ietf.org mailing list.


7.  Informative References

   [NEWPREP]  "Newprep BoF Meeting Minutes", March 2010.

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

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [RFC3491]  Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
              Profile for Internationalized Domain Names (IDN)",
              RFC 3491, March 2003.

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, March 2003.

   [RFC3530]  Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
              Beame, C., Eisler, M., and D. Noveck, "Network File System
              (NFS) version 4 Protocol", RFC 3530, April 2003.

   [RFC3722]  Bakke, M., "String Profile for Internet Small Computer
              Systems Interface (iSCSI) Names", RFC 3722, April 2004.



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   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)",
              RFC 3748, June 2004.

   [RFC3920]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 3920, October 2004.

   [RFC3922]  Saint-Andre, P., "Mapping the Extensible Messaging and
              Presence Protocol (XMPP) to Common Presence and Instant
              Messaging (CPIM)", RFC 3922, October 2004.

   [RFC4011]  Waldbusser, S., Saperia, J., and T. Hongal, "Policy Based
              Management MIB", RFC 4011, March 2005.

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

   [RFC4279]  Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
              for Transport Layer Security (TLS)", RFC 4279,
              December 2005.

   [RFC4314]  Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
              RFC 4314, December 2005.

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

   [RFC4505]  Zeilenga, K., "Anonymous Simple Authentication and
              Security Layer (SASL) Mechanism", RFC 4505, June 2006.

   [RFC4511]  Sermersheim, J., "Lightweight Directory Access Protocol
              (LDAP): The Protocol", RFC 4511, June 2006.

   [RFC4513]  Harrison, R., "Lightweight Directory Access Protocol
              (LDAP): Authentication Methods and Security Mechanisms",
              RFC 4513, June 2006.

   [RFC4518]  Zeilenga, K., "Lightweight Directory Access Protocol
              (LDAP): Internationalized String Preparation", RFC 4518,
              June 2006.

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

   [RFC4643]  Vinocur, J. and K. Murchison, "Network News Transfer
              Protocol (NNTP) Extension for Authentication", RFC 4643,
              October 2006.




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   [RFC4683]  Park, J., Lee, J., Lee, H., Park, S., and T. Polk,
              "Internet X.509 Public Key Infrastructure Subject
              Identification Method (SIM)", RFC 4683, October 2006.

   [RFC4690]  Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
              Recommendations for Internationalized Domain Names
              (IDNs)", RFC 4690, September 2006.

   [RFC4790]  Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
              Application Protocol Collation Registry", RFC 4790,
              March 2007.

   [RFC4954]  Siemborski, R. and A. Melnikov, "SMTP Service Extension
              for Authentication", RFC 4954, July 2007.

   [RFC5034]  Siemborski, R. and A. Menon-Sen, "The Post Office Protocol
              (POP3) Simple Authentication and Security Layer (SASL)
              Authentication Mechanism", RFC 5034, July 2007.

   [RFC5051]  Crispin, M., "i;unicode-casemap - Simple Unicode Collation
              Algorithm", RFC 5051, October 2007.

   [RFC5054]  Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
              "Using the Secure Remote Password (SRP) Protocol for TLS
              Authentication", RFC 5054, November 2007.

   [RFC5122]  Saint-Andre, P., "Internationalized Resource Identifiers
              (IRIs) and Uniform Resource Identifiers (URIs) for the
              Extensible Messaging and Presence Protocol (XMPP)",
              RFC 5122, February 2008.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [RFC5336]  Yao, J. and W. Mao, "SMTP Extension for Internationalized
              Email Addresses", RFC 5336, September 2008.

   [RFC5456]  Spencer, M., Capouch, B., Guy, E., Miller, F., and K.
              Shumard, "IAX: Inter-Asterisk eXchange Version 2",
              RFC 5456, February 2010.

   [RFC5661]  Shepler, S., Eisler, M., and D. Noveck, "Network File
              System (NFS) Version 4 Minor Version 1 Protocol",
              RFC 5661, January 2010.

   [RFC5802]  Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,



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              "Salted Challenge Response Authentication Mechanism
              (SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010.

   [RFC5804]  Melnikov, A. and T. Martin, "A Protocol for Remotely
              Managing Sieve Scripts", RFC 5804, 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.

   [RFC5892]  Faltstrom, P., "The Unicode Code Points and
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5892, 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.

   [UAX15]    "Unicode Standard Annex #15: Unicode Normalization Forms",
              UAX 15, September 2009.


Authors' Addresses

   Marc Blanchet
   Viagenie
   2600 boul. Laurier, suite 625
   Quebec, QC  G1V 4W1
   Canada

   Email: Marc.Blanchet@viagenie.ca
   URI:   http://viagenie.ca


   Andrew Sullivan
   519 Maitland St.
   London, ON  N6B 2Z5
   Canada

   Email: ajs@crankycanuck.ca




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