Stringprep Revision Problem Statement
draft-ietf-precis-problem-statement-04

Network Working Group                                        M. Blanchet
Internet-Draft                                                  Viagenie
Intended status: Informational                               A. Sullivan
Expires: July 16, 2012                                  January 13, 2012

                 Stringprep Revision Problem Statement
               draft-ietf-precis-problem-statement-04.txt

Abstract

   Using Unicode codepoints in protocol strings that expect comparison
   with other strings 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 needs to
   be designed.  This document outlines the issues to be faced by those
   designing a Stringprep replacement.

Status of this Memo

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   carefully, as they describe your rights and restrictions with respect
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Issues raised during newprep BOF . . . . . . . . . . . . . . .  5
   3.  Major Topics for Consideration . . . . . . . . . . . . . . . .  6
     3.1.  Comparison . . . . . . . . . . . . . . . . . . . . . . . .  6
       3.1.1.  Types of Identifiers . . . . . . . . . . . . . . . . .  6
       3.1.2.  Effect of comparison . . . . . . . . . . . . . . . . .  7
     3.2.  Dealing with characters  . . . . . . . . . . . . . . . . .  7
       3.2.1.  Case folding, case sensitivity, and case
               preservation . . . . . . . . . . . . . . . . . . . . .  7
       3.2.2.  Stringprep and NFKC  . . . . . . . . . . . . . . . . .  7
       3.2.3.  Character mapping  . . . . . . . . . . . . . . . . . .  8
       3.2.4.  Prohibited characters  . . . . . . . . . . . . . . . .  8
       3.2.5.  Internal structure, delimiters, and special
               characters . . . . . . . . . . . . . . . . . . . . . .  9
     3.3.  Where the data comes from and where it goes  . . . . . . .  9
       3.3.1.  User input and the source of protocol elements . . . .  9
       3.3.2.  User output  . . . . . . . . . . . . . . . . . . . . . 10
       3.3.3.  Operations . . . . . . . . . . . . . . . . . . . . . . 10
       3.3.4.  Some useful classes of strings . . . . . . . . . . . . 11
   4.  Considerations for Stringprep replacement  . . . . . . . . . . 12
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   7.  Discussion home for this draft . . . . . . . . . . . . . . . . 12
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 13
   Appendix A.  Protocols known to be using Stringprep  . . . . . . . 16
   Appendix B.  Detailed discussion of protocols under
                consideration . . . . . . . . . . . . . . . . . . . . 17
   Appendix C.  Changes between versions  . . . . . . . . . . . . . . 17
     C.1.  00 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     C.2.  01 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     C.3.  02 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     C.4.  03 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18

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1.  Introduction

   Internationalizing Domain Names in Applications (IDNA2003) [RFC3490],
   [RFC3491], [RFC3492], [RFC3454] described a mechanism for encoding
   Unicode 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] (the latter of these relies on the former);
   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.  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 from that discussion were as follows:
   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.

   Protocols that use Stringprep profiles use strings for different

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   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 desirable 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.

3.  Major Topics for Consideration

   This section provides an overview of major topics that a Stringprep
   replacement needs to address.  The headings correspond roughly with
   categories under which known Stringprep-using protocol RFCs have been
   evaluated.  For the details of those evaluations, see Appendix A.

3.1.  Comparison

3.1.1.  Types of Identifiers

   Following [I-D.iab-identifier-comparison], we can organize
   identifiers into three classes in respect of how they may be compared
   with one another:

   Absolute Identifiers  Identifiers that can be compared byte-by-byte
      for equality.
   Definite Identifiers  Identifiers that have a well-defined comparison
      algorithm on which all parties agree.
   Indefinite Identifiers  Identifiers that have no single comparison
      algorithm on which all parties agree.

   Definite Identifiers include cases like the comparison of Unicode
   code points in different encodings: they do not match byte for byte,
   but can all be converted to a single encoding which then does match
   byte for byte.  Indefinite Identifiers are sometimes algorithmically
   comparable by well-specified subsets of parties.  For more discussion
   of these categories, see [I-D.iab-identifier-comparison].

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   The section on treating the existing known cases, Appendix A uses
   these categories.

3.1.2.  Effect of comparison

   The three classes of comparison style outlined in Section 3.1.1 may
   have different effects when applied.  It is necessary to evaluate the
   effects if a comparison results in a false positive, and what the
   effects are if a comparison results in a false negative, especially
   in terms of the consequences to security and usability.

3.2.  Dealing with characters

   This section outlines a range of issues having to do with characters
   in the target protocols, and spends some effort to outline the ways
   in which IDNA2008 might be a good analogy to other protocols, and
   ways in which it might be a poor one.

3.2.1.  Case folding, case sensitivity, and case preservation

   In IDNA2003, labels are always mapped to lower case before the
   Punycode transformation.  In IDNA2008, 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.
   Accordingly, existing protocols are to be evaluated according to the
   following criteria:

   1.  Does the protocol use case folding?  For all blocks of code
       points, or just for certain subsets?
   2.  Is the system or protocol case sensitive?
   3.  Does the system or protocol preserve case?

3.2.2.  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,
   NFKC 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

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   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.

   For the purposes of evaluating an existing example of Stringprep use,
   it is helpful to know whether it uses no normalization, NFKC, or NFC.

3.2.3.  Character mapping

   Along with the case mapping issues raised in Section 3.2.1, there is
   the question of whether some characters are mapped either to other
   characters or to nothing during Stringprep.  [RFC3454], Section 3,
   outlines a number of characters that are mapped to nothing, and also
   permits Stringprep profiles to define their own mappings.

3.2.4.  Prohibited characters

   Along with case folding and other character mappings, many protocols
   have characters that are simply disallowed.  For example, control
   characters and special characters such as "@" or "/" may be
   prohibited in a protocol.

   One of the primary changes of IDNA2008 is in the way it approaches
   Unicode code points.  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 code points are disallowed,
   and then relies on Unicode properties to derive whether a given code
   point actually is allowed in the protocol.

   Moreover, there is more than one class of "allowed in the protocol".
   While some code points are disallowed outright, 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 (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 goal is to provide the widest possible repertoire of code
   points possible and consistent with the traditional DNS, trusting to
   the operators of individual zones to make sensible (and usually more
   restrictive) policies for their zones.

   IDNA2008 may be a poor model for what other protocols ought to do in
   this case, because it is designed to support an old protocol that is
   designed to operate on the scale of the entire Internet.  Moreover,
   IDNA2008 is intended to be deployed without any change to the base

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   DNS protocol.  Other protocols may aim at deployment in more local
   environments, or may have protocol version negotiation built in.

3.2.5.  Internal structure, delimiters, and special characters

   IDNA2008 has a special problem with delimiters, because the delimiter
   "character" in the DNS wire format is not really part of the data.
   In DNS, labels are not separated exactly; instead, a label carries
   with it an indicator that says how long the label is.  When the label
   is presented in presentation format as part of a fully qualified
   domain name, the label separator FULL STOP, U+002E (.) is used to
   break up the labels.  But because that label separator does not
   travel with the wire format of the domain name, there is no way to
   encode a different, "internationalized" separator in IDNA2008.

   Other protocols may include characters with similar special meaning
   within the protocol.  Common characters for these purposes include
   FULL STOP, U+002E (.); COMMERCIAL AT, U+0040 (@); HYPHEN-MINUS,
   U+002D (-); SOLIDUS, U+002F (/); and LOW LINE, U+005F (_).  The mere
   inclusion of such a character in the protocol is not enough for it to
   be considered similar to another protocol using the same character;
   instead, handling of the character must be taken into consideration
   as well.

   An important issue to tackle here is whether it is valuable to map to
   or from these special characters as part of the Stringprep
   replacement.  In some locales, the analogue to FULL STOP, U+002E is
   some other character, and users may expect to be able to substitute
   their normal stop for FULL STOP, U+002E. At the same time, there are
   predictability arguments in favour of treating names with FULL STOP,
   U+002E in them just the way they are treated under IDNA2008.

3.3.  Where the data comes from and where it goes

3.3.1.  User input and the source of protocol elements

   Some protocol elements are provided by users, and others are not.
   Those that are not may presumably be subject to greater restrictions,
   whereas those that users provide likely need to permit the broadest
   range of code points.  The following questions are helpful:

   1.  Do users input the strings directly?
   2.  If so, how? (keyboard, stylus, voice, copy-paste, etc.)
   3.  Where do we place the dividing line between user interface and
       protocol? (see [RFC5895])

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3.3.2.  User output

   Just as only some protocol elements are expected to be entered
   directly by users, only some protocol elements are intended to be
   consumed directly by users.  It is important to know how users are
   expected to be able to consume the protocol elements, because
   different environments present different challenges.  An element that
   is only ever delivered as part of a vCard remains in machine-readable
   format, so the problem of visual confusion is not a great one.  Is
   the protocol element published as part of a vCard, a web directory,
   on a business card, or on "the side of a bus"?  Do users use the
   protocol element as an identifier (which means that they might enter
   it again in some other context)?

3.3.3.  Operations

   Some strings are useful as part of the protocol but are not used as
   input to other operations (for instance, purely informative or
   descriptive text).  Other strings are used directly as input to other
   operations (such as cryptographic hash functions), or are used
   together with other strings to (such as concatenating a string with
   some others to form a unique identifier).

3.3.3.1.  String classes

   Strings often have a similar function in different protocols.  For
   instance, many different protocols contain user identifiers or
   passwords.  A single profile for all such uses might be desirable.

   Often, a string in a protocol is effectively a protocol element from
   another protocol.  For instance, different systems might use the same
   credentials database for authentication.

3.3.3.2.  Community considerations

   A Stringprep replacement that does anything more than just update
   Stringprep to the latest version of Unicode will probably entail some
   changes.  It is important to identify the willingness of the
   protocol-using community to accept backwards-incompatible changes.
   By the same token, it is important to evaluate the desire of the
   community for features not available under Stringprep.

3.3.3.3.  What to do about Unicode changes

   IDNA2008 uses an algorithm to derive the validity of a Unicode code
   point for use under IDNA2008.  It does this by using the properties
   of each code point to test its validity.

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   This approach depends crucially on the idea that code points, once
   valid for a protocol profile, will not later be made invalid.  That
   is not a guarantee currently provided by Unicode.  Properties of code
   points may change between versions of Unicode.  Rarely, such a change
   could cause a given code point to become invalid under a protocol
   profile, even though the code point would be valid with an earlier
   version of Unicode.  This is not merely a theoretical possibility,
   because it has occurred ([I-D.faltstrom-5892bis]).

   Accordingly, a Stringprep replacement that intends to be Unicode
   version agnostic will need to work out a mechanism to address cases
   where incompatible changes occur because of new Unicode versions.

3.3.4.  Some useful classes of strings

   With the above considerations in hand, we can usefully classify
   strings into the following categories, inspired by those outlined in
   [I-D.saintandre-xmpp-i18n]:

   Domainy strings  Strings that are intended for use in a domain name
      slot, as defined in [RFC5890].  Note that domainy strings could be
      used outside a domain name slot: the question here is what the
      eventual intended use for the string is, and not whether the
      string is actually functioning as a domain name at any moment.
   Namey strings  Strings that are intended for use as identifiers but
      that are not domainy strings.  Namey strings are normally public
      data within the protocol where they are used: these are intended
      as identifiers that can be passed around to identify something.
   Secretish strings  Strings that are intended for use as passwords or
      passphrases or other such type of token.  Secretish strings are
      normally not public data within the protocol where they are used:
      they function as a token for authorization, and normally should
      not be shared publicly.
   Protocolish strings  Strings that are intended to be used by the
      protocol as free-form strings, but that have some significant
      handling within the protocol.  For instance, a protocol slot that
      allows free-form text where case is not preserved would need to
      have case mapping rules applied; in this case, the string would be
      a protocolish string.
   String blobs  Elements of the protocol that look like strings to
      users, but that are passed around in the protocol unchanged and
      that cannot be used for comparison or other purposes.  In effect,
      these are strings that are part of a protocol payload, and are not
      themselves part of the protocol at all.

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4.  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 according to the
      considerations that make up the headings in Section 3
   o  It is reasonable to limit scope to Unicode code points, and rule
      the mapping of data from other character encodings outside the
      scope of this effort.
   o  Recommendations for handling protocol incompatibilities resulting
      from changes to Unicode are required.

   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
   of backward incompatibility.

5.  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.

6.  IANA Considerations

   This document has no actions for IANA.

7.  Discussion home for this draft

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

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8.  Acknowledgements

   This document is the product of the PRECIS IETF Working Group, and
   participants in that Working Group were helpful in addressing issues
   with the text.

   Specific contributions came from David Black, Alan DeKok, Bill
   McQuillan, Alexey Melnikov, Peter Saint-Andre, Dave Thaler, and
   Yoshiro Yoneya.

   Dave Thaler provided the "buckets" insight in Section 3.1.1, central
   to the organization of the problem.

9.  Informative References

   [I-D.faltstrom-5892bis]
              Faltstrom, P. and P. Hoffman, "The Unicode code points and
              IDNA - Unicode 6.0", draft-faltstrom-5892bis-05 (work in
              progress), June 2011.

   [I-D.iab-identifier-comparison]
              Thaler, D., "Issues in Identifier Comparison for Security
              Purposes", draft-iab-identifier-comparison-00 (work in
              progress), July 2011.

   [I-D.saintandre-xmpp-i18n]
              Saint-Andre, P., "Internationalized Addresses in XMPP",
              draft-saintandre-xmpp-i18n-03 (work in progress),
              March 2011.

   [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.

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   [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.

   [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.

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   [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.

   [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.

   [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

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              System (NFS) Version 4 Minor Version 1 Protocol",
              RFC 5661, January 2010.

   [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.

   [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.

   [RFC5895]  Resnick, P. and P. Hoffman, "Mapping Characters for
              Internationalized Domain Names in Applications (IDNA)
              2008", RFC 5895, September 2010.

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

Appendix A.  Protocols known to be using Stringprep

   The known cases are here described in two ways.  The types of
   identifiers the protocol uses is first called out in the ID type
   column (from Section 3.1.1), using the short forms "a" for Absolute,
   "d" for Definite, and "i" for Indefinite.  Next, there is a column
   that contains an "i" if the protocol string comes from user input, an
   "o" if the protocol string becomes user-facing output, "b" if both
   are true, and "n" if neither is true.  The remaining columns have an
   "x" if and only if the protocol uses that class, as described in
   Section 3.3.4.  Values marked "-" indicate that an answer is not

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   useful; in this case, see detailed discussion in Appendix B.

   +------+--------+-------+-------+-------+---------+---------+------+
   |  RFC | IDtype | User? | Dom'y | Nam'y | Sec'ish | Pro'ish | Blob |
   +------+--------+-------+-------+-------+---------+---------+------+
   | 3722 |    a   |   o   |       |   x   |    x    |    x    |      |
   | 3748 |    -   |   -   |   -   |   x   |    -    |    -    |   -  |
   | 3920 |   a,d  |   b   |       |   x   |         |    x    |      |
   | 4314 |   a,d  |   b   |       |   x   |    x    |    x    |      |
   +------+--------+-------+-------+-------+---------+---------+------+

                                  Table 1

   [[anchor21: The table still needs to be filled in, I am aware.
   --ajs@anvilwalrusden.com]]

Appendix B.  Detailed discussion of protocols under consideration

   Below are detailed reviews of the protocols under consideration
   (where such reviews are available). [[anchor22: These are to be cut
   and pasted from the wiki. --ajs@anvilwalrusden.com]]

Appendix C.  Changes between versions

   Note to RFC Editor: This section should be removed prior to
   publication.

C.1.  00

   First WG version.  Based on
   draft-blanchet-precis-problem-statement-00.

C.2.  01

   o  Made clear that the document is talking only about Unicode code
      points, and not any particular encoding.
   o  Substantially reorganized the document along the lines of the
      review template at <http://trac.tools.ietf.org/wg/precis/trac/
      wiki/StringprepReviewTemplate>.
   o  Included specific questions for each topic for consideration.
   o  Moved spot for individual protocol review to appendix.  Not
      populated yet.

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C.3.  02

   o  Cleared up details of comparison classes
   o  Added a section on changes in Unicode

C.4.  03

   o  Aligned comparison discussion with identifier discussion from
      draft-iab-identifier-comparison-00
   o  Added section on classes of strings ("Namey" and so on)

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@anvilwalrusden.com

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