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RFC 6885 - Stringprep Revision and Problem Statement for the Pre


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Internet Engineering Task Force (IETF)                       M. Blanchet
Request for Comments: 6885                                      Viagenie
Category: Informational                                      A. Sullivan
ISSN: 2070-1721                                                Dyn, Inc.
                                                              March 2013

               Stringprep Revision and Problem Statement
for the Preparation and Comparison of Internationalized Strings (PRECIS)

Abstract

   If a protocol expects to compare two strings and is prepared only for
   those strings to be ASCII, then using Unicode code points in those
   strings requires they be prepared somehow.  Internationalizing Domain
   Names in Applications (here called 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

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6885.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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 . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Keywords . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Stringprep Profiles Limitations  . . . . . . . . . . . . . . .  6
   5.  Major Topics for Consideration . . . . . . . . . . . . . . . .  8
     5.1.  Comparison . . . . . . . . . . . . . . . . . . . . . . . .  8
       5.1.1.  Types of Identifiers . . . . . . . . . . . . . . . . .  8
       5.1.2.  Effect of Comparison . . . . . . . . . . . . . . . . .  8
     5.2.  Dealing with Characters  . . . . . . . . . . . . . . . . .  9
       5.2.1.  Case Folding, Case Sensitivity, and Case
               Preservation . . . . . . . . . . . . . . . . . . . . .  9
       5.2.2.  Stringprep and NFKC  . . . . . . . . . . . . . . . . .  9
       5.2.3.  Character Mapping  . . . . . . . . . . . . . . . . . . 10
       5.2.4.  Prohibited Characters  . . . . . . . . . . . . . . . . 10
       5.2.5.  Internal Structure, Delimiters, and Special
               Characters . . . . . . . . . . . . . . . . . . . . . . 10
       5.2.6.  Restrictions Because of Glyph Similarity . . . . . . . 11
     5.3.  Where the Data Comes from and Where It Goes  . . . . . . . 11
       5.3.1.  User Input and the Source of Protocol Elements . . . . 11
       5.3.2.  User Output  . . . . . . . . . . . . . . . . . . . . . 12
       5.3.3.  Operations . . . . . . . . . . . . . . . . . . . . . . 12
   6.  Considerations for Stringprep Replacement  . . . . . . . . . . 13
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 15
   Appendix A.  Classification of Stringprep Profiles . . . . . . . . 19
   Appendix B.  Evaluation of Stringprep Profiles . . . . . . . . . . 19
     B.1.  iSCSI Stringprep Profile: RFC 3720, RFC 3721, RFC 3722 . . 19
     B.2.  SMTP/POP3/ManageSieve Stringprep Profiles: RFC 4954,
           RFC 5034, RFC 5804 . . . . . . . . . . . . . . . . . . . . 21
     B.3.  IMAP Stringprep Profiles for Usernames: RFC 4314, RFC
           5738 . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     B.4.  IMAP Stringprep Profiles for Passwords: RFC 5738 . . . . . 26
     B.5.  Anonymous SASL Stringprep Profiles: RFC 4505 . . . . . . . 28
     B.6.  XMPP Stringprep Profiles for Nodeprep: RFC 3920  . . . . . 30
     B.7.  XMPP Stringprep Profiles for Resourceprep: RFC 3920  . . . 31
     B.8.  EAP Stringprep Profiles: RFC 3748  . . . . . . . . . . . . 33

1.  Introduction

   Internationalizing Domain Names in Applications (here called
   IDNA2003) [RFC3490] [RFC3491] [RFC3492] and [RFC3454] describes a
   mechanism for encoding Unicode labels that make up the
   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 is used
   by other protocols with similar needs but with different constraints
   than IDNA2003.

   Stringprep defines a framework within which protocols define their
   Stringprep profiles.  Some known IETF specifications using Stringprep
   are listed below:

   o  The Nameprep profile [RFC3490] for use in Internationalized Domain
      Names (IDNs);

   o  The Inter-Asterisk eXchange (IAX) using Nameprep [RFC5456];

   o  NFSv4 [RFC3530] and NFSv4.1 [RFC5661];

   o  The Internet Small Computer System Interface (iSCSI) profile
      [RFC3722] for use in iSCSI names;

   o  The Extensible Authentication Protocol (EAP) [RFC3748];

   o  The Nodeprep and Resourceprep profiles [RFC3920] (which was
      obsoleted by [RFC6120]) for use in the Extensible Messaging and
      Presence Protocol (XMPP), and the XMPP to Common Presence and
      Instant Messaging (CPIM) mapping [RFC3922] (the latter of these
      relies on the former);

   o  The Internationalized Resource Identifier (IRI) and URI in XMPP
      [RFC5122];

   o  The Policy MIB profile [RFC4011] for use in the Simple Network
      Management Protocol (SNMP);

   o  Transport Layer Security (TLS) [RFC4279];

   o  The Lightweight Directory Access Protocol (LDAP) profile [RFC4518]
      for use with LDAP [RFC4511] and its authentication methods
      [RFC4513];

   o  PKIX subject identification using LDAPprep [RFC4683];

   o  PKIX Certificate Revocation List (CRL) using LDAPprep [RFC5280];

   o  The Simple Authentication and Security Layer (SASL) [RFC4422] and
      SASLprep profile [RFC4013] for use in SASL;

   o  Plain SASL using SASLprep [RFC4616];

   o  SMTP Auth using SASLprep [RFC4954];

   o  The Post Office Protocol (POP3) Auth using SASLprep [RFC5034];

   o  TLS Secure Remote Password (SRP) using SASLprep [RFC5054];

   o  SASL Salted Challenge Response Authentication Mechanism (SCRAM)
      using SASLprep [RFC5802];

   o  Remote management of Sieve using SASLprep [RFC5804];

   o  The Network News Transfer Protocol (NNTP) using SASLprep
      [RFC4643];

   o  IMAP4 using SASLprep [RFC4314];

   o  The trace profile [RFC4505] for use with the SASL ANONYMOUS
      mechanism;

   o  Internet Application Protocol Collation Registry [RFC4790];

   o  The unicode-casemap Unicode Collation [RFC5051].

   However, a review (see [78PRECIS]) of these protocol specifications
   found that they are very similar and can be grouped into a short
   number of classes.  Moreover, many reuse the same Stringprep profile,
   such as the SASL one.

   IDNA2003 was replaced because of some limitations described in
   [RFC4690].  The new IDN specification, called IDNA2008 [RFC5890],
   [RFC5891], [RFC5892], [RFC5893] was designed based on the
   considerations found 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 code points is defined.
   That algorithm generates a stable and complete table of the supported
   Unicode code points for each Unicode version.  This algorithm uses an
   inclusion-based approach, instead of the exclusion-based approach of
   Stringprep/Nameprep.  That is, 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.

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

2.  Keywords

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

   This document uses various internationalization terms, which are
   defined and discussed in [RFC6365].

   Additionally, this document defines the following keyword:

      PRECIS: Preparation and Comparison of Internationalized Strings

3.  Conventions

   A single Unicode code point in this memo is denoted by "U+" followed
   by four to six hexadecimal digits, as used in [Unicode61],
   Appendix A.

4.  Stringprep Profiles Limitations

   During IETF 77 (March 2010), 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 Version 3.2 of Unicode.  Stringprep has not
      been updated to new versions of Unicode.  Therefore, the protocols
      using Stringprep are stuck at Unicode 3.2, and their
      specifications need to be updated to support new versions of
      Unicode.

   o  The protocols would like to not be bound to a specific version of
      Unicode, but rather have better Unicode version 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 normalization is based on
      and profiles may use Unicode Normalization Form KC (NFKC) [UAX15],
      while IDNA2008 mostly uses Unicode Normalization Form C (NFC)
      [UAX15].

   o  Identifiers are passed between protocols.  For example, the same
      username string of code points may be passed between SASL, XMPP,
      LDAP, and EAP.  Therefore, a common set of rules or classes of
      strings are preferred over specific rules for each protocol.
      Without real planning in advance, many Stringprep profiles reuse
      other profiles, so this goal was accomplished by accident with
      Stringprep.

   Protocols that use Stringprep profiles use strings for different
   purposes:

   o  XMPP uses a different Stringprep profile for each part of the XMPP
      address Jabber Identifier (JID): a localpart, which is similar to
      a username and used for authentication; a domainpart, which is a
      domain name; and a resourcepart, which is less restrictive than
      the localpart.

   o  iSCSI uses a Stringprep profile for the names of protocol
      participants (called initiators and targets).  The iSCSI Qualified
      Name (IQN) format of iSCSI names contains a reversed DNS domain
      name.

   o  SASL and LDAP use a Stringprep profile for usernames.

   o  LDAP uses a set of Stringprep profiles.

   The apparent judgement of the BOF attendees [NEWPREP] was 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.  Accordingly, the IESG formed the PRECIS
   working group to undertake the task.

   Notwithstanding the desire evident in [NEWPREP] and the chartering of
   a working group, IDNA2008 may be a poor model for what other
   protocols ought to do, 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 DNS protocol.  Other protocols may aim at
   deployment in more local environments, or may have protocol version
   negotiation built in.

5.  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.

5.1.  Comparison

5.1.1.  Types of Identifiers

   Following [ID-COMP], it is possible to 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 [ID-COMP].

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

5.1.2.  Effect of Comparison

   The three classes of comparison style outlined in Section 5.1.1 may
   have different effects when applied.  It is necessary to evaluate the
   effects if a comparison results in a false positive or a false
   negative, especially in terms of the consequences to security and
   usability.

5.2.  Dealing with Characters

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

5.2.1.  Case Folding, Case Sensitivity, and Case Preservation

   In IDNA2003, labels are always mapped to lowercase 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,
   uppercase 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 case 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?

5.2.2.  Stringprep and NFKC

   Stringprep profiles may use normalization.  If they do, they use NFKC
   [UAX15] (most profiles do).  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."  In general, it can be said that NFKC is more
   aggressive about finding matches between code points than NFC.  For
   things like the spelling of users' names, 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 that are otherwise
   similar, by canonicalizing half-width to full-width.  This mapping
   step can be crucial in practice.  A replacement for Stringprep
   depends on analyzing the different use profiles and considering
   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.

5.2.3.  Character Mapping

   Along with the case mapping issues raised in Section 5.2.1, 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.

5.2.4.  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, using the new inclusion-based approach (see
   Section 1).

   Because of the default assumption in IDNA2008 that a code point is
   not allowed by the protocol, it has more than one class of "allowed
   by the protocol"; this is unlike IDNA2003.  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 of
   IDNA2008 is to provide the widest repertoire of code points possible
   and consistent with the traditional DNS "LDH" (letters, digits,
   hyphen) rule (see [RFC0952]), trusting to the operators of individual
   zones to make sensible (and usually more restrictive) policies for
   their zones.

5.2.5.  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 displayed 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 favor of treating identifiers with FULL
   STOP, U+002E in them just the way they are treated under IDNA2008.

5.2.6.  Restrictions Because of Glyph Similarity

   Homoglyphs are similarly (or identically) rendered glyphs of
   different code points.  For DNS names, homoglyphs may enable
   phishing.  If a protocol requires some visual comparison by end-
   users, then the issue of homoglyphs is to be considered.  In the DNS
   context, these issues are documented in [RFC5894] and [RFC4690].
   However, IDNA2008 does not have a mechanism to deal with them,
   trusting DNS zone operators to enact sensible policies for the subset
   of Unicode they wish to support, given their user community.  A
   similar policy/protocol split may not be desirable in every protocol.

5.3.  Where the Data Comes from and Where It Goes

5.3.1.  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])

5.3.2.  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)?  (See also Section 5.2.6.)

5.3.3.  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).

5.3.3.1.  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.

5.3.3.2.  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.

5.3.3.3.  Unicode Incompatible 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.

   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 [RFC6452].

   Accordingly, as in IDNA2008, 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.

6.  Considerations for Stringprep Replacement

   The above suggests the following guidance:

   o  A Stringprep replacement should be defined.

   o  The replacement should take an approach similar to IDNA2008 (e.g.,
      by using properties of code points instead of whitelisting of code
      points), in that it enables better Unicode agility.

   o  Protocols share similar characteristics of strings.  Therefore,
      defining internationalization preparation algorithms for the
      smallest set of string classes may be sufficient for most cases,
      providing coherence among a set of related protocols or protocols
      where identifiers are exchanged.

   o  The sets of string classes need to be evaluated according to the
      considerations that make up the headings in Section 5

   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  The replacement ought to at least provide guidance to applications
      using the replacement on how to handle protocol incompatibilities
      resulting from changes to Unicode.  In an ideal world, the
      Stringprep replacement would handle the changes automatically, but
      it appears that such automatic handling would require magic and
      cannot be expected.

   o  Compatibility within each protocol between a technique that is
      Stringprep-based and the technique's replacement has to be
      considered very carefully.

   Existing deployments already depend on Stringprep profiles.
   Therefore, a replacement must 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);
   disagreement about what to do during the transition has resulted in
   different approaches to mapping.  Different implementers may make
   different decisions about what to do in such cases; this could have
   interoperability effects.  It is necessary to trade better support
   for different linguistic environments against the potential side
   effects of backward incompatibility.

7.  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.  Moreover, the Stringprep Security Considerations
   [RFC3454] Section applies.  See also the analysis in the subsections
   of Appendix B, below.

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, Simon
   Josefsson, Bill McQuillan, Alexey Melnikov, Peter Saint-Andre, Dave
   Thaler, and Yoshiro Yoneya.

   Dave Thaler provided the "buckets" insight in Section 5.1.1, central
   to the organization of the problem.

   Evaluations of Stringprep profiles that are included in Appendix B
   were done by David Black, Alexey Melnikov, Peter Saint-Andre, and
   Dave Thaler.

9.  Informative References

   [78PRECIS]   Blanchet, M., "PRECIS Framework", Proceedings of IETF
                78, July 2010, <http://www.ietf.org/proceedings/78/
                slides/precis-2.pdf>.

   [ID-COMP]    Thaler, D., Ed., "Issues in Identifier Comparison for
                Security Purposes", Work in Progress, March 2013.

   [NEWPREP]    "Newprep BoF Meeting Minutes", March 2010,
                <http://www.ietf.org/proceedings/77/minutes/
                newprep.txt>.

   [RFC0952]    Harrenstien, K., Stahl, M., and E. Feinler, "DoD
                Internet host table specification", RFC 952,
                October 1985.

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

   [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.

   [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.

   [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
                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.

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

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

   [RFC6452]    Faltstrom, P. and P. Hoffman, "The Unicode Code Points
                and Internationalized Domain Names for Applications
                (IDNA) - Unicode 6.0", RFC 6452, November 2011.

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

   [Unicode61]  The Unicode Consortium.  The Unicode Standard, Version
                6.1.0, (Mountain View, CA: The Unicode Consortium, 2012.
                ISBN 978-1-936213-02-3).
                <http://www.unicode.org/versions/Unicode6.1.0/>.

Appendix A.  Classification of Stringprep Profiles

   A number of the known cases of Stringprep use were evaluated during
   the preparation of this document.  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 5.1.1) 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.

                         +------+--------+-------+
                         |  RFC | IDtype | User? |
                         +------+--------+-------+
                         | 3722 |    a   |   b   |
                         | 3748 |    -   |   -   |
                         | 3920 |   a,d  |   b   |
                         | 4505 |    a   |   i   |
                         | 4314 |   a,d  |   b   |
                         | 4954 |   a,d  |   b   |
                         | 5034 |   a,d  |   b   |
                         | 5804 |   a,d  |   b   |
                         +------+--------+-------+

                                  Table 1

Appendix B.  Evaluation of Stringprep Profiles

   This section is a summary of evaluation of Stringprep profiles that
   was done to get a good understanding of the usage of Stringprep.
   This summary is by no means normative nor the actual evaluations
   themselves.  A template was used for reviewers to get a coherent view
   of all evaluations.

B.1.  iSCSI Stringprep Profile: RFC 3720, RFC 3721, RFC 3722

   Description:  An iSCSI session consists of an initiator (i.e., host
      or server that uses storage) communicating with a target (i.e., a
      storage array or other system that provides storage).  Both the
      iSCSI initiator and target are named by iSCSI names.  The iSCSI
      Stringprep profile is used for iSCSI names.

   How it is used:  iSCSI initiators and targets (see above).  They can
      also be used to identify SCSI ports (these are software entities
      in the iSCSI protocol, not hardware ports) and iSCSI logical units
      (storage volumes), although both are unusual in practice.

   What entities create these identifiers?  Generally, a human user (1)
      configures an automated system (2) that generates the names.
      Advance configuration of the system is required due to the
      embedded use of external unique identifier (from the DNS or IEEE).

   How is the string input in the system?  Keyboard and copy-paste are
      common.  Copy-paste is common because iSCSI names are long enough
      to be problematic for humans to remember, causing use of email,
      sneaker-net, text files, etc., to avoid mistype mistakes.

   Where do we place the dividing line between user interface and
      protocol?  The iSCSI protocol requires that all
      internationalization string preparation occur in the user
      interface.  The iSCSI protocol treats iSCSI names as opaque
      identifiers that are compared byte-by-byte for equality. iSCSI
      names are generally not checked for correct formatting by the
      protocol.

   What entities enforce the rules?  There are no iSCSI-specific
      enforcement entities, although the use of unique identifier
      information in the names relies on DNS registrars and the IEEE
      Registration Authority.

   Comparison:  Byte-by-byte.

   Case Folding, Sensitivity, Preservation:  Case folding is required
      for the code blocks specified in RFC 3454, Table B.2.  The overall
      iSCSI naming system (UI + protocol) is case-insensitive.

   What is the impact if the comparison results in a false positive?
      Potential access to the wrong storage.

      -  If the initiator has no access to the wrong storage, an
         authentication failure is the probable result.

      -  If the initiator has access to the wrong storage, the resulting
         misidentification could result in use of the wrong data and
         possible corruption of stored data.

   What is the impact if the comparison results in a false negative?
      Denial of authorized storage access.

   What are the security impacts?  iSCSI names may be used as the
      authentication identities for storage systems.  Comparison
      problems could result in authentication problems, although note
      that authentication failure ameliorates some of the false positive
      cases.

   Normalization:  NFKC, as specified by RFC 3454.

   Mapping:  Yes, as specified by Table B.1 in RFC 3454.

   Disallowed Characters:  Only the following characters are allowed:
      -  ASCII dash, dot, colon
      -  ASCII lowercase letters and digits
      -  Unicode lowercase characters as specified by RFC 3454.
      All other characters are disallowed.

   Which other strings or identifiers are these most similar to?
      None -- iSCSI names are unique to iSCSI.

   Are these strings or identifiers sometimes the same as strings or
      identifiers from other protocols?  No.

   Does the identifier have internal structure that needs to be
      respected?  Yes. ASCII dot, dash, and colon are used for internal
      name structure.  These are not reserved characters, in that they
      can occur in the name in locations other than those used for
      structuring purposes (e.g., only the first occurrence of a colon
      character is structural, others are not).

   How are users exposed to these strings?  How are they published?
      iSCSI names appear in server and storage system configuration
      interfaces.  They also appear in system logs.

   Is the string / identifier used as input to other operations?
      Effectively, no.  The rarely used port and logical unit names
      involve concatenation, which effectively extends a unique iSCSI
      name for a target to uniquely identify something within that
      target.

   How much tolerance for change from existing Stringprep approach?
      Good tolerance; the community would prefer that
      internationalization experts solve internationalization problems.

   How strong a desire for change (e.g., for Unicode agility)?  Unicode
      agility is desired, in principle, as long as nothing significant
      breaks.

B.2.  SMTP/POP3/ManageSieve Stringprep Profiles: RFC 4954, RFC 5034,
      RFC 5804

   Description:  Authorization identity (user identifier) exchanged
      during SASL authentication: AUTH (SMTP/POP3) or AUTHENTICATE
      (ManageSieve) command.

   How It's Used:  Used for proxy authorization, e.g., to [lawfully]
      impersonate a particular user after a privileged authentication.

   Who Generates It:
      -  Typically generated by email system administrators using some
         tools/conventions, sometimes from some backend database.
      -  In some setups, human users can register their own usernames
         (e.g., webmail self-registration).

   User Input Methods:
      -  typing or selecting from a list
      -  copy and paste
      -  voice input
      -  in configuration files or on the command line

   Enforcement:  Rules enforced by server / add-on service (e.g.,
      gateway service) on registration of account.

   Comparison Method:  "Type 1" (byte-for-byte) or "Type 2" (compare by
      a common algorithm that everyone agrees on (e.g., normalize and
      then compare the result byte-by-byte).

   Case Folding, Sensitivity, Preservation:  Most likely case-sensitive.
      Exact requirements on case-sensitivity/case-preservation depend on
      a specific implementation, e.g., an implementation might treat all
      user identifiers as case-insensitive (or case-insensitive for
      US-ASCII subset only).

   Impact of Comparison:  False positives: an unauthorized user is
      allowed email service access (login).  False negatives: an
      authorized user is denied email service access.

   Normalization:  NFKC (as per RFC 4013).

   Mapping:  (see Section 2 of RFC 4013 for the full list) Non-ASCII
      spaces are mapped to space, etc.

   Disallowed Characters:  (see Section 2 of RFC 4013 for the full list)
      Unicode Control characters, etc.

   String Classes:  Simple username.  See Section 2 of RFC 4013 for
      details on restrictions.  Note that some implementations allow
      spaces in these.  While implementations are not required to use a
      specific format, an authorization identity frequently has the same
      format as an email address (and Email Address Internationalization
      (EAI) email address in the future), or as a left hand side of an
      email address.  Note: whatever is recommended for SMTP/POP/

      ManageSieve authorization identity should also be used for IMAP
      authorization identities, as IMAP/POP3/SMTP/ManageSieve are
      frequently implemented together.

   Internal Structure:  None

   User Output:  Unlikely, but possible.  For example, if it is the same
      as an email address.

   Operations:  Sometimes concatenated with other data and then used as
      input to a cryptographic hash function.

   How much tolerance for change from existing Stringprep approach?  Not
      sure.

   Background Information:
      In RFC 5034, when describing the POP3 AUTH command:

         The authorization identity generated by the SASL exchange is a
         simple username, and SHOULD use the SASLprep profile (see
         [RFC4013]) of the StringPrep algorithm (see [RFC3454]) to
         prepare these names for matching.  If preparation of the
         authorization identity fails or results in an empty string
         (unless it was transmitted as the empty string), the server
         MUST fail the authentication.

      In RFC 4954, when describing the SMTP AUTH command:

         The authorization identity generated by this [SASL] exchange is
         a "simple username" (in the sense defined in [SASLprep]), and
         both client and server SHOULD (*) use the [SASLprep] profile of
         the [StringPrep] algorithm to prepare these names for
         transmission or comparison.  If preparation of the
         authorization identity fails or results in an empty string
         (unless it was transmitted as the empty string), the server
         MUST fail the authentication.

         (*) Note: Future revision of this specification may change this
         requirement to MUST.  Currently, the SHOULD is used in order to
         avoid breaking the majority of existing implementations.

      In RFC 5804, when describing the ManageSieve AUTHENTICATE command:

         The authorization identity generated by this [SASL] exchange is
         a "simple username" (in the sense defined in [SASLprep]), and
         both client and server MUST use the [SASLprep] profile of the
         [StringPrep] algorithm to prepare these names for transmission
         or comparison.  If preparation of the authorization identity
         fails or results in an empty string (unless it was transmitted
         as the empty string), the server MUST fail the authentication.

B.3.  IMAP Stringprep Profiles for Usernames: RFC 4314, RFC 5738

   Evaluation Note:  These documents have 2 types of strings (usernames
      and passwords), so there are two separate templates.

   Description:  "username" parameter to the IMAP LOGIN command,
      identifiers in IMAP Access Control List (ACL) commands.  Note that
      any valid username is also an IMAP ACL identifier, but IMAP ACL
      identifiers can include other things like the name of a group of
      users.

   How It's Used:  Used for authentication (Usernames), or in IMAP
      Access Control Lists (Usernames or Group names).

   Who Generates It:
      -  Typically generated by email system administrators using some
         tools/conventions, sometimes from some backend database.
      -  In some setups, human users can register own usernames (e.g.,
         webmail self-registration).

   User Input Methods:
      -  typing or selecting from a list
      -  copy and paste
      -  voice input
      -  in configuration files or on the command line

   Enforcement:  Rules enforced by server / add-on service (e.g.,
      gateway service) on registration of account.

   Comparison Method:  "Type 1" (byte-for-byte) or "Type 2" (compare by
      a common algorithm that everyone agrees on (e.g., normalize and
      then compare the result byte-by-byte).

   Case Folding, Sensitivity, Preservation:  Most likely case-sensitive.
      Exact requirements on case-sensitivity/case-preservation depend on
      a specific implementation, e.g., an implementation might treat all
      user identifiers as case-insensitive (or case-insensitive for
      US-ASCII subset only).

   Impact of Comparison:  False positives: an unauthorized user is
      allowed IMAP access (login), privileges improperly granted (e.g.,
      access to a specific mailbox, ability to manage ACLs for a
      mailbox).  False negatives: an authorized user is denied IMAP
      access, unable to use granted privileges (e.g., access to a
      specific mailbox, ability to manage ACLs for a mailbox).

   Normalization:  NFKC (as per RFC 4013)

   Mapping:  (see Section 2 of RFC 4013 for the full list) Non-ASCII
      spaces are mapped to space.

   Disallowed Characters:  (see Section 2 of RFC 4013 for the full list)
      Unicode Control characters, etc.

   String Classes:  Simple username.  See Section 2 of RFC 4013 for
      details on restrictions.  Note that some implementations allow
      spaces in these.  While IMAP implementations are not required to
      use a specific format, an IMAP username frequently has the same
      format as an email address (and EAI email address in the future),
      or as a left hand side of an email address.  Note: whatever is
      recommended for the IMAP username should also be used for
      ManageSieve, POP3 and SMTP authorization identities, as IMAP/POP3/
      SMTP/ManageSieve are frequently implemented together.

   Internal Structure:  None.

   User Output:  Unlikely, but possible.  For example, if it is the same
      as an email address, access control lists (e.g. in IMAP ACL
      extension), both when managing membership and listing membership
      of existing access control lists.  Often shows up as mailbox names
      (under Other Users IMAP namespace).

   Operations:  Sometimes concatenated with other data and then used as
      input to a cryptographic hash function.

   How much tolerance for change from existing Stringprep approach?  Not
      sure.  Non-ASCII IMAP usernames are currently prohibited by IMAP
      (RFC 3501).  However, they are allowed when used in IMAP ACL
      extension.

B.4.  IMAP Stringprep Profiles for Passwords: RFC 5738

   Description:  "Password" parameter to the IMAP LOGIN command.

   How It's Used:  Used for authentication (Passwords).

   Who Generates It:  Either generated by email system administrators
      using some tools/conventions, or specified by the human user.

   User Input Methods:
      -  typing or selecting from a list
      -  copy and paste
      -  voice input
      -  in configuration files or on the command line

   Enforcement:  Rules enforced by server / add-on service (e.g.,
      gateway service or backend database) on registration of account.

   Comparison Method:  "Type 1" (byte-for-byte).

   Case Folding, Sensitivity, Preservation:  Most likely case-sensitive.

   Impact of Comparison:  False positives: an unauthorized user is
      allowed IMAP access (login).  False negatives: an authorized user
      is denied IMAP access.

   Normalization:  NFKC (as per RFC 4013).

   Mapping:  (see Section 2 of RFC 4013 for the full list) Non-ASCII
      spaces are mapped to space.

   Disallowed Characters:  (see Section 2 of RFC 4013 for the full list)
      Unicode Control characters, etc.

   String Classes:  Currently defined as "simple username" (see Section
      2 of RFC 4013 for details on restrictions); however, this is
      likely to be a different class from usernames.  Note that some
      implementations allow spaces in these.  Password in all email
      related protocols should be treated in the same way.  Same
      passwords are frequently shared with web, IM, and etc.
      applications.

   Internal Structure:  None.

   User Output:  Text of email messages (e.g. in "you forgot your
      password" email messages), web page / directory, side of the bus /
      in ads -- possible.

   Operations:  Sometimes concatenated with other data and then used as
      input to a cryptographic hash function.  Frequently stored as is,
      or hashed.

   How much tolerance for change from existing Stringprep approach?  Not
      sure.  Non-ASCII IMAP passwords are currently prohibited by IMAP
      (RFC 3501); however, they are likely to be in widespread use.

   Background Information:
      RFC 5738, Section 5 ("UTF8=USER Capability"):

         If the "UTF8=USER" capability is advertised, that indicates the
         server accepts UTF-8 user names and passwords and applies
         SASLprep [RFC4013] to both arguments of the LOGIN command.  The
         server MUST reject UTF-8 that fails to comply with the formal
         syntax in RFC 3629 [RFC3629] or if it encounters Unicode
         characters listed in Section 2.3 of SASLprep RFC 4013
         [RFC4013].

      RFC 4314, Section 3 ("Access control management commands and
      responses"):

         Servers, when processing a command that has an identifier as a
         parameter (i.e., any of SETACL, DELETEACL, and LISTRIGHTS
         commands), SHOULD first prepare the received identifier using
         "SASLprep" profile [SASLprep] of the "stringprep" algorithm
         [Stringprep].  If the preparation of the identifier fails or
         results in an empty string, the server MUST refuse to perform
         the command with a BAD response.  Note that Section 6
         recommends additional identifier's verification steps.

      RFC 4314, Section 6 ("Security Considerations"):

         This document relies on [SASLprep] to describe steps required
         to perform identifier canonicalization (preparation).  The
         preparation algorithm in SASLprep was specifically designed
         such that its output is canonical, and it is well-formed.
         However, due to an anomaly [PR29] in the specification of
         Unicode normalization, canonical equivalence is not guaranteed
         for a select few character sequences.  Identifiers prepared
         with SASLprep can be stored and returned by an ACL server.  The
         anomaly affects ACL manipulation and evaluation of identifiers
         containing the selected character sequences.  These sequences,
         however, do not appear in well-formed text.  In order to
         address this problem, an ACL server MAY reject identifiers
         containing sequences described in [PR29] by sending the tagged

         BAD response.  This is in addition to the requirement to reject
         identifiers that fail SASLprep preparation as described in
         Section 3.

B.5.  Anonymous SASL Stringprep Profiles: RFC 4505

   Description:  RFC 4505 defines a "trace" field:

   Comparison:  this field is not intended for comparison (only used for
      logging)

   Case folding; case-sensitivity, preserve case:  No case folding/
      case-sensitive

   Do users input the strings directly?  Yes. Possibly entered in
      configuration UIs, or on a command line.  Can also be stored in
      configuration files.  The value can also be automatically
      generated by clients (e.g., a fixed string is used, or a user's
      email address).

   How users input strings?  Keyboard/voice, stylus (pick from a list).
      Copy-paste - possibly.

   Normalization:  None.

   Disallowed Characters:  Control characters are disallowed.  (See
      Section 3 of RFC 4505).

   Which other strings or identifiers are these most similar to?
      RFC 4505 says that the trace "should take one of two forms: an
      Internet email address, or an opaque string that does not contain
      the '@' (U+0040) character and that can be interpreted by the
      system administrator of the client's domain".  In practice, this
      is a free-form text, so it belongs to a different class from
      "email address" or "username".

   Are these strings or identifiers sometimes the same as strings or
      identifiers from other protocols (e.g., does an IM system
      sometimes use the same credentials database for authentication as
      an email system)?  Yes: see above.  However, there is no strong
      need to keep them consistent in the future.

   How are users exposed to these strings, how are they published?  No.
      However, the value can be seen in server logs.

   Impacts of false positives and false negatives:
      False positive: a user can be confused with another user.
      False negative: two distinct users are treated as the same user.
      But note that the trace field is not authenticated, so it can be
      easily falsified.

   Tolerance of changes in the community:  The community would be
      flexible.

   Delimiters:  No internal structure, but see comments above about
      frequent use of email addresses.

   Background Information:
      RFC 4505, Section 2 ("The Anonymous Mechanism"):

      The mechanism consists of a single message from the client to the
      server.  The client may include in this message trace information
      in the form of a string of [UTF-8]-encoded [Unicode] characters
      prepared in accordance with [StringPrep] and the "trace"
      stringprep profile defined in Section 3 of this document.  The
      trace information, which has no semantical value, should take one
      of two forms: an Internet email address, or an opaque string that
      does not contain the '@' (U+0040) character and that can be
      interpreted by the system administrator of the client's domain.
      For privacy reasons, an Internet email address or other
      information identifying the user should only be used with
      permission from the user.

      RFC 4505, Section 3 ('The "trace" Profile of "Stringprep"'):
      This section defines the "trace" profile of [StringPrep].  This
      profile is designed for use with the SASL ANONYMOUS Mechanism.
      Specifically, the client is to prepare the <message> production in
      accordance with this profile.

      The character repertoire of this profile is Unicode 3.2 [Unicode].

      No mapping is required by this profile.

      No Unicode normalization is required by this profile.

      The list of unassigned code points for this profile is that
      provided in Appendix A of [StringPrep].  Unassigned code points
      are not prohibited.

      Characters from the following tables of [StringPrep] are
      prohibited:

         - C.2.1 (ASCII control characters)
         - C.2.2 (Non-ASCII control characters)
         - C.3 (Private use characters)
         - C.4 (Non-character code points)
         - C.5 (Surrogate codes)
         - C.6 (Inappropriate for plain text)
         - C.8 (Change display properties are deprecated)
         - C.9 (Tagging characters)

   No additional characters are prohibited.

   This profile requires bidirectional character checking per Section 6
   of [StringPrep].

B.6.  XMPP Stringprep Profiles for Nodeprep: RFC 3920

   Description:  Localpart of JabberID ("JID"), as in:
      localpart@domainpart/resourcepart

   How It's Used:
      -  Usernames (e.g., stpeter@jabber.org)
      -  Chatroom names (e.g., precis@jabber.ietf.org)
      -  Publish-subscribe nodes
      -  Bot names

   Who Generates It:
      -  Typically, end users via an XMPP client
      -  Sometimes created in an automated fashion

   User Input Methods:
      -  typing
      -  copy and paste
      -  voice input
      -  clicking a URI/IRI

   Enforcement:  Rules enforced by server / add-on service (e.g.,
      chatroom service) on registration of account, creation of room,
      etc.

   Comparison Method:  "Type 2" (common algorithm)

   Case Folding, Sensitivity, Preservation:
      -  Strings are always folded to lowercase
      -  Case is not preserved

   Impact of Comparison:
      False positives:
      -  unable to authenticate at server (or authenticate to wrong
         account)
      -  add wrong person to buddy list
      -  join the wrong chatroom
      -  improperly grant privileges (e.g., chatroom admin)
      -  subscribe to wrong pubsub node
      -  interact with wrong bot
      -  allow communication with blocked entity

      False negatives:
      -  unable to authenticate
      -  unable to add someone to buddy list
      -  unable to join desired chatroom
      -  unable to use granted privileges (e.g., chatroom admin)
      -  unable to subscribe to desired pubsub node
      -  unable to interact with desired bot
      -  disallow communication with unblocked entity

   Normalization:  NFKC

   Mapping:  Spaces are mapped to nothing

   Disallowed Characters:  ",&,',/,:,<,>,@

   String Classes:
      -  Often similar to generic username
      -  Often similar to localpart of email address
      -  Sometimes same as localpart of email address

   Internal Structure:  None

   User Output:
      -  vCard
      -  email signature
      -  web page / directory
      -  text of message (e.g., in a chatroom)

   Operations:  Sometimes concatenated with other data and then used as
      input to a cryptographic hash function

B.7.  XMPP Stringprep Profiles for Resourceprep: RFC 3920

   Description:
      -  Resourcepart of JabberID ("JID"), as in:
         localpart@domainpart/resourcepart
      -  Typically free-form text

   How It's Used:
      -  Device / session names (e.g., stpeter@jabber.org/Home)
      -  Nicknames (e.g., precis@jabber.ietf.org/StPeter)

   Who Generates It:
      -  Often human users via an XMPP client
      -  Often generated in an automated fashion by client or server

   User Input Methods:
      -  typing
      -  copy and paste
      -  voice input
      -  clicking a URI/IRI

   Enforcement:  Rules enforced by server / add-on service (e.g.,
      chatroom service) on account login, joining a chatroom, etc.

   Comparison Method:  "Type 2" (byte-for-byte)

   Case Folding, Sensitivity, Preservation:
      -  Strings are never folded
      -  Case is preserved

   Impact of Comparison:
      False positives:
      -  interact with wrong device (e.g., for file transfer or voice
         call)
      -  interact with wrong chatroom participant
      -  improperly grant privileges (e.g., chatroom moderator)
      -  allow communication with blocked entity
      False negatives:
      -  unable to choose desired chatroom nickname
      -  unable to use granted privileges (e.g., chatroom moderator)
      -  disallow communication with unblocked entity

   Normalization:  NFKC

   Mapping:  Spaces are mapped to nothing

   Disallowed Characters:  None

   String Classes:  Basically a free-form identifier

   Internal Structure:  None

   User Output:
      -  text of message (e.g., in a chatroom)
      -  device names often not exposed to human users

   Operations:  Sometimes concatenated with other data and then used as
      input to a cryptographic hash function

B.8.  EAP Stringprep Profiles: RFC 3748

   Description:  RFC 3748, Section 5, references Stringprep, but the WG
      did not agree with the text (was added by IESG) and there are no
      known implementations that use Stringprep.  The main problem with
      that text is that the use of strings is a per-method concept, not
      a generic EAP concept and so RFC 3748 itself does not really use
      Stringprep, but individual EAP methods could.  As such, the
      answers to the template questions are mostly not applicable, but a
      few answers are universal across methods.  The list of IANA
      registered EAP methods is at
      <http://www.iana.org/assignments/eap-numbers/eap-numbers.xml>.

   Comparison Methods:  n/a (per-method)

   Case Folding, Case-Sensitivity, Case Preservation:  n/a (per-method)

   Impact of comparison:  A false positive results in unauthorized
      network access (and possibly theft of service if some else is
      billed).  A false negative results in lack of authorized network
      access (no connectivity).

   User input:  n/a (per-method)

   Normalization:  n/a (per-method)

   Mapping:  n/a (per-method)

   Disallowed characters:  n/a (per-method)

   String classes:  Although some EAP methods may use a syntax similar
      to other types of identifiers, EAP mandates that the actual values
      must not be assumed to be identifiers usable with anything else.

   Internal structure:  n/a (per-method)

   User output:  Identifiers are never human displayed except perhaps as
      they're typed by a human.

   Operations:  n/a (per-method)

   Community considerations:  There is no resistance to change for the
      base EAP protocol (as noted, the WG didn't want the existing
      text).  However, actual use of Stringprep, if any, within specific
      EAP methods may have resistance.  It is currently unknown whether
      any EAP methods use Stringprep.

Authors' Addresses

   Marc Blanchet
   Viagenie
   246 Aberdeen
   Quebec, QC  G1R 2E1
   Canada

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

   Andrew Sullivan
   Dyn, Inc.
   150 Dow St
   Manchester, NH  03101
   U.S.A.

   EMail: asullivan@dyn.com

 

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