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rfc:rfc6885

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.

Blanchet & Sullivan Informational [Page 1] RFC 6885 Stringprep Revision Problem Statement March 2013

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.

Blanchet & Sullivan Informational [Page 2] RFC 6885 Stringprep Revision Problem Statement March 2013

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

Blanchet & Sullivan Informational [Page 3] RFC 6885 Stringprep Revision Problem Statement March 2013

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];

Blanchet & Sullivan Informational [Page 4] RFC 6885 Stringprep Revision Problem Statement March 2013

 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.

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 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.

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 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

Blanchet & Sullivan Informational [Page 7] RFC 6885 Stringprep Revision Problem Statement March 2013

 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.

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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.

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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.

Blanchet & Sullivan Informational [Page 10] RFC 6885 Stringprep Revision Problem Statement March 2013

 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])

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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.

Blanchet & Sullivan Informational [Page 12] RFC 6885 Stringprep Revision Problem Statement March 2013

 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.

Blanchet & Sullivan Informational [Page 13] RFC 6885 Stringprep Revision Problem Statement March 2013

 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.

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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.

Blanchet & Sullivan Informational [Page 15] RFC 6885 Stringprep Revision Problem Statement March 2013

 [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.

Blanchet & Sullivan Informational [Page 16] RFC 6885 Stringprep Revision Problem Statement March 2013

 [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.

Blanchet & Sullivan Informational [Page 17] RFC 6885 Stringprep Revision Problem Statement March 2013

 [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/>.

Blanchet & Sullivan Informational [Page 18] RFC 6885 Stringprep Revision Problem Statement March 2013

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.

Blanchet & Sullivan Informational [Page 19] RFC 6885 Stringprep Revision Problem Statement March 2013

 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.
  1. If the initiator has no access to the wrong storage, an

authentication failure is the probable result.

  1. 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.

Blanchet & Sullivan Informational [Page 20] RFC 6885 Stringprep Revision Problem Statement March 2013

 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.

Blanchet & Sullivan Informational [Page 21] RFC 6885 Stringprep Revision Problem Statement March 2013

 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/

Blanchet & Sullivan Informational [Page 22] RFC 6885 Stringprep Revision Problem Statement March 2013

    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.

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    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).

Blanchet & Sullivan Informational [Page 24] RFC 6885 Stringprep Revision Problem Statement March 2013

 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.

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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.

Blanchet & Sullivan Informational [Page 26] RFC 6885 Stringprep Revision Problem Statement March 2013

 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

Blanchet & Sullivan Informational [Page 27] RFC 6885 Stringprep Revision Problem Statement March 2013

       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.

Blanchet & Sullivan Informational [Page 28] RFC 6885 Stringprep Revision Problem Statement March 2013

 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.

Blanchet & Sullivan Informational [Page 29] RFC 6885 Stringprep Revision Problem Statement March 2013

    Characters from the following tables of [StringPrep] are
    prohibited:
  1. C.2.1 (ASCII control characters)
  2. C.2.2 (Non-ASCII control characters)
  3. C.3 (Private use characters)
  4. C.4 (Non-character code points)
  5. C.5 (Surrogate codes)
  6. C.6 (Inappropriate for plain text)
  7. C.8 (Change display properties are deprecated)
  8. 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

Blanchet & Sullivan Informational [Page 30] RFC 6885 Stringprep Revision Problem Statement March 2013

 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

Blanchet & Sullivan Informational [Page 31] RFC 6885 Stringprep Revision Problem Statement March 2013

 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

Blanchet & Sullivan Informational [Page 32] RFC 6885 Stringprep Revision Problem Statement March 2013

 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)

Blanchet & Sullivan Informational [Page 33] RFC 6885 Stringprep Revision Problem Statement March 2013

 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

Blanchet & Sullivan Informational [Page 34]

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