Preparation, Enforcement, and Comparison of Internationalized Strings Representing Nicknames
RFC 8266



Internet Engineering Task Force (IETF)                    P. Saint-Andre
Request for Comments: 8266                                    Jabber.org
Obsoletes: 7700                                             October 2017
Category: Standards Track
ISSN: 2070-1721

 Preparation, Enforcement, and Comparison of Internationalized Strings
                         Representing Nicknames

Abstract

   This document describes methods for handling Unicode strings
   representing memorable, human-friendly names (called "nicknames",
   "display names", or "petnames") for people, devices, accounts,
   websites, and other entities.  This document obsoletes RFC 7700.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8266.

Copyright Notice

   Copyright (c) 2017 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
   (https://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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Nickname Profile  . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Rules . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Preparation . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Enforcement . . . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Comparison  . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Use in Application Protocols  . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
     6.1.  Authentication and Authorization  . . . . . . . . . . . .   9
     6.2.  Reuse of PRECIS . . . . . . . . . . . . . . . . . . . . .  10
     6.3.  Reuse of Unicode  . . . . . . . . . . . . . . . . . . . .  10
     6.4.  Visually Similar Characters . . . . . . . . . . . . . . .  10
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Changes from RFC 7700  . . . . . . . . . . . . . . .  12
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

1.1.  Overview

   A number of technologies and applications provide the ability for a
   person to choose a memorable, human-friendly name in a communications
   context or to set such a name for another entity such as a device,
   account, contact, or website.  Such names are variously called
   "nicknames" (e.g., in chat room applications), "display names" (e.g.,
   in Internet mail), or "petnames" (see [PETNAME-SYSTEMS]); for
   consistency, these are all called "nicknames" in this document.

   Nicknames are commonly supported in technologies for textual chat
   rooms, such as:

   o  Internet Relay Chat (IRC) [RFC2811]

   o  The Message Session Relay Protocol (MSRP) [RFC4975] [RFC7701]

   o  Centralized Conferencing (XCON) [RFC5239] [XCON-SYSTEM]

   o  The Extensible Messaging and Presence Protocol (XMPP) [RFC6120]
      [XEP-0045]

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   Recent chat room technologies also allow internationalized nicknames
   because they support code points from outside the ASCII range
   [RFC20], typically by means of the Unicode coded character set
   [Unicode].  Although such nicknames tend to be used primarily for
   display purposes, they are sometimes used for programmatic purposes
   as well (e.g., kicking users out of a chat room or avoiding nickname
   conflicts).

   A similar usage enables a person to set their own preferred display
   name or to set a preferred display name for another user (e.g., the
   "display-name" construct in the Internet message format [RFC5322] and
   the <nick/> element in XMPP [XEP-0172]).

   Memorable, human-friendly names are also used in contexts other than
   personal messaging, such as names for devices (e.g., in a network
   visualization application), websites (e.g., for bookmarks in a web
   browser), accounts (e.g., in a web interface for a list of payees in
   a bank account), people (e.g., in a contact list application), and
   the like.

   The rules specified in this document can be applied in all of the
   foregoing contexts.

   It is important to understand that a nickname is a personally
   memorable name or handle for something that has a more stable,
   underlying identity, such as a URI or a file path.  To ensure secure
   operation of applications that use nicknames, authentication and
   authorization decisions MUST be made on the basis of the thing's
   identity, not its nickname.

   To increase the likelihood that memorable, human-friendly names will
   work in ways that make sense for typical users throughout the world,
   this document defines rules for handling nicknames in terms of the
   preparation, enforcement, and comparison of internationalized strings
   (PRECIS) framework specification [RFC8264].

1.2.  Terminology

   Many important terms used in this document are defined in [RFC8264],
   [RFC6365], and [Unicode].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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2.  Nickname Profile

2.1.  Rules

   The following rules apply within the Nickname profile of the PRECIS
   FreeformClass defined in the PRECIS framework specification
   [RFC8264].

   1.  Width Mapping Rule: There is no width mapping rule (such a rule
       is not necessary because width mapping is performed as part of
       normalization using Normalization Form KC (NFKC) as specified
       below).

   2.  Additional Mapping Rule: The additional mapping rule consists of
       the following sub-rules.

       a.  Map any instances of non-ASCII space to SPACE (U+0020); a
           non-ASCII space is any Unicode code point having a general
           category of "Zs", naturally with the exception of SPACE
           (U+0020).  (The inclusion of only ASCII space prevents
           confusion with various non-ASCII space code points, many of
           which are difficult to reproduce across different input
           methods.)

       b.  Remove any instances of the ASCII space character at the
           beginning or end of a nickname (e.g., "stpeter " is mapped to
           "stpeter").

       c.  Map interior sequences of more than one ASCII space character
           to a single ASCII space character (e.g., "St  Peter" is
           mapped to "St Peter").

   3.  Case Mapping Rule: Apply the Unicode toLowerCase() operation, as
       defined in the Unicode Standard [Unicode].  In applications that
       prohibit conflicting nicknames, this rule helps to reduce the
       possibility of confusion by ensuring that nicknames differing
       only by case (e.g., "stpeter" vs. "StPeter") would not be
       presented to a human user at the same time.  (As explained below,
       this is typically appropriate only for comparison, not for
       enforcement.)

   4.  Normalization Rule: Apply Unicode Normalization Form KC.  Because
       NFKC is more "aggressive" in finding matches than other
       normalization forms (in the terminology of Unicode, it performs
       both canonical and compatibility decomposition before recomposing
       code points), this rule helps to reduce the possibility of
       confusion by increasing the number of code points that would

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       match; for example, the character "Ⅳ" (ROMAN NUMERAL FOUR,
       U+2163) would match the combination of "I" (LATIN CAPITAL LETTER
       I, U+0049) and "V" (LATIN CAPITAL LETTER V, U+0056).

   5.  Directionality Rule: There is no directionality rule.  The "Bidi
       Rule" (defined in [RFC5893]) and similar rules are unnecessary
       and inapplicable to nicknames, because it is perfectly acceptable
       for a given nickname to be presented differently in different
       layout systems (e.g., a user interface that is configured to
       handle primarily a right-to-left script versus an interface that
       is configured to handle primarily a left-to-right script), as
       long as the presentation is consistent in any given layout
       system.

   Implementation experience has shown that applying the rules for the
   Nickname profile is not an idempotent procedure for all code points.
   Therefore, an implementation SHOULD apply the rules repeatedly until
   the output string is stable; if the output string does not stabilize
   after reapplying the rules three (3) additional times after the first
   application, the implementation SHOULD terminate application of the
   rules and reject the input string as invalid.

2.2.  Preparation

   An entity that prepares an input string for subsequent enforcement
   according to this profile MUST ensure that the string consists only
   of Unicode code points that conform to the FreeformClass string class
   defined in [RFC8264].

2.3.  Enforcement

   An entity that performs enforcement according to this profile MUST
   prepare an input string as described in Section 2.2 and MUST also
   apply the following rules specified in Section 2.1 in the order
   shown:

   1.  Additional Mapping Rule
   2.  Normalization Rule

   Note: An entity SHOULD apply the Case Mapping Rule only during
   comparison.

   After all of the foregoing rules have been enforced, the entity MUST
   ensure that the nickname is not zero bytes in length (this is done
   after enforcing the rules to prevent applications from mistakenly
   omitting a nickname entirely, because when internationalized strings
   are accepted a non-empty sequence of characters can result in a zero-
   length nickname after canonicalization).

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   The result of the foregoing operations is an output string that
   conforms to the Nickname profile.  Until an implementation produces
   such an output string, it MUST NOT treat the string as conforming (in
   particular, it MUST NOT assume that an input string is conforming
   before the enforcement operation has been completed).

2.4.  Comparison

   An entity that performs comparison of two strings according to this
   profile MUST prepare each input string as specified in Section 2.2
   and MUST apply the following rules specified in Section 2.1 in the
   order shown:

   1.  Additional Mapping Rule
   2.  Case Mapping Rule
   3.  Normalization Rule

   The two strings are to be considered equivalent if and only if they
   are an exact octet-for-octet match (sometimes called "bit-string
   identity").

   Until an implementation determines whether two strings are to be
   considered equivalent, it MUST NOT treat them as equivalent (in
   particular, it MUST NOT assume that two input strings are equivalent
   before the comparison operation has been completed).

3.  Examples

   The following examples illustrate a small number of nicknames that
   are consistent with the format defined above, along with the output
   string resulting from application of the PRECIS rules for comparison
   purposes (note that the characters "<" and ">" are used to delineate
   the actual nickname and are not part of the nickname strings).

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    +---------------------------+-------------------------------------+
    | #  | Nickname             | Output for Comparison               |
    +---------------------------+-------------------------------------+
    | 1  | <Foo>                | <foo>                               |
    +---------------------------+-------------------------------------+
    | 2  | <foo>                | <foo>                               |
    +---------------------------+-------------------------------------+
    | 3  | <Foo Bar>            | <foo bar>                           |
    +---------------------------+-------------------------------------+
    | 4  | <foo bar>            | <foo bar>                           |
    +---------------------------+-------------------------------------+
    | 5  | <Σ>                  | σ (GREEK SMALL LETTER SIGMA,        |
    |    |                      | U+03C3)                             |
    +---------------------------+-------------------------------------+
    | 6  | <σ>                  | σ (GREEK SMALL LETTER SIGMA,        |
    |    |                      | U+03C3)                             |
    +---------------------------+-------------------------------------+
    | 7  | <ς>                  | ς (GREEK SMALL LETTER FINAL SIGMA,  |
    |    |                      | U+03C2)                             |
    +---------------------------+-------------------------------------+
    | 8  | <ϔ>                  | ϋ (GREEK SMALL LETTER UPSILON       |
    |    |                      | WITH DIALYTIKA, U+03CB)             |
    +---------------------------+-------------------------------------+
    | 9  | <∞>                  | ∞ (INFINITY, U+221E)                |
    +---------------------------+-------------------------------------+
    | 10 | <Richard Ⅳ>         | <richard iv>                        |
    +---------------------------+-------------------------------------+

                   Table 1: A Sample of Legal Nicknames

   Regarding examples 5, 6, and 7: applying the Unicode toLowerCase()
   operation to the character "Σ" (GREEK CAPITAL LETTER SIGMA, U+03A3)
   results in the character "σ" (GREEK SMALL LETTER SIGMA, U+03C3);
   however, the toLowerCase() operation does not modify the character
   "ς" (GREEK SMALL LETTER FINAL SIGMA, U+03C2).  Therefore, the
   comparison operation defined in Section 2.4 would result in matching
   of the nicknames in examples 5 and 6 but not the nicknames in
   examples 5 and 7 or 6 and 7.

   Regarding example 8: this is an instance where applying the rules for
   the Nickname profile is not an idempotent procedure (see
   Section 2.1).  In particular:

   1.  Applying toLowerCase() to the character "ϔ" (GREEK UPSILON WITH
       DIARESIS AND HOOK SYMBOL, U+03D4) results in no changes, and
       applying NFKC to that character results in the character "Ϋ"
       (GREEK CAPITAL LETTER UPSILON WITH DIALYTIKA, U+03AB).

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   2.  Applying toLowerCase() to "Ϋ" (GREEK CAPITAL LETTER UPSILON WITH
       DIALYTIKA, U+03AB) results in the character "ϋ" (GREEK SMALL
       LETTER UPSILON WITH DIALYTIKA, U+03CB), and applying NFKC to that
       character results in no changes.

   Regarding example 9: symbol characters such as "∞" (INFINITY, U+221E)
   are allowed by the PRECIS FreeformClass and thus can be used in
   nicknames.

   Regarding example 10: applying the Unicode toLowerCase() operation to
   the character "Ⅳ" (ROMAN NUMERAL FOUR, U+2163) results in the
   character "ⅳ" (SMALL ROMAN NUMERAL FOUR, U+2173), and applying NFKC
   to the character "ⅳ" (SMALL ROMAN NUMERAL FOUR, U+2173) results in
   the characters "i" (LATIN SMALL LETTER I, U+0069) and "v" (LATIN
   SMALL LETTER V, U+0076).

4.  Use in Application Protocols

   This specification defines only the PRECIS-based rules for handling
   of nickname strings.  It is the responsibility of an application
   protocol (e.g., MSRP, XCON, or XMPP) or application definition to
   specify the protocol slots in which nickname strings can appear, the
   entities that are expected to enforce the rules governing nickname
   strings, and the point during protocol processing or interface
   handling when the rules need to be enforced.  See Section 6 of
   [RFC8264] for guidelines about using PRECIS profiles in applications.

   Above and beyond the PRECIS-based rules specified here, application
   protocols can also define application-specific rules governing
   nickname strings (rules regarding the minimum or maximum length of
   nicknames, further restrictions on allowable code points or character
   ranges, safeguards to mitigate the effects of visually similar
   characters, etc.).

   Naturally, application protocols can also specify rules governing the
   actual use of nicknames in applications (reserved nicknames,
   authorization requirements for using nicknames, whether certain
   nicknames can be prohibited, handling of duplicates, the relationship
   between nicknames and underlying identifiers such as SIP URIs or
   Jabber IDs, etc.).

   Entities that enforce the rules specified in this document are
   encouraged to be liberal in what they accept by following this
   procedure:

   1.  Where possible, map characters (e.g., through width mapping,
       additional mapping, case mapping, or normalization) and accept
       the mapped string.

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   2.  If mapping is not possible (e.g., because a character is
       disallowed in the FreeformClass), reject the string.

5.  IANA Considerations

   IANA has added the following entry to the "PRECIS Profiles" registry:

   Name:  Nickname.

   Base Class:  FreeformClass.

   Applicability:  Nicknames or display names in messaging and text
      conferencing technologies; petnames for devices, accounts, and
      people; and other uses of nicknames, display names, or petnames.

   Replaces:  None.

   Width Mapping Rule:  None (handled via NFKC).

   Additional Mapping Rule:  Map non-ASCII space characters to SPACE
      (U+0020), strip leading and trailing space characters, and map
      interior sequences of multiple space characters to a single
      instance of SPACE (U+0020).

   Case Mapping Rule:  Map uppercase and titlecase code points to
      lowercase using the Unicode toLowerCase() operation.

   Normalization Rule:  NFKC.

   Directionality Rule:  None.

   Enforcement:  To be specified by applications.

   Specification:  RFC 8266.

6.  Security Considerations

6.1.  Authentication and Authorization

   It is important to understand that a nickname is a personally
   memorable name or handle for something that has a more stable,
   underlying identity, such as a URI or a file path.  To ensure secure
   operation of applications that use nicknames, authentication and
   authorization decisions MUST be made on the basis of the thing's
   identity, not its nickname.

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6.2.  Reuse of PRECIS

   The security considerations described in [RFC8264] apply to the
   FreeformClass string class used in this document for nicknames.

6.3.  Reuse of Unicode

   The security considerations described in [UTS39] apply to the use of
   Unicode code points in nicknames.

6.4.  Visually Similar Characters

   [RFC8264] describes some of the security considerations related to
   visually similar characters, also called "confusable characters" or
   "confusables", and provides some examples of such characters.

   Although the mapping rules defined in Section 2 of this document are
   designed, in part, to reduce the possibility of confusion about
   nicknames, this document does not provide more-detailed
   recommendations regarding the handling of visually similar
   characters, such as those provided in [UTS39].

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5893]  Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts
              for Internationalized Domain Names for Applications
              (IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010,
              <https://www.rfc-editor.org/info/rfc5893>.

   [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in
              Internationalization in the IETF", BCP 166, RFC 6365,
              DOI 10.17487/RFC6365, September 2011,
              <https://www.rfc-editor.org/info/rfc6365>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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   [RFC8264]  Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
              Preparation, Enforcement, and Comparison of
              Internationalized Strings in Application Protocols",
              RFC 8264, DOI 10.17487/RFC8264, October 2017,
              <https://www.rfc-editor.org/info/rfc8264>.

   [Unicode]  The Unicode Consortium, "The Unicode Standard",
              <http://www.unicode.org/versions/latest/>.

   [UTS39]    Unicode Technical Standard #39, "Unicode Security
              Mechanisms", edited by Mark Davis and Michel Suignard,
              <http://unicode.org/reports/tr39/>.

7.2.  Informative References

   [Err4570]  RFC Errata, Erratum ID 4570, RFC 7700,
              <https://www.rfc-editor.org/errata/eid4570>.

   [PETNAME-SYSTEMS]
              Stiegler, M., "An Introduction to Petname Systems",
              updated June 2010, February 2005,
              <http://www.skyhunter.com/marcs/petnames/
              IntroPetNames.html>.

   [RFC20]    Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <https://www.rfc-editor.org/info/rfc20>.

   [RFC2811]  Kalt, C., "Internet Relay Chat: Channel Management",
              RFC 2811, DOI 10.17487/RFC2811, April 2000,
              <https://www.rfc-editor.org/info/rfc2811>.

   [RFC4975]  Campbell, B., Ed., Mahy, R., Ed., and C. Jennings, Ed.,
              "The Message Session Relay Protocol (MSRP)", RFC 4975,
              DOI 10.17487/RFC4975, September 2007,
              <https://www.rfc-editor.org/info/rfc4975>.

   [RFC5239]  Barnes, M., Boulton, C., and O. Levin, "A Framework for
              Centralized Conferencing", RFC 5239, DOI 10.17487/RFC5239,
              June 2008, <https://www.rfc-editor.org/info/rfc5239>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
              March 2011, <https://www.rfc-editor.org/info/rfc6120>.

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   [RFC7700]  Saint-Andre, P., "Preparation, Enforcement, and Comparison
              of Internationalized Strings Representing Nicknames",
              RFC 7700, DOI 10.17487/RFC7700, December 2015,
              <https://www.rfc-editor.org/info/rfc7700>.

   [RFC7701]  Niemi, A., Garcia-Martin, M., and G. Sandbakken, "Multi-
              party Chat Using the Message Session Relay Protocol
              (MSRP)", RFC 7701, DOI 10.17487/RFC7701, December 2015,
              <https://www.rfc-editor.org/info/rfc7701>.

   [XCON-SYSTEM]
              Barnes, M., Boulton, C., and S. Loreto, "Chatrooms within
              a Centralized Conferencing (XCON) System", Work in
              Progress, draft-boulton-xcon-session-chat-08, July 2012.

   [XEP-0045]
              Saint-Andre, P., "Multi-User Chat", XSF XEP 0045,
              September 2017,
              <https://xmpp.org/extensions/xep-0045.html>.

   [XEP-0172]
              Saint-Andre, P. and V. Mercier, "User Nickname", XSF
              XEP 0172, March 2012,
              <https://xmpp.org/extensions/xep-0172.html>.

Appendix A.  Changes from RFC 7700

   The following changes were made from [RFC7700].

   o  Addressed [Err4570] by removing the directionality rule from
      Sections 2.3 and 2.4.

   o  In accordance with working group discussions and updates to
      [RFC8264], removed the use of the Unicode toCaseFold() operation
      in favor of the Unicode toLowerCase() operation.

   o  Clarified several editorial matters.

   o  Updated references.

Acknowledgements

   Thanks to William Fisher for his implementation feedback, especially
   regarding idempotence.

   Thanks to Sam Whited for his feedback and for submitting [Err4570].

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   See [RFC7700] for acknowledgements related to the specification that
   this document supersedes.

Author's Address

   Peter Saint-Andre
   Jabber.org
   P.O. Box 787
   Parker, CO  80134
   United States of America

   Phone: +1 720 256 6756
   Email: stpeter@jabber.org
   URI:   https://www.jabber.org/

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