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rfc:rfc8266



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.

Saint-Andre Standards Track [Page 1] RFC 8266 PRECIS: Nicknames October 2017

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]

Saint-Andre Standards Track [Page 2] RFC 8266 PRECIS: Nicknames October 2017

 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.

Saint-Andre Standards Track [Page 3] RFC 8266 PRECIS: Nicknames October 2017

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

Saint-Andre Standards Track [Page 4] RFC 8266 PRECIS: Nicknames October 2017

     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).

Saint-Andre Standards Track [Page 5] RFC 8266 PRECIS: Nicknames October 2017

 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).

Saint-Andre Standards Track [Page 6] RFC 8266 PRECIS: Nicknames October 2017

  +---------------------------+-------------------------------------+
  | #  | 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).

Saint-Andre Standards Track [Page 7] RFC 8266 PRECIS: Nicknames October 2017

 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.

Saint-Andre Standards Track [Page 8] RFC 8266 PRECIS: Nicknames October 2017

 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.

Saint-Andre Standards Track [Page 9] RFC 8266 PRECIS: Nicknames October 2017

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>.

Saint-Andre Standards Track [Page 10] RFC 8266 PRECIS: Nicknames October 2017

 [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>.

Saint-Andre Standards Track [Page 11] RFC 8266 PRECIS: Nicknames October 2017

 [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].

Saint-Andre Standards Track [Page 12] RFC 8266 PRECIS: Nicknames October 2017

 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/

Saint-Andre Standards Track [Page 13]

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