Internet Engineering Task Force (IETF) P. Saint-Andre
Request for Comments: 8266 Jabber.org
Obsoletes: 7700 September 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
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Nickname Profile . . . . . . . . . . . . . . . . . . . . . . 3 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, e.g., such as:
o Internet Relay Chat (IRC) [RFC2811] and multi-party chat
technologies based on the Extensible Messaging and Presence Protocol
(XMPP) [RFC6120] [XEP-0045], the
o The Message Session Relay Protocol (MSRP) [RFC4975] [RFC7701], and [RFC7701]
o Centralized Conferencing (XCON) [RFC5239] [XCON-SYSTEM]. [XCON-SYSTEM]
o The Extensible Messaging and Presence Protocol (XMPP) [RFC6120]
[XEP-0045]
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.
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
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).
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).
+---------------------------+-------------------------------------+
| # | 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).
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.
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.
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>.
[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, September 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>.
[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, May
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].
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/