Preparation, Enforcement, and Comparison of Internationalized Strings Representing Nicknames
draft-ietf-precis-7700bis-06

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

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This Internet-Draft will expire on September 28, 2017.

Copyright Notice

Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

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

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., Internet Relay Chat [RFC2811] and multi-party chat technologies based on the Extensible Messaging and Presence Protocol (XMPP) [RFC6120] [XEP-0045], the Message Session Relay Protocol (MSRP) [RFC4975] [RFC7701], and Centralized Conferencing (XCON) [RFC5239] [XCON-SYSTEM]. 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 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 [XEP-0172] in XMPP).

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.

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 preparing, enforcing, and comparing internationalized nicknames.

1.2. Terminology

Many important terms used in this document are defined in [RFC7564], [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 [RFC2119].

2. Nickname Profile

2.1. Rules

The following rules apply within the Nickname profile of the PRECIS FreeformClass.

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.
   1. Map any instances of non-ASCII space to ASCII space (U+0020); a non-ASCII space is any Unicode code point having a general category of "Zs", naturally with the exception of U+0020.
   2. Remove any instances of the ASCII space character at the beginning or end of a nickname (e.g., "stpeter " is mapped to "stpeter").
   3. 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 toLower() 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 (e.g., U+2163 ROMAN NUMERAL FOUR would match the combination of U+0049 LATIN CAPITAL LETTER I and U+0056 LATIN CAPITAL LETTER V).
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.

2.2. Preparation

An entity that prepares a 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 [RFC7564].

2.3. Enforcement

An entity that performs enforcement according to this profile MUST prepare a 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 NOT apply the Case Mapping Rule during enforcement, because typically it is appropriate 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).

2.4. Comparison

An entity that performs comparison of two strings according to this profile MUST prepare each 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").

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 (note that the characters < and > are used to delineate the actual nickname and are not part of the nickname strings).

Table 1: A Sample of Legal Nicknames

+---------------------------+-----------------------------------+
| # | Nickname              | Output for Comparison             |
+---------------------------+-----------------------------------+
| 1 | <Foo>                 | <foo>                             |
+---------------------------+-----------------------------------+
| 2 | <foo>                 | <foo>                             |
+---------------------------+-----------------------------------+
| 3 | <Foo Bar>             | <foo bar>                         |
+---------------------------+-----------------------------------+
| 4 | <foo bar>             | <foo bar>                         |
+---------------------------+-----------------------------------+
| 5 | <&#x3A3;>             | GREEK SMALL LETTER SIGMA (U+03C3) |
+---------------------------+-----------------------------------+
| 6 | <&#x3C3;>             | GREEK SMALL LETTER SIGMA (U+03C3) |
+---------------------------+-----------------------------------+
| 7 | <&#x3C2;>             | GREEK SMALL LETTER FINAL SIGMA    | 
|   |                       | (U+03C2)                          |
+---------------------------+-----------------------------------+
| 8 | <&#x265A;>            | BLACK CHESS KING (U+265A)         |
+---------------------------+-----------------------------------+
| 9 | <Richard &#x2163;>    | <richard iv>                      |
+---------------------------+-----------------------------------+
      

Regarding examples 5, 6, and 7: applying the Unicode toLower() operation to GREEK CAPITAL LETTER SIGMA (U+03A3) results in GREEK SMALL LETTER SIGMA (U+03C3), however the toLower() operation does not modify 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: symbol characters such as BLACK CHESS KING (U+265A) are allowed by the PRECIS FreeformClass and thus can be used in nicknames. Regarding example 9: applying the Unicode toLower() operation to ROMAN NUMERAL FOUR (U+2163) results in SMALL ROMAN NUMERAL FOUR (U+2173), and applying NFKC to SMALL ROMAN NUMERAL FOUR (U+2173) results in LATIN SMALL LETTER I (U+0069) LATIN SMALL LETTER V (U+0086).

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 when in protocol processing or interface handling the rules need to be enforced. See Section 6 of [RFC7564] 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.

Implementation experience has shown that applying the rules for the Nickname profile is not an idempotent procedure for all code points. Therefore, implementations might need to apply the rules more than once to a nickname string.

5. IANA Considerations

The IANA shall add the following entry to the PRECIS Profiles Registry:

Name:

Nickname

Base Class:

FreeformClass

Applicability:

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

Replaces:

None

Width Mapping Rule:

None (handled via NFKC)

Additional Mapping Rule:

Map non-ASCII space characters to ASCII space, strip leading and trailing space characters, map interior sequences of multiple space characters to a single ASCII space.

Case Mapping Rule:

Map uppercase and titlecase code points to lowercase using the Unicode toLower() operation.

Normalization Rule:

NFKC

Directionality Rule:

None

Enforcement:

To be specified by applications.

Specification:

RFC 7700 (this document)

6. Security Considerations

6.1. Reuse of PRECIS

The security considerations described in [RFC7564] apply to the FreeformClass string class used in this document for nicknames.

6.2. Reuse of Unicode

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

6.3. Visually Similar Characters

[RFC7564] describes some of the security considerations related to visually similar characters, also called "confusable characters" or "confusables".

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

7.2. Informative References

Appendix A. Changes from RFC 7700

The following changes were made from [RFC7700].

• Addressed [Err4570] by removing the directionality rule and adding the normalization rule to Section 2.3.
• In accordance with working group discussions and updates to [RFC7564], removed the use of the Unicode CaseFold() operation in favor of the Unicode toLower() operation.
• Clarified several editorial matters.
• Updated references.

Appendix B. 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