draft-ietf-precis-framework-06.txt   draft-ietf-precis-framework-07.txt 
PRECIS P. Saint-Andre PRECIS P. Saint-Andre
Internet-Draft Cisco Systems, Inc. Internet-Draft Cisco Systems, Inc.
Obsoletes: 3454 (if approved) M. Blanchet Obsoletes: 3454 (if approved) M. Blanchet
Intended status: Standards Track Viagenie Intended status: Standards Track Viagenie
Expires: March 27, 2013 September 23, 2012 Expires: September 27, 2013 March 26, 2013
PRECIS Framework: Preparation and Comparison of Internationalized PRECIS Framework: Preparation and Comparison of Internationalized
Strings in Application Protocols Strings in Application Protocols
draft-ietf-precis-framework-06 draft-ietf-precis-framework-07
Abstract Abstract
Application protocols using Unicode code points in protocol strings Application protocols using Unicode code points in protocol strings
need to prepare such strings in order to perform comparison need to properly prepare such strings in order to perform valid
operations (e.g., for purposes of authentication or authorization). comparison operations (e.g., for purposes of authentication or
This document defines a framework enabling application protocols to authorization). This document defines a framework enabling
perform the preparation and comparison of internationalized strings application protocols to perform the preparation and comparison of
(a.k.a. "PRECIS") in a way that depends on the properties of Unicode internationalized strings (a.k.a. "PRECIS") in a way that depends on
code points and thus is agile with respect to versions of Unicode. the properties of Unicode code points and thus is agile with respect
As a result, this framework provides a more sustainable approach to to versions of Unicode. As a result, this framework provides a more
the handling of internationalized strings than the previous sustainable approach to the handling of internationalized strings
framework, known as Stringprep (RFC 3454). A specification that than the previous framework, known as Stringprep (RFC 3454). A
reuses this framework can either directly use the base string classes specification that reuses this framework can either directly use the
or subclass the base string classes as needed. This framework takes PRECIS string classes or subclass the PRECIS string classes as
an approach similar to the revised internationalized domain names in needed. This framework takes an approach similar to the revised
applications (IDNA) technology (RFC 5890, RFC 5891, RFC 5892, RFC internationalized domain names (IDNs) in applications (IDNA)
5893, RFC 5894) and thus adheres to the high-level design goals technology (RFC 5890, RFC 5891, RFC 5892, RFC 5893, RFC 5894) and
described in RFC 4690, albeit for application technologies other than thus adheres to the high-level design goals described in the IAB's
the Domain Name System (DNS). This document obsoletes RFC 3454. recommendations regarding IDNs (RFC 4690), albeit for application
technologies other than the Domain Name System (DNS). This document
obsoletes RFC 3454.
Status of this Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 27, 2013.
This Internet-Draft will expire on March 27, 2013.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. String Classes . . . . . . . . . . . . . . . . . . . . . . . . 5 3. String Classes . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. NameClass . . . . . . . . . . . . . . . . . . . . . . . . 7 3.2. IdentifierClass . . . . . . . . . . . . . . . . . . . . . 7
3.2.1. Valid . . . . . . . . . . . . . . . . . . . . . . . . 7 3.3. FreeformClass . . . . . . . . . . . . . . . . . . . . . . 9
3.2.2. Disallowed . . . . . . . . . . . . . . . . . . . . . . 8 4. Use of PRECIS String Classes . . . . . . . . . . . . . . . . 11
3.2.3. Unassigned . . . . . . . . . . . . . . . . . . . . . . 8 4.1. Principles . . . . . . . . . . . . . . . . . . . . . . . 11
3.2.4. Normalization . . . . . . . . . . . . . . . . . . . . 8 4.2. Subclassing . . . . . . . . . . . . . . . . . . . . . . . 11
3.2.5. Casemapping . . . . . . . . . . . . . . . . . . . . . 8 4.3. Building Application-Layer Constructs . . . . . . . . . . 12
3.2.6. Additional Mappings . . . . . . . . . . . . . . . . . 8 4.4. A Note about Spaces . . . . . . . . . . . . . . . . . . . 13
3.2.7. Directionality . . . . . . . . . . . . . . . . . . . . 8 5. Code Point Properties . . . . . . . . . . . . . . . . . . . . 13
3.3. FreeClass . . . . . . . . . . . . . . . . . . . . . . . . 9 6. Category Definitions Used to Calculate Derived Property Value 15
3.3.1. Valid . . . . . . . . . . . . . . . . . . . . . . . . 9 6.1. LetterDigits (A) . . . . . . . . . . . . . . . . . . . . 16
3.3.2. Disallowed . . . . . . . . . . . . . . . . . . . . . . 9 6.2. Unstable (B) . . . . . . . . . . . . . . . . . . . . . . 16
3.3.3. Unassigned . . . . . . . . . . . . . . . . . . . . . . 9 6.3. IgnorableProperties (C) . . . . . . . . . . . . . . . . . 16
3.3.4. Normalization . . . . . . . . . . . . . . . . . . . . 9 6.4. IgnorableBlocks (D) . . . . . . . . . . . . . . . . . . . 17
3.3.5. Casemapping . . . . . . . . . . . . . . . . . . . . . 10 6.5. LDH (E) . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.3.6. Additional Mappings . . . . . . . . . . . . . . . . . 10 6.6. Exceptions (F) . . . . . . . . . . . . . . . . . . . . . 17
3.3.7. Directionality . . . . . . . . . . . . . . . . . . . . 10 6.7. BackwardCompatible (G) . . . . . . . . . . . . . . . . . 18
4. Use of PRECIS String Classes . . . . . . . . . . . . . . . . . 10 6.8. JoinControl (H) . . . . . . . . . . . . . . . . . . . . . 19
4.1. Principles . . . . . . . . . . . . . . . . . . . . . . . . 10 6.9. OldHangulJamo (I) . . . . . . . . . . . . . . . . . . . . 19
4.2. Subclassing . . . . . . . . . . . . . . . . . . . . . . . 11 6.10. Unassigned (J) . . . . . . . . . . . . . . . . . . . . . 19
4.3. A Note about Spaces . . . . . . . . . . . . . . . . . . . 11 6.11. ASCII7 (K) . . . . . . . . . . . . . . . . . . . . . . . 20
5. Code Point Properties . . . . . . . . . . . . . . . . . . . . 12 6.12. Controls (L) . . . . . . . . . . . . . . . . . . . . . . 20
6. Category Definitions Used to Calculate Derived Property 6.13. PrecisIgnorableProperties (M) . . . . . . . . . . . . . . 20
Value . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 6.14. Spaces (N) . . . . . . . . . . . . . . . . . . . . . . . 20
6.1. LetterDigits (A) . . . . . . . . . . . . . . . . . . . . . 14 6.15. Symbols (O) . . . . . . . . . . . . . . . . . . . . . . . 21
6.2. Unstable (B) . . . . . . . . . . . . . . . . . . . . . . . 15 6.16. Punctuation (P) . . . . . . . . . . . . . . . . . . . . . 21
6.3. IgnorableProperties (C) . . . . . . . . . . . . . . . . . 15 6.17. HasCompat (Q) . . . . . . . . . . . . . . . . . . . . . . 21
6.4. IgnorableBlocks (D) . . . . . . . . . . . . . . . . . . . 15 6.18. OtherLetterDigits (R) . . . . . . . . . . . . . . . . . . 21
6.5. LDH (E) . . . . . . . . . . . . . . . . . . . . . . . . . 15 7. Calculation of the Derived Property . . . . . . . . . . . . . 21
6.6. Exceptions (F) . . . . . . . . . . . . . . . . . . . . . . 15 8. Code Points . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.7. BackwardCompatible (G) . . . . . . . . . . . . . . . . . . 17 9. Security Considerations . . . . . . . . . . . . . . . . . . . 23
6.8. JoinControl (H) . . . . . . . . . . . . . . . . . . . . . 17 9.1. General Issues . . . . . . . . . . . . . . . . . . . . . 23
6.9. OldHangulJamo (I) . . . . . . . . . . . . . . . . . . . . 17 9.2. Use of the IdentifierClass . . . . . . . . . . . . . . . 24
6.10. Unassigned (J) . . . . . . . . . . . . . . . . . . . . . . 18 9.3. Use of the FreeformClass . . . . . . . . . . . . . . . . 24
6.11. ASCII7 (K) . . . . . . . . . . . . . . . . . . . . . . . . 18 9.4. Local Character Set Issues . . . . . . . . . . . . . . . 24
6.12. Controls (L) . . . . . . . . . . . . . . . . . . . . . . . 18 9.5. Visually Similar Characters . . . . . . . . . . . . . . . 24
6.13. PrecisIgnorableProperties (M) . . . . . . . . . . . . . . 18 9.6. Security of Passwords . . . . . . . . . . . . . . . . . . 26
6.14. Spaces (N) . . . . . . . . . . . . . . . . . . . . . . . . 19 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
6.15. Symbols (O) . . . . . . . . . . . . . . . . . . . . . . . 19 10.1. PRECIS Derived Property Value Registry . . . . . . . . . 27
6.16. Punctuation (P) . . . . . . . . . . . . . . . . . . . . . 19 10.2. PRECIS Base Classes Registry . . . . . . . . . . . . . . 27
6.17. HasCompat (Q) . . . . . . . . . . . . . . . . . . . . . . 19 10.3. PRECIS Subclasses Registry . . . . . . . . . . . . . . . 28
6.18. OtherLetterDigits (R) . . . . . . . . . . . . . . . . . . 19 10.4. PRECIS Usage Registry . . . . . . . . . . . . . . . . . 30
7. Calculation of the Derived Property . . . . . . . . . . . . . 20 11. Interoperability Considerations . . . . . . . . . . . . . . . 31
8. Code Points . . . . . . . . . . . . . . . . . . . . . . . . . 21 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 31
9. Security Considerations . . . . . . . . . . . . . . . . . . . 21 12.1. Normative References . . . . . . . . . . . . . . . . . . 32
9.1. General Issues . . . . . . . . . . . . . . . . . . . . . . 21 12.2. Informative References . . . . . . . . . . . . . . . . . 32
9.2. Use of the NameClass . . . . . . . . . . . . . . . . . . . 21 Appendix A. Codepoint Table . . . . . . . . . . . . . . . . . . 34
9.3. Use of the FreeClass . . . . . . . . . . . . . . . . . . . 21 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 69
9.4. Local Character Set Issues . . . . . . . . . . . . . . . . 22 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 70
9.5. Visually Similar Characters . . . . . . . . . . . . . . . 22
9.6. Security of Passwords . . . . . . . . . . . . . . . . . . 24
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
10.1. PRECIS Derived Property Value Registry . . . . . . . . . . 25
10.2. PRECIS Base Classes Registry . . . . . . . . . . . . . . . 25
10.3. PRECIS Subclasses Registry . . . . . . . . . . . . . . . . 26
10.4. PRECIS Usage Registry . . . . . . . . . . . . . . . . . . 27
11. Interoperability Considerations . . . . . . . . . . . . . . . 28
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
12.1. Normative References . . . . . . . . . . . . . . . . . . . 29
12.2. Informative References . . . . . . . . . . . . . . . . . . 29
Appendix A. Codepoint Table . . . . . . . . . . . . . . . . . . . 31
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 66
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 67
1. Introduction 1. Introduction
As described in [I-D.ietf-precis-problem-statement], many IETF As described in the PRECIS problem statement [RFC6885], many IETF
protocols have used the Stringprep framework [RFC3454] as the basis protocols have used the Stringprep framework [RFC3454] as the basis
for preparing and comparing protocol strings that contain Unicode for preparing and comparing protocol strings that contain Unicode
code points [UNICODE]. The Stringprep framework was developed during code points [UNICODE] outside the ASCII range [RFC20]. The
work on the original technology for internationalized domain names Stringprep framework was developed during work on the original
(IDNs), here called "IDNA2003" [RFC3490], and Nameprep [RFC3491] was technology for internationalized domain names (IDNs), here called
the Stringprep profile for IDNs. At the time, Stringprep was "IDNA2003" [RFC3490], and Nameprep [RFC3491] was the Stringprep
designed as a general framework so that other application protocols profile for IDNs. At the time, Stringprep was designed as a general
could define their own Stringprep profiles for the preparation and framework so that other application protocols could define their own
comparison of strings and identifiers, and a number of application Stringprep profiles for the preparation and comparison of strings and
protocols did define such profiles. identifiers. Indeed, a number of application protocols defined such
profiles.
After the publication of [RFC3454] in 2002, several significant After the publication of [RFC3454] in 2002, several significant
issues arose with the use of Stringprep in the IDN case, as issues arose with the use of Stringprep in the IDN case, as
documented in [RFC4690] (most significantly, Stringprep was tied to documented in the IAB's recommendations regarding IDNs [RFC4690]
Unicode version 3.2). Therefore, the new IDNA specifications, here (most significantly, Stringprep was tied to Unicode version 3.2).
called "IDNA2008" ([RFC5890], [RFC5891], [RFC5892], [RFC5893], Therefore, the new IDNA specifications, here called "IDNA2008"
[RFC5894]), no longer use Stringprep and Nameprep. This migration ([RFC5890], [RFC5891], [RFC5892], [RFC5893], [RFC5894]), no longer
away from Stringprep for IDNs has prompted other "customers" of use Stringprep and Nameprep. This migration away from Stringprep for
Stringprep to consider new approaches to the preparation and IDNs has prompted other "customers" of Stringprep to consider new
comparison of internationalized strings (a.k.a. "PRECIS"), as approaches to the preparation and comparison of internationalized
described in [I-D.ietf-precis-problem-statement]. strings (a.k.a. "PRECIS"), as described in [RFC6885].
This document defines a framework for a post-Stringprep approach to This document defines a framework for a post-Stringprep approach to
the preparation and comparison of internationalized strings in the preparation and comparison of internationalized strings in
application protocols, based on several principles: application protocols, based on several principles:
1. Define a small set of base string classes appropriate for common 1. Define a small set of string classes appropriate for common
application protocol constructs such as usernames and free-form application protocol constructs such as usernames and free-form
strings. strings.
2. Define each base string class in terms of Unicode code points and 2. Define each PRECIS string class in terms of Unicode code points
their properties so that an algorithm can be used to determine and their properties so that an algorithm can be used to
whether each code point or character category is valid, determine whether each code point or character category is valid,
disallowed, or unassigned. disallowed, or unassigned.
3. Define string classes in terms of allowable code points, so that 3. Define string classes in terms of allowable code points, so that
any code points not explicitly allowed are forbidden. any code points not explicitly allowed are forbidden.
4. Enable application protocols to subclass the base string classes 4. Enable application protocols to subclass the PRECIS string
if needed, mainly to disallow particular code points that are classes if needed, mainly to disallow particular code points that
currently disallowed in the relevant application protocol (e.g., are currently disallowed in the relevant application protocol
characters with special or reserved meaning, such as "@" and "/" (e.g., characters with special or reserved meaning, such as "@"
when used as separators within identifiers). and "/" when used as separators within identifiers).
5. Leave various mapping operations (e.g., case preservation or 5. Leave various mapping operations (e.g., case preservation or
lowercasing, Unicode normalization, mapping of certain characters lowercasing, Unicode normalization, mapping of certain characters
to nothing, handling of right-to-left characters) as the to other characters or to nothing, handling of full-width and
responsibility of application protocols, as was done for IDNA2008 half-width characters, handling of right-to-left characters) as
through an IDNA-specific mapping document [RFC5895]. the responsibility of application protocols, as was done for
IDNA2008 through an IDNA-specific mapping document [RFC5895].
It is expected that this framework will yield the following benefits: It is expected that this framework will yield the following benefits:
o Application protocols will be more version-agile with regard to o Application protocols will be more version-agile with regard to
the Unicode database. the Unicode database.
o Implementers will be able to share code point tables and software o Implementers will be able to share code point tables and software
code across application protocols, most likely by means of code across application protocols, most likely by means of
software libraries. software libraries.
o End users will be able to acquire more accurate expectations about o End users will be able to acquire more accurate expectations about
the code points that are acceptable in various contexts. Given the code points that are acceptable in various contexts. Given
this more uniform set of string classes, it is also expected that this more uniform set of string classes, it is also expected that
copy/paste operations between software implementing different copy/paste operations between software implementing different
application protocols will be more predictable and coherent. application protocols will be more predictable and coherent.
Although this framework is similar to IDNA2008 and borrows some of Although this framework is similar to IDNA2008 and borrows some of
the character categories defined in [RFC5892], it defines additional the character categories defined in [RFC5892], it defines additional
string classes and character categories to meet the needs of common string classes and character categories to meet the needs of common
application protocols. application protocols.
skipping to change at page 5, line 32 skipping to change at page 5, line 19
application protocols will be more predictable and coherent. application protocols will be more predictable and coherent.
Although this framework is similar to IDNA2008 and borrows some of Although this framework is similar to IDNA2008 and borrows some of
the character categories defined in [RFC5892], it defines additional the character categories defined in [RFC5892], it defines additional
string classes and character categories to meet the needs of common string classes and character categories to meet the needs of common
application protocols. application protocols.
2. Terminology 2. Terminology
Many important terms used in this document are defined in [RFC5890], Many important terms used in this document are defined in [RFC5890],
[RFC6365], [I-D.ietf-precis-problem-statement], and [UNICODE]. [RFC6365], [RFC6885], and [UNICODE].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in "OPTIONAL" in this document are to be interpreted as described in
[RFC2119]. [RFC2119].
3. String Classes 3. String Classes
3.1. Overview 3.1. Overview
IDNA2008 essentially defines a base string class of internationalized IDNA2008 essentially defines a string class of internationalized
domain name, although it does not use the term "string class". (This domain name (IDN), although it does not use the term "string class".
document does not define a string class for domain names, and (This document does not define a string class for domain names, and
application protocols are strongly encouraged to use IDNA2008 as the application protocols are strongly encouraged to use IDNA2008 as the
appropriate method to prepare domain names and hostnames.) Because appropriate method to prepare domain names and hostnames.) Because
the IDN string class is designed to meet the particular requirements the IDN string class is designed to meet the particular requirements
of the Domain Name System (DNS), additional string classes are needed of the Domain Name System (DNS), additional string classes are needed
for non-DNS applications. for non-DNS applications.
Starting in 2010, various "customers" of Stringprep began to discuss Starting in 2010, various "customers" of Stringprep began to discuss
the need to define a post-Stringprep approach to the preparation and the need to define a post-Stringprep approach to the preparation and
comparison of internationalized strings. As a result of analyzing comparison of internationalized strings. As a result of analyzing
existing Stringprep profiles, this community concluded that most existing Stringprep profiles, this community concluded that most
existing uses could be addressed by two base string classes: existing uses could be addressed by two string classes:
NameClass: a sequence of letters, numbers, and symbols that is used IdentifierClass: a sequence of letters, numbers, and symbols that is
to identify or address a network entity such as a user account, a used to identify or address a network entity such as a user
venue (e.g., a chatroom), an information source (e.g., a data account, a venue (e.g., a chatroom), an information source (e.g.,
feed), or a collection of data (e.g., a file); the intent is that a data feed), or a collection of data (e.g., a file); the intent
this class will be very safe for use in a wide variety of is that this class will be very safe for use in a wide variety of
application protocols, with the result that safety has been application protocols, with the result that safety has been
prioritized over inclusiveness for this class. prioritized over inclusiveness for this class.
FreeClass: a sequence of letters, numbers, symbols, spaces, and FreeformClass: a sequence of letters, numbers, symbols, spaces, and
other code points that is used for free-form strings, including other code points that is used for free-form strings, including
passwords as well as display elements such as human-friendly passwords as well as display elements such as human-friendly
nicknames in chatrooms; the intent is that this class will allow nicknames in chatrooms; the intent is that this class will allow
nearly any Unicode character, with the result that inclusiveness nearly any Unicode character, with the result that inclusiveness
has been prioritized over safety for this class (e.g., protocol has been prioritized over safety for this class (e.g., protocol
designers, application developers, service providers, and end designers, application developers, service providers, and end
users might not understand or be able to enter all of the users might not understand or be able to enter all of the
characters that can be included in the FreeClass). characters that can be included in the FreeformClass).
Although members of the community discussed the possibility of Although members of the community discussed the possibility of
defining other bases string classes (e.g., a class falling somewhere defining other PRECIS string classes (e.g., a class falling somewhere
between the NameClass and the FreeClass), they concluded that the between the IdentifierClass and the FreeformClass), they concluded
NameClass would be a safe choice meeting the needs of many or even that the IdentifierClass would be a safe choice meeting the needs of
most application protocols, and that protocols needing a wider range many or even most application protocols, and that protocols needing a
of Unicode characters could use the FreeClass directly or subclass it wider range of Unicode characters could use the FreeformClass
if needed. directly or subclass it if needed.
The following subsections discuss the NameClass and FreeClass in more The following subsections discuss the IdentifierClass and
detail, with reference to the dimensions described in Section 3 of FreeformClass in more detail, with reference to the dimensions
[I-D.ietf-precis-problem-statement]. (Naturally, future documents described in Section 3 of [RFC6885]. (Naturally, future documents
can define base string classes beyond the NameClass and FreeClass; can define PRECIS string classes beyond the IdentifierClass and
see Section 10.2.) Each string class (or a particular usage thereof) FreeformClass; see Section 10.2.) Each string class (or a particular
is defined by the following behavioral rules: usage thereof) is defined by the following behavioral rules:
Valid: defines which code points and character categories are Valid: defines which code points and character categories are
treated as valid input to the string. treated as valid input to the string.
Disallowed: defines which code points and character categories are Disallowed: defines which code points and character categories are
treated as disallowed for the string. treated as disallowed for the string.
Unassigned: defines application behavior in the presence of code Unassigned: defines application behavior in the presence of code
points that are unassigned, i.e. unknown for the version of points that are unassigned, i.e. unknown for the version of
Unicode the application is built upon. Unicode the application is built upon.
Normalization: defines which Unicode normalization form (D, KD, C, Normalization: defines which Unicode normalization form (D, KD, C,
or KC) is to be applied (see [UAX15]). or KC) is to be applied (see [UAX15]).
Casemapping: specifies if casemapping is performed (instead of case Case Mapping: specifies if case mapping is performed (instead of
preservation), and how the mapping is done (e.g., mapping case preservation) on uppercase and titlecase characters, and how
uppercase and titlecase characters to their lowercase the mapping is done (e.g., mapping uppercase and titlecase
characters to their lowercase equivalents).
Width Mapping: specifies if width mapping is performed on fullwidth
and halfwidth characters, and how the mapping is done (e.g.,
mapping fullwidth and halfwidth characters to their decomposition
equivalents). equivalents).
Directionality: defines application behavior in the presence of code Directionality: defines application behavior in the presence of code
points that have directionality, in particular right-to-left code points that have directionality, in particular right-to-left code
points as defined in the Unicode database (see [UAX9]). points as defined in the Unicode database (see [UAX9]).
This document defines the valid, disallowed, and unassigned rules for This document defines the valid, disallowed, and unassigned rules for
the NameClass and FreeClass. Application protocols that use these the IdentifierClass and FreeformClass. Application protocols that
string classes are responsible for defining the normalization, use these string classes are responsible for defining the
casemapping, and directionality rules, as well as any additional normalization, case mapping, width mapping, and directionality rules,
mappings to be applied (e.g., mapping fullwidth and halfwidth as well as any additional mappings to be applied
characters to their decomposition equivalents).
NOTE: In order to ensure proper comparison, any normalization MUST be Note: In order to ensure proper comparison, any normalization MUST be
completed before the application of additional mappings or the completed before the application of additional mappings or the
process of checking whether a code point is valid, disallowed, or process of checking whether a code point is valid, disallowed, or
unassigned. unassigned.
3.2. NameClass 3.2. IdentifierClass
Most application technologies need strings that can be used to refer Most application technologies need strings that can be used to refer
to, include, or communicate things like usernames, file names, data to, include, or communicate protocol strings like usernames, file
feed names, and chatroom names. We group such things into a class names, data feed identifiers, and chatroom names. We group such
called "NameClass" having the following features. strings into a class called "IdentifierClass" having the following
features.
3.2.1. Valid 3.2.1. Valid
o Code points traditionally used as letters and numbers in writing o Code points traditionally used as letters and numbers in writing
systems, i.e., the LetterDigits ("A") category first defined in systems, i.e., the LetterDigits ("A") category first defined in
[RFC5892] and listed here under Section 6.1. [RFC5892] and listed here under Section 6.1.
o Code points in the range U+0021 through U+007E, i.e., the ASCII7 o Code points in the range U+0021 through U+007E, i.e., the ASCII7
("K") rule defined under Section 6.11. These code points are ("K") rule defined under Section 6.11. These code points are
"grandfathered" into PRECIS and thus are valid even if they would "grandfathered" into PRECIS and thus are valid even if they would
otherwise be disallowed according to the property-based rules otherwise be disallowed according to the property-based rules
specified in the next section. specified in the next section.
Although the PRECIS IdentifierClass re-uses the LetterDigits category
from IDNA2008, the range of characters allowed in the IdentifierClass
is wider than the range of characters allowed in IDNA2008. The main
reason is that IDNA2008 applies the Unstable category before the
LetterDigits category, thus disallowing uppercase characters, whereas
the IdentifierClass does not apply the Unstable category.
3.2.2. Disallowed 3.2.2. Disallowed
o Control characters, i.e., the Controls ("L") category defined o Control characters, i.e., the Controls ("L") category defined
under Section 6.12. under Section 6.12.
o Ignorable characters, i.e., the PrecisIgnorableProperties ("M") o Ignorable characters, i.e., the PrecisIgnorableProperties ("M")
category defined under Section 6.13. category defined under Section 6.13.
o Space characters, i.e., the Spaces ("N") category defined under o Space characters, i.e., the Spaces ("N") category defined under
Section 6.14. Section 6.14.
o Symbol characters, i.e., the Symbols ("O") category defined under o Symbol characters, i.e., the Symbols ("O") category defined under
Section 6.15. Section 6.15.
o Punctuation characters, i.e., the Punctuation ("P") category o Punctuation characters, i.e., the Punctuation ("P") category
defined under Section 6.16. defined under Section 6.16.
o Any character that has a compatibility equivalent, i.e., the o Any character that has a compatibility equivalent, i.e., the
HasCompat ("Q") category defined under Section 6.17. These code HasCompat ("Q") category defined under Section 6.17. These code
points are disallowed even if they would otherwise be valid points are disallowed even if they would otherwise be valid
according to the property-based rules specified in the previous according to the property-based rules specified in the previous
section. section.
o Letters and digits other than the "traditional" letters and digits o Letters and digits other than the "traditional" letters and digits
allowed in IDNs, i.e., the OtherLetterDigits ("R") category allowed in IDNs, i.e., the OtherLetterDigits ("R") category
defined under Section 6.18. defined under Section 6.18.
3.2.3. Unassigned 3.2.3. Unassigned
Any code points that are not yet assigned in the Unicode character Any code points that are not yet assigned in the Unicode character
set SHALL be considered Unassigned for purposes of the NameClass. set SHALL be considered Unassigned for purposes of the
IdentifierClass.
3.2.4. Normalization 3.2.4. Normalization
The Unicode normalization form MUST be specified by each application The Unicode normalization form MUST be specified by each application
protocol that uses or subclasses the NameClass. protocol that uses or subclasses the IdentifierClass.
However, in accordance with [RFC5198], normalization form C (NFC) is However, in accordance with [RFC5198], normalization form C (NFC) is
RECOMMENDED. RECOMMENDED.
3.2.5. Casemapping 3.2.5. Case Mapping
The casemapping rule MUST be specified by each application protocol The case mapping rule MUST be specified by each application protocol
that uses or subclasses the NameClass. that uses or subclasses the IdentifierClass.
3.2.6. Additional Mappings 3.2.6. Width Mapping
The width mapping rule MUST be specified by each application protocol
that uses or subclasses the IdentifierClass.
3.2.7. Additional Mappings
Additional mapping rules (if any) MUST be specified by each Additional mapping rules (if any) MUST be specified by each
application protocol that uses or subclasses the NameClass. application protocol that uses or subclasses the IdentifierClass.
3.2.7. Directionality 3.2.8. Directionality
The directionality rule MUST be specified by each application The directionality rule MUST be specified by each application
protocol that uses or subclasses the NameClass. protocol that uses or subclasses the IdentifierClass.
3.3. FreeClass 3.3. FreeformClass
Some application technologies need strings that can be used in a Some application technologies need strings that can be used in a
free-form way, e.g., as a password in an authentication exchange (see free-form way, e.g., as a password in an authentication exchange (see
[I-D.melnikov-precis-saslprepbis] or a nickname in a chatroom (see [I-D.ietf-precis-saslprepbis] or a nickname in a chatroom (see
[I-D.ietf-precis-nickname]). We group such things into a class [I-D.ietf-precis-nickname]). We group such things into a class
called "FreeClass" having the following features. called "FreeformClass" having the following features.
NOTE: Consult Section 9.6 for relevant security considerations when Note: Consult Section 9.6 for relevant security considerations when
strings conforming to the FreeClass, or a subclass thereof, are used strings conforming to the FreeformClass, or a subclass thereof, are
as passwords. used as passwords.
3.3.1. Valid 3.3.1. Valid
o Traditional letters and numbers, i.e., the LetterDigits ("A") o Traditional letters and numbers, i.e., the LetterDigits ("A")
category first defined in [RFC5892] and listed here under category first defined in [RFC5892] and listed here under
Section 6.1. Section 6.1.
o Letters and digits other than the "traditional" letters and digits o Letters and digits other than the "traditional" letters and digits
allowed in IDNs, i.e., the OtherLetterDigits ("R") category allowed in IDNs, i.e., the OtherLetterDigits ("R") category
defined under Section 6.18. defined under Section 6.18.
o Code points in the range U+0021 through U+007E, i.e., the ASCII7 o Code points in the range U+0021 through U+007E, i.e., the ASCII7
("K") rule defined under Section 6.11. ("K") rule defined under Section 6.11.
o Any character that has a compatibility equivalent, i.e., the o Any character that has a compatibility equivalent, i.e., the
HasCompat ("Q") category defined under Section 6.17. HasCompat ("Q") category defined under Section 6.17.
o Space characters, i.e., the Spaces ("N") category defined under o Space characters, i.e., the Spaces ("N") category defined under
Section 6.14. Section 6.14.
o Symbol characters, i.e., the Symbols ("O") category defined under o Symbol characters, i.e., the Symbols ("O") category defined under
Section 6.15. Section 6.15.
o Punctuation characters, i.e., the Punctuation ("P") category o Punctuation characters, i.e., the Punctuation ("P") category
defined under Section 6.16. defined under Section 6.16.
3.3.2. Disallowed 3.3.2. Disallowed
o Control characters, i.e., the Controls ("L") category defined o Control characters, i.e., the Controls ("L") category defined
under Section 6.12. under Section 6.12.
o Ignorable characters, i.e., the PrecisIgnorableProperties ("M") o Ignorable characters, i.e., the PrecisIgnorableProperties ("M")
category defined under Section 6.13. category defined under Section 6.13.
3.3.3. Unassigned 3.3.3. Unassigned
Any code points that are not yet assigned in the Unicode character Any code points that are not yet assigned in the Unicode character
set SHALL be considered Unassigned for purposes of the FreeClass. set SHALL be considered Unassigned for purposes of the FreeformClass.
3.3.4. Normalization 3.3.4. Normalization
The Unicode normalization form MUST be specified by each application The Unicode normalization form MUST be specified by each application
protocol that uses or subclasses the FreeClass. protocol that uses or subclasses the FreeformClass.
However, in accordance with [RFC5198], normalization form C (NFC) is However, in accordance with [RFC5198], normalization form C (NFC) is
RECOMMENDED. RECOMMENDED.
3.3.5. Casemapping 3.3.5. Case Mapping
The casemapping rule MUST be specified by each application protocol The case mapping rule MUST be specified by each application protocol
that uses or subclasses the FreeClass. that uses or subclasses the FreeformClass.
In general, the combination of case preservation and case-insensitive In general, the combination of case preservation and case-insensitive
comparison of internationalized strings is NOT RECOMMENDED; instead, comparison of internationalized strings is NOT RECOMMENDED; instead,
application protocols SHOULD either (a) not preserve case but perform application protocols SHOULD either (a) not preserve case but perform
case-insensitive comparison or (b) preserve case but perform case- case-insensitive comparison or (b) preserve case but perform case-
sensitive comparison. sensitive comparison.
In order to maximize entropy and minimize the potential for false In order to maximize entropy and minimize the potential for false
positives, it is NOT RECOMMENDED for application protocols to map positives, it is NOT RECOMMENDED for application protocols to map
uppercase and titlecase code points to their lowercase equivalents uppercase and titlecase code points to their lowercase equivalents
when strings conforming to the FreeClass, or a subclass thereof, are when strings conforming to the FreeformClass, or a subclass thereof,
used in passwords; instead, it is RECOMMENDED to preserve the case of are used in passwords; instead, it is RECOMMENDED to preserve the
all code points contained in such strings and then perform case- case of all code points contained in such strings and then perform
sensitive comparison. See also the related discussion in case-sensitive comparison. See also the related discussion in
[I-D.melnikov-precis-saslprepbis]. [I-D.ietf-precis-saslprepbis].
3.3.6. Additional Mappings 3.3.6. Width Mapping
The width mapping rule MUST be specified by each application protocol
that uses or subclasses the FreeformClass.
Because one aspect of Unicode normalization form KC is width mapping,
a PRECIS usage or subclass that uses NFKC does not need to specify
width mapping. However, if NFC is used then the usage or subclass
needs to specify whether to apply width mapping; in this case, width
mapping is in general RECOMMENDED because allowing fullwidth and
halfwidth characters to remain unmapped to their decomposition
equivalents would violate the principle of least user surprise. For
more information about the concept of width in East Asian scripts
within Unicode, see for instance [UAX11].
3.3.7. Additional Mappings
Additional mapping rules (if any) MUST be specified by each Additional mapping rules (if any) MUST be specified by each
application protocol that uses or subclasses the FreeClass. application protocol that uses or subclasses the FreeformClass.
3.3.7. Directionality 3.3.8. Directionality
The directionality rule MUST be specified by each application The directionality rule MUST be specified by each application
protocol that uses or subclasses the FreeClass. protocol that uses or subclasses the FreeformClass.
4. Use of PRECIS String Classes 4. Use of PRECIS String Classes
4.1. Principles 4.1. Principles
This document defines the valid, disallowed, and unassigned rules. This document defines the valid, disallowed, and unassigned rules.
Application protocols that use the PRECIS string classes MUST define Application protocols that use the PRECIS string classes MUST define
the normalization, casemapping, and directionality rules. Such the normalization, case mapping, width mapping, and directionality
definitions MUST at a minimum specify the following: rules. Such definitions MUST at a minimum specify the following:
Normalization: Which Unicode normalization form (D, KD, C, or KC) is Normalization: Which Unicode normalization form (D, KD, C, or KC) is
to be applied (see [UAX15] for background information); in to be applied (see [UAX15] for background information); in
accordance with [RFC5198], NFC is RECOMMENDED. accordance with [RFC5198], NFC is RECOMMENDED.
Casemapping: Whether uppercase and titlecase code points are to be Case Mapping: Whether uppercase and titlecase code points are to be
(a) preserved or (b) mapped to lowercase. (a) preserved or (b) mapped to lowercase.
Additional Mappings: Whether additional mappings are to be applied, Additional Mappings: Whether additional mappings are to be applied,
such as mapping fullwidth and halfwidth characters to their such as mapping fullwidth and halfwidth characters to their
decomposition mappings, mapping non-ASCII space characters to decomposition equivalents, mapping non-ASCII space characters to
ASCII space, or mapping certain characters to nothing. ASCII space, or mapping certain characters to nothing.
Directionality: Whether any instance of the class that contains a Directionality: Whether any instance of the class that contains a
right-to-left code point is to be considered a right-to-left right-to-left code point is to be considered a right-to-left
string, or whether some other rule is to be applied (e.g., the string, or whether some other rule is to be applied (e.g., the
"Bidi Rule" from [RFC5893]). "Bidi Rule" from [RFC5893]).
4.2. Subclassing 4.2. Subclassing
Application protocols are allowed to subclass the PRECIS string
classes specified in this document. As the word "subclass" implies,
a subclass MUST NOT add as valid any code points or character
categories that are disallowed by the relevant PRECIS string class.
However, a subclass MAY do either of the following:
Application protocols are allowed to subclass the base string classes 1. Exclude specific code points that are included in the relevant
specified in this document. As the word "subclass" implies, a PRECIS string class.
subclass MUST NOT add as valid any code points or character
categories that are disallowed by the base string class. However, a
subclass MAY do either of the following:
1. Exclude specific code points that are included in the base string
class.
2. Exclude characters matching certain Unicode properties (e.g., 2. Exclude characters matching certain Unicode properties (e.g.,
math symbols) that are included in the base string class. math symbols) that are included in the relevant PRECIS string
class.
As a result, code points that are defined as valid for the base As a result, code points that are defined as valid for the PRECIS
string class being subclassed will be defined as disallowed for the string class being subclassed will be defined as disallowed for the
subclass. subclass.
Application protocols that subclass the PRECIS string classes MUST Application protocols that subclass the PRECIS string classes MUST
register with the IANA as described under Section 10.3. register with the IANA as described under Section 10.3.
It is RECOMMENDED for subclass names to be of the form It is RECOMMENDED for subclass names to be of the form
"SubclassBaseClass", where the "Subclass" string is a differentiator "SubclassBaseClass", where the "Subclass" string is a differentiator
and "BaseClass" is the name of the base class being subclassed; for and "BaseClass" is the name of the PRECIS string class being
example, the subclass of the NameClass used for localparts in the subclassed; for example, the subclass of the IdentifierClass used for
Extensible Messaging and Presence Protocol (XMPP) is named localparts in the Extensible Messaging and Presence Protocol (XMPP)
"LocalpartNameClass" [I-D.ietf-xmpp-6122bis]. is named "LocalpartIdentifierClass" [I-D.ietf-xmpp-6122bis].
4.3. A Note about Spaces 4.3. Building Application-Layer Constructs
The NameClass does not allow spaces of any kind (even ASCII space, Sometimes, an application-layer construct does not map directly to
U+0020). This might be counter-intuitive, given that spaces are one of the PRECIS string classes. Consider, for example, the "simple
included between family names and personal names when representing user name" construct in the Simple Authentication and Security Layer
the full names of people (and full names might be used as usernames). (SASL) [RFC4422]. Depending on the deployment, a simple user name
The consensus of the PRECIS Working Group is that spaces are might take the form of a user's full name (e.g., the user's personal
problematic for many reasons, for example because in some locales name followed by a space and then the user's family name). Such a
some devices are known to generate a character other than ASCII space simple user name cannot be defined as an instance of the
(such as ZERO WIDTH JOINER, U+200D) when a user performs an action IdentifierClass, since space characters are not allowed in the
like hit the space bar on a keyboard. Working Group participants IdentifierClass; however, it could be defined using a space-separated
also raised concerns about the fact that spaces are not always sequence of IdentifierClass instances, as in the following pseudo-
visible, and that many Unicode characters might be confusable with ABNF [RFC5234]:
ASCII space.
Although some existing protocols, such as the Simple Authentication fullname = namepart [1*(1*SP namepart)]
and Security Layer (SASL) [RFC4422], might be used in ways that allow namepart = 1*(idpoint)
a username to include spaces, the sense of the Working Group was that ;
such protocols could define an application-layer construct that ; an "idpoint" is a UTF-8 encoded Unicode code point
consists of instances of the PRECIS NameClass separated from each ; that conforms to the PRECIS IdentifierClass
other by instances of the ASCII space character (U+0020). One
consequence of this approach might be to effectively discourage the Similar techniques could be used to define many application-layer
use of ASCII space (or, even more problematically, non-ASCII space constructs, say of the form "user@domain" or "/path/to/file".
characters) in newer application protocols; given the challenges
involved in properly handling space characters in usernames, 4.4. A Note about Spaces
identifiers, and other protocol strings, the Working Group considered
With regard to the IdentiferClass, the consensus of the PRECIS
Working Group was that spaces are problematic for many reasons,
including:
o Many Unicode characters are confusable with ASCII space.
o Even if non-ASCII space characters are mapped to ASCII space
(U+0020), space characters are often not rendered in user
interfaces, leading to the possibility that human user might
consider a string containing spaces to be equivalent to the same
string without spaces.
o In some locales, some devices are known to generate a character
other than ASCII space (such as ZERO WIDTH JOINER, U+200D) when a
user performs an action like hit the space bar on a keyboard.
One consequence of disallowing space characters in the
IdentifierClass might be to effectively discourage the use of ASCII
space (or, even more problematically, non-ASCII space characters)
within identifiers created in newer application protocols; given the
challenges involved in properly handling space characters in
identifiers and other protocol strings, the Working Group considered
this to be a feature, not a bug. this to be a feature, not a bug.
However, the FreeformClass does allow spaces, which enables
application protocols to define subclasses of the FreeformClass that
are more flexible than any profiles of the IdentifierClass.
5. Code Point Properties 5. Code Point Properties
In order to implement the string classes described above, this In order to implement the string classes described above, this
document does the following: document does the following:
1. Reviews and classifies the collections of code points in the 1. Reviews and classifies the collections of code points in the
Unicode character set by examining various code point properties. Unicode character set by examining various code point properties.
2. Defines an algorithm for determining a derived property value, 2. Defines an algorithm for determining a derived property value,
which can vary depending on the string class being used by the which can vary depending on the string class being used by the
relevant application protocol. relevant application protocol.
This document is not intended to specify precisely how derived This document is not intended to specify precisely how derived
property values are to be applied in protocol strings. That property values are to be applied in protocol strings. That
information is the responsibility of the protocol specification that information is the responsibility of the protocol specification that
uses or subclasses a base string class from this document. uses or subclasses a PRECIS string class from this document.
The value of the property is to be interpreted as follows. The value of the property is to be interpreted as follows.
PROTOCOL VALID Those code points that are allowed to be used in any PROTOCOL VALID Those code points that are allowed to be used in any
PRECIS string class (NameClass and FreeClass). Code points with PRECIS string class (IdentifierClass and FreeformClass). Code
this property value are permitted for general use in any string points with this property value are permitted for general use in
class. The abbreviated term PVALID is used to refer to this value any string class. The abbreviated term PVALID is used to refer to
in the remainder of this document. this value in the remainder of this document.
SPECIFIC CLASS PROTOCOL VALID Those code points that are allowed to SPECIFIC CLASS PROTOCOL VALID Those code points that are allowed to
be used in specific string classes. Code points with this be used in specific string classes. Code points with this
property value are permitted for use in specific string classes. property value are permitted for use in specific string classes.
In the remainder of this document, the abbreviated term *_PVAL is In the remainder of this document, the abbreviated term *_PVAL is
used, where * = (NAME | FREE), i.e., either FREE_PVAL or used, where * = (NAME | FREE), i.e., either FREE_PVAL or
NAME_PVAL. SAFE_PVAL.
CONTEXTUAL RULE REQUIRED Some characteristics of the character, such CONTEXTUAL RULE REQUIRED Some characteristics of the character, such
as its being invisible in certain contexts or problematic in as its being invisible in certain contexts or problematic in
others, require that it not be used in labels unless specific others, require that it not be used in labels unless specific
other characters or properties are present. The abbreviated term other characters or properties are present. The abbreviated term
CONTEXT is used to refer to this value in the remainder of this CONTEXT is used to refer to this value in the remainder of this
document. As in IDNA2008, there are two subdivisions of document. As in IDNA2008, there are two subdivisions of
CONTEXTUAL RULE REQUIRED, the first for Join_controls (called CONTEXTUAL RULE REQUIRED, the first for Join_controls (called
CONTEXTJ) and the second for other characters (called CONTEXTO). CONTEXTJ) and the second for other characters (called CONTEXTO).
DISALLOWED Those code points that must not permitted in any PRECIS DISALLOWED Those code points that must not permitted in any PRECIS
string class. string class.
SPECIFIC CLASS DISALLOWED Those code points that are not to be SPECIFIC CLASS DISALLOWED Those code points that are not to be
included in a specific string class. Code points with this included in a specific string class. Code points with this
property value are not permitted in one of the string classes but property value are not permitted in one of the string classes but
might be permitted in others. In the remainder of this document, might be permitted in others. In the remainder of this document,
the abbreviated term *_DIS is used, where * = (NAME | FREE), i.e., the abbreviated term *_DIS is used, where * = (NAME | FREE), i.e.,
either FREE_DIS or NAME_DIS. either FREE_DIS or SAFE_DIS.
UNASSIGNED Those code points that are not designated (i.e. are UNASSIGNED Those code points that are not designated (i.e. are
unassigned) in the Unicode Standard. unassigned) in the Unicode Standard.
The mechanisms described here allow determination of the value of the The mechanisms described here allow determination of the value of the
property for future versions of Unicode (including characters added property for future versions of Unicode (including characters added
after Unicode 5.2 or 6.1 depending on the category, since some after Unicode 5.2 or 6.1 depending on the category, since some
categories in this document are reused from IDNA2008 and therefore categories in this document are reused from IDNA2008 and therefore
were defined at the time of Unicode 5.2). Changes in Unicode were defined at the time of Unicode 5.2). Changes in Unicode
properties that do not affect the outcome of this process do not properties that do not affect the outcome of this process do not
affect this framework. For example, a character can have its Unicode affect this framework. For example, a character can have its Unicode
General_Category value [UNICODE] change from So to Sm, or from Lo to General_Category value [UNICODE] change from So to Sm, or from Lo to
skipping to change at page 14, line 25 skipping to change at page 15, line 38
1. Characters are placed in one or more character categories either 1. Characters are placed in one or more character categories either
(1) based on core properties defined by the Unicode Standard or (1) based on core properties defined by the Unicode Standard or
(2) by treating the code point as an exception and addressing the (2) by treating the code point as an exception and addressing the
code point based on its code point value. These categories are code point based on its code point value. These categories are
not mutually exclusive. not mutually exclusive.
2. Set operations are used with these categories to determine the 2. Set operations are used with these categories to determine the
values for a property that is specific to a given string class. values for a property that is specific to a given string class.
These operations are specified under Section 7. These operations are specified under Section 7.
(NOTE: Unicode property names and property value names might have (Note: Unicode property names and property value names might have
short abbreviations, such as "gc" for the General_Category property short abbreviations, such as "gc" for the General_Category property
and "Ll" for the Lowercase_Letter property value of the gc property.) and "Ll" for the Lowercase_Letter property value of the gc property.)
In the following specification of character categories, the operation In the following specification of character categories, the operation
that returns the value of a particular Unicode character property for that returns the value of a particular Unicode character property for
a code point is designated by using the formal name of that property a code point is designated by using the formal name of that property
(from the Unicode PropertyAliases.txt [1]) followed by '(cp)' for (from the Unicode PropertyAliases.txt [1]) followed by '(cp)' for
"code point". For example, the value of the General_Category "code point". For example, the value of the General_Category
property for a code point is indicated by General_Category(cp). property for a code point is indicated by General_Category(cp).
The first ten categories (A-J) shown below were previously defined The first ten categories (A-J) shown below were previously defined
for IDNA2008 and are copied directly from [RFC5892]. Some of these for IDNA2008 and are copied directly from [RFC5892]. Some of these
categories are reused in PRECIS and some of them are not; however, categories are reused in PRECIS and some of them are not; however,
the lettering of categories is retained to prevent overlap and to the lettering of categories is retained to prevent overlap and to
ease implementation of both IDNA2008 and PRECIS in a single software ease implementation of both IDNA2008 and PRECIS in a single software
application. The next seven categories (K-Q) are specific to PRECIS. application. The next seven categories (K-Q) are specific to PRECIS.
6.1. LetterDigits (A) 6.1. LetterDigits (A)
NOTE: This category is defined in [RFC5892] and copied here for use Note: This category is defined in [RFC5892] and copied here for use
in PRECIS. in PRECIS.
A: General_Category(cp) is in {Ll, Lu, Lm, Lo, Mn, Mc, Nd} A: General_Category(cp) is in {Ll, Lu, Lm, Lo, Mn, Mc, Nd}
These rules identify characters commonly used in mnemonics and often These rules identify characters commonly used in mnemonics and often
informally described as "language characters". informally described as "language characters".
For more information, see section 4.5 of [UNICODE]. For more information, see section 4.5 of [UNICODE].
The categories used in this rule are: The categories used in this rule are:
skipping to change at page 15, line 9 skipping to change at page 16, line 21
in PRECIS. in PRECIS.
A: General_Category(cp) is in {Ll, Lu, Lm, Lo, Mn, Mc, Nd} A: General_Category(cp) is in {Ll, Lu, Lm, Lo, Mn, Mc, Nd}
These rules identify characters commonly used in mnemonics and often These rules identify characters commonly used in mnemonics and often
informally described as "language characters". informally described as "language characters".
For more information, see section 4.5 of [UNICODE]. For more information, see section 4.5 of [UNICODE].
The categories used in this rule are: The categories used in this rule are:
o Ll - Lowercase_Letter o Ll - Lowercase_Letter
o Lu - Uppercase_Letter o Lu - Uppercase_Letter
o Lm - Modifier_Letter o Lm - Modifier_Letter
o Lo - Other_Letter o Lo - Other_Letter
o Mn - Nonspacing_Mark o Mn - Nonspacing_Mark
o Mc - Spacing_Mark o Mc - Spacing_Mark
o Nd - Decimal_Number o Nd - Decimal_Number
6.2. Unstable (B) 6.2. Unstable (B)
NOTE: This category is defined in [RFC5892] but not used in PRECIS. Note: This category is defined in [RFC5892] but not used in PRECIS.
6.3. IgnorableProperties (C) 6.3. IgnorableProperties (C)
NOTE: This category is defined in [RFC5892] but not used in PRECIS. Note: This category is defined in [RFC5892] but not used in PRECIS.
See the "PrecisIgnorableProperties (M)" category below for a more See the "PrecisIgnorableProperties (M)" category below for a more
inclusive category used in PRECIS identifiers. inclusive category used in PRECIS identifiers.
6.4. IgnorableBlocks (D) 6.4. IgnorableBlocks (D)
NOTE: This category is defined in [RFC5892] but not used in PRECIS. Note: This category is defined in [RFC5892] but not used in PRECIS.
6.5. LDH (E) 6.5. LDH (E)
NOTE: This category is defined in [RFC5892] but not used in PRECIS. Note: This category is defined in [RFC5892] but not used in PRECIS.
See the "ASCII7 (K)" category below for a more inclusive category See the "ASCII7 (K)" category below for a more inclusive category
used in PRECIS identifiers. used in PRECIS identifiers.
6.6. Exceptions (F) 6.6. Exceptions (F)
NOTE: This category is defined in [RFC5892] and used in PRECIS to Note: This category is defined in [RFC5892] and used in PRECIS to
ensure consistent treatment of the relevant code points. ensure consistent treatment of the relevant code points.
F: cp is in {00B7, 00DF, 0375, 03C2, 05F3, 05F4, 0640, 0660, F: cp is in {00B7, 00DF, 0375, 03C2, 05F3, 05F4, 0640, 0660,
0661, 0662, 0663, 0664, 0665, 0666, 0667, 0668, 0661, 0662, 0663, 0664, 0665, 0666, 0667, 0668,
0669, 06F0, 06F1, 06F2, 06F3, 06F4, 06F5, 06F6, 0669, 06F0, 06F1, 06F2, 06F3, 06F4, 06F5, 06F6,
06F7, 06F8, 06F9, 06FD, 06FE, 07FA, 0F0B, 3007, 06F7, 06F8, 06F9, 06FD, 06FE, 07FA, 0F0B, 3007,
302E, 302F, 3031, 3032, 3033, 3034, 3035, 303B, 302E, 302F, 3031, 3032, 3033, 3034, 3035, 303B,
30FB} 30FB}
This category explicitly lists code points for which the category This category explicitly lists code points for which the category
skipping to change at page 17, line 12 skipping to change at page 18, line 39
3031; DISALLOWED # VERTICAL KANA REPEAT MARK 3031; DISALLOWED # VERTICAL KANA REPEAT MARK
3032; DISALLOWED # VERTICAL KANA REPEAT WITH VOICED SOUND MARK 3032; DISALLOWED # VERTICAL KANA REPEAT WITH VOICED SOUND MARK
3033; DISALLOWED # VERTICAL KANA REPEAT MARK UPPER HALF 3033; DISALLOWED # VERTICAL KANA REPEAT MARK UPPER HALF
3034; DISALLOWED # VERTICAL KANA REPEAT WITH VOICED SOUND MARK 3034; DISALLOWED # VERTICAL KANA REPEAT WITH VOICED SOUND MARK
UPPER HA UPPER HA
3035; DISALLOWED # VERTICAL KANA REPEAT MARK LOWER HALF 3035; DISALLOWED # VERTICAL KANA REPEAT MARK LOWER HALF
303B; DISALLOWED # VERTICAL IDEOGRAPHIC ITERATION MARK 303B; DISALLOWED # VERTICAL IDEOGRAPHIC ITERATION MARK
6.7. BackwardCompatible (G) 6.7. BackwardCompatible (G)
NOTE: This category is defined in [RFC5892] and copied here for use Note: This category is defined in [RFC5892] and copied here for use
in PRECIS. Because of how the PRECIS string classes are defined, in PRECIS. Because of how the PRECIS string classes are defined,
only changes that would result in code points being added to or only changes that would result in code points being added to or
removed from the LetterDigits ("A") category would result in removed from the LetterDigits ("A") category would result in
backward-incompatible modifications to code point assignments. backward-incompatible modifications to code point assignments.
Therefore, management of this category is handled via the processes Therefore, management of this category is handled via the processes
specified in [RFC5892]. specified in [RFC5892].
G: cp is in {} G: cp is in {}
This category includes the code points for which property values in This category includes the code points for which property values in
versions of Unicode after 5.2 have changed in such a way that the versions of Unicode after 5.2 have changed in such a way that the
derived property value would no longer be PVALID or DISALLOWED. If derived property value would no longer be PVALID or DISALLOWED. If
changes are made to future versions of Unicode so that code points changes are made to future versions of Unicode so that code points
might change property value from PVALID or DISALLOWED, then this might change property value from PVALID or DISALLOWED, then this
table can be updated and keep special exception values so that the table can be updated and keep special exception values so that the
property values for code points stay stable. property values for code points stay stable.
6.8. JoinControl (H) 6.8. JoinControl (H)
skipping to change at page 17, line 32 skipping to change at page 19, line 14
This category includes the code points for which property values in This category includes the code points for which property values in
versions of Unicode after 5.2 have changed in such a way that the versions of Unicode after 5.2 have changed in such a way that the
derived property value would no longer be PVALID or DISALLOWED. If derived property value would no longer be PVALID or DISALLOWED. If
changes are made to future versions of Unicode so that code points changes are made to future versions of Unicode so that code points
might change property value from PVALID or DISALLOWED, then this might change property value from PVALID or DISALLOWED, then this
table can be updated and keep special exception values so that the table can be updated and keep special exception values so that the
property values for code points stay stable. property values for code points stay stable.
6.8. JoinControl (H) 6.8. JoinControl (H)
NOTE: This category is defined in [RFC5892] and copied here for use Note: This category is defined in [RFC5892] and copied here for use
in PRECIS. in PRECIS.
H: Join_Control(cp) = True H: Join_Control(cp) = True
This category consists of Join Control characters (i.e., they are not This category consists of Join Control characters (i.e., they are not
in LetterDigits (Section 6.1) but are still required in strings under in LetterDigits (Section 6.1) but are still required in strings under
some circumstances). some circumstances).
6.9. OldHangulJamo (I) 6.9. OldHangulJamo (I)
NOTE: This category is defined in [RFC5892] and copied here for use Note: This category is defined in [RFC5892] and copied here for use
in PRECIS. in PRECIS.
I: Hangul_Syllable_Type(cp) is in {L, V, T} I: Hangul_Syllable_Type(cp) is in {L, V, T}
This category consists of all conjoining Hangul Jamo (Leading Jamo, This category consists of all conjoining Hangul Jamo (Leading Jamo,
Vowel Jamo, and Trailing Jamo). Vowel Jamo, and Trailing Jamo).
Elimination of conjoining Hangul Jamos from the set of PVALID Elimination of conjoining Hangul Jamos from the set of PVALID
characters results in restricting the set of Korean PVALID characters characters results in restricting the set of Korean PVALID characters
just to preformed, modern Hangul syllable characters. Old Hangul just to preformed, modern Hangul syllable characters. Old Hangul
syllables, which must be spelled with sequences of conjoining Hangul syllables, which must be spelled with sequences of conjoining Hangul
Jamos, are not PVALID for string classes. Jamos, are not PVALID for string classes.
6.10. Unassigned (J) 6.10. Unassigned (J)
NOTE: This category is defined in [RFC5892] and copied here for use Note: This category is defined in [RFC5892] and copied here for use
in PRECIS. in PRECIS.
J: General_Category(cp) is in {Cn} and J: General_Category(cp) is in {Cn} and
Noncharacter_Code_Point(cp) = False Noncharacter_Code_Point(cp) = False
This category consists of code points in the Unicode character set This category consists of code points in the Unicode character set
that are not (yet) assigned. It should be noted that Unicode that are not (yet) assigned. It should be noted that Unicode
distinguishes between 'unassigned code points' and 'unassigned distinguishes between 'unassigned code points' and 'unassigned
characters'. The unassigned code points are all but (Cn - characters'. The unassigned code points are all but (Cn -
Noncharacters), while the unassigned *characters* are all but (Cn + Noncharacters), while the unassigned *characters* are all but (Cn +
skipping to change at page 20, line 10 skipping to change at page 21, line 45
letters and digits other than the "traditional" letters and digits letters and digits other than the "traditional" letters and digits
grouped under the LetterDigits (A) class (see Section 6.1). grouped under the LetterDigits (A) class (see Section 6.1).
R: General_Category(cp) is in {Lt, Nl, No, Me} R: General_Category(cp) is in {Lt, Nl, No, Me}
7. Calculation of the Derived Property 7. Calculation of the Derived Property
Possible values of the derived property are: Possible values of the derived property are:
o PVALID o PVALID
o NAME_PVAL
o SAFE_PVAL
o FREE_PVAL o FREE_PVAL
o CONTEXTJ o CONTEXTJ
o CONTEXTO o CONTEXTO
o DISALLOWED o DISALLOWED
o NAME_DIS
o SAFE_DIS
o FREE_DIS o FREE_DIS
o UNASSIGNED o UNASSIGNED
NOTE: The value of the derived property calculated can depend on the Note: The value of the derived property calculated can depend on the
string class; for example, if an identifier used in an application string class; for example, if an identifier used in an application
protocol is defined as using or subclassing the PRECIS NameClass then protocol is defined as using or subclassing the PRECIS
a space character such as U+0020 would be assigned to NAME_DIS, IdentifierClass then a space character such as U+0020 would be
whereas if an identifier is defined as using or subclassing the assigned to SAFE_DIS, whereas if an identifier is defined as using or
PRECIS FreeClass then the character would be assigned to FREE_PVAL. subclassing the PRECIS FreeformClass then the character would be
For the sake of brevity, the designation "FREE_PVAL" is used in the assigned to FREE_PVAL. For the sake of brevity, the designation
code point tables, instead of the longer designation "NAME_DIS or "FREE_PVAL" is used in the code point tables, instead of the longer
FREE_PVAL". In practice, the derived properties NAME_PVAL and designation "SAFE_DIS or FREE_PVAL". In practice, the derived
FREE_DIS are not used in this specification, since every NAME_PVAL properties SAFE_PVAL and FREE_DIS are not used in this specification,
code point is PVALID and every FREE_DIS code point is DISALLOWED. since every SAFE_PVAL code point is PVALID and every FREE_DIS code
point is DISALLOWED.
The algorithm to calculate the value of the derived property is as The algorithm to calculate the value of the derived property is as
follows. (NOTE: Use of the name of a rule (such as "Exception") follows. (Note: Use of the name of a rule (such as "Exception")
implies the set of code points that the rule defines, whereas the implies the set of code points that the rule defines, whereas the
same name as a function call (such as "Exception(cp)") implies the same name as a function call (such as "Exception(cp)") implies the
value that the code point has in the Exceptions table.) value that the code point has in the Exceptions table.)
If .cp. .in. Exceptions Then Exceptions(cp); If .cp. .in. Exceptions Then Exceptions(cp);
Else If .cp. .in. BackwardCompatible Then BackwardCompatible(cp); Else If .cp. .in. BackwardCompatible Then BackwardCompatible(cp);
Else If .cp. .in. Unassigned Then UNASSIGNED; Else If .cp. .in. Unassigned Then UNASSIGNED;
Else If .cp. .in. ASCII7 Then PVALID; Else If .cp. .in. ASCII7 Then PVALID;
Else If .cp. .in. JoinControl Then CONTEXTJ; Else If .cp. .in. JoinControl Then CONTEXTJ;
Else If .cp. .in. PrecisIgnorableProperties Then DISALLOWED; Else If .cp. .in. PrecisIgnorableProperties Then DISALLOWED;
Else If .cp. .in. Controls Then DISALLOWED; Else If .cp. .in. Controls Then DISALLOWED;
Else If .cp. .in. OldHangulJamo Then DISALLOWED; Else If .cp. .in. OldHangulJamo Then DISALLOWED;
Else If .cp. .in. LetterDigits Then PVALID; Else If .cp. .in. LetterDigits Then PVALID;
Else If .cp. .in. OtherLetterDigits Then NAME_DIS or FREE_PVAL; Else If .cp. .in. OtherLetterDigits Then SAFE_DIS or FREE_PVAL;
Else If .cp. .in. Spaces Then NAME_DIS or FREE_PVAL; Else If .cp. .in. Spaces Then SAFE_DIS or FREE_PVAL;
Else If .cp. .in. Symbols Then NAME_DIS or FREE_PVAL; Else If .cp. .in. Symbols Then SAFE_DIS or FREE_PVAL;
Else If .cp. .in. Punctuation Then NAME_DIS or FREE_PVAL; Else If .cp. .in. Punctuation Then SAFE_DIS or FREE_PVAL;
Else If .cp. .in. HasCompat Then NAME_DIS or FREE_PVAL; Else If .cp. .in. HasCompat Then SAFE_DIS or FREE_PVAL;
Else DISALLOWED; Else DISALLOWED;
8. Code Points 8. Code Points
The Categories and Rules defined under Section 6 and Section 7 apply The Categories and Rules defined under Section 6 and Section 7 apply
to all Unicode code points. The table in Appendix A shows, for to all Unicode code points. The table in Appendix A shows, for
illustrative purposes, the consequences of the categories and illustrative purposes, the consequences of the categories and
classification rules, and the resulting property values. classification rules, and the resulting property values.
The list of code points that can be found in Appendix A is non- The list of code points that can be found in Appendix A is non-
skipping to change at page 21, line 35 skipping to change at page 24, line 5
string is connected to the wrong account or online resource based on string is connected to the wrong account or online resource based on
different interpretations of the string. different interpretations of the string.
Specifications of application protocols that use this framework are Specifications of application protocols that use this framework are
encouraged to describe how internationalized strings are used in the encouraged to describe how internationalized strings are used in the
protocol, including the security implications of any false positives protocol, including the security implications of any false positives
and false negatives that might result from various comparison and false negatives that might result from various comparison
operations. For some helpful guidelines, refer to operations. For some helpful guidelines, refer to
[I-D.iab-identifier-comparison], [RFC5890], [UTR36], and [UTR39]. [I-D.iab-identifier-comparison], [RFC5890], [UTR36], and [UTR39].
9.2. Use of the NameClass 9.2. Use of the IdentifierClass
Strings that conform to the NameClass and any subclass thereof are Strings that conform to the IdentifierClass and any subclass thereof
intended to be relatively safe for use in a broad range of are intended to be relatively safe for use in a broad range of
applications, primarily because they include only letters, digits, applications, primarily because they include only letters, digits,
and "grandfathered" non-space characters from the ASCII range; thus and "grandfathered" non-space characters from the ASCII range; thus
they exclude spaces, characters with compatibility equivalents, and they exclude spaces, characters with compatibility equivalents, and
almost all symbols and punctuation marks. However, because such almost all symbols and punctuation marks. However, because such
strings can still include so-called confusable characters (see strings can still include so-called confusable characters (see
Section 9.5, protocol designers and implementers are encouraged to Section 9.5, protocol designers and implementers are encouraged to
pay close attention to the security considerations described pay close attention to the security considerations described
elsewhere in this document. elsewhere in this document.
9.3. Use of the FreeClass 9.3. Use of the FreeformClass
Strings that conform to the FreeClass and many subclasses thereof can Strings that conform to the FreeformClass and many subclasses thereof
include virtually any Unicode character. This makes the FreeClass can include virtually any Unicode character. This makes the
quite expressive, but also problematic from the perspective of FreeformClass quite expressive, but also problematic from the
possible user confusion. Protocol designers are hereby warned that perspective of possible user confusion. Protocol designers are
the FreeClass contains codepoints they might not understand, and are hereby warned that the FreeformClass contains codepoints they might
encouraged to use or subclass the NameClass wherever feasible; not understand, and are encouraged to use or subclass the
however, if an application protocol requires more code points than IdentifierClass wherever feasible; however, if an application
are allowed by the NameClass, protocol designers are encouraged to protocol requires more code points than are allowed by the
define a subclass of the FreeClass that restricts the allowable code IdentifierClass, protocol designers are encouraged to define a
subclass of the FreeformClass that restricts the allowable code
points as tightly as possible. (The working group considered the points as tightly as possible. (The working group considered the
option of allowing superclasses as well as subclasses of PRECIS option of allowing superclasses as well as subclasses of PRECIS
string classes, but decided against allowing superclasses to reduce string classes, but decided against allowing superclasses to reduce
the likelihood of security and interoperability problems.) the likelihood of security and interoperability problems.)
9.4. Local Character Set Issues 9.4. Local Character Set Issues
When systems use local character sets other than ASCII and Unicode, When systems use local character sets other than ASCII and Unicode,
these specifications leave the problem of converting between the these specifications leave the problem of converting between the
local character set and Unicode up to the application or local local character set and Unicode up to the application or local
skipping to change at page 24, line 25 skipping to change at page 26, line 45
existence of such communities and encourages due caution when existence of such communities and encourages due caution when
presenting unfamiliar scripts or characters to human users.) presenting unfamiliar scripts or characters to human users.)
9.6. Security of Passwords 9.6. Security of Passwords
Two goals of passwords are to maximize the amount of entropy and to Two goals of passwords are to maximize the amount of entropy and to
minimize the potential for false positives. These goals can be minimize the potential for false positives. These goals can be
achieved in part by allowing a wide range of code points and by achieved in part by allowing a wide range of code points and by
ensuring that passwords are handled in such a way that code points ensuring that passwords are handled in such a way that code points
are not compared aggressively. Therefore, it is NOT RECOMMENDED for are not compared aggressively. Therefore, it is NOT RECOMMENDED for
application protocols to subclass the FreeClass for use in passwords application protocols to subclass the FreeformClass for use in
in a way that removes entire categories (e.g., by disallowing symbols passwords in a way that removes entire categories (e.g., by
or punctuation). Furthermore, it is NOT RECOMMENDED for application disallowing symbols or punctuation). Furthermore, it is NOT
protocols to map uppercase and titlecase code points to their RECOMMENDED for application protocols to map uppercase and titlecase
lowercase equivalents in such strings; instead, it is RECOMMENDED to code points to their lowercase equivalents in such strings; instead,
preserve the case of all code points contained in such strings and to it is RECOMMENDED to preserve the case of all code points contained
compare them in a case-sensitive manner. in such strings and to compare them in a case-sensitive manner.
That said, software implementers need to be aware that there exist That said, software implementers need to be aware that there exist
tradeoffs between entropy and usability. For example, allowing a tradeoffs between entropy and usability. For example, allowing a
user to establish a password containing "uncommon" code points might user to establish a password containing "uncommon" code points might
make it difficult for the user to access a service when using an make it difficult for the user to access a service when using an
unfamiliar or constrained input device. unfamiliar or constrained input device.
Some application protocols use passwords directly, whereas others Some application protocols use passwords directly, whereas others
reuse technologies that themselves process passwords (one example of reuse technologies that themselves process passwords (one example of
such a technology is the Simple Authentication and Security Layer such a technology is the Simple Authentication and Security Layer
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The IESG is to be notified if backward-incompatible changes to the The IESG is to be notified if backward-incompatible changes to the
table of derived properties are discovered or if other problems arise table of derived properties are discovered or if other problems arise
during the process of creating the table of derived property values during the process of creating the table of derived property values
or during expert review. Changes to the rules defined under or during expert review. Changes to the rules defined under
Section 6 and Section 7) require IETF Review, as described in Section 6 and Section 7) require IETF Review, as described in
[RFC5226]. [RFC5226].
10.2. PRECIS Base Classes Registry 10.2. PRECIS Base Classes Registry
IANA is requested to create a registry of PRECIS base string classes. IANA is requested to create a registry of PRECIS string classes. In
In accordance with [RFC5226], the registration policy is "RFC accordance with [RFC5226], the registration policy is "RFC Required".
Required".
The registration template is as follows: The registration template is as follows:
Base Class: [the name of the base class] Base Class: [the name of the PRECIS string class]
Description: [a brief description of the base class and its intended
use, e.g., "A sequence of letters, numbers, and symbols that is Description: [a brief description of the PRECIS string class and its
used to identify or address a network entity."] intended use, e.g., "A sequence of letters, numbers, and symbols
that is used to identify or address a network entity."]
Normalization: [which Unicode normalization form is applied, e.g., Normalization: [which Unicode normalization form is applied, e.g.,
"NFC"; or "Application Specific" if to be defined by protocols "NFC"; or "Application Specific" if to be defined by protocols
that use the base class] that use the PRECIS string class]
Casemapping: [the behavioral rule for handling of case, e.g., "Map
Case Mapping: [the behavioral rule for handling of case, e.g., "Map
uppercase and titlecase characters to lowercase."; or "Application uppercase and titlecase characters to lowercase."; or "Application
Specific" if to be defined by protocols that use the base class] Specific" if to be defined by protocols that use the PRECIS string
class]
Additional Mappings: [any additional mappings are required or Additional Mappings: [any additional mappings are required or
recommended, e.g., "Map non-ASCII space characters to ASCII recommended, e.g., "Map non-ASCII space characters to ASCII
space."; or "Application Specific" if to be defined by protocols space."; or "Application Specific" if to be defined by protocols
that use the base class] that use the PRECIS string class]
Directionality: [the behavioral rule for handling of right-to-left Directionality: [the behavioral rule for handling of right-to-left
code points, e.g., "The 'Bidi Rule' defined in RFC 5893 applies."; code points, e.g., "The 'Bidi Rule' defined in RFC 5893 applies.";
or "Application Specific" if to be defined by protocols that use or "Application Specific" if to be defined by protocols that use
the base class] the PRECIS string class]
Specification: [the RFC number] Specification: [the RFC number]
The initial registrations are as follows: The initial registrations are as follows:
Base Class: FreeClass. Base Class: FreeformClass.
Description: A sequence of letters, numbers, symbols, spaces, and Description: A sequence of letters, numbers, symbols, spaces, and
other code points that is used for free-form strings. other code points that is used for free-form strings.
Normalization: Application Specific. Normalization: Application Specific.
Casemapping: Application Specific. Case Mapping: Application Specific.
Additional Mappings: Application Specific. Additional Mappings: Application Specific.
Directionality: Application Specific. Directionality: Application Specific.
Specification: RFC XXXX. [Note to RFC Editor: please change XXXX to Specification: RFC XXXX. [Note to RFC Editor: please change XXXX to
the number issued for this specification.] the number issued for this specification.]
Base Class: NameClass. Base Class: IdentifierClass.
Description: A sequence of letters, numbers, and symbols that is Description: A sequence of letters, numbers, and symbols that is
used to identify or address a network entity. used to identify or address a network entity.
Normalization: Application Specific. Normalization: Application Specific.
Casemapping: Application Specific. Case Mapping: Application Specific.
Additional Mappings: Application Specific. Additional Mappings: Application Specific.
Directionality: Application Specific. Directionality: Application Specific.
Specification: RFC XXXX. [Note to RFC Editor: please change XXXX to Specification: RFC XXXX. [Note to RFC Editor: please change XXXX to
the number issued for this specification.] the number issued for this specification.]
10.3. PRECIS Subclasses Registry 10.3. PRECIS Subclasses Registry
IANA is requested to create a registry of subclasses that use the IANA is requested to create a registry of subclasses that use the
PRECIS base string classes. In accordance with [RFC5226], the PRECIS string classes. In accordance with [RFC5226], the
registration policy is "Expert Review". This policy was chosen in registration policy is "Expert Review". This policy was chosen in
order to ensure that "customers" of PRECIS receive appropriate order to ensure that "customers" of PRECIS receive appropriate
guidance regarding the sometimes complex and subtle guidance regarding the sometimes complex and subtle
internationalization issues related to subclassing of PRECIS base internationalization issues related to subclassing of PRECIS string
classes. classes.
The registration template is as follows: The registration template is as follows:
Subclass: [the name of the subclass] Subclass: [the name of the subclass]
Base Class: [which base class is being subclassed]
Base Class: [which PRECIS string class is being subclassed]
Exclusions: [a brief description of the specific code points that Exclusions: [a brief description of the specific code points that
are excluded or of the properties based on which characters are are excluded or of the properties based on which characters are
excluded, e.g., "Eight legacy characters in the ASCII range" or excluded, e.g., "Eight legacy characters in the ASCII range" or
"Any character that has a compatibility equivalent, i.e., the "Any character that has a compatibility equivalent, i.e., the
HasCompat category"] HasCompat category"]
Specification: [a pointer to relevant documentation, such as an RFC Specification: [a pointer to relevant documentation, such as an RFC
or Internet-Draft] or Internet-Draft]
In order to request a review, the registrant shall send a completed In order to request a review, the registrant shall send a completed
template to the precis@ietf.org list or its designated successor. template to the precis@ietf.org list or its designated successor.
Factors to focus on while reviewing subclass registrations include Factors to focus on while reviewing subclass registrations include
the following: the following:
o Is the problem well-defined? o Is the problem well-defined?
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Specification: [a pointer to relevant documentation, such as an RFC Specification: [a pointer to relevant documentation, such as an RFC
or Internet-Draft] or Internet-Draft]
In order to request a review, the registrant shall send a completed In order to request a review, the registrant shall send a completed
template to the precis@ietf.org list or its designated successor. template to the precis@ietf.org list or its designated successor.
Factors to focus on while reviewing subclass registrations include Factors to focus on while reviewing subclass registrations include
the following: the following:
o Is the problem well-defined? o Is the problem well-defined?
o Is it clear what applications will use this subclass? o Is it clear what applications will use this subclass?
o Would an existing base class or subclass solve the problem?
o Would an existing PRECIS string class or subclass solve the
problem?
o Are the defined exclusions a reasonable solution to the problem o Are the defined exclusions a reasonable solution to the problem
for the relevant applications? for the relevant applications?
o Is the subclass clearly defined? o Is the subclass clearly defined?
o Does the subclass reduce the degree to which human users could be o Does the subclass reduce the degree to which human users could be
surprised by application behavior (the "principle of least user surprised by application behavior (the "principle of least user
surprise")? surprise")?
o Is the subclass based on an appropriate dividing line between user o Is the subclass based on an appropriate dividing line between user
interface (culture, context, intent, locale, device limitations, interface (culture, context, intent, locale, device limitations,
etc.) and the use of conformant strings in protocol elements? etc.) and the use of conformant strings in protocol elements?
o Does the subclass introduce any new security concerns (e.g., false o Does the subclass introduce any new security concerns (e.g., false
positives for authentication or authorization)? positives for authentication or authorization)?
10.4. PRECIS Usage Registry 10.4. PRECIS Usage Registry
IANA is requested to create a registry of application protocols that IANA is requested to create a registry of application protocols that
use the base string classes. The registry will include one entry for use the PRECIS string classes. The registry will include one entry
each use (e.g., if a protocol uses both the NameClass and the for each use (e.g., if a protocol uses both the IdentifierClass and
FreeClass then the specification for that protocol would submit two the FreeformClass then the specification for that protocol would
registrations). In accordance with [RFC5226], the registration submit two registrations). In accordance with [RFC5226], the
policy is "Expert Review". This policy was chosen in order to ensure registration policy is "Expert Review". This policy was chosen in
that "customers" of PRECIS receive appropriate guidance regarding the order to ensure that "customers" of PRECIS receive appropriate
sometimes complex and subtle internationalization issues related to guidance regarding the sometimes complex and subtle
use of PRECIS base classes. internationalization issues related to use of PRECIS string classes.
The registration template is as follows: The registration template is as follows:
Applicability: [the specific protocol elements to which this usage Applicability: [the specific protocol elements to which this usage
applies, e.g., "Localparts in XMPP addresses."] applies, e.g., "Localparts in XMPP addresses."]
Base Class: [the base string class that is being used or subclassed]
Subclass: [whether the protocol has defined a subclass of the base Base Class: [the PRECIS string class that is being used or
class and, if so, the name of the subclass, e.g., "Yes, subclassed]
LocalpartNameClass."]
Subclass: [whether the protocol has defined a subclass of the PRECIS
string class and, if so, the name of the subclass, e.g., "Yes,
LocalpartIdentifierClass."]
Replaces: [the Stringprep profile that this PRECIS usage replaces, Replaces: [the Stringprep profile that this PRECIS usage replaces,
if any] if any]
Normalization: [which Unicode normalization form is applied, e.g., Normalization: [which Unicode normalization form is applied, e.g.,
"NFC"] "NFC"]
Casemapping: [the behavioral rule for handling of case, e.g., "Map
Case Mapping: [the behavioral rule for handling of case, e.g., "Map
uppercase and titlecase characters to lowercase."] uppercase and titlecase characters to lowercase."]
Width Mapping: [the behavioral rule for handling of width, e.g.,
"Map fullwidth and halfwidth characters to their decomposition
equivalents."]
Additional Mappings: [any additional mappings are required or Additional Mappings: [any additional mappings are required or
recommended, e.g., "Map non-ASCII space characters to ASCII recommended, e.g., "Map non-ASCII space characters to ASCII
space."] space."]
Directionality: [the behavioral rule for handling of right-to-left Directionality: [the behavioral rule for handling of right-to-left
code points, e.g., "The 'Bidi Rule' defined in RFC 5893 applies."] code points, e.g., "The 'Bidi Rule' defined in RFC 5893 applies."]
Specification: [a pointer to relevant documentation, such as an RFC Specification: [a pointer to relevant documentation, such as an RFC
or Internet-Draft] or Internet-Draft]
In order to request a review, the registrant shall send a completed In order to request a review, the registrant shall send a completed
template to the precis@ietf.org list or its designated successor. template to the precis@ietf.org list or its designated successor.
Factors to focus on while reviewing usage registrations include the Factors to focus on while reviewing usage registrations include the
following: following:
o Does the specification define what kinds of applications are o Does the specification define what kinds of applications are
skipping to change at page 28, line 21 skipping to change at page 31, line 16
or Internet-Draft] or Internet-Draft]
In order to request a review, the registrant shall send a completed In order to request a review, the registrant shall send a completed
template to the precis@ietf.org list or its designated successor. template to the precis@ietf.org list or its designated successor.
Factors to focus on while reviewing usage registrations include the Factors to focus on while reviewing usage registrations include the
following: following:
o Does the specification define what kinds of applications are o Does the specification define what kinds of applications are
involved and the protocol elements to which this usage applies? involved and the protocol elements to which this usage applies?
o Is there a base class or subclass that would be more appropriate
to use? o Is there a PRECIS string class or subclass that would be more
o Are the normalization, casemapping, additional mapping, and appropriate to use?
directionality rules appropriate for the intended use?
o Are the normalization, case mapping, width mapping, additional
mapping, and directionality rules appropriate for the intended
use?
o Does the usage reduce the degree to which human users could be o Does the usage reduce the degree to which human users could be
surprised by application behavior (the "principle of least user surprised by application behavior (the "principle of least user
surprise")? surprise")?
o Is the usage based on an appropriate dividing line between user o Is the usage based on an appropriate dividing line between user
interface (culture, context, intent, locale, device limitations, interface (culture, context, intent, locale, device limitations,
etc.) and the use of conformant strings in protocol elements? etc.) and the use of conformant strings in protocol elements?
o Does the usage introduce any new security concerns (e.g., false o Does the usage introduce any new security concerns (e.g., false
positives for authentication or authorization)? positives for authentication or authorization)?
11. Interoperability Considerations 11. Interoperability Considerations
Although strings that are consumed in PRECIS-based application Although strings that are consumed in PRECIS-based application
protocols are often encoded using UTF-8 [RFC3629], the exact encoding protocols are often encoded using UTF-8 [RFC3629], the exact encoding
is a matter for the application protocol that reuses PRECIS, not for is a matter for the application protocol that reuses PRECIS, not for
the PRECIS framework. the PRECIS framework.
skipping to change at page 29, line 6 skipping to change at page 32, line 6
range. If two (or more) applications need to interoperate when range. If two (or more) applications need to interoperate when
exchanging data (e.g., for the purpose of authenticating a username exchanging data (e.g., for the purpose of authenticating a username
or password), they will naturally need have in common at least one or password), they will naturally need have in common at least one
coded character set (as defined by [RFC6365]). Establishing such a coded character set (as defined by [RFC6365]). Establishing such a
baseline is a matter for the application protocol that reuses PRECIS, baseline is a matter for the application protocol that reuses PRECIS,
not for the PRECIS framework. not for the PRECIS framework.
12. References 12. References
12.1. Normative References 12.1. Normative References
[RFC20] Cerf, V., "ASCII format for network interchange", RFC 20,
October 1969.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network [RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, March 2008. Interchange", RFC 5198, March 2008.
[UNICODE] The Unicode Consortium, "The Unicode Standard, Version [UNICODE] The Unicode Consortium, "The Unicode Standard, Version
6.1", 2012, 6.2", 2012,
<http://www.unicode.org/versions/Unicode6.1.0/>. <http://www.unicode.org/versions/Unicode6.2.0/>.
12.2. Informative References 12.2. Informative References
[I-D.iab-identifier-comparison] [I-D.iab-identifier-comparison]
Thaler, D., "Issues in Identifier Comparison for Security Thaler, D., "Issues in Identifier Comparison for Security
Purposes", draft-iab-identifier-comparison-04 (work in Purposes", draft-iab-identifier-comparison-09 (work in
progress), August 2012. progress), March 2013.
[I-D.ietf-precis-nickname] [I-D.ietf-precis-nickname]
Saint-Andre, P., "Preparation and Comparison of Saint-Andre, P., "Preparation and Comparison of
Nicknames", draft-ietf-precis-nickname-01 (work in Nicknames", draft-ietf-precis-nickname-05 (work in
progress), September 2012. progress), November 2012.
[I-D.ietf-precis-problem-statement] [I-D.ietf-precis-saslprepbis]
Sullivan, A. and M. Blanchet, "Stringprep Revision Problem Saint-Andre, P. and A. Melnikov, "Username and Password
Statement", draft-ietf-precis-problem-statement-08 (work Preparation Algorithms", draft-ietf-precis-saslprepbis-00
in progress), September 2012. (work in progress), February 2012.
[I-D.ietf-xmpp-6122bis] [I-D.ietf-xmpp-6122bis]
Saint-Andre, P., "Extensible Messaging and Presence Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Address Format", Protocol (XMPP): Address Format", draft-ietf-xmpp-
draft-ietf-xmpp-6122bis-03 (work in progress), 6122bis-05 (work in progress), November 2013.
August 2012.
[I-D.melnikov-precis-saslprepbis]
Melnikov, A. and P. Saint-Andre, "Username and Password
Preparation Algorithms",
draft-melnikov-precis-saslprepbis-03 (work in progress),
September 2012.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson, [RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)", "Remote Authentication Dial In User Service (RADIUS)", RFC
RFC 2865, June 2000. 2865, June 2000.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454, Internationalized Strings ("stringprep")", RFC 3454,
December 2002. December 2002.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello, [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)", "Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003. RFC 3490, March 2003.
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep [RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)", Profile for Internationalized Domain Names (IDN)", RFC
RFC 3491, March 2003. 3491, March 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003. 10646", STD 63, RFC 3629, November 2003.
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and [RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006. Security Layer (SASL)", RFC 4422, June 2006.
[RFC4510] Zeilenga, K., "Lightweight Directory Access Protocol [RFC4510] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Technical Specification Road Map", RFC 4510, (LDAP): Technical Specification Road Map", RFC 4510, June
June 2006. 2006.
[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and [RFC4690] Klensin, J., Faltstrom, P., Karp, C., IAB, "Review and
Recommendations for Internationalized Domain Names Recommendations for Internationalized Domain Names
(IDNs)", RFC 4690, September 2006. (IDNs)", RFC 4690, September 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008. May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008. (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5890] Klensin, J., "Internationalized Domain Names for [RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework", Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010. RFC 5890, August 2010.
[RFC5891] Klensin, J., "Internationalized Domain Names in [RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010. Applications (IDNA): Protocol", RFC 5891, August 2010.
skipping to change at page 31, line 13 skipping to change at page 34, line 13
Rationale", RFC 5894, August 2010. Rationale", RFC 5894, August 2010.
[RFC5895] Resnick, P. and P. Hoffman, "Mapping Characters for [RFC5895] Resnick, P. and P. Hoffman, "Mapping Characters for
Internationalized Domain Names in Applications (IDNA) Internationalized Domain Names in Applications (IDNA)
2008", RFC 5895, September 2010. 2008", RFC 5895, September 2010.
[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in [RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365, Internationalization in the IETF", BCP 166, RFC 6365,
September 2011. September 2011.
[RFC6885] Blanchet, M. and A. Sullivan, "Stringprep Revision and
Problem Statement for the Preparation and Comparison of
Internationalized Strings (PRECIS)", RFC 6885, March 2013.
[UAX11] The Unicode Consortium, "Unicode Standard Annex #11: East
Asian Width", September 2012,
<http://unicode.org/reports/tr11/>.
[UAX15] The Unicode Consortium, "Unicode Standard Annex #15: [UAX15] The Unicode Consortium, "Unicode Standard Annex #15:
Unicode Normalization Forms", September 2010, Unicode Normalization Forms", August 2012,
<http://unicode.org/reports/tr15/>. <http://unicode.org/reports/tr15/>.
[UAX9] The Unicode Consortium, "Unicode Standard Annex #9: [UAX9] The Unicode Consortium, "Unicode Standard Annex #9:
Unicode Bidirectional Algorithm", September 2010, Unicode Bidirectional Algorithm", September 2012,
<http://unicode.org/reports/tr9/>. <http://unicode.org/reports/tr9/>.
[UTR36] The Unicode Consortium, "Unicode Technical Report #36: [UTR36] The Unicode Consortium, "Unicode Technical Report #36:
Unicode Security Considerations", August 2010, Unicode Security Considerations", July 2012,
<http://unicode.org/reports/tr36/>. <http://unicode.org/reports/tr36/>.
[UTR39] The Unicode Consortium, "Unicode Technical Report #39: [UTR39] The Unicode Consortium, "Unicode Technical Report #39:
Unicode Security Mechanisms", August 2010, Unicode Security Mechanisms", July 2012,
<http://unicode.org/reports/tr39/>. <http://unicode.org/reports/tr39/>.
URIs
[1] <http://unicode.org/Public/UNIDATA/PropertyAliases.txt>
[2] <http://unicode.org/Public/UNIDATA/DerivedCoreProperties.txt>
Appendix A. Codepoint Table Appendix A. Codepoint Table
WARNING: The following table is incomplete and very likely contains WARNING: The following table is incomplete and very likely contains
errors! errors!
If one applies the property calculation rules from Section 7 to the If one applies the property calculation rules from Section 7 to the
code points 0x0000 to 0x10FFFF in Unicode 6.1, the result is as shown code points 0x0000 to 0x10FFFF in Unicode 6.1, the result is as shown
in the following table, in Unicode Character Database (UCD) format. in the following table, in Unicode Character Database (UCD) format.
The columns of the table are as follows: The columns of the table are as follows:
skipping to change at page 31, line 46 skipping to change at page 34, line 48
WARNING: The following table is incomplete and very likely contains WARNING: The following table is incomplete and very likely contains
errors! errors!
If one applies the property calculation rules from Section 7 to the If one applies the property calculation rules from Section 7 to the
code points 0x0000 to 0x10FFFF in Unicode 6.1, the result is as shown code points 0x0000 to 0x10FFFF in Unicode 6.1, the result is as shown
in the following table, in Unicode Character Database (UCD) format. in the following table, in Unicode Character Database (UCD) format.
The columns of the table are as follows: The columns of the table are as follows:
1. The code point or codepoint range. 1. The code point or codepoint range.
2. The assignment for the code point or range, where the value is 2. The assignment for the code point or range, where the value is
one of PVALID, DISALLOWED, UNASSIGNED, CONTEXTO, CONTEXTJ, or one of PVALID, DISALLOWED, UNASSIGNED, CONTEXTO, CONTEXTJ, or
FREE_PVAL (which includes NAME_DIS). FREE_PVAL (which includes SAFE_DIS).
3. The name or names for the code point or range. 3. The name or names for the code point or range.
This table is non-normative, and included only for illustrative This table is non-normative, and included only for illustrative
purposes. Please note that the strings displayed in the third column purposes. Please note that the strings displayed in the third column
are not necessarily the formal name of the code point (as defined in are not necessarily the formal name of the code point (as defined in
[UNICODE]) because the fixed width of the RFC format necessitated [UNICODE]) because the fixed width of the RFC format necessitated
truncation of many names. truncation of many names.
0000..001F ; DISALLOWED # <control> 0000..001F ; DISALLOWED # <control>
0020 ; FREE_PVAL # SPACE 0020 ; FREE_PVAL # SPACE
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