PRECIS P. Saint-Andre
Internet-Draft &yet
Obsoletes: 3454 (if approved) M. Blanchet
Intended status: Standards Track Viagenie
Expires: March 6, 2015 September 2, 2014
PRECIS Framework: Preparation and Comparison of Internationalized
Strings in Application Protocols
draft-ietf-precis-framework-18
Abstract
Application protocols using Unicode characters in protocol strings
need to properly prepare such strings in order to perform valid
comparison operations (e.g., for purposes of authentication or
authorization). This document defines a framework enabling
application protocols to perform the preparation and comparison of
internationalized strings ("PRECIS") in a way that depends on the
properties of Unicode characters and thus is agile with respect to
versions of Unicode. As a result, this framework provides a more
sustainable approach to the handling of internationalized strings
than the previous framework, known as Stringprep (RFC 3454). This
document obsoletes RFC 3454.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 6, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. String Classes . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. IdentifierClass . . . . . . . . . . . . . . . . . . . . . 7
3.2.1. Valid . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2.2. Contextual Rule Required . . . . . . . . . . . . . . 8
3.2.3. Disallowed . . . . . . . . . . . . . . . . . . . . . 8
3.2.4. Unassigned . . . . . . . . . . . . . . . . . . . . . 9
3.2.5. Examples . . . . . . . . . . . . . . . . . . . . . . 9
3.3. FreeformClass . . . . . . . . . . . . . . . . . . . . . . 9
3.3.1. Valid . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3.2. Contextual Rule Required . . . . . . . . . . . . . . 10
3.3.3. Disallowed . . . . . . . . . . . . . . . . . . . . . 10
3.3.4. Unassigned . . . . . . . . . . . . . . . . . . . . . 10
3.3.5. Examples . . . . . . . . . . . . . . . . . . . . . . 10
4. Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Principles . . . . . . . . . . . . . . . . . . . . . . . 11
4.1.1. Width Mapping . . . . . . . . . . . . . . . . . . . . 11
4.1.2. Additional Mappings . . . . . . . . . . . . . . . . . 12
4.1.3. Case Mapping . . . . . . . . . . . . . . . . . . . . 12
4.1.4. Normalization . . . . . . . . . . . . . . . . . . . . 12
4.1.5. Directionality . . . . . . . . . . . . . . . . . . . 12
4.1.6. Exclusions . . . . . . . . . . . . . . . . . . . . . 13
4.2. Building Application-Layer Constructs . . . . . . . . . . 13
4.3. A Note about Spaces . . . . . . . . . . . . . . . . . . . 14
5. Order of Operations . . . . . . . . . . . . . . . . . . . . . 14
6. Code Point Properties . . . . . . . . . . . . . . . . . . . . 15
7. Category Definitions Used to Calculate Derived Property . . . 18
7.1. LetterDigits (A) . . . . . . . . . . . . . . . . . . . . 18
7.2. Unstable (B) . . . . . . . . . . . . . . . . . . . . . . 18
7.3. IgnorableProperties (C) . . . . . . . . . . . . . . . . . 19
7.4. IgnorableBlocks (D) . . . . . . . . . . . . . . . . . . . 19
7.5. LDH (E) . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.6. Exceptions (F) . . . . . . . . . . . . . . . . . . . . . 19
7.7. BackwardCompatible (G) . . . . . . . . . . . . . . . . . 19
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7.8. JoinControl (H) . . . . . . . . . . . . . . . . . . . . . 19
7.9. OldHangulJamo (I) . . . . . . . . . . . . . . . . . . . . 20
7.10. Unassigned (J) . . . . . . . . . . . . . . . . . . . . . 20
7.11. ASCII7 (K) . . . . . . . . . . . . . . . . . . . . . . . 20
7.12. Controls (L) . . . . . . . . . . . . . . . . . . . . . . 20
7.13. PrecisIgnorableProperties (M) . . . . . . . . . . . . . . 20
7.14. Spaces (N) . . . . . . . . . . . . . . . . . . . . . . . 21
7.15. Symbols (O) . . . . . . . . . . . . . . . . . . . . . . . 21
7.16. Punctuation (P) . . . . . . . . . . . . . . . . . . . . . 21
7.17. HasCompat (Q) . . . . . . . . . . . . . . . . . . . . . . 21
7.18. OtherLetterDigits (R) . . . . . . . . . . . . . . . . . . 21
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
8.1. PRECIS Derived Property Value Registry . . . . . . . . . 21
8.2. PRECIS Base Classes Registry . . . . . . . . . . . . . . 22
8.3. PRECIS Profiles Registry . . . . . . . . . . . . . . . . 22
9. Security Considerations . . . . . . . . . . . . . . . . . . . 24
9.1. General Issues . . . . . . . . . . . . . . . . . . . . . 24
9.2. Use of the IdentifierClass . . . . . . . . . . . . . . . 25
9.3. Use of the FreeformClass . . . . . . . . . . . . . . . . 25
9.4. Local Character Set Issues . . . . . . . . . . . . . . . 26
9.5. Visually Similar Characters . . . . . . . . . . . . . . . 26
9.6. Security of Passwords . . . . . . . . . . . . . . . . . . 28
10. Interoperability Considerations . . . . . . . . . . . . . . . 29
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
11.1. Normative References . . . . . . . . . . . . . . . . . . 29
11.2. Informative References . . . . . . . . . . . . . . . . . 30
11.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
1. Introduction
As described in the problem statement for the preparation and
comparison of internationalized strings ("PRECIS") [RFC6885], many
IETF protocols have used the Stringprep framework [RFC3454] as the
basis for preparing and comparing protocol strings that contain
Unicode characters [Unicode7.0] outside the ASCII range [RFC20]. The
Stringprep framework was developed during work on the original
technology for internationalized domain names (IDNs), here called
"IDNA2003" [RFC3490], and Nameprep [RFC3491] was the Stringprep
profile for IDNs. At the time, Stringprep was designed as a general
framework so that other application protocols could define their own
Stringprep profiles for the preparation and comparison of strings and
identifiers. Indeed, a number of application protocols defined such
profiles.
After the publication of [RFC3454] in 2002, several significant
issues arose with the use of Stringprep in the IDN case, as
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documented in the IAB's recommendations regarding IDNs [RFC4690]
(most significantly, Stringprep was tied to Unicode version 3.2).
Therefore, the newer IDNA specifications, here called "IDNA2008"
([RFC5890], [RFC5891], [RFC5892], [RFC5893], [RFC5894]), no longer
use Stringprep and Nameprep. This migration away from Stringprep for
IDNs has prompted other "customers" of Stringprep to consider new
approaches to the preparation and comparison of internationalized
strings, as described in [RFC6885].
This document defines a framework for a post-Stringprep approach to
the preparation and comparison of internationalized strings in
application protocols, based on several principles:
1. Define a small set of string classes that specify the Unicode
characters (i.e., specific "code points") appropriate for common
application protocol constructs.
2. Define each PRECIS string class in terms of Unicode code points
and their properties so that an algorithm can be used to
determine whether each code point or character category is (a)
valid, (b) allowed in certain contexts, (c) disallowed, or (d)
unassigned.
3. Use an "inclusion model" such that a string class consists only
of code points that are explicitly allowed, with the result that
any code point not explicitly allowed is forbidden.
4. Enable application protocols to define profiles of the PRECIS
string classes, addressing matters such as width mapping, case
folding and other forms of character mapping, Unicode
normalization, directionality, and further excluded code points
or character categories.
Whereas the string classes define the "baseline" code points for a
range of applications, profiling enables application protocols to
further restrict the allowable code points beyond those specified for
the relevant string class (e.g., characters with special or reserved
meaning, such as "@" and "/" when used as separators within
identifiers) and to apply the string classes in ways that are
appropriate for constructs such as usernames and passwords
[I-D.ietf-precis-saslprepbis], nicknames [I-D.ietf-precis-nickname],
the localparts of instant messaging addresses
[I-D.ietf-xmpp-6122bis], and free-form strings
[I-D.ietf-xmpp-6122bis]. Profiles are responsible for defining the
handling of right-to-left characters as well as various mapping
operations of the kind also discussed for IDNs in [RFC5895], such as
case preservation or lowercasing, Unicode normalization, mapping of
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certain characters to other characters or to nothing, and mapping of
full-width and half-width characters.
When an application applies a profile of a PRECIS string class, it
can achieve the following objectives:
a. Determine if a given string conforms to the profile (e.g. to
determine if it is allowed for use in the relevant "slot"
specified by an application protocol).
b. Determine if any two given strings are equivalent (e.g., to make
an access decision for purposes of authentication or
authorization as further described in [RFC6943]).
It is expected that this framework will yield the following benefits:
o Application protocols will be agile with regard to Unicode
versions.
o Implementers will be able to share code point tables and software
code across application protocols, most likely by means of
software libraries.
o End users will be able to acquire more accurate expectations about
the characters that are acceptable in various contexts. Given
this more uniform set of string classes, it is also expected that
copy/paste operations between software implementing different
application protocols will be more predictable and coherent.
Although this framework is similar to IDNA2008 and borrows some of
the character categories defined in [RFC5892], it defines additional
character categories to meet the needs of common application
protocols.
The character categories and calculation rules defined under
Section 7 and Section 6 are normative and apply to all Unicode code
points. The code point table that results from applying the
character categories and calculation rules to the latest version of
Unicode are provided in an IANA registry.
2. Terminology
Many important terms used in this document are defined in [RFC5890],
[RFC6365], [RFC6885], and [Unicode7.0]. The terms "left-to-right"
(LTR) and "right-to-left" (RTL) are defined in Unicode Standard Annex
#9 [UAX9].
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As of the date of writing, the version of Unicode published by the
Unicode Consortium is 6.3 [Unicode7.0]; however, PRECIS is not tied
to a specific version of Unicode. The latest version of Unicode is
always available [UnicodeCurrent].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
3. String Classes
3.1. Overview
Starting in 2010, various "customers" of Stringprep began to discuss
the need to define a post-Stringprep approach to the preparation and
comparison of internationalized strings other than IDNs. This
community analyzed the existing Stringprep profiles and also weighed
the costs and benefits of defining a relatively small set of Unicode
characters that would minimize the potential for user confusion
caused by visually similar characters (and thus be relatively "safe")
vs. defining a much larger set of Unicode characters that would
maximize the potential for user creativity (and thus be relatively
"expressive"). As a result, the community concluded that most
existing uses could be addressed by two string classes:
IdentifierClass: a sequence of letters, numbers, and some symbols
that is used to identify or address a network entity such as a
user account, a venue (e.g., a chatroom), an information source
(e.g., a data feed), or a collection of data (e.g., a file); the
intent is that this class will minimize user confusion in a wide
variety of application protocols, with the result that safety has
been prioritized over expressiveness for this class.
FreeformClass: a sequence of letters, numbers, symbols, spaces, and
other characters that is used for free-form strings, including
passwords as well as display elements such as human-friendly
nicknames in chatrooms; the intent is that this class will allow
nearly any Unicode character, with the result that expressiveness
has been prioritized over safety for this class (e.g., protocol
designers, application developers, service providers, and end
users might not understand or be able to enter all of the
characters that can be included in the FreeformClass - see
Section 9.3 for details).
Future specifications might define additional PRECIS string classes,
such as a class that falls somewhere between the IdentifierClass and
the FreeformClass. At this time, it is not clear how useful such a
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class would be. In any case, because application developers are able
to define profiles of PRECIS string classes, a protocol needing a
construct between the IdentiferClass and the FreeformClass could
define a restricted profile of the FreeformClass if needed.
The following subsections discuss the IdentifierClass and
FreeformClass in more detail, with reference to the dimensions
described in Section 3 of [RFC6885]. Each string class is defined by
the following behavioral rules:
Valid: Defines which code points and character categories are
treated as valid input to the string.
Contextual Rule Required: Defines which code points and character
categories are treated as allowed only if the requirements of a
contextual rule are met (i.e., either CONTEXTJ or CONTEXTO).
Disallowed: Defines which code points and character categories need
to be excluded from the string.
Unassigned: Defines application behavior in the presence of code
points that are unknown (i.e., not yet designated) for the version
of Unicode used by the application.
This document defines the valid, contextual rule required,
disallowed, and unassigned rules for the IdentifierClass and
FreeformClass. As described under Section 4, profiles of these
string classes are responsible for defining the width mapping,
additional mappings, case mapping, normalization, directionality, and
exclusion rules.
3.2. IdentifierClass
Most application technologies need strings that can be used to refer
to, include, or communicate protocol strings like usernames, file
names, data feed identifiers, and chatroom names. We group such
strings into a class called "IdentifierClass" having the following
features.
3.2.1. Valid
o Code points traditionally used as letters and numbers in writing
systems, i.e., the LetterDigits ("A") category first defined in
[RFC5892] and listed here under Section 7.1.
o Code points in the range U+0021 through U+007E, i.e., the
(printable) ASCII7 ("K") rule defined under Section 7.11. These
code points are "grandfathered" into PRECIS and thus are valid
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even if they would otherwise be disallowed according to the
property-based rules specified in the next section.
Note: 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. Contextual Rule Required
o A number of characters from the Exceptions ("F") category defined
under Section 7.6 (see Section 7.6 for a full list).
o Joining characters, i.e., the JoinControl ("H") category defined
under Section 7.8.
3.2.3. Disallowed
o Old Hangul Jamo characters, i.e., the OldHangulJamo ("I") category
defined under Section 7.9.
o Control characters, i.e., the Controls ("L") category defined
under Section 7.12.
o Ignorable characters, i.e., the PrecisIgnorableProperties ("M")
category defined under Section 7.13.
o Space characters, i.e., the Spaces ("N") category defined under
Section 7.14.
o Symbol characters, i.e., the Symbols ("O") category defined under
Section 7.15.
o Punctuation characters, i.e., the Punctuation ("P") category
defined under Section 7.16.
o Any character that has a compatibility equivalent, i.e., the
HasCompat ("Q") category defined under Section 7.17. These code
points are disallowed even if they would otherwise be valid
according to the property-based rules specified in the previous
section.
o Letters and digits other than the "traditional" letters and digits
allowed in IDNs, i.e., the OtherLetterDigits ("R") category
defined under Section 7.18.
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3.2.4. Unassigned
Any code points that are not yet designated in the Unicode character
set are considered Unassigned for purposes of the IdentifierClass,
and such code points are to be treated as Disallowed.
3.2.5. Examples
As described in the Introduction to this document, the string classes
do not handle all issues related to string preparation and comparison
(such as case mapping); instead, such issues are handled at the level
of profiles. Examples for two profiles of the IdentifierClass can be
found in [I-D.ietf-precis-saslprepbis] (the UsernameIdentifierClass
profile) and in [I-D.ietf-xmpp-6122bis] (the JIDlocalIdentifierClass
profile).
3.3. FreeformClass
Some application technologies need strings that can be used in a
free-form way, e.g., as a password in an authentication exchange (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
called "FreeformClass" having the following features.
Security Warning: As mentioned, the FreeformClass prioritizes
expressiveness over safety; Section 9.3 describes some of the
security hazards involved with using or profiling the
FreeformClass.
Security Warning: Consult Section 9.6 for relevant security
considerations when strings conforming to the FreeformClass, or a
profile thereof, are used as passwords.
3.3.1. Valid
o Traditional letters and numbers, i.e., the LetterDigits ("A")
category first defined in [RFC5892] and listed here under
Section 7.1.
o Letters and digits other than the "traditional" letters and digits
allowed in IDNs, i.e., the OtherLetterDigits ("R") category
defined under Section 7.18.
o Code points in the range U+0021 through U+007E, i.e., the
(printable) ASCII7 ("K") rule defined under Section 7.11.
o Any character that has a compatibility equivalent, i.e., the
HasCompat ("Q") category defined under Section 7.17.
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o Space characters, i.e., the Spaces ("N") category defined under
Section 7.14.
o Symbol characters, i.e., the Symbols ("O") category defined under
Section 7.15.
o Punctuation characters, i.e., the Punctuation ("P") category
defined under Section 7.16.
3.3.2. Contextual Rule Required
o A number of characters from the Exceptions ("F") category defined
under Section 7.6 (see Section 7.6 for a full list).
o Joining characters, i.e., the JoinControl ("H") category defined
under Section 7.8.
3.3.3. Disallowed
o Old Hangul Jamo characters, i.e., the OldHangulJamo ("I") category
defined under Section 7.9.
o Control characters, i.e., the Controls ("L") category defined
under Section 7.12.
o Ignorable characters, i.e., the PrecisIgnorableProperties ("M")
category defined under Section 7.13.
3.3.4. Unassigned
Any code points that are not yet designated in the Unicode character
set are considered Unassigned for purposes of the FreeformClass, and
such code points are to be treated as Disallowed.
3.3.5. Examples
As described in the Introduction to this document, the string classes
do not handle all issues related to string preparation and comparison
(such as case mapping); instead, such issues are handled at the level
of profiles. Examples for two profiles of the FreeformClass can be
found in [I-D.ietf-precis-nickname] (the NicknameFreeformClass
profile) and in [I-D.ietf-xmpp-6122bis] (the
JIDresourceIdentifierClass profile).
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4. Profiles
4.1. Principles
This framework document defines the valid, contextual-rule-required,
disallowed, and unassigned rules for the IdentifierClass and the
FreeformClass. A profile of a PRECIS string class MUST define the
width mapping, additional mappings (if any), case mapping,
normalization, directionality, and exclusion rules. A profile MAY
also restrict the allowable characters above and beyond the
definition of the relevant PRECIS string class (but MUST NOT add as
valid any code points or character categories that are disallowed by
the relevant PRECIS string class). These matters are discussed in
the following subsections.
Profiles of the PRECIS string classes are registered with the IANA as
described under Section 8.3. Profile names use the following
convention: they are of the form "ProfilenameBaseClass", where the
"Profilename" string is a differentiator and "BaseClass" is the name
of the PRECIS string class being profiled; for example, the profile
of the IdentifierClass used for localparts of Jabber Identifiers
(JIDs) in the Extensible Messaging and Presence Protocol (XMPP) is
named "JIDlocalIdentifierClass" [I-D.ietf-xmpp-6122bis].
4.1.1. Width Mapping
The width mapping rule of a profile specifies whether width mapping
is performed on fullwidth and halfwidth characters, and how the
mapping is done. Typically such mapping consists of mapping
fullwidth and halfwidth characters, i.e., code points with a
Decomposition Type of Wide or Narrow, to their decomposition
mappings; as an example, FULLWIDTH DIGIT ZERO (U+FF10) would be
mapped to DIGIT ZERO (U+0030).
The normalization form specified by a profile (see below) has an
impact on the need for width mapping. Because width mapping is
performed as a part of compatibility decomposition, a profile
employing either normalization form KD (NFKD) or normalization form
KC (NFKC) does not need to specify width mapping. However, if
Unicode normalization form C (NFC) is used then the profile 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 compatibility variants would
violate the principle of least user surprise. For more information
about the concept of width in East Asian scripts within Unicode, see
Unicode Standard Annex #11 [UAX11].
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4.1.2. Additional Mappings
The additional mappings rule of a profile specifies whether
additional mappings are to be applied, such as mapping of delimiter
characters and mapping of special characters (e.g., non-ASCII space
characters to ASCII space or certain characters to nothing).
4.1.3. Case Mapping
The case mapping rule of a profile specifies whether case mapping is
performed (instead of case preservation) on uppercase and titlecase
characters, and how the mapping is done (e.g., mapping uppercase and
titlecase characters to their lowercase equivalents).
If case mapping is desired (instead of case preservation), it is
RECOMMENDED to use Unicode Default Case Folding as defined in Chapter
3 of the Unicode Standard [Unicode7.0].
Note: Unicode Default Case Folding is not designed to handle
various localization issues (such as so-called "dotless i" in
several Turkic languages). The PRECIS mappings document
[I-D.ietf-precis-mappings] describes these issues in greater
detail and defines a "local case mapping" method that handles some
locale-dependent and context-dependent mappings.
In order to maximize entropy and minimize the potential for false
positives, it is NOT RECOMMENDED for application protocols to map
uppercase and titlecase code points to their lowercase equivalents
when strings conforming to the FreeformClass, or a profile thereof,
are used in passwords; instead, it is RECOMMENDED to preserve the
case of all code points contained in such strings and then perform
case-sensitive comparison. See also the related discussion in
[I-D.ietf-precis-saslprepbis].
4.1.4. Normalization
The normalization rule of a profile specifies which Unicode
normalization form (D, KD, C, or KC) is to be applied (see Unicode
Standard Annex #15 [UAX15] for background information).
In accordance with [RFC5198], normalization form C (NFC) is
RECOMMENDED.
4.1.5. Directionality
The directionality rule of a profile specifies how to treat strings
containing left-to-right (LTR) and right-to-left (RTL) characters
(see Unicode Standard Annex #9 [UAX9]). A profile usually specifies
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a directionality rule that restricts strings to be entirely LTR
strings or entirely RTL strings and defines the allowable sequences
of characters in LTR and RTL strings. Possible rules include, but
are not limited to, (a) considering any string that contains a right-
to-left code point to be a right-to-left string, or (b) applying the
"Bidi Rule" from [RFC5893].
Mixed-direction strings are not directly supported by the PRECIS
framework itself, since there is currently no widely accepted and
implemented solution for the safe display of mixed-direction strings.
An application protocol that uses the PRECIS framework (or an
extension to the framework) could define better ways to present
mixed-direction strings; however, that work is outside the scope of
this framework and would likely require a great deal of careful
research into the problems of displaying bidirectional text.
4.1.6. Exclusions
The exclusions rule of a profile specifies whether the profile
excludes additional code points or character categories above and
beyond those excluded by the string class being profiled. That is, a
profile MAY do either of the following:
1. Exclude specific code points that are allowed by the relevant
string class.
2. Exclude characters matching certain Unicode properties (e.g.,
math symbols) that are included in the relevant PRECIS string
class.
As a result of such exclusions, code points that are defined as valid
for the PRECIS string class being profiled will be defined as
disallowed for the profile.
4.2. Building Application-Layer Constructs
Sometimes, an application-layer construct does not map in a
straightforward manner to one of the base string classes or a profile
thereof. Consider, for example, the "simple user name" construct in
the Simple Authentication and Security Layer (SASL) [RFC4422].
Depending on the deployment, a simple user name might take the form
of a user's full name (e.g., the user's personal name followed by a
space and then the user's family name). Such a simple user name
cannot be defined as an instance of the IdentifierClass or a profile
thereof, since space characters are not allowed in the
IdentifierClass; however, it could be defined using a space-separated
sequence of IdentifierClass instances, as in the following pseudo-
ABNF [RFC5234]:
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fullname = namepart *(1*SP namepart)
namepart = 1*idpoint
;
; an "idpoint" is a UTF-8 encoded Unicode code point
; that conforms to the PRECIS IdentifierClass
Similar techniques could be used to define many application-layer
constructs, say of the form "user@domain" or "/path/to/file".
4.3. A Note about Spaces
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 a 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 their use within
identifiers created in newer application protocols; given the
challenges involved in properly handling space characters (especially
non-ASCII space characters) in identifiers and other protocol
strings, the Working Group considered this to be a feature, not a
bug.
However, the FreeformClass does allow spaces, which enables
application protocols to define profiles of the FreeformClass that
are more flexible than any profiles of the IdentifierClass. In
addition, as explained in the previous section, application protocols
can also define application-layer constructs containing spaces.
5. Order of Operations
To ensure proper comparison, the following order of operations is
REQUIRED:
1. Width mapping
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2. Optionally, additional mappings such as mapping of delimiters
(e.g., characters such as '@', ':', '/', '+', and '-') and
special handling of certain characters or classes of characters
(e.g., mapping of non-ASCII spaces to ASCII space or mapping of
control characters to nothing); the PRECIS mappings document
[I-D.ietf-precis-mappings] describes such mappings in more detail
3. Case mapping as described under Section 4.1.3 of this document
4. Normalization
5. Behavioral rules for determining whether a code point is valid,
allowed under a contextual rule, disallowed, or unassigned
As already described, the width mapping, additional mappings, case
mapping, and normalization operations are specified for each profile,
whereas the behavioral rules are specified for each string class.
Some of the logic behind this order is provided under Section 4.1.1
(see also the PRECIS mappings document [I-D.ietf-precis-mappings]).
6. Code Point Properties
In order to implement the string classes described above, this
document does the following:
1. Reviews and classifies the collections of code points in the
Unicode character set by examining various code point properties.
2. Defines an algorithm for determining a derived property value,
which can vary depending on the string class being used by the
relevant application protocol.
This document is not intended to specify precisely how derived
property values are to be applied in protocol strings. That
information is the responsibility of the protocol specification that
uses or profiles a PRECIS string class from this document. The value
of the property is to be interpreted as follows.
PROTOCOL VALID Those code points that are allowed to be used in any
PRECIS string class (currently, IdentifierClass and
FreeformClass). Code points with this property value are
permitted for general use in any string class. The abbreviated
term "PVALID" is used to refer to this value in the remainder of
this document.
SPECIFIC CLASS PROTOCOL VALID Those code points that are allowed to
be used in specific string classes. Code points with this
property value are permitted for use in specific string classes.
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In the remainder of this document, the abbreviated term *_PVAL is
used, where * = (ID | FREE), i.e., either "FREE_PVAL" or
"ID_PVAL". In practice, the derived property ID_PVAL is not used
in this specification, since every ID_PVAL code point is PVALID.
CONTEXTUAL RULE REQUIRED Some characteristics of the character, such
as its being invisible in certain contexts or problematic in
others, require that it not be used in labels unless specific
other characters or properties are present. As in IDNA2008, there
are two subdivisions of CONTEXTUAL RULE REQUIRED, the first for
Join_controls (called "CONTEXTJ") and the second for other
characters (called "CONTEXTO"). A character with the derived
property value CONTEXTJ or CONTEXTO MUST NOT be used unless an
appropriate rule has been established and the context of the
character is consistent with that rule. The most notable of the
CONTEXTUAL RULE REQUIRED characters are the Join Control
characters U+200D ZERO WIDTH JOINER and U+200C ZERO WIDTH NON-
JOINER, which have a derived property value of CONTEXTJ. See
Appendix A of [RFC5892] for more information.
DISALLOWED Those code points that are not permitted in any PRECIS
string class.
SPECIFIC CLASS DISALLOWED Those code points that are not to be
included in a specific string class. Code points with this
property value are not permitted in one of the string classes but
might be permitted in others. In the remainder of this document,
the abbreviated term *_DIS is used, where * = (ID | FREE), i.e.,
either "FREE_DIS" or "ID_DIS". In practice, the derived property
FREE_DIS is not used in this specification, since every FREE_DIS
code point is DISALLOWED.
UNASSIGNED Those code points that are not designated (i.e. are
unassigned) in the Unicode Standard.
To summarize, the assigned values of the derived property are:
o PVALID
o FREE_PVAL
o CONTEXTJ
o CONTEXTO
o DISALLOWED
o UNASSIGNED
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The algorithm to calculate the value of the derived property is as
follows:
If .cp. .in. Exceptions Then Exceptions(cp);
Else If .cp. .in. BackwardCompatible Then BackwardCompatible(cp);
Else If .cp. .in. Unassigned Then UNASSIGNED;
Else If .cp. .in. ASCII7 Then PVALID;
Else If .cp. .in. JoinControl Then CONTEXTJ;
Else If .cp. .in. OldHangulJamo Then DISALLOWED;
Else If .cp. .in. PrecisIgnorableProperties Then DISALLOWED;
Else If .cp. .in. Controls Then DISALLOWED;
Else If .cp. .in. HasCompat Then ID_DIS or FREE_PVAL;
Else If .cp. .in. LetterDigits Then PVALID;
Else If .cp. .in. OtherLetterDigits Then ID_DIS or FREE_PVAL;
Else If .cp. .in. Spaces Then ID_DIS or FREE_PVAL;
Else If .cp. .in. Symbols Then ID_DIS or FREE_PVAL;
Else If .cp. .in. Punctuation Then ID_DIS or FREE_PVAL;
Else DISALLOWED;
The value of the derived property calculated can depend on the string
class; for example, if an identifier used in an application protocol
is defined as profiling the PRECIS IdentifierClass then a space
character such as U+0020 would be assigned to ID_DIS, whereas if an
identifier is defined as profiling the PRECIS FreeformClass then the
character would be assigned to FREE_PVAL. For the sake of brevity,
the designation "FREE_PVAL" is used in the code point tables, instead
of the longer designation "ID_DIS or FREE_PVAL". In practice, the
derived properties ID_PVAL and FREE_DIS are not used in this
specification, since every ID_PVAL code point is PVALID and every
FREE_DIS code point is DISALLOWED.
Use of the name of a rule (such as "Exceptions") implies the set of
code points that the rule defines, whereas the same name as a
function call (such as "Exceptions(cp)") implies the value that the
code point has in the Exceptions table.
The mechanisms described here allow determination of the value of the
property for future versions of Unicode (including characters added
after Unicode 5.2 or 7.0 depending on the category, since some
categories in this document are reused from IDNA2008 and therefore
were defined at the time of Unicode 5.2). Changes in Unicode
properties that do not affect the outcome of this process therefore
do not affect this framework. For example, a character can have its
Unicode General_Category value (see Chapter 4 of the Unicode Standard
[Unicode7.0]) change from So to Sm, or from Lo to Ll, without
affecting the algorithm results. Moreover, even if such changes were
to result, the BackwardCompatible list (Section 7.7) can be adjusted
to ensure the stability of the results.
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7. Category Definitions Used to Calculate Derived Property
The derived property obtains its value based on a two-step procedure:
1. Characters are placed in one or more character categories either
(1) based on core properties defined by the Unicode Standard or
(2) by treating the code point as an exception and addressing the
code point based on its code point value. These categories are
not mutually exclusive.
2. Set operations are used with these categories to determine the
values for a property specific to a given string class. These
operations are specified under Section 6.
Note: Unicode property names and property value names might have
short abbreviations, such as "gc" for the General_Category
property and "Ll" for the Lowercase_Letter property value of the
gc property.
In the following specification of character categories, the operation
that returns the value of a particular Unicode character property for
a code point is designated by using the formal name of that property
(from the Unicode PropertyAliases.txt [1]) followed by '(cp)' for
"code point". For example, the value of the General_Category
property for a code point is indicated by General_Category(cp).
The first ten categories (A-J) shown below were previously defined
for IDNA2008 and are copied directly from [RFC5892] to ease the
understanding of how PRECIS handles various characters. Some of
these categories are reused in PRECIS and some of them are not;
however, the lettering of categories is retained to prevent overlap
and to ease implementation of both IDNA2008 and PRECIS in a single
software application. The next eight categories (K-R) are specific
to PRECIS.
7.1. LetterDigits (A)
This category is defined in Secton 2.1 of [RFC5892] and is included
by reference for use in PRECIS.
7.2. Unstable (B)
This category is defined in Secton 2.2 of [RFC5892] but not used in
PRECIS.
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7.3. IgnorableProperties (C)
This category is defined in Secton 2.3 of [RFC5892] but not used in
PRECIS.
Note: See the "PrecisIgnorableProperties (M)" category below for a
more inclusive category used in PRECIS identifiers.
7.4. IgnorableBlocks (D)
This category is defined in Secton 2.4 of [RFC5892] but not used in
PRECIS.
7.5. LDH (E)
This category is defined in Secton 2.5 of [RFC5892] but not used in
PRECIS.
Note: See the "ASCII7 (K)" category below for a more inclusive
category used in PRECIS identifiers.
7.6. Exceptions (F)
This category is defined in Secton 2.6 of [RFC5892] and is included
by reference for use in PRECIS.
7.7. BackwardCompatible (G)
This category is defined in Secton 2.7 of [RFC5892] and is included
by reference for use in PRECIS.
Note: Because of how the PRECIS string classes are defined, only
changes that would result in code points being added to or removed
from the LetterDigits ("A") category would result in backward-
incompatible modifications to code point assignments. Therefore,
management of this category is handled via the processes specified in
[RFC5892]. At the time of this writing (and also at the time that
RFC 5892 was published), this category consisted of the empty set;
however, that is subject to change as described in RFC 5892.
7.8. JoinControl (H)
This category is defined in Secton 2.8 of [RFC5892] and is included
by reference for use in PRECIS.
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7.9. OldHangulJamo (I)
This category is defined in Secton 2.9 of [RFC5892] and is included
by reference for use in PRECIS.
7.10. Unassigned (J)
This category is defined in Secton 2.10 of [RFC5892] and is included
by reference for use in PRECIS.
7.11. ASCII7 (K)
This PRECIS-specific category consists of all printable, non-space
characters from the 7-bit ASCII range. By applying this category,
the algorithm specified under Section 6 exempts these characters from
other rules that might be applied during PRECIS processing, on the
assumption that these code points are in such wide use that
disallowing them would be counter-productive.
K: cp is in {0021..007E}
7.12. Controls (L)
L: Control(cp) = True
7.13. PrecisIgnorableProperties (M)
This PRECIS-specific category is used to group code points that are
discouraged from use in PRECIS string classes.
M: Default_Ignorable_Code_Point(cp) = True or
Noncharacter_Code_Point(cp) = True
The definition for Default_Ignorable_Code_Point can be found in the
DerivedCoreProperties.txt [2] file, and at the time of Unicode 7.0 is
as follows:
Other_Default_Ignorable_Code_Point
+ Cf (Format characters)
+ Variation_Selector
- White_Space
- FFF9..FFFB (Annotation Characters)
- 0600..0604, 06DD, 070F, 110BD (exceptional Cf characters
that should be visible)
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7.14. Spaces (N)
This PRECIS-specific category is used to group code points that are
space characters.
N: General_Category(cp) is in {Zs}
7.15. Symbols (O)
This PRECIS-specific category is used to group code points that are
symbols.
O: General_Category(cp) is in {Sm, Sc, Sk, So}
7.16. Punctuation (P)
This PRECIS-specific category is used to group code points that are
punctuation characters.
P: General_Category(cp) is in {Pc, Pd, Ps, Pe, Pi, Pf, Po}
7.17. HasCompat (Q)
This PRECIS-specific category is used to group code points that have
compatibility equivalents as explained in Chapter 2 and Chapter 3 of
the Unicode Standard [Unicode7.0].
Q: toNFKC(cp) != cp
The toNFKC() operation returns the code point in normalization form
KC. For more information, see Section 5 of Unicode Standard Annex
#15 [UAX15].
7.18. OtherLetterDigits (R)
This PRECIS-specific category is used to group code points that are
letters and digits other than the "traditional" letters and digits
grouped under the LetterDigits (A) class (see Section 7.1).
R: General_Category(cp) is in {Lt, Nl, No, Me}
8. IANA Considerations
8.1. PRECIS Derived Property Value Registry
IANA is requested to create a PRECIS-specific registry with the
Derived Properties for the versions of Unicode that are released
after (and including) version 7.0. The derived property value is to
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be calculated in cooperation with a designated expert [RFC5226]
according to the rules specified under Section 7 and Section 6.
The IESG is to be notified if backward-incompatible changes to the
table of derived properties are discovered or if other problems arise
during the process of creating the table of derived property values
or during expert review. Changes to the rules defined under
Section 7 and Section 6 require IETF Review.
8.2. PRECIS Base Classes Registry
IANA is requested to create a registry of PRECIS string classes. In
accordance with [RFC5226], the registration policy is "RFC Required".
The registration template is as follows:
Base Class: [the name of the PRECIS string class]
Description: [a brief description of the PRECIS string class and its
intended use, e.g., "A sequence of letters, numbers, and symbols
that is used to identify or address a network entity."]
Specification: [the RFC number]
The initial registrations are as follows:
Base Class: FreeformClass.
Description: A sequence of letters, numbers, symbols, spaces, and
other code points that is used for free-form strings.
Specification: Section 3.3 of this document.
[Note to RFC Editor: please change "this document"
to the RFC number issued for this specification.]
Base Class: IdentifierClass.
Description: A sequence of letters, numbers, and symbols that is
used to identify or address a network entity.
Specification: Section 3.2 of this document.
[Note to RFC Editor: please change "this document"
to the RFC number issued for this specification.]
8.3. PRECIS Profiles Registry
IANA is requested to create a registry of profiles that use the
PRECIS string classes. In accordance with [RFC5226], the
registration policy is "Expert Review". This policy was chosen in
order to ease the burden of registration while ensuring that
"customers" of PRECIS receive appropriate guidance regarding the
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sometimes complex and subtle internationalization issues related to
profiles of PRECIS string classes.
The registration template is as follows:
Name: [the name of the profile]
Applicability: [the specific protocol elements to which this profile
applies, e.g., "Localparts in XMPP addresses."]
Base Class: [which PRECIS string class is being profiled]
Replaces: [the Stringprep profile that this PRECIS profile replaces,
if any]
Width Mapping: [the behavioral rule for handling of width, e.g.,
"Map fullwidth and halfwidth characters to their compatibility
variants."]
Additional Mappings: [any additional mappings are required or
recommended, e.g., "Map non-ASCII space characters to ASCII
space."]
Case Mapping: [the behavioral rule for handling of case, e.g.,
"Unicode Default Case Folding"]
Normalization: [which Unicode normalization form is applied, e.g.,
"NFC"]
Directionality: [the behavioral rule for handling of right-to-left
code points, e.g., "The 'Bidi Rule' defined in RFC 5893 applies."]
Exclusions: [a brief description of the specific code points or
characters categories are excluded, e.g., "Eight legacy characters
in the ASCII range" or "Any character that has a compatibility
equivalent, i.e., the HasCompat category"]
Enforcement: [which entities enforce the rules, and when that
enforcement occurs during protocol operations]
Specification: [a pointer to relevant documentation, such as an RFC
or Internet-Draft]
In order to request a review, the registrant shall send a completed
template to the precis@ietf.org list or its designated successor.
Factors to focus on while defining profiles and reviewing profile
registrations include the following:
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o Is the problem being addressed by this profile well-defined?
o Does the specification define what kinds of applications are
involved and the protocol elements to which this profile applies?
o Would an existing PRECIS string class or profile solve the
problem?
o Is the profile clearly defined?
o Is the profile based on an appropriate dividing line between user
interface (culture, context, intent, locale, device limitations,
etc.) and the use of conformant strings in protocol elements?
o Are the width mapping, case mapping, additional mappings,
normalization, exclusion, and directionality rules appropriate for
the intended use?
o Does the profile explain which entities enforce the rules, and
when such enforcement occurs during protocol operations?
o Does the profile reduce the degree to which human users could be
surprised or confused by application behavior (the "principle of
least user surprise")?
o Does the profile introduce any new security concerns such as those
described under Section 9 of this document (e.g., false positives
for authentication or authorization)?
9. Security Considerations
9.1. General Issues
If input strings that appear "the same" to users are programmatically
considered to be distinct in different systems, or if input strings
that appear distinct to users are programmatically considered to be
"the same" in different systems, then users can be confused. Such
confusion can have security implications, such as the false positives
and false negatieves discussed in [RFC6943]. One starting goal of
work on the PRECIS framework was to limit the number of times that
users are confused (consistent with the "principle of least
astonishment"). Unfortunately, this goal has been difficult to
achieve given the large number of application protocols already in
existence, each with its own conventions regarding allowable
characters (see for example [I-D.saintandre-username-interop] with
regard to various username constructs). Despite these difficulties,
profiles should not be multiplied beyond necessity. In particular,
application protocol designers should think long and hard before
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defining a new profile instead of using one that has already been
defined, and if they decide to define a new profile then they should
clearly explain their reasons for doing so.
The security of applications that use this framework can depend in
part on the proper preparation and comparison of internationalized
strings. For example, such strings can be used to make
authentication and authorization decisions, and the security of an
application could be compromised if an entity providing a given
string is connected to the wrong account or online resource based on
different interpretations of the string.
Specifications of application protocols that use this framework are
strongly encouraged to describe how internationalized strings are
used in the protocol, including the security implications of any
false positives and false negatives that might result from various
comparison operations. For some helpful guidelines, refer to
[RFC6943], [RFC5890], [UTR36], and [UTS39].
9.2. Use of the IdentifierClass
Strings that conform to the IdentifierClass and any profile thereof
are intended to be relatively safe for use in a broad range of
applications, primarily because they include only letters, digits,
and "grandfathered" non-space characters from the ASCII range; thus
they exclude spaces, characters with compatibility equivalents, and
almost all symbols and punctuation marks. However, because such
strings can still include so-called confusable characters (see
Section 9.5), protocol designers and implementers are encouraged to
pay close attention to the security considerations described
elsewhere in this document.
9.3. Use of the FreeformClass
Strings that conform to the FreeformClass and many profiles thereof
can include virtually any Unicode character. This makes the
FreeformClass quite expressive, but also problematic from the
perspective of possible user confusion. Protocol designers are
hereby warned that the FreeformClass contains codepoints they might
not understand, and are encouraged to profile the IdentifierClass
wherever feasible; however, if an application protocol requires more
code points than are allowed by the IdentifierClass, protocol
designers are encouraged to define a profile of the FreeformClass
that restricts the allowable code points as tightly as possible.
(The PRECIS Working Group considered the option of allowing
superclasses as well as profiles of PRECIS string classes, but
decided against allowing superclasses to reduce the likelihood of
security and interoperability problems.)
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9.4. Local Character Set Issues
When systems use local character sets other than ASCII and Unicode,
this specification leaves the problem of converting between the local
character set and Unicode up to the application or local system. If
different applications (or different versions of one application)
implement different rules for conversions among coded character sets,
they could interpret the same name differently and contact different
application servers or other network entities. This problem is not
solved by security protocols, such as Transport Layer Security (TLS)
[RFC5246] and the Simple Authentication and Security Layer (SASL)
[RFC4422], that do not take local character sets into account.
9.5. Visually Similar Characters
Some characters are visually similar and thus can cause confusion
among humans. Such characters are often called "confusable
characters" or "confusables".
The problem of confusable characters is not necessarily caused by the
use of Unicode code points outside the ASCII range. For example, in
some presentations and to some individuals the string "ju1iet"
(spelled with DIGIT ONE, U+0031, as the third character) might appear
to be the same as "juliet" (spelled with LATIN SMALL LETTER L,
U+006C), especially on casual visual inspection. This phenomenon is
sometimes called "typejacking".
However, the problem is made more serious by introducing the full
range of Unicode code points into protocol strings. For example, the
characters U+13DA U+13A2 U+13B5 U+13AC U+13A2 U+13AC U+13D2 from the
Cherokee block look similar to the ASCII characters "STPETER" as they
might appear when presented using a "creative" font family.
In some examples of confusable characters, it is unlikely that the
average human could tell the difference between the real string and
the fake string. (Indeed, there is no programmatic way to
distinguish with full certainty which is the fake string and which is
the real string; in some contexts, the string formed of Cherokee
characters might be the real string and the string formed of ASCII
characters might be the fake string.) Because PRECIS-compliant
strings can contain almost any properly-encoded Unicode code point,
it can be relatively easy to fake or mimic some strings in systems
that use the PRECIS framework. The fact that some strings are easily
confused introduces security vulnerabilities of the kind that have
also plagued the World Wide Web, specifically the phenomenon known as
phishing.
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Despite the fact that some specific suggestions about identification
and handling of confusable characters appear in the Unicode Security
Considerations [UTR36] and the Unicode Security Mechanisms [UTS39],
it is also true (as noted in [RFC5890]) that "there are no
comprehensive technical solutions to the problems of confusable
characters". Because it is impossible to map visually similar
characters without a great deal of context (such as knowing the font
families used), the PRECIS framework does nothing to map similar-
looking characters together, nor does it prohibit some characters
because they look like others.
Nevertheless, specifications for application protocols that use this
framework are strongly encouraged to describe how confusable
characters can be abused to compromise the security of systems that
use the protocol in question, along with any protocol-specific
suggestions for overcoming those threats. In particular, software
implementations and service deployments that use PRECIS-based
technologies are strongly encouraged to define and implement
consistent policies regarding the registration, storage, and
presentation of visually similar characters. The following
recommendations are appropriate:
1. An application service SHOULD define a policy that specifies the
scripts or blocks of characters that the service will allow to be
registered (e.g., in an account name) or stored (e.g., in a file
name). Such a policy SHOULD be informed by the languages and
scripts that are used to write registered account names; in
particular, to reduce confusion, the service SHOULD forbid
registration or storage of strings that contain characters from
more than one script and SHOULD restrict registrations to
characters drawn from a very small number of scripts (e.g.,
scripts that are well-understood by the administrators of the
service, to improve manageability).
2. User-oriented application software SHOULD define a policy that
specifies how internationalized strings will be presented to a
human user. Because every human user of such software has a
preferred language or a small set of preferred languages, the
software SHOULD gather that information either explicitly from
the user or implicitly via the operating system of the user's
device. Furthermore, because most languages are typically
represented by a single script or a small set of scripts, and
because most scripts are typically contained in one or more
blocks of characters, the software SHOULD warn the user when
presenting a string that mixes characters from more than one
script or block, or that uses characters outside the normal range
of the user's preferred language(s). (Such a recommendation is
not intended to discourage communication across different
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communities of language users; instead, it recognizes the
existence of such communities and encourages due caution when
presenting unfamiliar scripts or characters to human users.)
The challenges inherent in supporting the full range of Unicode code
points have in the past led some to hope for a way to
programmatically negotiate more restrictive ranges based on locale,
script, or other relevant factors, to tag the locale associated with
a particular string, etc. As a general-purpose internationalization
technology, the PRECIS framework does not include such mechanisms.
9.6. Security of Passwords
Two goals of passwords are to maximize the amount of entropy and to
minimize the potential for false positives. These goals can be
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
are not compared aggressively. Therefore, it is NOT RECOMMENDED for
application protocols to profile the FreeformClass for use in
passwords in a way that removes entire categories (e.g., by
disallowing symbols or punctuation). Furthermore, it is NOT
RECOMMENDED for application protocols to map uppercase and titlecase
code points to their lowercase equivalents in such strings; instead,
it is RECOMMENDED to preserve the case of all code points contained
in such strings and to compare them in a case-sensitive manner.
That said, software implementers need to be aware that there exist
tradeoffs between entropy and usability. For example, allowing a
user to establish a password containing "uncommon" code points might
make it difficult for the user to access a service when using an
unfamiliar or constrained input device.
Some application protocols use passwords directly, whereas others
reuse technologies that themselves process passwords (one example of
such a technology is the Simple Authentication and Security Layer
[RFC4422]). Moreover, passwords are often carried by a sequence of
protocols with backend authentication systems or data storage systems
such as RADIUS [RFC2865] and LDAP [RFC4510]. Developers of
application protocols are encouraged to look into reusing these
profiles instead of defining new ones, so that end-user expectations
about passwords are consistent no matter which application protocol
is used.
In protocols that provide passwords as input to a cryptographic
algorithm such as a hash function, the client will need to perform
proper preparation of the password before applying the algorithm,
since the password is not available to the server in plaintext form.
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Further discussion of password handling can be found in
[I-D.ietf-precis-saslprepbis].
10. Interoperability Considerations
Although strings that are consumed in PRECIS-based application
protocols are often encoded using UTF-8 [RFC3629], the exact encoding
is a matter for the application protocol that uses PRECIS, not for
the PRECIS framework.
It is known that some existing systems are unable to support the full
Unicode character set, or even any characters outside the ASCII
range. If two (or more) applications need to interoperate when
exchanging data (e.g., for the purpose of authenticating a username
or password), they will naturally need to have in common at least one
coded character set (as defined by [RFC6365]). Establishing such a
baseline is a matter for the application protocol that uses PRECIS,
not for the PRECIS framework.
Three Unicode code points underwent changes in their GeneralCategory
between Unicode 5.2 (current at the time IDNA2008 was originally
published) and Unicode 6.0, as described in [RFC6452]. Implementers
might need to be aware that the treatment of these characters differs
depending on which version of Unicode is available on the system that
is using IDNA2008 or PRECIS, and that other such differences are
possible between the version of Unicode current at the time of this
writing (7.0) and future versions.
11. References
11.1. 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
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, March 2008.
[Unicode7.0]
The Unicode Consortium, "The Unicode Standard, Version
6.0.0", 2014,
.
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11.2. Informative References
[I-D.ietf-precis-mappings]
Yoneya, Y. and T. NEMOTO, "Mapping characters for PRECIS
classes", draft-ietf-precis-mappings-08 (work in
progress), June 2014.
[I-D.ietf-precis-nickname]
Saint-Andre, P., "Preparation and Comparison of
Nicknames", draft-ietf-precis-nickname-09 (work in
progress), January 2014.
[I-D.ietf-precis-saslprepbis]
Saint-Andre, P. and A. Melnikov, "Username and Password
Preparation Algorithms", draft-ietf-precis-saslprepbis-07
(work in progress), March 2014.
[I-D.ietf-xmpp-6122bis]
Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Address Format", draft-ietf-xmpp-
6122bis-12 (work in progress), March 2014.
[I-D.saintandre-username-interop]
Saint-Andre, P., "An Interoperable Subset of Characters
for Internationalized Usernames", draft-saintandre-
username-interop-03 (work in progress), March 2014.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)", RFC
2865, June 2000.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)", RFC
3491, March 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.
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[RFC4510] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Technical Specification Road Map", RFC 4510, June
2006.
[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
Recommendations for Internationalized Domain Names
(IDNs)", RFC 4690, September 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
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
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010.
[RFC5892] Faltstrom, P., "The Unicode Code Points and
Internationalized Domain Names for Applications (IDNA)",
RFC 5892, August 2010.
[RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
Internationalized Domain Names for Applications (IDNA)",
RFC 5893, August 2010.
[RFC5894] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Background, Explanation, and
Rationale", RFC 5894, August 2010.
[RFC5895] Resnick, P. and P. Hoffman, "Mapping Characters for
Internationalized Domain Names in Applications (IDNA)
2008", RFC 5895, September 2010.
[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365,
September 2011.
[RFC6452] Faltstrom, P. and P. Hoffman, "The Unicode Code Points and
Internationalized Domain Names for Applications (IDNA) -
Unicode 6.0", RFC 6452, November 2011.
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[RFC6885] Blanchet, M. and A. Sullivan, "Stringprep Revision and
Problem Statement for the Preparation and Comparison of
Internationalized Strings (PRECIS)", RFC 6885, March 2013.
[RFC6943] Thaler, D., "Issues in Identifier Comparison for Security
Purposes", RFC 6943, May 2013.
[UAX9] The Unicode Consortium, "Unicode Standard Annex #9:
Unicode Bidirectional Algorithm", September 2012,
.
[UAX11] The Unicode Consortium, "Unicode Standard Annex #11: East
Asian Width", September 2012,
.
[UAX15] The Unicode Consortium, "Unicode Standard Annex #15:
Unicode Normalization Forms", August 2012,
.
[UnicodeCurrent]
The Unicode Consortium, "The Unicode Standard",
2014-present, .
[UTR36] The Unicode Consortium, "Unicode Technical Report #36:
Unicode Security Considerations", July 2012,
.
[UTS39] The Unicode Consortium, "Unicode Technical Standard #39:
Unicode Security Mechanisms", July 2012,
.
11.3. URIs
[1] http://unicode.org/Public/UNIDATA/PropertyAliases.txt
[2] http://unicode.org/Public/UNIDATA/DerivedCoreProperties.txt
Appendix A. Acknowledgements
The authors would like to acknowledge the comments and contributions
of the following individuals during working group discussion: David
Black, Edward Burns, Dan Chiba, Mark Davis, Alan DeKok, Martin
Duerst, Patrik Faltstrom, Ted Hardie, Joe Hildebrand, Bjoern
Hoehrmann, Paul Hoffman, Jeffrey Hutzelman, Simon Josefsson, John
Klensin, Alexey Melnikov, Takahiro Nemoto, Yoav Nir, Mike Parker,
Pete Resnick, Andrew Sullivan, Dave Thaler, Yoshiro Yoneya, and
Florian Zeitz.
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Charlie Kaufman, Tom Taylor, and Tim Wicinski reviewed the document
on behalf of the Security Directorate, the General Area Review Team,
and the Operations and Management Directorate, respectively.
During IESG review, Alissa Cooper, Stephen Farrell, and Barry Leiba
provided comments that led to further improvements.
Some algorithms and textual descriptions have been borrowed from
[RFC5892]. Some text regarding security has been borrowed from
[RFC5890], [I-D.ietf-precis-saslprepbis], and
[I-D.ietf-xmpp-6122bis].
Peter Saint-Andre wishes to acknowledge Cisco Systems, Inc., for
employing him during his work on earlier versions of this document.
Authors' Addresses
Peter Saint-Andre
&yet
P.O. Box 787
Parker, CO 80134
USA
Email: peter@andyet.net
Marc Blanchet
Viagenie
246 Aberdeen
Quebec, QC G1R 2E1
Canada
Email: Marc.Blanchet@viagenie.ca
URI: http://www.viagenie.ca/
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