PRECIS Framework: Preparation, Enforcement, and Comparison of Internationalized Strings in Application Protocols&yetpeter@andyet.comhttps://andyet.com/Viagenie246 AberdeenQuebecQCG1R 2E1CanadaMarc.Blanchet@viagenie.cahttp://www.viagenie.ca/internationalizationi18nStringprepApplication protocols using Unicode characters in protocol strings
need to properly handle such strings in order to enforce
internationalization rules for strings placed in various protocol slots
(such as addresses and identifiers) and 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, enforcement, 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.Application protocols using Unicode characters in protocol strings need to properly handle such
strings in order to enforce internationalization rules for strings
placed in various protocol slots (such as addresses and identifiers) and
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, enforcement,
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 described in the PRECIS problem statement , many IETF protocols have used the Stringprep
framework as the basis for preparing,
enforcing, and comparing protocol strings that contain Unicode
characters, especially characters outside the ASCII range . The Stringprep framework was developed during work on
the original technology for internationalized domain names (IDNs), here
called "IDNA2003" , and Nameprep 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. Indeed, a number
of application protocols defined such profiles.After the publication of in 2002, several
significant issues arose with the use of Stringprep in the IDN case, as
documented in the IAB's recommendations regarding IDNs (most significantly, Stringprep was tied to Unicode
version 3.2). Therefore, the newer IDNA specifications, here called
"IDNA2008" (, , , , ),
no longer use Stringprep and Nameprep. This migration away from
Stringprep for IDNs prompted other "customers" of Stringprep to consider
new approaches to the preparation, enforcement, and comparison of
internationalized strings, as described in .This document defines a framework for a post-Stringprep approach to
the preparation, enforcement, and comparison of internationalized
strings in application protocols, based on several principles:Define a small set of string classes that specify the Unicode
characters (i.e., specific "code points") appropriate for common
application protocol constructs.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.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.Enable application protocols to define profiles of the PRECIS
string classes if necessary (addressing matters such as width
mapping, case mapping, Unicode normalization, and directionality)
but strongly discourage the multiplication of profiles beyond
necessity in order to avoid violations of the "Principle of Least
Astonishment".It is expected that this framework will yield the following
benefits:Application protocols will be agile with regard to Unicode
versions.Implementers will be able to share code point tables and software
code across application protocols, most likely by means of software
libraries.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.Whereas the string classes define the "baseline" code points for a
range of applications, profiling enables application protocols to apply
the string classes in ways that are appropriate for common constructs
such as usernames , opaque
strings such as passwords ,
and nicknames . 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 , such as case preservation or lowercasing,
Unicode normalization, mapping of certain characters to other characters
or to nothing, and mapping of fullwidth and halfwidth characters.When an application applies a profile of a PRECIS string class, it
transforms an input string (which might or might not be conforming) into
an output string that definitively conforms to the profile. In
particular, this document focuses on the resulting ability to achieve
the following objectives:Enforcing all the rules of a profile for a single output
string (e.g., to determine if a string can be included in a protocol
slot, communicated to another entity within a protocol, stored in a
retrieval system, etc.).Comparing two output strings to determine if they are equivalent,
typically through octet-for-octet matching to test for
"bit&nbhy;string identity" (e.g., to make an access decision for
purposes of authentication or authorization as further described
in ).The opportunity to define profiles naturally introduces the
possibility of a proliferation of profiles, thus potentially mitigating
the benefits of common code and violating user expectations. See for a discussion of this important topic.In addition, it is extremely important for protocol designers and
application developers to understand that the transformation of an input
string to an output string is rarely reversible. As one relatively
simple example, case mapping would transform an input string of
"StPeter" to "stpeter", and information about the capitalization of the
first and third characters would be lost. Similar considerations apply
to other forms of mapping and normalization.Although this framework is similar to IDNA2008 and includes by
reference some of the character categories defined in , it defines additional character categories to meet
the needs of common application protocols other than DNS.The character categories and calculation rules defined under
Sections
and 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 can be found in an IANA
registry.Many important terms used in this document are defined in , , ,
and . The terms "left-to-right" (LTR) and
"right-to-left" (RTL) are defined in Unicode Standard Annex #9 .As of the date of writing, the version of Unicode published by the
Unicode Consortium is 7.0 ; however, PRECIS
is not tied to a specific version of Unicode. The latest version of
Unicode is always available .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 .This document distinguishes between three different actions that an
entity can take with regard to a string:Enforcement entails applying all of the rules specified for a
particular string class or profile thereof to an individual string,
for the purpose of determining if the string can be used in a given
protocol slot.Comparison entails applying all of the rules specified for a
particular string class or profile thereof to two separate strings,
for the purpose of determining if the two strings are equivalent.Preparation entails only ensuring that the characters in an
individual string are allowed by the underlying PRECIS string
class.In most cases, authoritative entities such as servers are responsible
for enforcement, whereas subsidiary entities such as clients are
responsible only for preparation. The rationale for this distinction is
that clients might not have the facilities (in terms of device memory and
processing power) to enforce all the rules regarding internationalized
strings (such as width mapping and Unicode normalization), although they
can more easily limit the repertoire of characters they offer to an end
user. By contrast, it is assumed that a server would have more capacity
to enforce the rules, and in any case acts as an authority regarding
allowable strings in protocol slots such as addresses and endpoint
identifiers. In addition, a client cannot necessarily be trusted to
properly generate such strings, especially for security-sensitive contexts
such as authentication and authorization.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: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.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 for devices or for participants in a
chatroom; 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. Note well that 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 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 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 IdentifierClass 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 5 of . Each string
class is defined by the following behavioral rules:Defines which code points are treated as
valid for the string.Defines which code points
are treated as allowed only if the requirements of a contextual
rule are met (i.e., either CONTEXTJ or CONTEXTO).Defines which code points need to be
excluded from the string.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 , profiles
of these string classes are responsible for defining the width
mapping, additional mappings, case mapping, normalization, and
directionality rules.Most application technologies need strings that can be used to
refer to, include, or communicate protocol strings like usernames,
filenames, data feed identifiers, and chatroom names. We group such
strings into a class called "IdentifierClass" having the following
features.Code points traditionally used as letters and numbers in
writing systems, i.e., the LetterDigits ("A") category first
defined in and listed here under .Code points in the range U+0021 through U+007E, i.e., the
(printable) ASCII7 ("K") category defined under .
These code points are "grandfathered" into PRECIS and thus are
valid even if they would otherwise be disallowed according to
the property-based rules specified in the next section.Note: Although the PRECIS IdentifierClass
reuses 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.A number of characters from the Exceptions ("F") category
defined under (see for a
full list).Joining characters, i.e., the JoinControl ("H") category
defined under .Old Hangul Jamo characters, i.e., the OldHangulJamo ("I")
category defined under .Control characters, i.e., the Controls ("L") category defined
under .Ignorable characters, i.e., the PrecisIgnorableProperties
("M") category defined under .Space characters, i.e., the Spaces ("N") category defined
under .Symbol characters, i.e., the Symbols ("O") category defined
under .Punctuation characters, i.e., the Punctuation ("P") category
defined under .Any character that has a compatibility equivalent, i.e., the
HasCompat ("Q") category defined under .
These code points are disallowed even if they would otherwise be
valid according to the property-based rules specified in the
previous section.Letters and digits other than the "traditional" letters and
digits allowed in IDNs, i.e., the OtherLetterDigits ("R")
category defined under .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. See .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 profiles of the
IdentifierClass can be found in
(the UsernameCaseMapped and UsernameCasePreserved profiles).Some application technologies need strings that can be used in a
free-form way, e.g., as a password in an authentication exchange (see
) or a nickname in a
chatroom (see ). We group
such things into a class called "FreeformClass" having the following
features.Security Warning: As mentioned, the
FreeformClass prioritizes expressiveness over safety; describes some of the security
hazards involved with using or profiling the
FreeformClass.Security Warning: Consult for relevant security considerations
when strings conforming to the FreeformClass, or a profile thereof,
are used as passwords.Traditional letters and numbers, i.e., the LetterDigits ("A")
category first defined in and listed
here under .Letters and digits other than the "traditional" letters and
digits allowed in IDNs, i.e., the OtherLetterDigits ("R")
category defined under .Code points in the range U+0021 through U+007E, i.e., the
(printable) ASCII7 ("K") category defined under .Any character that has a compatibility equivalent, i.e., the
HasCompat ("Q") category defined under .Space characters, i.e., the Spaces ("N") category defined
under .Symbol characters, i.e., the Symbols ("O") category defined
under .Punctuation characters, i.e., the Punctuation ("P") category
defined under .A number of characters from the Exceptions ("F") category
defined under (see for a
full list).Joining characters, i.e., the JoinControl ("H") category
defined under .Old Hangul Jamo characters, i.e., the OldHangulJamo ("I")
category defined under .Control characters, i.e., the Controls ("L") category defined
under .Ignorable characters, i.e., the PrecisIgnorableProperties
("M") category defined under .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.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 profiles of the
FreeformClass can be found in
(the OpaqueString profile) and
(the Nickname profile).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, and directionality 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 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 . Profile names use
the following convention: they are of the form "Profilename of
BaseClass", 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 FreeformClass used for opaque strings
such as passwords is the OpaqueString profile .The risk of profile proliferation is significant because having too
many profiles will result in different behavior across various
applications, thus violating what is known in user interface design as
the "Principle of Least Astonishment".Indeed, we already have too many profiles. Ideally we would have
at most two or three profiles. Unfortunately, numerous application
protocols exist with their own quirks regarding protocol strings.
Domain names, email addresses, instant messaging addresses, chatroom
nicknames, filenames, authentication identifiers, passwords, and other
strings are already out there in the wild and need to be supported in
existing application protocols such as DNS, SMTP, the
Extensible Messaging and Presence Protocol (XMPP),
Internet Relay Chat (IRC), NFS, the Internet Small Computer System
Interface (iSCSI), the Extensible Authentication Protocol (EAP),
and the Simple Authentication and Security Layer (SASL), among
others.Nevertheless, profiles must not be multiplied beyond necessity.To help prevent profile proliferation, this document recommends
sensible defaults for the various options offered to profile creators
(such as width mapping and Unicode normalization). In addition, the
guidelines for designated experts provided under are meant to encourage a high level of due
diligence regarding new profiles.The width mapping rule of a profile specifies whether width
mapping is performed on the characters of a string, 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 (as is recommended) 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
Astonishment". For more information about the concept of width in
East Asian scripts within Unicode, see Unicode Standard Annex #11
.The additional mapping rule of a profile specifies whether
additional mappings are performed on the characters of a string, such
as:Mapping of delimiter characters (such as '@', ':', '/', '+',
and '-')Mapping of special characters (e.g., non-ASCII space
characters to ASCII space or control characters to nothing).The PRECIS mappings document describes such mappings in more
detail.The case mapping rule of a profile specifies whether case mapping
(instead of case preservation) is performed on the characters of a
string, and how the mapping is applied (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 the
Unicode Standard (at the time of this
writing, the algorithm is specified in Chapter 3 of ).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 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 and
in .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 for background
information).In accordance with , normalization form C
(NFC) is RECOMMENDED.The directionality rule of a profile specifies how to treat
strings containing what are often called "right-to-left" (RTL)
characters (see Unicode Standard Annex #9 ).
RTL characters come from scripts that are normally written from
right to left and are considered by Unicode to, themselves, have
right-to-left directionality. Some strings containing RTL
characters also contain "left-to-right" (LTR) characters, such as
numerals, as well as characters without directional properties.
Consequently, such strings are known as "bidirectional strings".Presenting bidirectional strings in different layout systems
(e.g., a user interface that is configured to handle primarily an
RTL script vs. an interface that is configured to handle primarily
an LTR script) can yield display results that, while predictable to
those who understand the display rules, are counter-intuitive to
casual users. In particular, the same bidirectional string (in
PRECIS terms) might not be presented in the same way to users of
those different layout systems, even though the presentation is
consistent within any particular layout system. In some
applications, these presentation differences might be considered
problematic and thus the application designers might wish to
restrict the use of bidirectional strings by specifying a
directionality rule. In other applications, these presentation
differences might not be considered problematic (this especially
tends to be true of more "free-form" strings) and thus no
directionality rule is needed.The PRECIS framework does not directly address how to deal with
bidirectional strings across all string classes and profiles, and
does not define any new directionality rules, since at present there
is no widely accepted and implemented solution for the safe display
of arbitrary bidirectional strings beyond the Unicode bidirectional
algorithm . Although rules for management and
display of bidirectional strings have been defined for domain name
labels and similar identifiers through the "Bidi Rule" specified in
the IDNA2008 specification on right-to-left scripts , those rules are quite restrictive and are not
necessarily applicable to all bidirectional strings.The authors of a PRECIS profile might believe that they need to
define a new directionality rule of their own. Because of the
complexity of the issues involved, such a belief is almost always
misguided, even if the authors have done a great deal of careful
research into the challenges of displaying bidirectional strings.
This document strongly suggests that profile authors who are
thinking about defining a new directionality rule think again, and
instead consider using the "Bidi Rule" (for
profiles based on the IdentifierClass) or following the Unicode
bidirectional algorithm (for profiles based on
the FreeformClass or in situations where the IdentifierClass is not
appropriate).With regard to the IdentifierClass, the consensus of the PRECIS
Working Group was that spaces are problematic for many reasons,
including the following:Many Unicode characters are confusable with ASCII space.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.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 hitting 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 with properly handling space characters
(especially non-ASCII space characters) in identifiers and other
protocol strings, the PRECIS 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 , application
protocols can also define application-layer constructs containing
spaces.Although PRECIS has been designed with applications in mind,
internationalization is not suddenly made easy through the use of
PRECIS. Application developers still need to give some thought to how
they will use the PRECIS string classes, or profiles thereof, in their
applications. This section provides some guidelines to application
developers (and to expert reviewers of application protocol
specifications).Don't define your own profile unless absolutely necessary (see
). Existing profiles have
been designed for wide reuse. It is highly likely that an existing
profile will meet your needs, especially given the ability to
specify further excluded characters () and to build application-layer
constructs (see ).Do specify:
Exactly which entities are responsible for preparation,
enforcement, and comparison of internationalized strings
(e.g., servers or clients).Exactly when those entities need to complete their tasks
(e.g., a server might need to enforce the rules of a profile
before allowing a client to gain network access).Exactly which protocol slots need to be checked against
which profiles (e.g., checking the address of a message's
intended recipient against the UsernameCaseMapped profile
of the
IdentifierClass, or checking the password of a user against
the OpaqueString profile of the
FreeformClass).
See and for definitions of these matters for several applications.An application protocol that uses a profile MAY specify particular
code points that are not allowed in relevant slots within that
application protocol, above and beyond those excluded by the string
class or profile.That is, an application protocol MAY do either of the
following:Exclude specific code points that are allowed by the relevant
string class.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 or profile will be defined as
disallowed for the relevant protocol slot.Typically, such exclusions are defined for the purpose of
backward compatibility with legacy formats within an application
protocol. These are defined for application protocols, not profiles,
in order to prevent multiplication of profiles beyond necessity (see
).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) . 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 ABNF from :Similar techniques could be used to define many application-layer
constructs, say of the form "user@domain" or "/path/to/file".To ensure proper comparison, the rules specified for a particular
string class or profile MUST be applied in the following order:Width Mapping RuleAdditional Mapping RuleCase Mapping RuleNormalization RuleDirectionality RuleBehavioral rules for determining whether a code point is valid,
allowed under a contextual rule, disallowed, or unassignedAs already described, the width mapping, additional mapping, case
mapping, normalization, and directionality rules 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 (see also the PRECIS mappings
document ).In order to implement the string classes described above, this
document does the following:Reviews and classifies the collections of code points in the
Unicode character set by examining various code point
properties.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.Those code points that are allowed to
be used in any PRECIS string class (currently, IdentifierClass and
FreeformClass). The abbreviated term "PVALID" is used to refer to
this value in the remainder of this document.Those code points that
are allowed to be used in specific string classes. 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.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&nbhy;JOINER, which
have a derived property value of CONTEXTJ. See Appendix A of for more information.Those code points that are not permitted in
any PRECIS string class.Those code points that are
not to be included in one of the string classes but that 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.Those code points that are not designated
(i.e., are unassigned) in the Unicode Standard.The algorithm to calculate the value of the derived property is as
follows (implementations MUST NOT modify the order of operations within
this algorithm, since doing so would cause inconsistent results across
implementations):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 herein, 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 mentioned in this document are simply pointers to 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 (at the time of this writing,
see Chapter 4 of ) 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 can be adjusted to ensure the stability of the results.The derived property obtains its value based on a two-step
procedure: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.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 .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 file
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 referenced from 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.This category is defined in Section 2.1 of and is included by reference for use in
PRECIS.This category is defined in Section 2.2 of . However, it is not used in PRECIS.This category is defined in Section 2.3 of . However, it is not used in PRECIS.Note: See the PrecisIgnorableProperties ("M") category below
for a more inclusive category used in PRECIS identifiers.This category is defined in Section 2.4 of . However, it is not used in PRECIS.This category is defined in Section 2.5 of . However, it is not used in PRECIS.Note: See the ASCII7 ("K") category below for a more
inclusive category used in PRECIS identifiers.This category is defined in Section 2.6 of and is included by reference for use in
PRECIS.This category is defined in Section 2.7 of and is included by reference for use in
PRECIS.Note: Management of this category is handled via the processes
specified in . 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.This category is defined in Section 2.8 of and is included by reference for use in
PRECIS.This category is defined in Section 2.9 of and is included by reference for use in
PRECIS.This category is defined in Section 2.10 of and is included by reference for use in
PRECIS.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 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.This PRECIS-specific category consists of all control
characters.This PRECIS-specific category is used to group code points that
are discouraged from use in PRECIS string classes.The definition for Default_Ignorable_Code_Point can be found in
the DerivedCoreProperties.txt file .
This PRECIS-specific category is used to group code points that
are space characters.This PRECIS-specific category is used to group code points that
are symbols.This PRECIS-specific category is used to group code points that
are punctuation characters.This PRECIS-specific category is used to group code points that
have compatibility equivalents as explained in the Unicode Standard
(at the time of this writing, see Chapters 2 and 3 of ).The toNFKC() operation returns the code point in normalization
form KC. For more information, see Section 5 of Unicode Standard
Annex #15 .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 ).Experience with internationalization in application protocols has
shown that protocol designers and application developers usually do not
understand the subtleties and tradeoffs involved with
internationalization and that they need considerable guidance in making
reasonable decisions with regard to the options before them.Therefore:Protocol designers are strongly encouraged to question the
assumption that they need to define new profiles, since existing
profiles are designed for wide reuse (see
for further discussion).Those who persist in defining new profiles are strongly
encouraged to clearly explain a strong justification for doing so,
and to publish a stable specification that provides all of the
information described under .The designated experts for profile registration requests ought to
seek answers to all of the questions provided under and to encourage applicants to provide a
stable specification documenting the profile (even though the
registration policy for PRECIS profiles is Expert Review and a
stable specification is not strictly required).Developers of applications that use PRECIS are strongly
encouraged to apply the guidelines provided under and to seek out the advice of the designated experts
or other knowledgeable individuals in doing so.All parties are strongly encouraged to help prevent the
multiplication of profiles beyond necessity, as described under
, and to use PRECIS in ways
that will minimize user confusion and insecure application
behavior.Internationalization can be difficult and contentious; designated
experts, profile registrants, and application developers are strongly
encouraged to work together in a spirit of good faith and mutual
understanding to achieve rough consensus on profile registration
requests and the use of PRECIS in particular applications. They are
also encouraged to bring additional expertise into the discussion if
that would be helpful in adding perspective or otherwise resolving
issues.IANA has created and now maintains the "PRECIS Derived Property
Value" registry that records the derived properties for the versions
of Unicode that are released after
(and including) version 7.0. The derived property value is to be
calculated in cooperation with a designated expert according to the rules specified under
Sections
and .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
Sections
and
require IETF Review.IANA has created the "PRECIS Base Classes" registry.
In accordance with , the registration policy
is "RFC Required".The registration template is as follows:[the name of the PRECIS string
class][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."][the RFC number]The initial registrations are as follows:IANA has created the "PRECIS Profiles" registry to identify
profiles that use the
PRECIS string classes. In accordance with ,
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
sometimes complex and subtle internationalization issues related to
profiles of PRECIS string classes.The registration template is as follows:[the name of the profile][which PRECIS string class is being
profiled][the specific protocol elements to
which this profile applies, e.g., "Localparts in XMPP
addresses."][the Stringprep profile that this PRECIS
profile replaces, if any][the behavioral rule for
handling of width, e.g., "Map fullwidth and halfwidth characters
to their compatibility variants."][any additional mappings
that are required or recommended, e.g., "Map non-ASCII space
characters to ASCII space."][the behavioral rule for handling
of case, e.g., "Unicode Default Case Folding"][which Unicode normalization
form is applied, e.g., "NFC"][the behavioral rule for
handling of right-to-left code points, e.g., "The 'Bidi Rule'
defined in RFC 5893 applies."][which entities enforce the rules, and
when that enforcement occurs during protocol operations][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:Would an existing PRECIS string class or profile solve the
problem? If not, why not? (See for related considerations.)Is the problem being addressed by this profile well defined?Does the specification define what kinds of applications are
involved and the protocol elements to which this profile
applies?Is the profile clearly defined?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?Are the width mapping, case mapping, additional mappings,
normalization, and directionality rules appropriate for the
intended use?Does the profile explain which entities enforce the rules, and
when such enforcement occurs during protocol operations?Does the profile reduce the degree to which human users could
be surprised or confused by application behavior (the "Principle
of Least Astonishment")?Does the profile introduce any new security concerns such as
those described under of this document
(e.g., false positives for authentication or authorization)?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 negatives discussed in . 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. Despite these difficulties, profiles
should not be multiplied beyond necessity (see ). In particular, application
protocol designers should think long and hard before 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, enforcement, 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 (again, see ).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
enforcement and comparison operations. For some helpful guidelines,
refer to , , , and .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 ), protocol designers and implementers
are encouraged to pay close attention to the security considerations
described elsewhere in this document.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 code points 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.)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)
and the Simple Authentication and Security
Layer (SASL) , that do not take local
character sets into account.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.Despite the fact that some specific suggestions about
identification and handling of confusable characters appear in the
Unicode Security Considerations and the Unicode
Security Mechanisms , it is also true (as noted
in ) 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: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
filename). 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).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 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.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
). Moreover, passwords are often carried by a
sequence of protocols with backend authentication systems or data
storage systems such as RADIUS and the
Lightweight Directory Access Protocol (LDAP) . 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.Further discussion of password handling can be found in .Although strings that are consumed in PRECIS-based application
protocols are often encoded using UTF-8 , 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 ).
Establishing such a baseline is a matter for the application protocol
that uses PRECIS, not for the PRECIS framework.Changes to the properties of Unicode code points can occur as the
Unicode Standard is modified from time to time. For example, three
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 . 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. Other such differences might arise
between the version of Unicode current at the time of this writing
(7.0) and future versions.As part of the review of Unicode 7.0 for IDNA, a question was
raised about a newly added code point that led to a re-analysis of the
normalization rules used by IDNA and inherited by this document (). Some of the general
issues are described in and pursued in
more detail in .At the time of writing, these issues have yet to be settled.
However, implementers need to be aware that this specification is
likely to be updated in the future to address these issues. The
potential changes include the following:The range of characters in the LetterDigits category
(Sections
and ) might be
narrowed.Some characters with special properties that are now allowed
might be excluded.More "Additional Mapping Rules" () might be defined.Alternative normalization methods might be added.Nevertheless, implementations and deployments that are sensitive to
the advice given in this specification are unlikely to encounter
significant problems as a consequence of these issues or potential
changes -- specifically, the advice to use the more restrictive
IdentifierClass whenever possible or, if using the FreeformClass, to
allow only a restricted set of characters, particularly avoiding
characters whose implications they do not actually understand.ASCII format for network interchangeKey words for use in RFCs to Indicate Requirement LevelsUnicode Format for Network InterchangeTerminology Used in Internationalization in the IETFThe Unicode StandardThe Unicode ConsortiumThe Unicode Standard, Version 7.0.0The Unicode ConsortiumDerivedCoreProperties-7.0.0.txtThe Unicode ConsortiumPropertyAliases-7.0.0.txtThe Unicode ConsortiumIAB Statement on Identifiers and Unicode 7.0.0Internet Architecture BoardMapping characters for PRECIS classesPreparation, Enforcement, and Comparison of Internationalized Strings
Representing NicknamesPreparation, Enforcement, and Comparison of Internationalized Strings
Representing Usernames and PasswordsExtensible Messaging and Presence Protocol (XMPP): Address FormatIDNA Update for Unicode 7.0.0Remote Authentication Dial In User Service (RADIUS)Preparation of Internationalized Strings ("stringprep")Internationalizing Domain Names in Applications (IDNA)Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN)UTF-8, a transformation format of ISO 10646Simple Authentication and Security Layer (SASL)Lightweight Directory Access Protocol (LDAP): Technical Specification Road MapReview and Recommendations for Internationalized Domain Names (IDNs)IABGuidelines for Writing an IANA Considerations Section in RFCsAugmented BNF for Syntax Specifications: ABNFThe Transport Layer Security (TLS) Protocol Version 1.2Internationalized Domain Names for Applications (IDNA): Definitions and Document FrameworkInternationalized Domain Names in Applications (IDNA): ProtocolThe Unicode Code Points and Internationalized Domain Names for Applications (IDNA)Right-to-Left Scripts for Internationalized Domain Names for Applications (IDNA)Internationalized Domain Names for Applications (IDNA): Background, Explanation, and RationaleMapping Characters for Internationalized Domain Names in Applications (IDNA) 2008The Unicode Code Points and Internationalized Domain Names for Applications (IDNA) - Unicode 6.0Stringprep Revision and Problem Statement for the Preparation and Comparison of Internationalized Strings (PRECIS)Issues in Identifier Comparison for Security PurposesUnicode Bidirectional AlgorithmUnicode Standard Annex #9East Asian WidthUnicode Standard Annex #11Unicode Normalization FormsUnicode Standard Annex #15Unicode Security ConsiderationsUnicode Technical Report #36Unicode Security MechanismsUnicode Technical Standard #39The 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.Special thanks are due to John Klensin and Patrik Faltstrom for their
challenging feedback and detailed reviews.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
. Some text regarding security has been
borrowed from , , and .Peter Saint-Andre wishes to acknowledge Cisco Systems, Inc., for
employing him during his work on earlier draft versions of this document.