11. IANA Considerations
11.1. PRECIS Derived Property Value Registry
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 [RFC5226] according to the rules specified under
Sections 8 and 9.
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 8 and 9 require IETF Review.
11.2. PRECIS Base Classes Registry
IANA has created the "PRECIS Base Classes" registry. 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 4.3 of RFC 7564.
Base Class: IdentifierClass.
Description: A sequence of letters, numbers, and symbols that is
used to identify or address a network entity.
Specification: Section 4.2 of RFC 7564.
11.3. PRECIS Profiles Registry
IANA has created the "PRECIS Profiles" registry to identify 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
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]
Base Class: [which PRECIS string class is being profiled]
Applicability: [the specific protocol elements to which this profile
applies, e.g., "Localparts in XMPP addresses."]
Replaces: [the Stringprep profile that this PRECIS profile replaces,
Width Mapping Rule: [the behavioral rule for handling of width,
e.g., "Map fullwidth and halfwidth characters to their
Additional Mapping Rule: [any additional mappings that are required
or recommended, e.g., "Map non-ASCII space characters to ASCII
Case Mapping Rule: [the behavioral rule for handling of case, e.g.,
"Unicode Default Case Folding"]
Normalization Rule: [which Unicode normalization form is applied,
Directionality Rule: [the behavioral rule for handling of right-to-
left code points, e.g., "The 'Bidi Rule' defined in RFC 5893
Enforcement: [which entities enforce the rules, and when that
enforcement occurs during protocol operations]
Specification: [a pointer to relevant documentation, such as an RFC
In order to request a review, the registrant shall send a completed
template to the email@example.com list or its designated successor.
Factors to focus on while defining profiles and reviewing profile
registrations include the following:
o Would an existing PRECIS string class or profile solve the
problem? If not, why not? (See Section 5.1 for related
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 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, and directionality rules appropriate for the
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
o Does the profile introduce any new security concerns such as those
described under Section 12 of this document (e.g., false positives
for authentication or authorization)?
12. Security Considerations
12.1. 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 negatives 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. Despite these difficulties, profiles should not be
multiplied beyond necessity (see Section 5.1). 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 [RFC6943]).
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 [RFC6943], [RFC5890], [UTR36], and [UTS39].
12.2. 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 12.5), protocol designers and implementers are encouraged to
pay close attention to the security considerations described
elsewhere in this document.
12.3. 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 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.)
12.4. 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.
12.5. 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
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
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).
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
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.
12.6. 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 the Lightweight Directory Access
Protocol (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.
Further discussion of password handling can be found in
13. 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.
13.2. Character Sets
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.
13.3. Unicode Versions
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 [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. Other such differences might arise between the
version of Unicode current at the time of this writing (7.0) and
13.4. Potential Changes to Handling of Certain Unicode Code Points
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
(Section 5.2.4). Some of the general issues are described in
[IAB-Statement] and pursued in more detail in [IDNA-Unicode].
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:
o The range of characters in the LetterDigits category
(Sections 4.2.1 and 9.1) might be narrowed.
o Some characters with special properties that are now allowed might
o More "Additional Mapping Rules" (Section 5.2.2) might be defined.
o 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.
14.1. Normative References
[RFC20] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,
[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365,
DOI 10.17487/RFC6365, September 2011,
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
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
[RFC5892]. Some text regarding security has been borrowed from
[RFC5890], [PRECIS-Users-Pwds], and [XMPP-Addr-Format].
Peter Saint-Andre wishes to acknowledge Cisco Systems, Inc., for
employing him during his work on earlier draft versions of this
Quebec, QC G1R 2E1