    Internet Engineering Task Force (IETF)                    P. Saint-Andre
    Request for Comments: 7564                                          &yet
    Obsoletes: 3454                                              M. Blanchet
    Category: Standards Track                                       Viagenie
    ISSN: 2070-1721                                                 May 2015

         PRECIS Framework: Preparation, Enforcement, and Comparison of
               Internationalized Strings in Application Protocols

    Abstract

       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 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 is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 5741.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       http://www.rfc-editor.org/info/rfc7564.

    Saint-Andre & Blanchet       Standards Track                    [Page 1]

    RFC 7564                    PRECIS Framework                    May 2015

    Copyright Notice

       Copyright (c) 2015 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

       This document is subject to BCP 78 and the IETF Trust's Legal
       Provisions Relating to IETF Documents
       (http://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

    Table of Contents

       1. Introduction ....................................................4
       2. Terminology .....................................................7
       3. Preparation, Enforcement, and Comparison ........................7
       4. String Classes ..................................................8
          4.1. Overview ...................................................8
          4.2. IdentifierClass ............................................9
               4.2.1. Valid ...............................................9
               4.2.2. Contextual Rule Required ...........................10
               4.2.3. Disallowed .........................................10
               4.2.4. Unassigned .........................................11
               4.2.5. Examples ...........................................11
          4.3. FreeformClass .............................................11
               4.3.1. Valid ..............................................11
               4.3.2. Contextual Rule Required ...........................12
               4.3.3. Disallowed .........................................12
               4.3.4. Unassigned .........................................12
               4.3.5. Examples ...........................................12
       5. Profiles .......................................................13
          5.1. Profiles Must Not Be Multiplied beyond Necessity ..........13
          5.2. Rules .....................................................14
               5.2.1. Width Mapping Rule .................................14
               5.2.2. Additional Mapping Rule ............................14
               5.2.3. Case Mapping Rule ..................................14
               5.2.4. Normalization Rule .................................15
               5.2.5. Directionality Rule ................................15
          5.3. A Note about Spaces .......................................16
       6. Applications ...................................................17
          6.1. How to Use PRECIS in Applications .........................17
          6.2. Further Excluded Characters ...............................18
          6.3. Building Application-Layer Constructs .....................18
       7. Order of Operations ............................................19

    Saint-Andre & Blanchet       Standards Track                    [Page 2]

    RFC 7564                    PRECIS Framework                    May 2015

       8. Code Point Properties ..........................................20
       9. Category Definitions Used to Calculate Derived Property ........22
          9.1. LetterDigits (A) ..........................................23
          9.2. Unstable (B) ..............................................23
          9.3. IgnorableProperties (C) ...................................23
          9.4. IgnorableBlocks (D) .......................................23
          9.5. LDH (E) ...................................................23
          9.6. Exceptions (F) ............................................23
          9.7. BackwardCompatible (G) ....................................23
          9.8. JoinControl (H) ...........................................24
          9.9. OldHangulJamo (I) .........................................24
          9.10. Unassigned (J) ...........................................24
          9.11. ASCII7 (K) ...............................................24
          9.12. Controls (L) .............................................24
          9.13. PrecisIgnorableProperties (M) ............................24
          9.14. Spaces (N) ...............................................25
          9.15. Symbols (O) ..............................................25
          9.16. Punctuation (P) ..........................................25
          9.17. HasCompat (Q) ............................................25
          9.18. OtherLetterDigits (R) ....................................25
       10. Guidelines for Designated Experts .............................26
       11. IANA Considerations ...........................................27
          11.1. PRECIS Derived Property Value Registry ...................27
          11.2. PRECIS Base Classes Registry .............................27
          11.3. PRECIS Profiles Registry .................................28
       12. Security Considerations .......................................29
          12.1. General Issues ...........................................29
          12.2. Use of the IdentifierClass ...............................30
          12.3. Use of the FreeformClass .................................30
          12.4. Local Character Set Issues ...............................31
          12.5. Visually Similar Characters ..............................31
          12.6. Security of Passwords ....................................33
       13. Interoperability Considerations ...............................34
          13.1. Encoding .................................................34
          13.2. Character Sets ...........................................34
          13.3. Unicode Versions .........................................34
          13.4. Potential Changes to Handling of Certain Unicode
                Code Points ..............................................34
       14. References ....................................................35
          14.1. Normative References .....................................35
          14.2. Informative References ...................................36
       Acknowledgements ..................................................40
       Authors' Addresses ................................................40

    Saint-Andre & Blanchet       Standards Track                    [Page 3]

    RFC 7564                    PRECIS Framework                    May 2015

    1.  Introduction

       Application protocols using Unicode characters [Unicode] 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 [RFC6885], many IETF
       protocols have used the Stringprep framework [RFC3454] as the basis
       for preparing, enforcing, and comparing protocol strings that contain
       Unicode characters, especially characters 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.  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
       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 prompted other "customers" of Stringprep to consider new
       approaches to the preparation, enforcement, and comparison of
       internationalized strings, as described in [RFC6885].

    Saint-Andre & Blanchet       Standards Track                    [Page 4]

    RFC 7564                    PRECIS Framework                    May 2015

       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:

       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 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:

       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.

       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 [PRECIS-Users-Pwds], opaque strings such
       as passwords [PRECIS-Users-Pwds], and nicknames [PRECIS-Nickname].
       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

    Saint-Andre & Blanchet       Standards Track                    [Page 5]

    RFC 7564                    PRECIS Framework                    May 2015

       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:

       a.  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.).

       b.  Comparing two output strings to determine if they are equivalent,
           typically through octet-for-octet matching to test for
           "bit-string identity" (e.g., to make an access decision for
           purposes of authentication or authorization as further described
           in [RFC6943]).

       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 Section 5 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 [RFC5892], 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 8 and 9 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.

    Saint-Andre & Blanchet       Standards Track                    [Page 6]

    RFC 7564                    PRECIS Framework                    May 2015

    2.  Terminology

       Many important terms used in this document are defined in [RFC5890],
       [RFC6365], [RFC6885], and [Unicode].  The terms "left-to-right" (LTR)
       and "right-to-left" (RTL) are defined in Unicode Standard Annex #9
       [UAX9].

       As of the date of writing, the version of Unicode published by the
       Unicode Consortium is 7.0 [Unicode7.0]; however, PRECIS is not tied
       to a specific version of Unicode.  The latest version of Unicode is
       always available [Unicode].

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
       "OPTIONAL" in this document are to be interpreted as described in
       [RFC2119].

    3.  Preparation, Enforcement, and Comparison

       This document distinguishes between three different actions that an
       entity can take with regard to a string:

       o  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.

       o  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.

       o  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

    Saint-Andre & Blanchet       Standards Track                    [Page 7]

    RFC 7564                    PRECIS Framework                    May 2015

       client cannot necessarily be trusted to properly generate such
       strings, especially for security-sensitive contexts such as
       authentication and authorization.

    4.  String Classes

    4.1.  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 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 Section 12.3 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
       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.

    Saint-Andre & Blanchet       Standards Track                    [Page 8]

    RFC 7564                    PRECIS Framework                    May 2015

       The following subsections discuss the IdentifierClass and
       FreeformClass in more detail, with reference to the dimensions
       described in Section 5 of [RFC6885].  Each string class is defined by
       the following behavioral rules:

       Valid:  Defines which code points are treated as valid for the
          string.

       Contextual Rule Required:  Defines which code points 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 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 5, profiles of these
       string classes are responsible for defining the width mapping,
       additional mappings, case mapping, normalization, and directionality
       rules.

    4.2.  IdentifierClass

       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.

    4.2.1.  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 9.1.

       o  Code points in the range U+0021 through U+007E, i.e., the
          (printable) ASCII7 ("K") category defined under Section 9.11.
          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.

    Saint-Andre & Blanchet       Standards Track                    [Page 9]

    RFC 7564                    PRECIS Framework                    May 2015

          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.

    4.2.2.  Contextual Rule Required

       o  A number of characters from the Exceptions ("F") category defined
          under Section 9.6 (see Section 9.6 for a full list).

       o  Joining characters, i.e., the JoinControl ("H") category defined
          under Section 9.8.

    4.2.3.  Disallowed

       o  Old Hangul Jamo characters, i.e., the OldHangulJamo ("I") category
          defined under Section 9.9.

       o  Control characters, i.e., the Controls ("L") category defined
          under Section 9.12.

       o  Ignorable characters, i.e., the PrecisIgnorableProperties ("M")
          category defined under Section 9.13.

       o  Space characters, i.e., the Spaces ("N") category defined under
          Section 9.14.

       o  Symbol characters, i.e., the Symbols ("O") category defined under
          Section 9.15.

       o  Punctuation characters, i.e., the Punctuation ("P") category
          defined under Section 9.16.

       o  Any character that has a compatibility equivalent, i.e., the
          HasCompat ("Q") category defined under Section 9.17.  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 9.18.

    Saint-Andre & Blanchet       Standards Track                   [Page 10]

    RFC 7564                    PRECIS Framework                    May 2015

    4.2.4.  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.  See
       Section 9.10.

    4.2.5.  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 profiles of the IdentifierClass can be
       found in [PRECIS-Users-Pwds] (the UsernameCaseMapped and
       UsernameCasePreserved profiles).

    4.3.  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
       [PRECIS-Users-Pwds]) or a nickname in a chatroom (see
       [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 12.3 describes some of the
          security hazards involved with using or profiling the
          FreeformClass.

          Security Warning: Consult Section 12.6 for relevant security
          considerations when strings conforming to the FreeformClass, or a
          profile thereof, are used as passwords.

    4.3.1.  Valid

       o  Traditional letters and numbers, i.e., the LetterDigits ("A")
          category first defined in [RFC5892] and listed here under
          Section 9.1.

       o  Letters and digits other than the "traditional" letters and digits
          allowed in IDNs, i.e., the OtherLetterDigits ("R") category
          defined under Section 9.18.

       o  Code points in the range U+0021 through U+007E, i.e., the
          (printable) ASCII7 ("K") category defined under Section 9.11.

       o  Any character that has a compatibility equivalent, i.e., the
          HasCompat ("Q") category defined under Section 9.17.

    Saint-Andre & Blanchet       Standards Track                   [Page 11]

    RFC 7564                    PRECIS Framework                    May 2015

       o  Space characters, i.e., the Spaces ("N") category defined under
          Section 9.14.

       o  Symbol characters, i.e., the Symbols ("O") category defined under
          Section 9.15.

       o  Punctuation characters, i.e., the Punctuation ("P") category
          defined under Section 9.16.

    4.3.2.  Contextual Rule Required

       o  A number of characters from the Exceptions ("F") category defined
          under Section 9.6 (see Section 9.6 for a full list).

       o  Joining characters, i.e., the JoinControl ("H") category defined
          under Section 9.8.

    4.3.3.  Disallowed

       o  Old Hangul Jamo characters, i.e., the OldHangulJamo ("I") category
          defined under Section 9.9.

       o  Control characters, i.e., the Controls ("L") category defined
          under Section 9.12.

       o  Ignorable characters, i.e., the PrecisIgnorableProperties ("M")
          category defined under Section 9.13.

    4.3.4.  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.

    4.3.5.  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 profiles of the FreeformClass can be found
       in [PRECIS-Users-Pwds] (the OpaqueString profile) and
       [PRECIS-Nickname] (the Nickname profile).

    Saint-Andre & Blanchet       Standards Track                   [Page 12]

    RFC 7564                    PRECIS Framework                    May 2015

    5.  Profiles

       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 Section 11.3.  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 [PRECIS-Users-Pwds].

    5.1.  Profiles Must Not Be Multiplied beyond Necessity

       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 Section 10 are meant
       to encourage a high level of due diligence regarding new profiles.

    Saint-Andre & Blanchet       Standards Track                   [Page 13]

    RFC 7564                    PRECIS Framework                    May 2015

    5.2.  Rules

    5.2.1.  Width Mapping Rule

       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
       [UAX11].

    5.2.2.  Additional Mapping Rule

       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 [PRECIS-Mappings] describes such
       mappings in more detail.

    5.2.3.  Case Mapping Rule

       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).

    Saint-Andre & Blanchet       Standards Track                   [Page 14]

    RFC 7564                    PRECIS Framework                    May 2015

       If case mapping is desired (instead of case preservation), it is
       RECOMMENDED to use Unicode Default Case Folding as defined in the
       Unicode Standard [Unicode] (at the time of this writing, the
       algorithm is specified in Chapter 3 of [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
          [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
       Section 12.6 and in [PRECIS-Users-Pwds].

    5.2.4.  Normalization Rule

       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.

    5.2.5.  Directionality Rule

       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 [UAX9]).  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

    Saint-Andre & Blanchet       Standards Track                   [Page 15]

    RFC 7564                    PRECIS Framework                    May 2015

       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 [UAX9].  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 [RFC5893], 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" [RFC5893] (for profiles based on the
       IdentifierClass) or following the Unicode bidirectional algorithm
       [UAX9] (for profiles based on the FreeformClass or in situations
       where the IdentifierClass is not appropriate).

    5.3.  A Note about Spaces

       With regard to the IdentifierClass, the consensus of the PRECIS
       Working Group was that spaces are problematic for many reasons,
       including the following:

       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.

    Saint-Andre & Blanchet       Standards Track                   [Page 16]

    RFC 7564                    PRECIS Framework                    May 2015

       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 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 Section 6.3, application protocols can also
       define application-layer constructs containing spaces.

    6.  Applications

    6.1.  How to Use PRECIS in Applications

       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).

       o  Don't define your own profile unless absolutely necessary (see
          Section 5.1).  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 (Section 6.2) and to build application-layer
          constructs (see Section 6.3).

       o  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).

    Saint-Andre & Blanchet       Standards Track                   [Page 17]

    RFC 7564                    PRECIS Framework                    May 2015

          *  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
             [PRECIS-Users-Pwds] of the IdentifierClass, or checking the
             password of a user against the OpaqueString profile
             [PRECIS-Users-Pwds] of the FreeformClass).

          See [PRECIS-Users-Pwds] and [XMPP-Addr-Format] for definitions of
          these matters for several applications.

    6.2.  Further Excluded Characters

       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:

       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 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
       Section 5.1).

    6.3.  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

    Saint-Andre & Blanchet       Standards Track                   [Page 18]

    RFC 7564                    PRECIS Framework                    May 2015

       IdentifierClass; however, it could be defined using a space-separated
       sequence of IdentifierClass instances, as in the following ABNF
       [RFC5234] from [PRECIS-Users-Pwds]:

          username   = userpart *(1*SP userpart)
          userpart   = 1*(idbyte)
                       ;
                       ; an "idbyte" is a byte used to represent a
                       ; UTF-8 encoded Unicode code point that can be
                       ; contained in a string 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".

    7.  Order of Operations

       To ensure proper comparison, the rules specified for a particular
       string class or profile MUST be applied in the following order:

       1.  Width Mapping Rule

       2.  Additional Mapping Rule

       3.  Case Mapping Rule

       4.  Normalization Rule

       5.  Directionality Rule

       6.  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 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
       Section 5.2.1 (see also the PRECIS mappings document
       [PRECIS-Mappings]).

    Saint-Andre & Blanchet       Standards Track                   [Page 19]

    RFC 7564                    PRECIS Framework                    May 2015

    8.  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).  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.  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.

    Saint-Andre & Blanchet       Standards Track                   [Page 20]

    RFC 7564                    PRECIS Framework                    May 2015

       SPECIFIC CLASS DISALLOWED  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.

       UNASSIGNED  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):

       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 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.

    Saint-Andre & Blanchet       Standards Track                   [Page 21]

    RFC 7564                    PRECIS Framework                    May 2015

       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 [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 9.7) can
       be adjusted to ensure the stability of the results.

    9.  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 8.

          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 [PropertyAliases] 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 [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.

    Saint-Andre & Blanchet       Standards Track                   [Page 22]

    RFC 7564                    PRECIS Framework                    May 2015

    9.1.  LetterDigits (A)

       This category is defined in Section 2.1 of [RFC5892] and is included
       by reference for use in PRECIS.

    9.2.  Unstable (B)

       This category is defined in Section 2.2 of [RFC5892].  However, it is
       not used in PRECIS.

    9.3.  IgnorableProperties (C)

       This category is defined in Section 2.3 of [RFC5892].  However, it is
       not used in PRECIS.

       Note: See the PrecisIgnorableProperties ("M") category below for a
       more inclusive category used in PRECIS identifiers.

    9.4.  IgnorableBlocks (D)

       This category is defined in Section 2.4 of [RFC5892].  However, it is
       not used in PRECIS.

    9.5.  LDH (E)

       This category is defined in Section 2.5 of [RFC5892].  However, it is
       not used in PRECIS.

       Note: See the ASCII7 ("K") category below for a more inclusive
       category used in PRECIS identifiers.

    9.6.  Exceptions (F)

       This category is defined in Section 2.6 of [RFC5892] and is included
       by reference for use in PRECIS.

    9.7.  BackwardCompatible (G)

       This category is defined in Section 2.7 of [RFC5892] and is included
       by reference for use in PRECIS.

       Note: 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.

    Saint-Andre & Blanchet       Standards Track                   [Page 23]

    RFC 7564                    PRECIS Framework                    May 2015

    9.8.  JoinControl (H)

       This category is defined in Section 2.8 of [RFC5892] and is included
       by reference for use in PRECIS.

    9.9.  OldHangulJamo (I)

       This category is defined in Section 2.9 of [RFC5892] and is included
       by reference for use in PRECIS.

    9.10.  Unassigned (J)

       This category is defined in Section 2.10 of [RFC5892] and is included
       by reference for use in PRECIS.

    9.11.  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 8 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}

    9.12.  Controls (L)

       This PRECIS-specific category consists of all control characters.

       L: Control(cp) = True

    9.13.  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 file [DerivedCoreProperties].

    Saint-Andre & Blanchet       Standards Track                   [Page 24]

    RFC 7564                    PRECIS Framework                    May 2015

    9.14.  Spaces (N)

       This PRECIS-specific category is used to group code points that are
       space characters.

       N: General_Category(cp) is in {Zs}

    9.15.  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}

    9.16.  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}

    9.17.  HasCompat (Q)

       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 [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].

    9.18.  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 9.1).

       R: General_Category(cp) is in {Lt, Nl, No, Me}

    Saint-Andre & Blanchet       Standards Track                   [Page 25]

    RFC 7564                    PRECIS Framework                    May 2015

    10.  Guidelines for Designated Experts

       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:

       o  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 Section 5 for further
          discussion).

       o  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 Section 11.3.

       o  The designated experts for profile registration requests ought to
          seek answers to all of the questions provided under Section 11.3
          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).

       o  Developers of applications that use PRECIS are strongly encouraged
          to apply the guidelines provided under Section 6 and to seek out
          the advice of the designated experts or other knowledgeable
          individuals in doing so.

       o  All parties are strongly encouraged to help prevent the
          multiplication of profiles beyond necessity, as described under
          Section 5.1, 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.

    Saint-Andre & Blanchet       Standards Track                   [Page 26]

    RFC 7564                    PRECIS Framework                    May 2015

    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.

    Saint-Andre & Blanchet       Standards Track                   [Page 27]

    RFC 7564                    PRECIS Framework                    May 2015

    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,
          if any]

       Width Mapping Rule:  [the behavioral rule for handling of width,
          e.g., "Map fullwidth and halfwidth characters to their
          compatibility variants."]

       Additional Mapping Rule:  [any additional mappings that are required
          or recommended, e.g., "Map non-ASCII space characters to ASCII
          space."]

       Case Mapping Rule:  [the behavioral rule for handling of case, e.g.,
          "Unicode Default Case Folding"]

       Normalization Rule:  [which Unicode normalization form is applied,
          e.g., "NFC"]

       Directionality Rule:  [the behavioral rule for handling of right-to-
          left code points, e.g., "The 'Bidi Rule' defined in RFC 5893
          applies."]

       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.

    Saint-Andre & Blanchet       Standards Track                   [Page 28]

    RFC 7564                    PRECIS Framework                    May 2015

       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
          considerations.)

       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
          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 Astonishment")?

       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

    Saint-Andre & Blanchet       Standards Track                   [Page 29]

    RFC 7564                    PRECIS Framework                    May 2015

       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.

    Saint-Andre & Blanchet       Standards Track                   [Page 30]

    RFC 7564                    PRECIS Framework                    May 2015

       (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

    Saint-Andre & Blanchet       Standards Track                   [Page 31]

    RFC 7564                    PRECIS Framework                    May 2015

       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 [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

    Saint-Andre & Blanchet       Standards Track                   [Page 32]

    RFC 7564                    PRECIS Framework                    May 2015

           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.

    Saint-Andre & Blanchet       Standards Track                   [Page 33]

    RFC 7564                    PRECIS Framework                    May 2015

       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
       [PRECIS-Users-Pwds].

    13.  Interoperability Considerations

    13.1.  Encoding

       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
       future versions.

    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].

    Saint-Andre & Blanchet       Standards Track                   [Page 34]

    RFC 7564                    PRECIS Framework                    May 2015

       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
          be excluded.

       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.  References

    14.1.  Normative References

       [RFC20]    Cerf, V., "ASCII format for network interchange", STD 80,
                  RFC 20, DOI 10.17487/RFC0020, October 1969,
                  <http://www.rfc-editor.org/info/rfc20>.

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <http://www.rfc-editor.org/info/rfc2119>.

       [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
                  Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,
                  <http://www.rfc-editor.org/info/rfc5198>.

       [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in
                  Internationalization in the IETF", BCP 166, RFC 6365,
                  DOI 10.17487/RFC6365, September 2011,
                  <http://www.rfc-editor.org/info/rfc6365>.

    Saint-Andre & Blanchet       Standards Track                   [Page 35]

    RFC 7564                    PRECIS Framework                    May 2015

       [Unicode]  The Unicode Consortium, "The Unicode Standard",
                  <http://www.unicode.org/versions/latest/>.

       [Unicode7.0]
                  The Unicode Consortium, "The Unicode Standard, Version
                  7.0.0", (Mountain View, CA: The Unicode Consortium, 2014
                  ISBN 978-1-936213-09-2),
                  <http://www.unicode.org/versions/Unicode7.0.0/>.

    14.2.  Informative References

       [DerivedCoreProperties]
                  The Unicode Consortium, "DerivedCoreProperties-7.0.0.txt",
                  Unicode Character Database, February 2014,
                  <http://www.unicode.org/Public/UCD/latest/ucd/
                  DerivedCoreProperties.txt>.

       [IAB-Statement]
                  Internet Architecture Board, "IAB Statement on Identifiers
                  and Unicode 7.0.0", February 2015, <https://www.iab.org/
                  documents/correspondence-reports-documents/
                  2015-2/iab-statement-on-identifiers-and-unicode-7-0-0/>.

       [IDNA-Unicode]
                  Klensin, J. and P. Faltstrom, "IDNA Update for Unicode
                  7.0.0", Work in Progress,
                  draft-klensin-idna-5892upd-unicode70-04, March 2015.

       [PRECIS-Mappings]
                  Yoneya, Y. and T. Nemoto, "Mapping characters for PRECIS
                  classes", Work in Progress, draft-ietf-precis-mappings-10,
                  May 2015.

       [PRECIS-Nickname]
                  Saint-Andre, P., "Preparation, Enforcement, and Comparison
                  of Internationalized Strings Representing Nicknames", Work
                  in Progress, draft-ietf-precis-nickname-17, April 2015.

       [PRECIS-Users-Pwds]
                  Saint-Andre, P. and A. Melnikov, "Preparation,
                  Enforcement, and Comparison of Internationalized Strings
                  Representing Usernames and Passwords", Work in Progress,
                  draft-ietf-precis-saslprepbis-17, May 2015.

    Saint-Andre & Blanchet       Standards Track                   [Page 36]

    RFC 7564                    PRECIS Framework                    May 2015

       [PropertyAliases]
                  The Unicode Consortium, "PropertyAliases-7.0.0.txt",
                  Unicode Character Database, November 2013,
                  <http://www.unicode.org/Public/UCD/latest/ucd/
                  PropertyAliases.txt>.

       [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
                  "Remote Authentication Dial In User Service (RADIUS)",
                  RFC 2865, DOI 10.17487/RFC2865, June 2000,
                  <http://www.rfc-editor.org/info/rfc2865>.

       [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
                  Internationalized Strings ("stringprep")", RFC 3454,
                  DOI 10.17487/RFC3454, December 2002,
                  <http://www.rfc-editor.org/info/rfc3454>.

       [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
                  "Internationalizing Domain Names in Applications (IDNA)",
                  RFC 3490, DOI 10.17487/RFC3490, March 2003,
                  <http://www.rfc-editor.org/info/rfc3490>.

       [RFC3491]  Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
                  Profile for Internationalized Domain Names (IDN)",
                  RFC 3491, DOI 10.17487/RFC3491, March 2003,
                  <http://www.rfc-editor.org/info/rfc3491>.

       [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
                  10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
                  2003, <http://www.rfc-editor.org/info/rfc3629>.

       [RFC4422]  Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple
                  Authentication and Security Layer (SASL)", RFC 4422,
                  DOI 10.17487/RFC4422, June 2006,
                  <http://www.rfc-editor.org/info/rfc4422>.

       [RFC4510]  Zeilenga, K., Ed., "Lightweight Directory Access Protocol
                  (LDAP): Technical Specification Road Map", RFC 4510,
                  DOI 10.17487/RFC4510, June 2006,
                  <http://www.rfc-editor.org/info/rfc4510>.

       [RFC4690]  Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
                  Recommendations for Internationalized Domain Names
                  (IDNs)", RFC 4690, DOI 10.17487/RFC4690, September 2006,
                  <http://www.rfc-editor.org/info/rfc4690>.

    Saint-Andre & Blanchet       Standards Track                   [Page 37]

    RFC 7564                    PRECIS Framework                    May 2015

       [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
                  IANA Considerations Section in RFCs", BCP 26, RFC 5226,
                  DOI 10.17487/RFC5226, May 2008,
                  <http://www.rfc-editor.org/info/rfc5226>.

       [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
                  Syntax Specifications: ABNF", STD 68, RFC 5234,
                  DOI 10.17487/RFC5234, January 2008,
                  <http://www.rfc-editor.org/info/rfc5234>.

       [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
                  (TLS) Protocol Version 1.2", RFC 5246,
                  DOI 10.17487/RFC5246, August 2008,
                  <http://www.rfc-editor.org/info/rfc5246>.

       [RFC5890]  Klensin, J., "Internationalized Domain Names for
                  Applications (IDNA): Definitions and Document Framework",
                  RFC 5890, DOI 10.17487/RFC5890, August 2010,
                  <http://www.rfc-editor.org/info/rfc5890>.

       [RFC5891]  Klensin, J., "Internationalized Domain Names in
                  Applications (IDNA): Protocol", RFC 5891,
                  DOI 10.17487/RFC5891, August 2010,
                  <http://www.rfc-editor.org/info/rfc5891>.

       [RFC5892]  Faltstrom, P., Ed., "The Unicode Code Points and
                  Internationalized Domain Names for Applications (IDNA)",
                  RFC 5892, DOI 10.17487/RFC5892, August 2010,
                  <http://www.rfc-editor.org/info/rfc5892>.

       [RFC5893]  Alvestrand, H., Ed., and C. Karp, "Right-to-Left Scripts
                  for Internationalized Domain Names for Applications
                  (IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010,
                  <http://www.rfc-editor.org/info/rfc5893>.

       [RFC5894]  Klensin, J., "Internationalized Domain Names for
                  Applications (IDNA): Background, Explanation, and
                  Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010,
                  <http://www.rfc-editor.org/info/rfc5894>.

       [RFC5895]  Resnick, P. and P. Hoffman, "Mapping Characters for
                  Internationalized Domain Names in Applications (IDNA)
                  2008", RFC 5895, DOI 10.17487/RFC5895, September 2010,
                  <http://www.rfc-editor.org/info/rfc5895>.

    Saint-Andre & Blanchet       Standards Track                   [Page 38]

    RFC 7564                    PRECIS Framework                    May 2015

       [RFC6452]  Faltstrom, P., Ed., and P. Hoffman, Ed., "The Unicode Code
                  Points and Internationalized Domain Names for Applications
                  (IDNA) - Unicode 6.0", RFC 6452, DOI 10.17487/RFC6452,
                  November 2011, <http://www.rfc-editor.org/info/rfc6452>.

       [RFC6885]  Blanchet, M. and A. Sullivan, "Stringprep Revision and
                  Problem Statement for the Preparation and Comparison of
                  Internationalized Strings (PRECIS)", RFC 6885,
                  DOI 10.17487/RFC6885, March 2013,
                  <http://www.rfc-editor.org/info/rfc6885>.

       [RFC6943]  Thaler, D., Ed., "Issues in Identifier Comparison for
                  Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
                  2013, <http://www.rfc-editor.org/info/rfc6943>.

       [UAX11]    Unicode Standard Annex #11, "East Asian Width", edited by
                  Ken Lunde. An integral part of The Unicode Standard,
                  <http://unicode.org/reports/tr11/>.

       [UAX15]    Unicode Standard Annex #15, "Unicode Normalization Forms",
                  edited by Mark Davis and Ken Whistler. An integral part of
                  The Unicode Standard, <http://unicode.org/reports/tr15/>.

       [UAX9]     Unicode Standard Annex #9, "Unicode Bidirectional
                  Algorithm", edited by Mark Davis, Aharon Lanin, and Andrew
                  Glass. An integral part of The Unicode Standard,
                  <http://unicode.org/reports/tr9/>.

       [UTR36]    Unicode Technical Report #36, "Unicode Security
                  Considerations", by Mark Davis and Michel Suignard,
                  <http://unicode.org/reports/tr36/>.

       [UTS39]    Unicode Technical Standard #39, "Unicode Security
                  Mechanisms", edited by Mark Davis and Michel Suignard,
                  <http://unicode.org/reports/tr39/>.

       [XMPP-Addr-Format]
                  Saint-Andre, P., "Extensible Messaging and Presence
                  Protocol (XMPP): Address Format", Work in Progress,
                  draft-ietf-xmpp-6122bis-22, May 2015.

    Saint-Andre & Blanchet       Standards Track                   [Page 39]

    RFC 7564                    PRECIS Framework                    May 2015

    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.

       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
       document.

    Authors' Addresses

       Peter Saint-Andre
       &yet

       EMail: peter@andyet.com
       URI:   https://andyet.com/

       Marc Blanchet
       Viagenie
       246 Aberdeen
       Quebec, QC  G1R 2E1
       Canada

       EMail: Marc.Blanchet@viagenie.ca
       URI:   http://www.viagenie.ca/

    Saint-Andre & Blanchet       Standards Track                   [Page 40]