





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.












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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



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   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








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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].
















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   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



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   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.





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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




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   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.





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   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.






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      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.






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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.



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   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).









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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.





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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).






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   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



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   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.




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   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).





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      *  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




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   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]).











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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.




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   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.



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   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.



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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.





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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].









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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}











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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.






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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.








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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.



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   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



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   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.





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   (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



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   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



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       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.





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   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].



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   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>.







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   [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.








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   [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>.







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   [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>.







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   [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.











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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/





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