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PRECIS                                                    P. Saint-Andre
Internet-Draft                                                      &yet
Obsoletes: 4013 (if approved)                                A. Melnikov
Intended status: Standards Track                               Isode Ltd
Expires: November 29, 2015                                  May 28, 2015


 Preparation, Enforcement, and Comparison of Internationalized Strings
                  Representing Usernames and Passwords
                    draft-ietf-precis-saslprepbis-18

Abstract

   This document describes updated methods for handling Unicode strings
   representing usernames and passwords.  The previous approach was
   known as SASLprep (RFC 4013) and was based on Stringprep (RFC 3454).
   The methods specified in this document provide a more sustainable
   approach to the handling of internationalized usernames and
   passwords.  The PRECIS framework, RFC 7564, obsoletes RFC 3454, and
   this document obsoletes RFC 4013.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 29, 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



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Usernames . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Definition  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  UsernameCaseMapped Profile  . . . . . . . . . . . . . . .   6
       3.2.1.  Preparation . . . . . . . . . . . . . . . . . . . . .   6
       3.2.2.  Enforcement . . . . . . . . . . . . . . . . . . . . .   6
       3.2.3.  Comparison  . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  UsernameCasePreserved Profile . . . . . . . . . . . . . .   7
       3.3.1.  Preparation . . . . . . . . . . . . . . . . . . . . .   7
       3.3.2.  Enforcement . . . . . . . . . . . . . . . . . . . . .   7
       3.3.3.  Comparison  . . . . . . . . . . . . . . . . . . . . .   8
     3.4.  Case Mapping vs. Case Preservation  . . . . . . . . . . .   8
     3.5.  Application-Layer Constructs  . . . . . . . . . . . . . .   9
     3.6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  Passwords . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Definition  . . . . . . . . . . . . . . . . . . . . . . .  11
     4.2.  OpaqueString Profile  . . . . . . . . . . . . . . . . . .  12
       4.2.1.  Preparation . . . . . . . . . . . . . . . . . . . . .  12
       4.2.2.  Enforcement . . . . . . . . . . . . . . . . . . . . .  12
       4.2.3.  Comparison  . . . . . . . . . . . . . . . . . . . . .  13
     4.3.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  13
   5.  Use in Application Protocols  . . . . . . . . . . . . . . . .  14
   6.  Migration . . . . . . . . . . . . . . . . . . . . . . . . . .  15
     6.1.  Usernames . . . . . . . . . . . . . . . . . . . . . . . .  15
     6.2.  Passwords . . . . . . . . . . . . . . . . . . . . . . . .  16
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     7.1.  UsernameCaseMapped Profile  . . . . . . . . . . . . . . .  17
     7.2.  UsernameCasePreserved Profile . . . . . . . . . . . . . .  18
     7.3.  OpaqueString Profile  . . . . . . . . . . . . . . . . . .  19
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
     8.1.  Password/Passphrase Strength  . . . . . . . . . . . . . .  19
     8.2.  Identifier Comparison . . . . . . . . . . . . . . . . . .  19
     8.3.  Reuse of PRECIS . . . . . . . . . . . . . . . . . . . . .  20
     8.4.  Reuse of Unicode  . . . . . . . . . . . . . . . . . . . .  20
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Appendix A.  Differences from RFC 4013  . . . . . . . . . . . . .  22
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23



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1.  Introduction

   Usernames and passwords are widely used for authentication and
   authorization on the Internet, either directly when provided in
   plaintext (as in the SASL PLAIN mechanism [RFC4616] or the HTTP Basic
   scheme [I-D.ietf-httpauth-basicauth-update]) or indirectly when
   provided as the input to a cryptographic algorithm such as a hash
   function (as in the SASL SCRAM mechanism [RFC5802] or the HTTP Digest
   scheme [I-D.ietf-httpauth-digest]).

   To increase the likelihood that the input and comparison of usernames
   and passwords will work in ways that make sense for typical users
   throughout the world, this document defines rules for preparing,
   enforcing, and comparing internationalized strings that represent
   usernames and passwords.  Such strings consist of characters from the
   Unicode character set [Unicode], with special attention to characters
   outside the ASCII range [RFC20].  The rules for handling such strings
   are specified through profiles of the string classes defined in the
   PRECIS framework specification [RFC7564].

   Profiles of the PRECIS framework enable software to handle Unicode
   characters outside the ASCII range in an automated way, so that such
   characters are treated carefully and consistently in application
   protocols.  In large measure, these profiles are designed to protect
   application developers from the potentially negative consequences of
   supporting the full range of Unicode characters.  For instance, in
   almost all application protocols it would be dangerous to treat the
   Unicode character SUPERSCRIPT ONE (U+0089) as equivalent to DIGIT ONE
   (U+0031), since that would result in false positives during
   comparison, authentication, and authorization (e.g., an attacker
   could easy spoof an account "user1@example.com").

   Whereas a naive use of Unicode would make such attacks trivially
   easy, the PRECIS profile defined here for usernames generally
   protects applications from inadvertently causing such problems.
   (Similar considerations apply to passwords, although here it is
   desirable to support a wider range of characters so as to maximize
   entropy for purposes of authentication.)

   The methods defined here might be applicable wherever usernames or
   passwords are used.  However, the methods are not intended for use in
   preparing strings that are not usernames (e.g., LDAP distinguished
   names), nor in cases where identifiers or secrets are not strings
   (e.g., keys and certificates) or require specialized handling.

   This document obsoletes RFC 4013 (the "SASLprep" profile of
   Stringprep [RFC3454]) but can be used by technologies other than the
   Simple Authentication and Security Layer (SASL) [RFC4422], such as



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   HTTP authentication as specified in
   [I-D.ietf-httpauth-basicauth-update] and [I-D.ietf-httpauth-digest].

   This document does not modify the handling of internationalized
   strings in usernames and passwords as prescribed by existing
   application protocols that use SASLprep.  If the community that uses
   such an application protocol wishes to modernize its handling of
   internationalized strings to use PRECIS instead of Stringprep, it
   needs to explicitly update the existing application protocol
   definition (one example is [I-D.ietf-xmpp-6122bis], which obsoletes
   [RFC6122]).  Non-coordinated updates to protocol implementations are
   discouraged because they can have a negative impact on
   interoperability and security.

2.  Terminology

   Many important terms used in this document are defined in [RFC5890],
   [RFC6365], [RFC7564], and [Unicode].  The term "non-ASCII space"
   refers to any Unicode code point having a general category of "Zs",
   with the exception of U+0020 (here called "ASCII space").

   As used here, the term "password" is not literally limited to a word;
   i.e., a password could be a passphrase consisting of more than one
   word, perhaps separated by spaces, punctuation, or other non-
   alphanumeric characters.

   Some SASL mechanisms (e.g., CRAM-MD5, DIGEST-MD5, and SCRAM) specify
   that the authentication identity used in the context of such
   mechanisms is a "simple user name" (see Section 2 of [RFC4422] as
   well as [RFC4013]).  Various application technologies also assume
   that the identity of a user or account takes the form of a username
   (e.g., authentication for the HyperText Transfer Protocol as
   specified in [I-D.ietf-httpauth-basicauth-update] and
   [I-D.ietf-httpauth-digest]), whether or not they use SASL.  Note well
   that the exact form of a username in any particular SASL mechanism or
   application technology is a matter for implementation and deployment,
   and that a username does not necessarily map to any particular
   application identifier (such as the localpart of an email address).

   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].








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3.  Usernames

3.1.  Definition

   This document specifies that a username is a string of Unicode code
   points [Unicode], encoded using UTF-8 [RFC3629], and structured as an
   ordered sequence of "userparts" (where the complete username can
   consist of a single userpart or a space-separated sequence of
   userparts).

   The syntax for a username is defined as follows using the Augmented
   Backus-Naur Form (ABNF) [RFC5234].

      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"
                   ;

   All code points and blocks not explicitly allowed in the PRECIS
   IdentifierClass are disallowed; this includes private use characters,
   surrogate code points, and the other code points and blocks that were
   defined as "Prohibited Output" in [RFC4013].  In addition, common
   constructions such as "user@example.com" (e.g., the Network Access
   Identifier from [RFC7542]) are allowed as usernames under this
   specification, as they were under [RFC4013].

      Implementation Note: The username construct defined in this
      document does not necessarily match what all deployed applications
      might refer to as a "username" or "userid", but instead provides a
      relatively safe subset of Unicode characters that can be used in
      existing SASL mechanisms and SASL-using application protocols, and
      even in most application protocols that do not currently use SASL.

   A username MUST NOT be zero bytes in length.  This rule is to be
   enforced after any normalization and mapping of code points.

   In protocols that provide usernames as input to a cryptographic
   algorithm such as a hash function, the client will need to perform
   proper preparation of the username before applying the algorithm.

   This specification defines two profiles for usernames: one that
   performs case mapping and one that performs case preservation (see
   further discussion under Section 3.4).




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3.2.  UsernameCaseMapped Profile

   The definition of the UsernameCaseMapped profile of the
   IdentifierClass is provided in the following sections, including
   detailed information about preparation, enforcement, and comparison
   (on the distinction between these actions, refer to [RFC7564]).

3.2.1.  Preparation

   An entity that prepares a string according to this profile MUST
   ensure that the string consists only of Unicode code points that
   conform to the "IdentifierClass" base string class defined in
   [RFC7564].  In addition, the string MUST be encoded as UTF-8
   [RFC3629].

3.2.2.  Enforcement

   An entity that performs enforcement according to this profile MUST
   prepare a string as described in the previous section and MUST also
   apply the rules specified below for the UsernameCaseMapped profile
   (these rules MUST be applied in the order shown).

   1.  Width Mapping Rule: Fullwidth and halfwidth characters MUST be
       mapped to their decomposition mappings (see Unicode Standard
       Annex #11 [UAX11]).

   2.  Additional Mapping Rule: There is no additional mapping rule.

   3.  Case Mapping Rule: Uppercase and titlecase characters MUST be
       mapped to their lowercase equivalents, preferably using 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], but the chapter number might change in
       a future version of the Unicode Standard); see further discussion
       in Section 3.4.

   4.  Normalization Rule: Unicode Normalization Form C (NFC) MUST be
       applied to all characters.

   5.  Directionality Rule: Applications MUST apply the "Bidi Rule"
       defined in [RFC5893] to strings that contain right-to-left
       characters (i.e., each of the six conditions of the Bidi Rule
       must be satisfied).








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3.2.3.  Comparison

   An entity that performs comparison of two strings according to this
   profile MUST prepare each string and enforce the rules specified in
   the previous two sections.  The two strings are to be considered
   equivalent if they are an exact octet-for-octet match (sometimes
   called "bit-string identity").

3.3.  UsernameCasePreserved Profile

   The definition of the UsernameCasePreserved profile of the
   IdentifierClass is provided in the following sections, including
   detailed information about preparation, enforcement, and comparison
   (on the distinction between these actions, refer to [RFC7564]).

3.3.1.  Preparation

   An entity that prepares a string according to this profile MUST
   ensure that the string consists only of Unicode code points that
   conform to the "IdentifierClass" base string class defined in
   [RFC7564].  In addition, the string MUST be encoded as UTF-8
   [RFC3629].

3.3.2.  Enforcement

   An entity that performs enforcement according to this profile MUST
   prepare a string as described in the previous section and MUST also
   apply the rules specified below for the UsernameCasePreserved profile
   (these rules MUST be applied in the order shown).

   1.  Width Mapping Rule: Fullwidth and halfwidth characters MUST be
       mapped to their decomposition mappings (see Unicode Standard
       Annex #11 [UAX11]).

   2.  Additional Mapping Rule: There is no additional mapping rule.

   3.  Case Mapping Rule: Uppercase and titlecase characters MUST NOT be
       mapped to their lowercase equivalents; see further discussion in
       Section 3.4.

   4.  Normalization Rule: Unicode Normalization Form C (NFC) MUST be
       applied to all characters.

   5.  Directionality Rule: Applications MUST apply the "Bidi Rule"
       defined in [RFC5893] to strings that contain right-to-left
       characters (i.e., each of the six conditions of the Bidi Rule
       must be satisfied).




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3.3.3.  Comparison

   An entity that performs comparison of two strings according to this
   profile MUST prepare each string and enforce the rules specified in
   the previous two sections.  The two strings are to be considered
   equivalent if they are an exact octet-for-octet match (sometimes
   called "bit-string identity").

3.4.  Case Mapping vs. Case Preservation

   In order to accommodate the widest range of username constructs in
   applications, this document defines two username profiles:
   UsernameCaseMapped and UsernameCasePreserved.  These two profiles
   differ only in the Case Mapping Rule, and are otherwise identical.

   Case mapping is a matter for the application protocol, protocol
   implementation, or end deployment.  In general, this document
   suggests that it is preferable to apply the UsernameCaseMapped
   profile and therefore perform case mapping, since not doing so can
   lead to false positives during authentication and authorization (as
   described in [RFC6943]) and can result in confusion among end users
   given the prevalence of case mapping in many existing protocols and
   applications.  However, there can be good reasons to apply the
   UsernameCasePreserved profile and thus not perform case mapping, such
   as backward compatibility with deployed infrastructure.

   In particular:

   o  SASL mechanisms that follow the recommendations in this document
      MUST specify whether and when case mapping is to be applied to
      authentication identifiers.  SASL mechanisms SHOULD delay any case
      mapping to the last possible moment, such as when doing a lookup
      by username, username comparisons, or generating a cryptographic
      salt from a username (if the last possible moment happens on the
      server, then decisions about case mapping can be a matter of
      deployment policy).  In keeping with [RFC4422], SASL mechanisms
      are not to apply this or any other profile to authorization
      identifiers.

   o  Application protocols that use SASL (such as IMAP [RFC3501] and
      XMPP [RFC6120]) and that directly re-use this profile MUST specify
      whether case mapping is to be applied to authorization
      identifiers.  Such "SASL application protocols" SHOULD delay any
      case mapping of authorization identifiers to the last possible
      moment, which happens to necessarily be on the server side (this
      enables decisions about case mapping to be a matter of deployment
      policy).  In keeping with [RFC4422], SASL application protocols




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      are not to apply this or any other profile to authentication
      identifiers.

   o  Application protocols that do not use SASL (such as HTTP
      authentication with the Basic and Digest schemes as specified in
      [I-D.ietf-httpauth-basicauth-update] and
      [I-D.ietf-httpauth-digest]) but that directly re-use this profile
      MUST specify whether and when case mapping is to be applied to
      authentication identifiers and authorization identifiers.  Such
      "non-SASL application protocols" SHOULD delay any case mapping to
      the last possible moment, such as when doing a lookup by username,
      username comparisons, or generating a cryptographic salt from a
      username (if the last possible moment happens on the server, then
      decisions about case mapping can be a matter of deployment
      policy).

   If the specification for a SASL mechanism, SASL application protocol,
   or non-SASL application protocol uses the UsernameCaseMapped profile,
   it MUST clearly describe whether case mapping is to be applied at the
   level of the protocol itself, implementations thereof, or service
   deployments (all of these approaches can be legitimate depending on
   the application in question).

3.5.  Application-Layer Constructs

   Both the UsernameCaseMapped and UsernameCasePreserved profiles enable
   an application protocol, implementation, or deployment to create
   application-layer constructs such as a space-separated set of names
   like "Firstname Middlename Lastname".  Although such a construct is
   not a PRECIS profile (since U+0020 SPACE is not allowed in the
   IdentifierClass), it can be created at the application layer because
   U+0020 SPACE can be used as a separator between instances of the
   PRECIS IdentifierClass (or a profile thereof).

3.6.  Examples

   The following examples illustrate a small number of userparts (not
   usernames) that are consistent with the format defined above (note
   that the characters < and > are used here to delineate the actual
   userparts and are not part of the userpart strings).











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   Table 1: A sample of legal userparts

   +--------------------------+---------------------------------+
   | # | Userpart             | Notes                           |
   +--------------------------+---------------------------------+
   | 1 | <juliet@example.com> | The at-sign is allowed in the   |
   |   |                      | PRECIS IdentifierClass          |
   +--------------------------+---------------------------------+
   | 2 | <fussball>           |                                 |
   +--------------------------+---------------------------------+
   | 3 | <fu&#xDF;ball>       | The third character is LATIN    |
   |   |                      | SMALL LETTER SHARP S (U+00DF)   |
   +--------------------------+---------------------------------+
   | 4 | <&#x3C0;>            | A userpart of GREEK SMALL       |
   |   |                      | LETTER PI (U+03C0)              |
   +--------------------------+---------------------------------+
   | 5 | <&#x3A3;>            | A userpart of GREEK CAPITAL     |
   |   |                      | LETTER SIGMA (U+03A3)           |
   +--------------------------+---------------------------------+
   | 6 | <&#x3C3;>            | A userpart of GREEK SMALL       |
   |   |                      | LETTER SIGMA (U+03C3)           |
   +--------------------------+---------------------------------+
   | 7 | <&#x3C2;>            | A userpart of GREEK SMALL       |
   |   |                      | LETTER FINAL SIGMA (U+03C2)     |
   +--------------------------+---------------------------------+

   Several points are worth noting.  Regarding examples 2 and 3:
   although in German the character eszett (LATIN SMALL LETTER SHARP S,
   U+00DF) can mostly be used interchangeably with the two characters
   "ss", the userparts in these examples are different and (if desired)
   a server would need to enforce a registration policy that disallows
   one of them if the other is registered.  Regarding examples 5, 6, and
   7: optional case-mapping of GREEK CAPITAL LETTER SIGMA (U+03A3) to
   lowercase (i.e., to GREEK SMALL LETTER SIGMA, U+03C3) during
   comparison would result in matching the userparts in examples 5 and
   6; however, because the PRECIS mapping rules do not account for the
   special status of GREEK SMALL LETTER FINAL SIGMA (U+03C2), the
   userparts in examples 5 and 7 or examples 6 and 7 would not be
   matched during comparison.

   The following examples illustrate strings that are not valid
   userparts (not usernames) because they violate the format defined
   above.








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   Table 2: A sample of strings that violate the userpart rule

   +--------------------------+---------------------------------+
   | # | Non-Userpart string  | Notes                           |
   +--------------------------+---------------------------------+
   | 8 | <foo bar>            | Space (U+0020) is disallowed in |
   |   |                      | the userpart                    |
   +--------------------------+---------------------------------+
   | 9 | <>                   | Zero-length userpart            |
   +--------------------------+---------------------------------+
   | 10| <henry&#x2163;>      | The sixth character is ROMAN    |
   |   |                      | NUMERAL FOUR (U+2163)           |
   +--------------------------+---------------------------------+
   | 11| <&#x265A;>           | A localpart of BLACK CHESS KING |
   |   |                      | (U+265A)                        |
   +--------------------------+---------------------------------+

   Here again, several points are worth noting.  Regarding example 10,
   the Unicode character ROMAN NUMERAL FOUR (U+2163) has a compatibility
   equivalent of the string formed of LATIN CAPITAL LETTER I (U+0049)
   and LATIN CAPITAL LETTER V (U+0056), but characters with
   compatibility equivalents are not allowed in the PRECIS
   IdentiferClass.  Regarding example 11: symbol characters such as
   BLACK CHESS KING (U+265A) are not allowed in the PRECIS
   IdentifierClass.

4.  Passwords

4.1.  Definition

   This document specifies that a password is a string of Unicode code
   points [Unicode], encoded using UTF-8 [RFC3629], and conformant to
   OpaqueString profile of the PRECIS FreeformClass specified below.

   The syntax for a password is defined as follows using the Augmented
   Backus-Naur Form (ABNF) [RFC5234].

      password   = 1*(freebyte)
                   ;
                   ; a "freebyte" 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 "FreeformClass"
                   ;

   All code points and blocks not explicitly allowed in the PRECIS
   FreeformClass are disallowed; this includes private use characters,




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   surrogate code points, and the other code points and blocks defined
   as "Prohibited Output" in Section 2.3 of RFC 4013.

   A password MUST NOT be zero bytes in length.  This rule is to be
   enforced after any normalization and mapping of code points.

      Note: Some existing systems allow an empty string in places where
      a password would be expected (e.g., command-line tools that might
      be called from an automated script, or servers that might need to
      be restarted without human intervention).  From the perspective of
      this document (and RFC 4013 before it), these empty strings are
      not passwords but are workarounds for the practical difficulty of
      using passwords in certain scenarios.  The prohibition on zero-
      length passwords is not a recommendation regarding password
      strength (since a password of only one byte is highly insecure),
      but is meant to prevent applications from mistakenly omitting a
      password entirely, since when internationalized characters are
      accepted a non-empty sequence of characters can result in a zero-
      length password after canonicalization.

   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.

4.2.  OpaqueString Profile

   The definition of the OpaqueString profile is provided in the
   following sections, including detailed information about preparation,
   enforcement, and comparison (on the distinction between these
   actions, refer to [RFC7564]).

4.2.1.  Preparation

   An entity that prepares a string according to this profile MUST
   ensure that the string consists only of Unicode code points that
   conform to the "FreeformClass" base string class defined in
   [RFC7564].  In addition, the string MUST be encoded as UTF-8
   [RFC3629].

4.2.2.  Enforcement

   An entity that performs enforcement according to this profile MUST
   prepare a string as described in the previous section and MUST also
   apply the rules specified below (these rules MUST be applied in the
   order shown).





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   1.  Width Mapping Rule: Fullwidth and halfwidth characters MUST NOT
       be mapped to their decomposition mappings (see Unicode Standard
       Annex #11 [UAX11]).

   2.  Additional Mapping Rule: Any instances of non-ASCII space MUST be
       mapped to ASCII space (U+0020); a non-ASCII space is any Unicode
       code point having a general category of "Zs", naturally with the
       exception of U+0020.

   3.  Case Mapping Rule: Uppercase and titlecase characters MUST NOT be
       mapped to their lowercase equivalents.

   4.  Normalization Rule: Unicode Normalization Form C (NFC) MUST be
       applied to all characters.

   5.  Directionality Rule: There is no directionality rule.  The "Bidi
       Rule" (defined in [RFC5893]) and similar rules are unnecessary
       and inapplicable to passwords, since they can reduce the range of
       characters that are allowed in a string and therefore reduce the
       amount of entropy that is possible in a password.  Such rules are
       intended to minimize the possibility that the same string will be
       displayed differently on a layout system set for right-to-left
       display and a layout system set for left-to-right display;
       however, passwords are typically not displayed at all and are
       rarely meant to be interoperable across different layout systems
       in the way that non-secret strings like domain names and
       usernames are.  Furthermore, it is perfectly acceptable for
       opaque strings other than passwords to be presented differently
       in different layout systems, as long as the presentation is
       consistent in any given layout system.

4.2.3.  Comparison

   An entity that performs comparison of two strings according to this
   profile MUST prepare each string and enforce the rules specified in
   the previous two sections.  The two strings are to be considered
   equivalent if they are an exact octet-for-octet match (sometimes
   called "bit-string identity").

4.3.  Examples

   The following examples illustrate a small number of passwords that
   are consistent with the format defined above (note that the
   characters < and > are used here to delineate the actual passwords
   and are not part of the password strings).






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   Table 3: A sample of legal passwords

   +------------------------------------+------------------------------+
   | # | Password                       | Notes                        |
   +------------------------------------+------------------------------+
   | 12| <correct horse battery staple> | ASCII space is allowed       |
   +------------------------------------+------------------------------+
   | 13| <Correct Horse Battery Staple> | Different from example 12    |
   +------------------------------------+------------------------------+
   | 14| <&#x3C0;&#xDF;&#xE5;>          | Non-ASCII letters are OK     |
   |   |                                | (e.g., GREEK SMALL LETTER    |
   |   |                                | PI, U+03C0)                  |
   +------------------------------------+------------------------------+
   | 15| <Jack of &#x2666;s>            | Symbols are OK (e.g., BLACK  |
   |   |                                | DIAMOND SUIT, U+2666)        |
   +------------------------------------+------------------------------+
   | 16| <foo&#x1680;bar>               | OGHAM SPACE MARK, U+1680, is |
   |   |                                | mapped to U+0020 and thus    |
   |   |                                | the full string is mapped to |
   |   |                                | <foo bar>                    |
   +------------------------------------+------------------------------+

   The following example illustrates a string that is not a valid
   password because it violates the format defined above.

   Table 4: A string that violates the password rules

   +------------------------------------+------------------------------+
   | # | Password                       | Notes                        |
   +------------------------------------+------------------------------+
   | 17| <my cat is a &#x9;by>          | Controls are disallowed      |
   +------------------------------------+------------------------------+

5.  Use in Application Protocols

   This specification defines only the PRECIS-based rules for handling
   of strings conforming to the UsernameCaseMapped and
   UsernameCasePreserved profiles of the PRECIS IdentifierClass, and
   strings conforming to the OpaqueString profile of the PRECIS
   FreeformClass.  It is the responsibility of an application protocol
   to specify the protocol slots in which such strings can appear, the
   entities that are expected to enforce the rules governing such
   strings, and when in protocol processing or interface handling the
   rules need to be enforced.  See Section 6 of [RFC7564] for guidelines
   about using PRECIS profiles in applications.

   Above and beyond the PRECIS-based rules specified here, application
   protocols can also define application-specific rules governing such



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   strings (rules regarding minimum or maximum length, further
   restrictions on allowable characters or character ranges, safeguards
   to mitigate the effects of visually similar characters, etc.),
   application-layer constructs (see Section 3.5), and related matters.

   Some PRECIS profile definitions encourage entities that enforce the
   rules to be liberal in what they accept.  However, for usernames and
   passwords such a policy can be problematic since it can lead to false
   positives.  An in-depth discussion can be found in "Issues in
   Identifier Comparison for Security Purposes" [RFC6943].

6.  Migration

   The rules defined in this specification differ slightly from those
   defined by the SASLprep specification [RFC4013].  The following
   sections describe these differences, along with their implications
   for migration, in more detail.

6.1.  Usernames

   Deployments that currently use SASLprep for handling usernames might
   need to scrub existing data when migrating to use of the rules
   defined in this specification.  In particular:

   o  SASLprep specified the use of Unicode Normalization Form KC
      (NFKC), whereas the UsernameCaseMapped and UsernameCasePreserved
      profiles employ Unicode Normalization Form C (NFC).  In practice
      this change is unlikely to cause significant problems, because
      NFKC provides methods for mapping Unicode code points with
      compatibility equivalents to those equivalents, whereas the PRECIS
      IdentifierClass entirely disallows Unicode code points with
      compatibility equivalents (i.e., during comparison NFKC is more
      "aggressive" about finding matches than NFC).  A few examples
      might suffice to indicate the nature of the problem:

      1.  U+017F LATIN SMALL LETTER LONG S is compatibility equivalent
          to U+0073 LATIN SMALL LETTER S

      2.  U+2163 ROMAN NUMERAL FOUR is compatibility equivalent to
          U+0049 LATIN CAPITAL LETTER I and U+0056 LATIN CAPITAL LETTER
          V

      3.  U+FB01 LATIN SMALL LIGATURE FI is compatibility equivalent to
          U+0066 LATIN SMALL LETTER F and U+0069 LATIN SMALL LETTER I

      Under SASLprep, the use of NFKC also handled the mapping of
      fullwidth and halfwidth code points to their decomposition
      mappings.



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      For migration purposes operators might want to search their
      database of usernames for names containing Unicode code points
      with compatibility equivalents and, where there is no conflict,
      map those code points to their equivalents.  Naturally, it is
      possible that during this process the operator will discover
      conflicting usernames (e.g., HENRYIV with the last two characters
      being U+0049 LATIN CAPITAL LETTER I and U+0056 LATIN CAPITAL
      LETTER V vs. "HENRYIV" with the last character being U+2163 ROMAN
      NUMERAL FOUR, which is compatibility equivalent to U+0049 and
      U+0056); in these cases the operator will need to determine how to
      proceed, for instance by disabling the account whose name contains
      a Unicode code point with a compatibility equivalent.  Such cases
      are probably rare, but it is important for operators to be aware
      of them.

   o  SASLprep mapped the "characters commonly mapped to nothing" from
      Appendix B.1 of [RFC3454]) to nothing, whereas the PRECIS
      IdentifierClass entirely disallows most of these characters, which
      correspond to the code points from the "M" category defined under
      Section 9.13 of [RFC7564] (with the exception of U+1806 MONGOLIAN
      TODO SOFT HYPHEN, which was "commonly mapped to nothing" in
      Unicode 3.2 but at the time of this writing does not have a
      derived property of Default_Ignorable_Code_Point in Unicode 7.0).
      For migration purposes, the operator might want to remove from
      usernames any code points contained in the PRECIS "M" category
      (e.g., U+00AD SOFT HYPHEN).  Because these code points would have
      been "mapped to nothing" in Stringprep, in practice a user would
      not notice the difference if upon migration to PRECIS the code
      points are removed.

   o  SASLprep allowed uppercase and titlecase characters, whereas the
      UsernameCaseMapped profile maps uppercase and titlecase characters
      to their lowercase equivalents (by contrast, the
      UsernameCasePreserved profile matches SASLprep in this regard).
      For migration purposes, the operator can either use the
      UsernameCaseMapped profile (thus losing the case information) or
      use the UsernameCasePreserved profile (thus ignoring case
      difference when comparing usernames).

6.2.  Passwords

   Depending on local service policy, migration from RFC 4013 to this
   specification might not involve any scrubbing of data (since
   passwords might not be stored in the clear anyway); however, service
   providers need to be aware of possible issues that might arise during
   migration.  In particular:





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   o  SASLprep specified the use of Unicode Normalization Form KC
      (NFKC), whereas the OpaqueString profile employs Unicode
      Normalization Form C (NFC).  Because NFKC is more aggressive about
      finding matches than NFC, in practice this change is unlikely to
      cause significant problems and indeed has the security benefit of
      probably resulting in fewer false positives when comparing
      passwords.  A few examples might suffice to indicate the nature of
      the problem:

      1.  U+017F LATIN SMALL LETTER LONG S is compatibility equivalent
          to U+0073 LATIN SMALL LETTER S

      2.  U+2163 ROMAN NUMERAL FOUR is compatibility equivalent to
          U+0049 LATIN CAPITAL LETTER I and U+0056 LATIN CAPITAL LETTER
          V

      3.  U+FB01 LATIN SMALL LIGATURE FI is compatibility equivalent to
          U+0066 LATIN SMALL LETTER F and U+0069 LATIN SMALL LETTER I

      Under SASLprep, the use of NFKC also handled the mapping of
      fullwidth and halfwidth code points to their decomposition
      mappings.  Although it is expected that code points with
      compatibility equivalents are rare in existing passwords, some
      passwords that matched when SASLprep was used might no longer work
      when the rules in this specification are applied.

   o  SASLprep mapped the "characters commonly mapped to nothing" from
      Appendix B.1 of [RFC3454]) to nothing, whereas the PRECIS
      FreeformClass entirely disallows such characters, which correspond
      to the code points from the "M" category defined under
      Section 9.13 of [RFC7564] (with the exception of U+1806 MONGOLIAN
      TODO SOFT HYPHEN, which was commonly mapped to nothing in Unicode
      3.2 but at the time of this writing is allowed by Unicode 7.0).
      In practice, this change will probably have no effect on
      comparison, but user-oriented software might reject such code
      points instead of ignoring them during password preparation.

7.  IANA Considerations

   The IANA shall add the following entries to the PRECIS Profiles
   Registry.

7.1.  UsernameCaseMapped Profile

   Name:  UsernameCaseMapped.

   Base Class:  IdentifierClass.




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   Applicability:  Usernames in security and application protocols.

   Replaces:  The SASLprep profile of Stringprep.

   Width Mapping Rule:  Map fullwidth and halfwidth characters to their
      decomposition mappings.

   Additional Mapping Rule:  None.

   Case Mapping Rule:  Map uppercase and titlecase characters to
      lowercase.

   Normalization Rule:  NFC.

   Directionality Rule:  The "Bidi Rule" defined in RFC 5893 applies.

   Enforcement:  To be defined by security or application protocols that
      use this profile.

   Specification:  RFC XXXX, Section 3.2.  [Note to RFC Editor: please
      change XXXX to the number issued for this specification.]

7.2.  UsernameCasePreserved Profile

   Name:  UsernameCasePreserved.

   Base Class:  IdentifierClass.

   Applicability:  Usernames in security and application protocols.

   Replaces:  The SASLprep profile of Stringprep.

   Width Mapping Rule:  Map fullwidth and halfwidth characters to their
      decomposition mappings.

   Additional Mapping Rule:  None.

   Case Mapping Rule:  None.

   Normalization Rule:  NFC.

   Directionality Rule:  The "Bidi Rule" defined in RFC 5893 applies.

   Enforcement:  To be defined by security or application protocols that
      use this profile.

   Specification:  RFC XXXX, Section 3.3.  [Note to RFC Editor: please
      change XXXX to the number issued for this specification.]



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7.3.  OpaqueString Profile

   Name:  OpaqueString.

   Base Class:  FreeformClass.

   Applicability:  Passwords and other opaque strings in security and
      application protocols.

   Replaces:  The SASLprep profile of Stringprep.

   Width Mapping Rule:  None.

   Additional Mapping Rule:  Map non-ASCII space characters to ASCII
      space.

   Case Mapping Rule:  None.

   Normalization Rule:  NFC.

   Directionality Rule:  None.

   Enforcement:  To be defined by security or application protocols that
      use this profile.

   Specification:  RFC XXXX, Section 4.2.  [Note to RFC Editor: please
      change XXXX to the number issued for this specification.]

8.  Security Considerations

8.1.  Password/Passphrase Strength

   The ability to include a wide range of characters in passwords and
   passphrases can increase the potential for creating a strong password
   with high entropy.  However, in practice, the ability to include such
   characters ought to be weighed against the possible need to reproduce
   them on various devices using various input methods.

8.2.  Identifier Comparison

   The process of comparing identifiers (such as SASL simple user names,
   authentication identifiers, and authorization identifiers) can lead
   to either false negatives or false positives, both of which have
   security implications.  A more detailed discussion can be found in
   [RFC6943].






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8.3.  Reuse of PRECIS

   The security considerations described in [RFC7564] apply to the
   "IdentifierClass" and "FreeformClass" base string classes used in
   this document for usernames and passwords, respectively.

8.4.  Reuse of Unicode

   The security considerations described in [UTS39] apply to the use of
   Unicode characters in usernames and passwords.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.

   [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in
              Internationalization in the IETF", BCP 166, RFC 6365,
              September 2011.

   [RFC7564]  Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
              Preparation, Enforcement, and Comparison of
              Internationalized Strings in Application Protocols", RFC
              7564, May 2015.

   [UAX11]    The Unicode Consortium, "Unicode Standard Annex #11: East
              Asian Width", September 2012,
              <http://unicode.org/reports/tr11/>.

   [Unicode7.0]
              The Unicode Consortium, "The Unicode Standard, Version
              7.0.0", 2014,
              <http://www.unicode.org/versions/Unicode7.0.0/>.

   [Unicode]  The Unicode Consortium, "The Unicode Standard",
              2015-present, <http://www.unicode.org/versions/latest/>.



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9.2.  Informative References

   [I-D.ietf-httpauth-basicauth-update]
              Reschke, J., "The 'Basic' HTTP Authentication Scheme",
              draft-ietf-httpauth-basicauth-update-07 (work in
              progress), February 2015.

   [I-D.ietf-httpauth-digest]
              Shekh-Yusef, R., Ahrens, D., and S. Bremer, "HTTP Digest
              Access Authentication", draft-ietf-httpauth-digest-19
              (work in progress), April 2015.

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

   [RFC20]    Cerf, V., "ASCII format for network interchange", RFC 20,
              October 1969.

   [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.

   [RFC4013]  Zeilenga, K., "SASLprep: Stringprep Profile for User Names
              and Passwords", RFC 4013, February 2005.

   [RFC4422]  Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
              Authentication and Security Layer (SASL)", RFC 4422, June
              2006.

   [RFC4616]  Zeilenga, K., "The PLAIN Simple Authentication and
              Security Layer (SASL) Mechanism", RFC 4616, August 2006.

   [RFC5802]  Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
              "Salted Challenge Response Authentication Mechanism
              (SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891, August 2010.

   [RFC5893]  Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5893, August 2010.




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   [RFC5894]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Background, Explanation, and
              Rationale", RFC 5894, August 2010.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, March 2011.

   [RFC6122]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Address Format", RFC 6122, March 2011.

   [RFC6943]  Thaler, D., "Issues in Identifier Comparison for Security
              Purposes", RFC 6943, May 2013.

   [RFC7542]  DeKok, A., "The Network Access Identifier", RFC 7542, May
              2015.

   [UTS39]    The Unicode Consortium, "Unicode Technical Standard #39:
              Unicode Security Mechanisms", July 2012,
              <http://unicode.org/reports/tr39/>.

Appendix A.  Differences from RFC 4013

   This document builds upon the PRECIS framework defined in [RFC7564],
   which differs fundamentally from the Stringprep technology [RFC3454]
   used in SASLprep [RFC4013].  The primary difference is that
   Stringprep profiles allowed all characters except those which were
   explicitly disallowed, whereas PRECIS profiles disallow all
   characters except those which are explicitly allowed (this "inclusion
   model" was originally used for internationalized domain names in
   [RFC5891]; see [RFC5894] for further discussion).  It is important to
   keep this distinction in mind when comparing the technology defined
   in this document to SASLprep [RFC4013].

   The following substantive modifications were made from RFC 4013.

   o  A single SASLprep algorithm was replaced by three separate
      algorithms: one for usernames with case mapping, one for usernames
      with case preservation, and one for passwords.

   o  The new preparation algorithms use PRECIS instead of a Stringprep
      profile.  The new algorithms work independenctly of Unicode
      versions.

   o  As recommended in the PRECIS framework, changed the Unicode
      normalization form from NFKC to NFC.

   o  Some Unicode code points that were mapped to nothing in RFC 4013
      are simply disallowed by PRECIS.



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Appendix B.  Acknowledgements

   This document borrows some text from [RFC4013] and [RFC6120].

   The following individuals provided helpful feedback on this document:
   Marc Blanchet, Ben Campbell, Alan DeKok, Joe Hildebrand, Jeffrey
   Hutzelman, Simon Josefsson, Jonathan Lennox, James Manger, Matt
   Miller, Chris Newman, Yutaka OIWA, Pete Resnick, Andrew Sullivan,
   Nico Williams, and Yoshiro YONEYA.  Nico Williams in particular
   deserves special recognition for providing text that was used in
   Section 3.4.  Thanks also to Takahiro NEMOTO and Yoshiro YONEYA for
   implementation feedback.

   Robert Sparks and Derek Atkins reviewed the document on behalf of the
   General Area Review Team and the Security Directorate, respectively.

   Stephen Farrell provided helpful input during IESG review.

   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/


   Alexey Melnikov
   Isode Ltd
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX
   UK

   Email: Alexey.Melnikov@isode.com












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