rfc8265.xml

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<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<?rfc compact="yes"?>
<?rfc subcompact="no"?>
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<rfc category="std" ipr="trust200902" number="8265" obsoletes="7613"
     submissionType="IETF" consensus="yes">

  <front>

    <title abbrev="PRECIS: Usernames and Passwords">Preparation, Enforcement, and Comparison of Internationalized Strings Representing Usernames and Passwords</title>

    <author initials="P." surname="Saint-Andre" fullname="Peter Saint-Andre">
      <organization>Jabber.org</organization>
      <address>
        <postal>
          <street>P.O. Box 787</street>
          <city>Parker</city>
          <region>CO</region>
          <code>80134</code>
          <country>USA</country>
        </postal>
        <phone>+1 720 256 6756</phone>
        <email>stpeter@jabber.org</email>
        <uri>https://www.jabber.org/</uri>
      </address>
    </author>

    <author initials="A." surname="Melnikov" fullname="Alexey Melnikov">
      <organization>Isode Ltd</organization>
      <address>
        <postal>
          <street>5 Castle Business Village</street>
          <street>36 Station Road</street>
          <city>Hampton</city>
          <region>Middlesex</region>
          <code>TW12 2BX</code>
          <country>United Kingdom</country>
        </postal>
        <email>Alexey.Melnikov@isode.com</email>
      </address>
    </author>

    <date month="September" year="2017"/>

    <keyword>Username</keyword>
    <keyword>Password</keyword>
    <keyword>Unicode</keyword>
    <keyword>Internationalization</keyword>
    <keyword>i18n</keyword>
    <keyword>Authentication</keyword>
    <keyword>SASLprep</keyword>

    <abstract>
      <t>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.  This
      document obsoletes RFC 7613.</t>
    </abstract>

  </front>

  <middle>

  <section title="Introduction" anchor="intro">

    <t>Usernames and passwords are widely used for authentication and
    authorization on the Internet, either directly when provided in plaintext
    (as in the PLAIN Simple Authentication and Security Layer (SASL)
    mechanism <xref target='RFC4616'/> and the HTTP Basic scheme
    <xref target='RFC7617'/>) or indirectly
    when provided as the input to a cryptographic algorithm such as a hash
    function (as in the Salted Challenge Response Authentication Mechanism
    (SCRAM) SASL mechanism <xref target='RFC5802'/> and the
    HTTP Digest scheme <xref target='RFC7616'/>).</t>

    <t>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 handling
    internationalized strings that represent
    usernames and passwords.  Such strings consist of code points from the
    Unicode coded character set <xref target="Unicode"/>, with special attention to
    code points outside the ASCII range <xref target='RFC20'/>.  The rules for
    handling such strings are specified through profiles of the string classes
    defined in the preparation, enforcement, and comparison of
    internationalized strings (PRECIS) framework specification <xref
    target='RFC8264'/>.</t>

    <t>Profiles of the PRECIS framework enable software to handle Unicode
    code points outside the ASCII range in an automated way, so that such
    code points 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 code points.  For instance, in almost
    all application protocols it would be dangerous to treat the Unicode
    code point "¹" (SUPERSCRIPT ONE, U+00B9) as equivalent to "1" (DIGIT ONE, U+0031),
    because that would result in false accepts during comparison,
    authentication, and authorization (e.g., an attacker could easily spoof an
    account "user1@example.com").</t>

    <t>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.)</t>

    <t>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., Lightweight Directory
    Access Protocol (LDAP) distinguished names), nor in cases where
    identifiers or secrets are not strings (e.g., keys and
    certificates) or require specialized handling.</t>

    <t>Although the historical predecessor of this document was the 
    SASLprep profile of Stringprep <xref target='RFC3454'/>), the 
    approach defined here can be used by technologies other than
    SASL <xref target='RFC4422'/>, such as HTTP authentication as
    specified in <xref target='RFC7617'/> and <xref
    target='RFC7616'/>.</t>

    <t>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 <xref
    target='RFC7622'/>).
    Non-coordinated updates to protocol implementations are discouraged
    because they can have a negative impact on interoperability and
    security.</t>

  </section>

  <section title="Terminology" anchor="terms">
    <t>A "username" or "user identifier" is a string of characters designating 
    an account on a computing device or system, often but not necessarily for 
    use by a person.  Although some devices and systems might allow a username 
    to be part or all of a person's name and a person might want their account
    designator to be part or all of their name, because of the complexities
    involved, that outcome is not guaranteed for all human names on all 
    computing devices or systems that follow the rules defined in this 
    specification.  Protocol designers and application developers who wish
    to allow a wider range of characters are encouraged to consider a
    separation between more restrictive account identifiers and more
    expressive display names or nicknames (see <xref target='RFC8266'/>).</t>

    <t>A "password" is a string of characters that allows access to a computing
    device or system, often associated with a particular username.  A password 
    is not literally limited to a word, because a password could be a passphrase 
    consisting of more than one word, perhaps separated by spaces, punctuation, 
    or other non-alphanumeric characters.</t>

    <t>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 <xref target='RFC4422'/> as well as
    <xref target='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 <xref
    target='RFC7617'/> and <xref
    target='RFC7616'/>), 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; note
    also that a username does not necessarily map to any particular application
    identifier.</t>

    <t>Many important terms used in this document are defined in <xref
    target='RFC5890'/>, <xref target='RFC6365'/>, <xref target='RFC8264'/>,
    and <xref target='Unicode'/>.  The term "non-ASCII space" refers to any
    Unicode code point having a Unicode general category of "Zs", naturally with the
    exception of SPACE (U+0020).</t>

    <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
    "SHALL&nbsp;NOT", "SHOULD", "SHOULD&nbsp;NOT", "RECOMMENDED",
    "NOT&nbsp;RECOMMENDED", "MAY", and "OPTIONAL" in this document
    are to be interpreted as described in BCP&nbsp;14
    <xref format="default" pageno="false" target="RFC2119"/>
    <xref format="default" pageno="false" target="RFC8174"/> when,
    and only when, they appear in all capitals, as shown here.</t>

  </section>

  <section title='Usernames' anchor='username'>

    <section title='Definition' anchor='username-definition'>
      <t>This document specifies that a username is a string of Unicode code
      points <xref target='Unicode'/> that is 
      structured as an ordered sequence of "userparts"
      and expressed in a standard Unicode Encoding Form (such as
      UTF-8 <xref target='RFC3629'/>).
      A userpart is allowed to contain only code points that
      are allowed by the PRECIS IdentifierClass defined in
      Section 4.2 of <xref target="RFC8264"/> and thus consists almost
      exclusively of letters and digits. A username can consist of a
      single userpart or a space-separated sequence of userparts.</t>

      <t>The syntax for a username is defined as follows, using the Augmented
      Backus-Naur Form (ABNF) <xref target="RFC5234"/>.</t>

      <figure>
        <artwork><![CDATA[
   username   = userpart *(1*SP userpart)
   userpart   = 1*(idpoint)
                ;
                ; an "idpoint" is a Unicode code point that
                ; can be contained in a string conforming to 
                ; the PRECIS IdentifierClass
                ;
        ]]></artwork>
      </figure>

      <t>All code points and blocks not explicitly allowed in the PRECIS
      IdentifierClass are disallowed; this includes private-use code points,
      surrogate code points, and the other code points and blocks that were
      defined as "Prohibited Output" in Section 2.3 of
      <xref target='RFC4013'/> (when corrected per <xref target="Err1812" />).
      In addition, common constructions such as "user@example.com" (e.g., the
      Network Access Identifier from <xref target='RFC7542'/>) are allowed as
      usernames under this specification, as they were under <xref
      target='RFC4013'/>.</t>

      <t><list style='empty'><t>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 code points that can be
      used in existing SASL mechanisms and in application protocols that use
      SASL, and even in most application protocols that do not currently use
      SASL.</t></list></t>

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

      <t>This specification defines two profiles for usernames: the
      UsernameCaseMapped profile performs case mapping and the 
      UsernameCasePreserved performs case preservation (see
      further discussion under <xref target='username-case'/>).</t>

      <t>In protocols that provide usernames as input to a cryptographic
      algorithm such as a hash function, the client will need to perform
      enforcement of the rules for the UsernameCaseMapped or
      UsernameCasePreserved profile before applying the algorithm.</t>

    </section>

    <section title='Case Mapping vs. Case Preservation' anchor='username-case'>
      <t>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 their use (or not) of the Case Mapping Rule, and are otherwise 
      identical.</t>

      <t>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, because not doing so can lead to false
      accepts during authentication and authorization (as described in <xref
      target='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.</t>

      <t>In particular:</t>
      <t>
        <list style='symbols'>
          <t>SASL mechanisms that follow the recommendations in this document
          MUST specify whether and when case mapping is to be applied to
          authentication identifiers.  Because case mapping results in
          information loss, in order to retain that information for as
          long as possible during processing, implementations SHOULD delay 
          any case mapping to the last possible moment, such as when doing a
          lookup by username, performing username comparisons, or generating a
          cryptographic salt from a username (if the last possible moment
          happens on a server, then decisions about case mapping can be a
          matter of service deployment policy).  In keeping with
          <xref target='RFC4422'/>, SASL mechanisms are not to apply this or
          any other profile to authorization identifiers, only to
          authentication identifiers.</t>

          <t>Application protocols that use SASL (such as IMAP <xref
          target='RFC3501'/> and the Extensible Messaging and Presence
          Protocol (XMPP) <xref target='RFC6120'/>) and that
          directly reuse this profile MUST specify whether or not
          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 service deployment
          policy).  In keeping with <xref target='RFC4422'/>, SASL
          application protocols are not to apply this or any other
          profile to authentication identifiers, only to authorization
          identifiers.</t>

          <t>Application protocols that do not use SASL (such as HTTP
          authentication with the HTTP Basic and Digest schemes as specified
          in <xref target='RFC7617'/> and <xref
          target='RFC7616'/>) but that directly reuse this
          profile MUST specify whether and when case mapping is to be applied
          to authentication identifiers or authorization identifiers, or both.
          Such "non-SASL application protocols" SHOULD delay any
          case mapping to the last possible moment, such as when doing a
          lookup by username, performing 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 service deployment policy).</t>

        </list>
      </t>
      <t>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
      (each of these approaches can be legitimate, depending on the
      application in question).</t> 

    </section>

    <section title='UsernameCaseMapped Profile' anchor='usernamecasemapped'>
      <section title="Rules" anchor="usernamecasemapped-rules">
        <t>The following rules are defined for use within the
        UsernameCaseMapped profile of the PRECIS IdentifierClass.</t>
        <t>
          <list style='numbers'>
            <t>Width Mapping Rule: Map fullwidth and halfwidth code points to
            their decomposition mappings (see Unicode Standard Annex #11 <xref
            target='UAX11'/>).</t>
            <t>Additional Mapping Rule: There is no additional mapping
            rule.</t>
            <t>Case Mapping Rule: Map uppercase and titlecase code points to
            their lowercase equivalents, preferably using the Unicode
            toLowerCase() operation as defined in the Unicode Standard <xref
            target='Unicode'/>; see further discussion in <xref
            target='username-case'/>.</t>
            <t>Normalization Rule: Apply Unicode Normalization Form C (NFC) to
            all strings.</t>
            <t>Directionality Rule: Apply the "Bidi Rule" defined in <xref
            target='RFC5893'/> to strings that contain right-to-left code
            points (i.e., each of the six conditions of the Bidi Rule must be
            satisfied); for strings that do not contain right-to-left code
            points, there is no special processing for directionality.</t>
          </list>
        </t>
      </section>
      <section title="Preparation" anchor="usernamecasemapped-preparation">
        <t>An entity that prepares an input string for subsequent enforcement
        according to this profile MUST proceed as follows (applying the steps
        in the order shown).</t>
        <t>
          <list style='numbers'>
            <t>Apply the width mapping rule specified in <xref
            target='usernamecasemapped-rules'/>.  It is necessary to apply the
            rule at this point because otherwise the PRECIS "HasCompat"
            category specified in Section 9.17 of <xref target='RFC8264'/>
            would forbid fullwidth and halfwidth code points.</t>
            <t>Ensure that the string consists only of Unicode code points
            that are explicitly allowed by the PRECIS IdentifierClass defined
            in Section 4.2 of <xref target='RFC8264'/>.</t>
          </list>
        </t>
      </section>
      <section title="Enforcement" anchor="usernamecasemapped-enforcement">
        <t>An entity that performs enforcement according to this profile MUST prepare an input string as described in <xref target='usernamecasemapped-preparation'/> and MUST also apply the following rules specified in <xref target='usernamecasemapped-rules'/> in the order shown:</t>
        <t>
          <list style="numbers">
            <t>Case Mapping Rule</t>
            <t>Normalization Rule</t>
            <t>Directionality Rule</t>
          </list>
        </t>
        <t>After all of the foregoing rules have been enforced, the entity MUST ensure that the username is not zero bytes in length (this is done after enforcing the rules to prevent applications from mistakenly omitting a username entirely, because when internationalized strings are accepted, a non-empty sequence of characters can result in a zero-length username after canonicalization).</t>
        <t>The result of the foregoing operations is an output string that conforms to the UsernameCaseMapped profile.  Until an implementation produces such an output string, it MUST NOT treat the string as conforming (in particular, it MUST NOT assume that an input string is conforming before the enforcement operation has been completed).</t>
      </section>
      <section title="Comparison" anchor="usernamecasemapped-comparison">
        <t>An entity that performs comparison of two strings according to this
        profile MUST prepare each string as specified in
        <xref target="usernamecasemapped-preparation"/> and then MUST
        enforce the rules specified in
        <xref target="usernamecasemapped-enforcement"/>.
        The two strings are to be considered equivalent if and only if they are an exact
        octet-for-octet match (sometimes called "bit-string identity").</t> 
        <t>Until an implementation determines whether two strings are to be
        considered equivalent, it MUST NOT treat them as equivalent (in
        particular, it MUST NOT assume that two input strings are equivalent 
        before the comparison operation has been completed).</t>
      </section>
    </section>

    <section title='UsernameCasePreserved Profile' anchor='usernamecasepreserved'>
      <section title="Rules" anchor="usernamecasepreserved-rules">
        <t>The following rules are defined for use within the
        UsernameCasePreserved profile of the PRECIS IdentifierClass.</t>
        <t>
          <list style='numbers'>
            <t>Width Mapping Rule: Map fullwidth and halfwidth code points to
            their decomposition mappings (see Unicode Standard Annex #11 <xref
            target='UAX11'/>).</t>
            <t>Additional Mapping Rule: There is no additional mapping
            rule.</t>
            <t>Case Mapping Rule: There is no case mapping rule.</t>
            <t>Normalization Rule: Apply Unicode Normalization Form C (NFC) to
            all strings.</t>
            <t>Directionality Rule: Apply the "Bidi Rule" defined in <xref
            target='RFC5893'/> to strings that contain right-to-left code
            points (i.e., each of the six conditions of the Bidi Rule must be
            satisfied); for strings that do not contain right-to-left code
            points, there is no special processing for directionality.</t>
          </list>
        </t>
      </section>

      <section title="Preparation" anchor="usernamecasepreserved-preparation">
        <t>An entity that prepares a string for subsequent enforcement
        according to this profile MUST proceed as follows (applying the steps
        in the order shown).</t>
        <t>
          <list style='numbers'>
            <t>Apply the width mapping rule specified in <xref
            target='usernamecasepreserved-rules'/>.
  It is necessary to apply
            the rule at this point because otherwise the PRECIS "HasCompat"
            category specified in Section 9.17 of <xref target='RFC8264'/>
            would forbid fullwidth and halfwidth code points.</t>
            <t>Ensure that the string consists only of Unicode code points
            that are explicitly allowed by the PRECIS IdentifierClass defined
            in Section 4.2 of <xref target='RFC8264'/>.</t>
          </list>
        </t>
      </section>

      <section title="Enforcement" anchor="usernamecasepreserved-enforcement">
        <t>An entity that performs enforcement according to this profile MUST
        prepare a string as described in <xref
        target='usernamecasepreserved-preparation'/> and MUST also apply the
        following rules specified in <xref
        target='usernamecasepreserved-rules'/> in the order shown:</t>
        <t>
          <list style="numbers">
            <t>Normalization Rule</t>
            <t>Directionality Rule</t>
          </list>
        </t>
        <t>After all of the foregoing rules have been enforced, the entity
        MUST ensure that the username is not zero bytes in length (this is
        done after enforcing the rules to prevent applications from mistakenly
        omitting a username entirely, because when internationalized strings
        are accepted, a non-empty sequence of characters can result in a
        zero-length username after canonicalization).</t>
        <t>The result of the foregoing operations is an output string that
        conforms to the UsernameCasePreserved profile.  Until an
        implementation produces such an output string, it MUST NOT treat the
        string as conforming (in particular, it MUST NOT assume that an input
        string is conforming before the enforcement operation has been
        completed).</t>
      </section>

      <section title="Comparison" anchor="usernamecasepreserved-comparison">
        <t>An entity that performs comparison of two strings according to this
        profile MUST prepare each string as specified in
        <xref target="usernamecasepreserved-preparation"/> and then MUST
        enforce the rules specified in
        <xref target="usernamecasepreserved-enforcement"/>.
        The two strings are to be considered equivalent if and only if they
        are an exact octet-for-octet match (sometimes called "bit-string
        identity").</t>
        <t>Until an implementation determines whether two strings are to be
        considered equivalent, it MUST NOT treat them as equivalent (in
        particular, it MUST NOT assume that two input strings are equivalent 
        before the comparison operation has been completed).</t>
      </section>
    </section>

    <section title="Application-Layer Constructs" anchor="username-app">
      <t>Both the UsernameCaseMapped and UsernameCasePreserved profiles enable
      an application protocol, implementation, or deployment to create
      application-layer constructs such as a username that is a
      space-separated set of userparts like "Firstname Middlename Lastname".
      Such a construct is not a profile of the PRECIS IdentifierClass, 
      because SPACE (U+0020) is not allowed in the IdentifierClass; however, it can be
      created at the application layer because SPACE (U+0020) can be used as a
      separator between instances of the PRECIS IdentifierClass (e.g.,
      userparts as defined in this specification).</t>

    </section>

    <section title="Examples" anchor="username-examples">
      <t>The following examples illustrate a small number of userparts (not
      usernames) that are consistent with the format defined above (note that
      the characters &quot;&lt;&quot; and &quot;&gt;&quot; are used here to
      delineate the actual userparts and are not part of the userpart
      strings).</t>

      <figure align="center">
        <artwork align="center"><![CDATA[
+--------------------------+---------------------------------+
| # | Userpart             | Notes                           |
+--------------------------+---------------------------------+
| 1 | <juliet@example.com> | The "at" sign ("@") is allowed  |
|   |                      | in the PRECIS IdentifierClass   |
+--------------------------+---------------------------------+
| 2 | <fussball>           |                                 |
+--------------------------+---------------------------------+
| 3 | <fußball>            | The third character is LATIN    |
|   |                      | SMALL LETTER SHARP S (U+00DF)   |
+--------------------------+---------------------------------+
| 4 | <π>                  | A userpart of GREEK SMALL       |
|   |                      | LETTER PI (U+03C0)              |
+--------------------------+---------------------------------+
| 5 | <Σ>                  | A userpart of GREEK CAPITAL     |
|   |                      | LETTER SIGMA (U+03A3)           |
+--------------------------+---------------------------------+
| 6 | <σ>                  | A userpart of GREEK SMALL       |
|   |                      | LETTER SIGMA (U+03C3)           |
+--------------------------+---------------------------------+
| 7 | <ς>                  | A userpart of GREEK SMALL       |
|   |                      | LETTER FINAL SIGMA (U+03C2)     |
+--------------------------+---------------------------------+
        ]]></artwork>
        <postamble>Table 1: A Sample of Legal Userparts</postamble>
      </figure>

      <t>Regarding examples 2 and 3:
      although in German writing the character eszett "ß" (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.</t>
      
      <t>Regarding examples 5, 6, and 7:
      optional case mapping of "Σ" (GREEK CAPITAL LETTER SIGMA, U+03A3) to
      the lowercase character "σ" (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 the character "ς" (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.</t>

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

      <figure align="center">
        <artwork align="center"><![CDATA[
+--------------------------+---------------------------------+
| # | Non-Userpart String  | Notes                           |
+--------------------------+---------------------------------+
| 8 | <foo bar>            | SPACE (U+0020) is disallowed in |
|   |                      | the userpart                    |
+--------------------------+---------------------------------+
| 9 | <>                   | Zero-length userpart            |
+--------------------------+---------------------------------+
| 10| <henryⅣ>             | The sixth character is ROMAN    |
|   |                      | NUMERAL FOUR (U+2163)           |
+--------------------------+---------------------------------+
| 11| <♚>                  | A userpart of BLACK CHESS KING  | 
|   |                      | (U+265A)                        |
+--------------------------+---------------------------------+
        ]]></artwork>
        <postamble>Table 2: A Sample of Strings That Violate the Userpart Rules</postamble>
      </figure>

      <t>Regarding example 8:
      although this is not a valid userpart, it is a valid username because
      it is a space-separated sequence of userparts.</t>
      
      <t>Regarding example 10:
      the character "Ⅳ" (ROMAN NUMERAL FOUR, U+2163) has a compatibility
      equivalent of the characters "I" (LATIN CAPITAL LETTER I, U+0049) and
      "V" (LATIN CAPITAL LETTER V, U+0056), but code points with compatibility
      equivalents are not allowed in the PRECIS IdentifierClass.</t>
      
      <t>Regarding example 11: 
      symbol characters such as "♚" (BLACK CHESS KING, U+265A) are not
      allowed in the PRECIS IdentifierClass.</t>

    </section>

  </section>

  <section title='Passwords' anchor='password'>

    <section title='Definition' anchor='password-definition'>
      <t>This document specifies that a password is a string of Unicode code
      points <xref target='Unicode'/> that is conformant to the OpaqueString 
      profile (specified below) of the PRECIS FreeformClass defined in
      Section 4.3 of <xref target="RFC8264"/> and expressed in a standard
      Unicode Encoding Form (such as UTF-8 <xref target='RFC3629'/>).</t>

      <t>The syntax for a password is defined as follows, using the Augmented
      Backus-Naur Form (ABNF) <xref target="RFC5234"/>.</t>

      <figure>
        <artwork><![CDATA[
   password   = 1*(freepoint)
                ;
                ; a "freepoint" is a Unicode code point that
                ; can be contained in a string conforming to 
                ; the PRECIS FreeformClass
                ;
        ]]></artwork>
      </figure>
      <t>All code points and blocks not explicitly allowed in the PRECIS
      FreeformClass are disallowed; this includes private-use code points,
      surrogate code points, and the other code points and blocks defined as
      "Prohibited Output" in Section 2.3 of <xref target='RFC4013'/> 
      (when corrected per <xref target="Err1812" />).</t>

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

      <t><list style='empty'><t>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.</t>

      <t><list style='empty'><t>Note: The prohibition of
      zero-length passwords is not a recommendation regarding password
      strength (because a password of only one byte is highly insecure) but is
      meant to prevent applications from mistakenly omitting a password
      entirely; such an outcome is possible when internationalized
      strings are accepted, because a non-empty sequence of
      characters can result in a zero-length password after
      canonicalization.</t></list></t>

      <t>In protocols that provide passwords as input to a cryptographic
      algorithm such as a hash function, the client will need to perform
      enforcement of the rules for the OpaqueString profile before
      applying the algorithm, because the password is not available to
      the server in plaintext form.</t>

    </section>

    <section title='OpaqueString Profile' anchor='opaquestring-profile'>
      <t>The definition of the OpaqueString profile is provided in the
      following sections, including detailed information about preparation,
      enforcement, and comparison (for details on the distinction
      between these actions, refer to <xref target='RFC8264'/>).</t>

      <section title="Preparation" anchor="opaquestring-profile-preparation">
        <t>An entity that prepares a string according to this profile MUST
        ensure that the string consists only of Unicode code points that
        are explicitly allowed by the FreeformClass string class defined 
        in <xref target='RFC8264'/>.</t>

      </section>
      <section title="Enforcement" anchor="opaquestring-profile-enforcement">
        <t>An entity that performs enforcement according to this profile MUST
        prepare a string as described in
        <xref target="opaquestring-profile-preparation"/> and MUST also
        apply the rules specified below for the OpaqueString profile
        (these rules MUST be applied in the order shown):</t>

        <t>
          <list style='numbers'>
            <t>Width Mapping Rule: Fullwidth and halfwidth code points MUST NOT
            be mapped to their decomposition mappings (see Unicode Standard
            Annex #11 <xref target='UAX11'/>).</t>

            <t>Additional Mapping Rule: Any instances of non-ASCII space MUST
            be mapped to SPACE (U+0020); a non-ASCII space is any
            Unicode code point having a Unicode general category of "Zs",
            with the exception of SPACE (U+0020).  As was the case in RFC 4013, 
            the inclusion of only SPACE (U+0020) prevents confusion with various 
            non-ASCII space code points, many of which are difficult to 
            reproduce across different input methods.</t>

            <t>Case Mapping Rule: There is no case mapping rule (because mapping
            uppercase and titlecase code points to their lowercase equivalents
            would lead to false accepts and thus to reduced security).</t>

            <t>Normalization Rule: Unicode Normalization Form C (NFC) MUST be
            applied to all strings.</t>

            <t>Directionality Rule: There is no directionality rule. The "Bidi
            Rule" (defined in <xref target='RFC5893'/>) and similar rules are
            unnecessary and inapplicable to passwords, because they can reduce
            the repertoire 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.</t>

          </list>
        </t>
        <t>The result of the foregoing operations is an output string that conforms to the OpaqueString profile.  Until an implementation produces such an output string, it MUST NOT treat the string as conforming (in particular, it MUST NOT assume that an input string is conforming before the enforcement operation has been completed).</t>
      </section>
      <section title="Comparison" anchor="opaquestring-profile-comparison">
        <t>An entity that performs comparison of two strings according to this
        profile MUST prepare each string as specified in
        <xref target="opaquestring-profile-preparation"/> and then MUST
        enforce the rules specified in
        <xref target="opaquestring-profile-enforcement"/>.
        The two strings are to be considered equivalent if and only if they are an exact
        octet-for-octet match (sometimes called "bit-string identity").</t> 
        <t>Until an implementation determines whether two strings are to be
        considered equivalent, it MUST NOT treat them as equivalent (in
        particular, it MUST NOT assume that two input strings are equivalent 
        before the comparison operation has been completed).</t>
        <t>See <xref target='security-password-comparison'/> regarding comparison
        of passwords and passphrases.</t>

      </section>
    </section>

    <section title="Examples" anchor="password-examples">
      <t>The following examples illustrate a small number of passwords that
      are consistent with the format defined above (note that the characters
      &quot;&lt;&quot; and &quot;&gt;&quot; are used here to delineate the
      actual passwords and are not part of the password strings).</t>

      <figure align="center">
        <artwork align="center"><![CDATA[
+------------------------------------+------------------------------+
| # | Password                       | Notes                        |
+------------------------------------+------------------------------+
| 12| <correct horse battery staple> | SPACE (U+0020) is allowed    |
+------------------------------------+------------------------------+
| 13| <Correct Horse Battery Staple> | Differs by case from         |
|   |                                | example 12                   |
+------------------------------------+------------------------------+
| 14| <πßå>                          | Non-ASCII letters are OK     |
|   |                                | (e.g., GREEK SMALL LETTER    |
|   |                                | PI (U+03C0))                 |
+------------------------------------+------------------------------+
| 15| <Jack of ♦s>                   | Symbols are OK (e.g., BLACK  |
|   |                                | DIAMOND SUIT (U+2666))       |
+------------------------------------+------------------------------+
| 16| <foo bar>                      | OGHAM SPACE MARK (U+1680) is |
|   |                                | mapped to SPACE (U+0020);    |
|   |                                | thus the full string is      |
|   |                                | mapped to <foo bar>          |
+------------------------------------+------------------------------+
        ]]></artwork>
        <postamble>Table 3: A Sample of Legal Passwords</postamble>
      </figure>

      <t>The following examples illustrate strings that are not valid
      passwords because they violate the format defined above.</t>

      <figure align="center">
        <artwork align="center"><![CDATA[
+------------------------------------+------------------------------+
| # | Password                       | Notes                        |
+------------------------------------+------------------------------+
| 17| <>                             | Zero-length passwords are    |
|   |                                | disallowed                   |
+------------------------------------+------------------------------+
| 18| <my cat is a &#x9;by>          | Control characters like TAB  |
|   |                                | (U+0009) are disallowed      |
+------------------------------------+------------------------------+
        ]]></artwork>
      <postamble>Table 4: A Sample of Strings That Violate the Password Rules</postamble>
     </figure>

     <t>Note: Following the "XML Notation" used in <xref target='RFC3987'/>, 
     in example 18 the character TAB (U+0009) in example 18 is represented 
     as &amp;#x9 because otherwise it could not be shown in running text.</t>

    </section>

  </section>

  <section title="Use in Application Protocols" anchor="use">
    <t>This specification defines only the PRECIS-based rules for the 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 at what points during
    protocol processing or interface handling the rules need to be enforced.
    See Section 6 of <xref target='RFC8264'/> for guidelines on using
    PRECIS profiles in applications.</t>

    <t>Above and beyond the PRECIS-based rules specified here, application
    protocols can also define application-specific rules governing such
    strings (rules regarding minimum or maximum length, further restrictions
    on allowable code points or character ranges, safeguards to mitigate the
    effects of visually similar characters, etc.), application-layer
    constructs (see <xref target='username-app'/>), and related matters.</t>

    <t>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, because it can lead to false
    accepts.  An in-depth discussion can be found in
    <xref target='RFC6943'/>.</t>

    <t>Applying the rules for any given PRECIS profile is not necessarily an 
    idempotent procedure for all code points. Therefore, an implementation 
    SHOULD apply the rules repeatedly until the output string is stable; 
    if the output string does not stabilize after reapplying the rules 
    three (3) additional times after the first application,
    the implementation SHOULD terminate application of the rules 
    and reject the input string as invalid.</t>

  </section>

  <section title="Migration" anchor='migration'>

      <t>The rules defined in this specification differ slightly from those
      defined by the SASLprep specification <xref target='RFC4013'/> (but 
      not from <xref target='RFC7613'/>).  In order to smooth the process 
      of migrating from SASLprep to the approach defined herein, the
      following sections describe these differences, along with their
      implications for migration, in more detail.</t>

    <section title='Usernames' anchor='migration-usernames'>
      <t>Deployments that currently use SASLprep for handling usernames might
      need to scrub existing data when they migrate to the rules defined
      in this specification.  In particular:</t>

      <t>
        <list style='symbols'>
          <t>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:

          <list style='numbers'>
            <t>"ſ" (LATIN SMALL LETTER LONG S, U+017F) is compatibility equivalent
            to "s" (LATIN SMALL LETTER S, U+0073).</t>

            <t>"Ⅳ" (ROMAN NUMERAL FOUR, U+2163) is compatibility equivalent
            to "I" (LATIN CAPITAL LETTER I, U+0049) and "V" (LATIN CAPITAL LETTER V,
            U+0056).</t>

            <t>"fi" (LATIN SMALL LIGATURE FI, U+FB01) is compatibility equivalent
            to "f" (LATIN SMALL LETTER F, U+0066) and "i" (LATIN SMALL LETTER I,
            U+0069).</t>

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

          <vspace blankLines='1'/>
          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;
          for instance, "HENRYIV" with the last two code points being LATIN CAPITAL
          LETTER I (U+0049) and LATIN CAPITAL LETTER V (U+0056) as opposed to
          "HENRYⅣ" with the last character being "Ⅳ" (ROMAN NUMERAL FOUR,
          U+2163), 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.</t>

          <t>SASLprep mapped the "characters commonly mapped to nothing"
          (from Appendix&nbsp;B.1 of <xref target='RFC3454'/>) to nothing,
          whereas the PRECIS IdentifierClass entirely disallows most of these
          code points, which correspond to the code points from the PRECIS
          "M" category defined under Section&nbsp;9.13 of <xref
          target='RFC8264'/>. For migration purposes, the operator might want
          to remove from usernames any code points contained in the PRECIS "M"
          category (e.g., SOFT HYPHEN (U+00AD)).  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.</t>

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

        </list>
      </t>
    </section>

    <section title='Passwords' anchor='migration-passwords'>
      <t>Depending on local service policy, migration from SASLprep to this
      specification might not involve any scrubbing of data (because 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:</t>

      <t>
        <list style='symbols'>
          <t>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 accepts when comparing
          passwords.  A few examples might suffice to indicate the nature of
          the problem:

          <list style='numbers'>
            <t>"ſ" (LATIN SMALL LETTER LONG S, U+017F) is compatibility equivalent
            to "s" (LATIN SMALL LETTER S, U+0073).</t>

            <t>"Ⅳ" (ROMAN NUMERAL FOUR, U+2163) is compatibility equivalent
            to "I" (LATIN CAPITAL LETTER I, U+0049) and "V" (LATIN CAPITAL LETTER V,
            U+0056).</t>

            <t>"fi" (LATIN SMALL LIGATURE FI, U+FB01) is compatibility equivalent
            to "f" (LATIN SMALL LETTER F, U+0066) and "i" (LATIN SMALL LETTER I,
            U+0069).</t>

          </list>
          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.</t>

          <t>SASLprep mapped the "characters commonly mapped to nothing"
          (from Appendix&nbsp;B.1 of <xref target='RFC3454'/>) to nothing,
          whereas the PRECIS FreeformClass entirely disallows such code
          points, which correspond to the code points from the PRECIS "M"
          category defined under Section&nbsp;9.13 of <xref
          target='RFC8264'/>.  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.</t>
        </list>
      </t>
    </section>

  </section>

  <section title="IANA Considerations" anchor="iana">

    <t>IANA has made the updates described below. </t>

    <section title="UsernameCaseMapped Profile" anchor="iana-usernamecasemapped">
      <t>
IANA has added the following entry to the "PRECIS Profiles"
   registry.
        <list style='hanging'>
          <t hangText='Name:'>UsernameCaseMapped</t>
          <t hangText='Base Class:'>IdentifierClass</t>
          <t hangText='Applicability:'>Usernames in security and application
          protocols.</t>

          <t hangText='Replaces:'>The SASLprep profile of Stringprep.</t>
          <t hangText='Width Mapping Rule:'>Map fullwidth and halfwidth
          code points to their decomposition mappings.</t>

          <t hangText='Additional Mapping Rule:'>None</t>
          <t hangText='Case Mapping Rule:'>Map uppercase and titlecase
          code points to lowercase.</t>

          <t hangText='Normalization Rule:'>NFC</t>
          <t hangText='Directionality Rule:'>The "Bidi Rule" defined in RFC
          5893 applies.</t>

          <t hangText='Enforcement:'>To be defined by security or application
          protocols that use this profile.</t>

          <t hangText='Specification:'>Section 3.3 of RFC 8265</t>

        </list>
      </t>
    </section>

    <section title="UsernameCasePreserved Profile" anchor="iana-usernamecasepreserved">
      <t>
IANA has added the following entry to the "PRECIS Profiles"
   registry.

        <list style='hanging'>
          <t hangText='Name:'>UsernameCasePreserved</t>
          <t hangText='Base Class:'>IdentifierClass</t>
          <t hangText='Applicability:'>Usernames in security and application
          protocols.</t>

          <t hangText='Replaces:'>The SASLprep profile of Stringprep.</t>
          <t hangText='Width Mapping Rule:'>Map fullwidth and halfwidth
          code points to their decomposition mappings.</t>

          <t hangText='Additional Mapping Rule:'>None</t>
          <t hangText='Case Mapping Rule:'>None</t>
          <t hangText='Normalization Rule:'>NFC</t>
          <t hangText='Directionality Rule:'>The "Bidi Rule" defined in RFC
          5893 applies.</t>

          <t hangText='Enforcement:'>To be defined by security or application
          protocols that use this profile.</t>

          <t hangText='Specification:'>Section 3.4 of RFC 8265</t>

        </list>
      </t>
    </section>

    <section title="OpaqueString Profile" anchor="iana-password">
      <t>

IANA has added the following entry to the "PRECIS Profiles"
   registry.

        <list style='hanging'>
          <t hangText='Name:'>OpaqueString</t>
          <t hangText='Base Class:'>FreeformClass</t>
          <t hangText='Applicability:'>Passwords and other opaque strings in
          security and application protocols.</t>

          <t hangText='Replaces:'>The SASLprep profile of Stringprep.</t>
          <t hangText='Width Mapping Rule:'>None</t>
          <t hangText='Additional Mapping Rule:'>Map non-ASCII space
          code points to ASCII space.</t>

          <t hangText='Case Mapping Rule:'>None</t>
          <t hangText='Normalization Rule:'>NFC</t>
          <t hangText='Directionality Rule:'>None</t>
          <t hangText='Enforcement:'>To be defined by security or application
          protocols that use this profile.</t>

          <t hangText='Specification:'>Section 4.2 of RFC 8265</t>

        </list>
      </t>
    </section>

    <section title="Stringprep Profile">
      <t>
      The Stringprep specification <xref target="RFC3454"/> did not
      provide for entries in the "Stringprep Profiles" registry to have
      any state except "Current" or "Not Current".  Because RFC 7613
      obsoleted RFC 4013, which registered the SASLprep profile of
      Stringprep, IANA previously marked that profile as "Not Current"
      and cited RFC 7613 as an additional reference.  IANA has modified
      the profile so that the current document is now cited as the additional reference.
      </t>
    </section>

  </section>

  <section title="Security Considerations" anchor="security">
    <section title="Password/Passphrase Strength" anchor="security-password-strength">
      <t>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.</t>

    </section>
    <section title="Password/Passphrase Comparison" anchor="security-password-comparison">
      <t>In systems that conform to modern best practices for security,
      verification of passwords during authentication will not use the 
      comparison defined in <xref target='opaquestring-profile-comparison'/>.
      Instead, because the system performs cryptographic calculations to verify 
      the password, it will prepare the password as defined in 
      <xref target='opaquestring-profile-preparation'/> and enforce the rules 
      as defined in <xref target='opaquestring-profile-enforcement'/> before 
      performing the relevant calculations.</t>

    </section>
    <section title="Identifier Comparison" anchor="security-id">
      <t>The process of comparing identifiers (such as SASL simple user names,
      authentication identifiers, and authorization identifiers) can lead to
      either false rejects or false accepts, both of which have security
      implications.  A more detailed discussion can be found in <xref
      target='RFC6943'/>.</t>

    </section>
    <section title="Reuse of PRECIS" anchor="security-precis">
      <t>The security considerations described in <xref target="RFC8264"/>
      apply to the IdentifierClass and FreeformClass string classes
      used in this document for usernames and passwords, respectively.</t>

    </section>
    <section title="Reuse of Unicode" anchor="security-unicode">
      <t>The security considerations described in <xref target='UTS39'/> apply
      to the use of Unicode code points in usernames and passwords.</t>

    </section>
  </section>

  </middle>

  <back>

    <references title="Normative References">

<?rfc include="reference.RFC.2119" ?>
<?rfc include="reference.RFC.3629" ?>
<?rfc include="reference.RFC.5234" ?>
<?rfc include="reference.RFC.5890" ?>
<?rfc include="reference.RFC.6365" ?>
<?rfc include="reference.RFC.8174" ?>

<!-- draft-ietf-precis-7564bis (RFC 8264) -->
<reference anchor="RFC8264" target="https://www.rfc-editor.org/info/rfc8264">
<front>
<title>PRECIS Framework: Preparation, Enforcement, and Comparison of Internationalized Strings in Application Protocols</title>
<author initials='P' surname='Saint-Andre' fullname='Peter Saint-Andre'>
    <organization />
</author>
<author initials='M' surname='Blanchet' fullname='Marc Blanchet'>
    <organization />
</author>
<date month='September' year='2017' />
</front>
<seriesInfo name="RFC" value="8264"/>
<seriesInfo name="DOI" value="10.17487/RFC8264"/>
</reference>

<reference anchor="UAX11" target='http://unicode.org/reports/tr11/'>
  <front>
    <title>East Asian Width</title>
    <author>
      <organization>Unicode Standard Annex #11</organization>
    </author>
  <date/>
  </front>
 <seriesInfo name='edited by Ken Lunde. ' value='An integral part of The
   Unicode Standard'/>
</reference>

<reference anchor="Unicode" target="http://www.unicode.org/versions/latest/">
  <front>
    <title>The Unicode Standard</title>
    <author>
      <organization>The Unicode Consortium</organization>
    </author>
    <date/>
  </front>
</reference>

    </references>

    <references title="Informative References">

<reference  anchor='RFC20' target='https://www.rfc-editor.org/info/rfc20'>
<front>
<title>ASCII format for network interchange</title>
<author initials='V.G.' surname='Cerf' fullname='V.G. Cerf'><organization /></author>
<date year='1969' month='October' />
</front>
<seriesInfo name='STD' value='80'/>
<seriesInfo name='RFC' value='20'/>
<seriesInfo name='DOI' value='10.17487/RFC0020'/>
</reference>

<?rfc include="reference.RFC.3454" ?>
<?rfc include="reference.RFC.3501" ?>
<?rfc include="reference.RFC.3987" ?>
<?rfc include="reference.RFC.4013" ?>
<?rfc include="reference.RFC.4422" ?>
<?rfc include="reference.RFC.4616" ?>
<?rfc include="reference.RFC.5802" ?>
<?rfc include="reference.RFC.5893" ?>
<?rfc include="reference.RFC.6120" ?>
<?rfc include="reference.RFC.6943" ?>
<?rfc include="reference.RFC.7542" ?>
<?rfc include="reference.RFC.7613" ?>
<?rfc include="reference.RFC.7616" ?>
<?rfc include="reference.RFC.7617" ?>
<?rfc include="reference.RFC.7622" ?>

<!-- draft-ietf-precis-7700bis (RFC 8266) -->
<reference anchor='RFC8266' target="https://www.rfc-editor.org/info/rfc8266">
<front>
<title>Preparation, Enforcement, and Comparison of Internationalized Strings Representing Nicknames</title>
<author initials='P' surname='Saint-Andre' fullname='Peter Saint-Andre'>
    <organization />
</author>
<date month='September' year='2017' />
</front>
<seriesInfo name="RFC" value="8266"/>
<seriesInfo name="DOI" value="10.17487/RFC8266"/>
</reference>

<reference anchor="UTS39" target='http://unicode.org/reports/tr39/'>
  <front>
    <title>Unicode Security Mechanisms</title>
    <author>
      <organization>Unicode Technical Standard #39</organization>
    </author>
  <date/>
  </front>
 <seriesInfo name='edited by Mark Davis' value='and Michel Suignard'/>
</reference>

<reference anchor="Err1812" quote-title="false" target="https://www.rfc-editor.org/errata/eid1812">
     <front>
       <title>Erratum ID 1812</title>
       <author>
         <organization>RFC Errata</organization>
       </author>
       <date month="" year="" />
     </front>
     <seriesInfo name="RFC" value="4013" />
   </reference>

    </references>

    <section title="Changes from RFC 7613" anchor="changes">
      <t>The following changes were made from <xref target='RFC7613'/>.</t>
      <t>
        <list style='symbols'>
          <t>Corrected the order of operations for the UsernameCaseMapped
          profile to ensure consistency with <xref target='RFC8264'/>.</t>
          <t>In accordance with working group discussions and updates to <xref
          target='RFC8264'/>, removed the use of the Unicode toCaseFold()
          operation in favor of the Unicode toLowerCase() operation.</t>
          <t>Modified the presentation (but not the content) of the rules.</t>
          <t>Removed UTF-8 as a mandatory encoding, because that is a matter
          for the application.</t>
          <t>Clarified several editorial matters.</t>
          <t>Updated references.</t>
        </list>
      </t>
      <t>See <xref target='RFC7613'/> for a description of the differences from <xref target='RFC4013'/>.</t>
    </section>

    <section title="Acknowledgements" anchor="acks" numbered="no">
      
      <t>Thanks to Christian Schudt and Sam Whited for their bug reports and
      feedback.</t>

      <t>See <xref target='RFC7613'/> for acknowledgements related to the
      specification that this document supersedes.</t>
      
    </section>

  </back>

</rfc>