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RFC 8265

 
 
 

Preparation, Enforcement, and Comparison of Internationalized Strings Representing Usernames and Passwords

Part 2 of 2, p. 13 to 26
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4.  Passwords

4.1.  Definition

   This document specifies that a password is a string of Unicode code
   points [Unicode] that is conformant to the OpaqueString profile
   (specified below) of the PRECIS FreeformClass defined in Section 4.3
   of [RFC8264] and expressed in a standard Unicode Encoding Form (such
   as UTF-8 [RFC3629]).

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

      password   = 1*(freepoint)
                   ;
                   ; a "freepoint" is a Unicode code point that
                   ; can be contained in a string conforming to
                   ; the PRECIS FreeformClass
                   ;

   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 [RFC4013] (when corrected
   per [Err1812]).

   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.

      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.

   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

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   the algorithm, because 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 (for details on the distinction between
   these actions, refer to [RFC8264]).

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 are
   explicitly allowed by the FreeformClass string class defined in
   [RFC8264].

4.2.2.  Enforcement

   An entity that performs enforcement according to this profile MUST
   prepare a string as described in Section 4.2.1 and MUST also apply
   the rules specified below for the OpaqueString profile (these rules
   MUST be applied in the order shown):

   1.  Width Mapping Rule: Fullwidth and halfwidth code points 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 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.

   3.  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).

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

   5.  Directionality Rule: There is no directionality rule.  The "Bidi
       Rule" (defined in [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

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       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.

   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).

4.2.3.  Comparison

   An entity that performs comparison of two strings according to this
   profile MUST prepare each string as specified in Section 4.2.1 and
   then MUST enforce the rules specified in Section 4.2.2.  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").

   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).

   See Section 8.2 regarding comparison of passwords and passphrases.

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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   +------------------------------------+------------------------------+
   | # | Password                       | Notes                        |
   +------------------------------------+------------------------------+
   | 12| <correct horse battery staple> | SPACE (U+0020) is allowed    |
   +------------------------------------+------------------------------+
   | 13| <Correct Horse Battery Staple> | Differs by case from         |
   |   |                                | example 12                   |
   +------------------------------------+------------------------------+
   | 14| <πβå>                          | 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>          |
   +------------------------------------+------------------------------+

                   Table 3: A Sample of Legal Passwords

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

   +------------------------------------+------------------------------+
   | # | Password                       | Notes                        |
   +------------------------------------+------------------------------+
   | 17| <>                             | Zero-length passwords are    |
   |   |                                | disallowed                   |
   +------------------------------------+------------------------------+
   | 18| <my cat is a &#x9;by>          | Control characters like TAB  |
   |   |                                | (U+0009) are disallowed      |
   +------------------------------------+------------------------------+

       Table 4: A Sample of Strings That Violate the Password Rules

   Note: Following the "XML Notation" used in [RFC3987], the character
   TAB (U+0009) in example 18 is represented as &#x9 because otherwise
   it could not be shown in running text.

5.  Use in Application Protocols

   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

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   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 [RFC8264]
   for guidelines on using PRECIS profiles in applications.

   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 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, because it can lead to
   false accepts.  An in-depth discussion can be found in [RFC6943].

   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.

6.  Migration

   The rules defined in this specification differ slightly from those
   defined by the SASLprep specification [RFC4013] (but not from
   [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.

6.1.  Usernames

   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:

   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

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      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.  "ſ" (LATIN SMALL LETTER LONG S, U+017F) is compatibility
          equivalent to "s" (LATIN SMALL LETTER S, U+0073).

      2.  "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).

      3.  "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).

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

      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.

   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 code points,
      which correspond to the code points from the PRECIS "M" category
      defined under Section 9.13 of [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.

   o  SASLprep allowed uppercase and titlecase code points, whereas the
      UsernameCaseMapped profile maps uppercase and titlecase code

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      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).

6.2.  Passwords

   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:

   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 accepts when comparing
      passwords.  A few examples might suffice to indicate the nature of
      the problem:

      1.  "ſ" (LATIN SMALL LETTER LONG S, U+017F) is compatibility
          equivalent to "s" (LATIN SMALL LETTER S, U+0073).

      2.  "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).

      3.  "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).

      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 code points, which
      correspond to the code points from the PRECIS "M" category defined
      under Section 9.13 of [RFC8264].  In practice, this change will
      probably have no effect on comparison, but user-oriented software

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      might reject such code points instead of ignoring them during
      password preparation.

7.  IANA Considerations

   IANA has made the updates described below.

7.1.  UsernameCaseMapped Profile

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

   Name:  UsernameCaseMapped.

   Base Class:  IdentifierClass.

   Applicability:  Usernames in security and application protocols.

   Replaces:  The SASLprep profile of Stringprep.

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

   Additional Mapping Rule:  None.

   Case Mapping Rule:  Map uppercase and titlecase code points 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:  Section 3.3 of RFC 8265.

7.2.  UsernameCasePreserved Profile

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

   Name:  UsernameCasePreserved.

   Base Class:  IdentifierClass.

   Applicability:  Usernames in security and application protocols.

   Replaces:  The SASLprep profile of Stringprep.

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   Width Mapping Rule:  Map fullwidth and halfwidth code points 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:  Section 3.4 of RFC 8265.

7.3.  OpaqueString Profile

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

   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 code points to SPACE
      (U+0020).

   Case Mapping Rule:  None.

   Normalization Rule:  NFC.

   Directionality Rule:  None.

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

   Specification:  Section 4.2 of RFC 8265.

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7.4.  Stringprep Profile

   The Stringprep specification [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.

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.  Password/Passphrase Comparison

   In systems that conform to modern best practices for security,
   verification of passwords during authentication will not use the
   comparison defined in Section 4.2.3.  Instead, because the system
   performs cryptographic calculations to verify the password, it will
   prepare the password as defined in Section 4.2.1 and enforce the
   rules as defined in Section 4.2.2 before performing the relevant
   calculations.

8.3.  Identifier Comparison

   The process of comparing identifiers (such as SASL simple usernames,
   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 [RFC6943].

8.4.  Reuse of PRECIS

   The security considerations described in [RFC8264] apply to the
   IdentifierClass and FreeformClass string classes used in this
   document for usernames and passwords, respectively.

8.5.  Reuse of Unicode

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

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9.  References

9.1.  Normative References

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

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

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

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

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8264]  Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
              Preparation, Enforcement, and Comparison of
              Internationalized Strings in Application Protocols",
              RFC 8264, DOI 10.17487/RFC8264, October 2017,
              <https://www.rfc-editor.org/info/rfc8264>.

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

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

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9.2.  Informative References

   [Err1812]  RFC Errata, Erratum ID 1812, RFC 4013,
              <https://www.rfc-editor.org/errata/eid1812>.

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

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

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
              <https://www.rfc-editor.org/info/rfc3501>.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
              January 2005, <https://www.rfc-editor.org/info/rfc3987>.

   [RFC4013]  Zeilenga, K., "SASLprep: Stringprep Profile for User Names
              and Passwords", RFC 4013, DOI 10.17487/RFC4013, February
              2005, <https://www.rfc-editor.org/info/rfc4013>.

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

   [RFC4616]  Zeilenga, K., Ed., "The PLAIN Simple Authentication and
              Security Layer (SASL) Mechanism", RFC 4616,
              DOI 10.17487/RFC4616, August 2006,
              <https://www.rfc-editor.org/info/rfc4616>.

   [RFC5802]  Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
              "Salted Challenge Response Authentication Mechanism
              (SCRAM) SASL and GSS-API Mechanisms", RFC 5802,
              DOI 10.17487/RFC5802, July 2010,
              <https://www.rfc-editor.org/info/rfc5802>.

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

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   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
              March 2011, <https://www.rfc-editor.org/info/rfc6120>.

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

   [RFC7542]  DeKok, A., "The Network Access Identifier", RFC 7542,
              DOI 10.17487/RFC7542, May 2015,
              <https://www.rfc-editor.org/info/rfc7542>.

   [RFC7613]  Saint-Andre, P. and A. Melnikov, "Preparation,
              Enforcement, and Comparison of Internationalized Strings
              Representing Usernames and Passwords", RFC 7613,
              DOI 10.17487/RFC7613, August 2015,
              <https://www.rfc-editor.org/info/rfc7613>.

   [RFC7616]  Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
              Digest Access Authentication", RFC 7616,
              DOI 10.17487/RFC7616, September 2015,
              <https://www.rfc-editor.org/info/rfc7616>.

   [RFC7617]  Reschke, J., "The 'Basic' HTTP Authentication Scheme",
              RFC 7617, DOI 10.17487/RFC7617, September 2015,
              <https://www.rfc-editor.org/info/rfc7617>.

   [RFC7622]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Address Format", RFC 7622,
              DOI 10.17487/RFC7622, September 2015,
              <https://www.rfc-editor.org/info/rfc7622>.

   [RFC8266]  Saint-Andre, P., "Preparation, Enforcement, and Comparison
              of Internationalized Strings Representing Nicknames",
              RFC 8266, DOI 10.17487/RFC8266, October 2017,
              <https://www.rfc-editor.org/info/rfc8266>.

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

Appendix A.  Changes from RFC 7613

   The following changes were made from [RFC7613].

   o  Corrected the order of operations for the UsernameCaseMapped
      profile to ensure consistency with [RFC8264].

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   o  In accordance with working group discussions and updates to
      [RFC8264], removed the use of the Unicode toCaseFold() operation
      in favor of the Unicode toLowerCase() operation.

   o  Modified the presentation (but not the content) of the rules.

   o  Removed UTF-8 as a mandatory encoding, because that is a matter
      for the application.

   o  Clarified several editorial matters.

   o  Updated references.

   See [RFC7613] for a description of the differences from [RFC4013].

Acknowledgements

   Thanks to Christian Schudt and Sam Whited for their bug reports and
   feedback.

   See [RFC7613] for acknowledgements related to the specification that
   this document supersedes.

Authors' Addresses

   Peter Saint-Andre
   Jabber.org
   P.O. Box 787
   Parker, CO  80134
   United States of America

   Phone: +1 720 256 6756
   Email: stpeter@jabber.org
   URI:   https://www.jabber.org/


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

   Email: Alexey.Melnikov@isode.com