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Internet Engineering Task Force (IETF) P. Saint-AndreRequest for Comments: 8265 Jabber.orgObsoletes:7613 A. MelnikovCategory: Standards Track Isode LtdISSN: 2070-1721 October 2017Preparation, Enforcement, and Comparison of Internationalized StringsRepresenting Usernames and PasswordsAbstract 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 obsoletesRFC 7613.Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available inSection 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc8265.Copyright Notice Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject toBCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.Saint-Andre & Melnikov Standards Track [Page 1]

RFC 8265             PRECIS: Usernames and Passwords        October 2017Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .32.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .43.  Usernames . . . . . . . . . . . . . . . . . . . . . . . . . .53.1.  Definition  . . . . . . . . . . . . . . . . . . . . . . .53.2.  Case Mapping vs. Case Preservation  . . . . . . . . . . .63.3.  UsernameCaseMapped Profile  . . . . . . . . . . . . . . .73.3.1.  Rules . . . . . . . . . . . . . . . . . . . . . . . .73.3.2.  Preparation . . . . . . . . . . . . . . . . . . . . .83.3.3.  Enforcement . . . . . . . . . . . . . . . . . . . . .83.3.4.  Comparison  . . . . . . . . . . . . . . . . . . . . .93.4.  UsernameCasePreserved Profile . . . . . . . . . . . . . .93.4.1.  Rules . . . . . . . . . . . . . . . . . . . . . . . .93.4.2.  Preparation . . . . . . . . . . . . . . . . . . . . .93.4.3.  Enforcement . . . . . . . . . . . . . . . . . . . . .103.4.4.  Comparison  . . . . . . . . . . . . . . . . . . . . .103.5.  Application-Layer Constructs  . . . . . . . . . . . . . .113.6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .114.  Passwords . . . . . . . . . . . . . . . . . . . . . . . . . .134.1.  Definition  . . . . . . . . . . . . . . . . . . . . . . .134.2.  OpaqueString Profile  . . . . . . . . . . . . . . . . . .144.2.1.  Preparation . . . . . . . . . . . . . . . . . . . . .144.2.2.  Enforcement . . . . . . . . . . . . . . . . . . . . .144.2.3.  Comparison  . . . . . . . . . . . . . . . . . . . . .154.3.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .155.  Use in Application Protocols  . . . . . . . . . . . . . . . .166.  Migration . . . . . . . . . . . . . . . . . . . . . . . . . .176.1.  Usernames . . . . . . . . . . . . . . . . . . . . . . . .176.2.  Passwords . . . . . . . . . . . . . . . . . . . . . . . .197.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .207.1.  UsernameCaseMapped Profile  . . . . . . . . . . . . . . .207.2.  UsernameCasePreserved Profile . . . . . . . . . . . . . .207.3.  OpaqueString Profile  . . . . . . . . . . . . . . . . . .217.4.  Stringprep Profile  . . . . . . . . . . . . . . . . . . .228.  Security Considerations . . . . . . . . . . . . . . . . . . .228.1.  Password/Passphrase Strength  . . . . . . . . . . . . . .228.2.  Password/Passphrase Comparison  . . . . . . . . . . . . .228.3.  Identifier Comparison . . . . . . . . . . . . . . . . . .228.4.  Reuse of PRECIS . . . . . . . . . . . . . . . . . . . . .228.5.  Reuse of Unicode  . . . . . . . . . . . . . . . . . . . .229.  References  . . . . . . . . . . . . . . . . . . . . . . . . .239.1.  Normative References  . . . . . . . . . . . . . . . . . .239.2.  Informative References  . . . . . . . . . . . . . . . . .24Appendix A.  Changes fromRFC 7613  . . . . . . . . . . . . . . .25   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .26   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .26Saint-Andre & Melnikov       Standards Track                    [Page 2]

RFC 8265             PRECIS: Usernames and Passwords        October 20171.  Introduction   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 [RFC4616] and the HTTP Basic scheme [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 [RFC5802] and the   HTTP Digest scheme [RFC7616]).   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 [Unicode], with special attention to code points outside the   ASCII range [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 [RFC8264].   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").   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., 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.Saint-Andre & Melnikov       Standards Track                    [Page 3]

RFC 8265             PRECIS: Usernames and Passwords        October 2017   Although the historical predecessor of this document was the SASLprep   profile of Stringprep [RFC3454]), the approach defined here can be   used by technologies other than SASL [RFC4422], such as HTTP   authentication as specified in [RFC7617] and [RFC7616].   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 [RFC7622]).  Non-coordinated updates to   protocol implementations are discouraged because they can have a   negative impact on interoperability and security.2.  Terminology   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 [RFC8266]).   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.   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 username" (seeSection 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   [RFC7617] and [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.Saint-Andre & Melnikov       Standards Track                    [Page 4]

RFC 8265             PRECIS: Usernames and Passwords        October 2017   Many important terms used in this document are defined in [RFC5890],   [RFC6365], [RFC8264], and [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).   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 inBCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all   capitals, as shown here.3.  Usernames3.1.  Definition   This document specifies that a username is a string of Unicode code   points [Unicode] that is structured as an ordered sequence of   "userparts" and expressed in a standard Unicode Encoding Form (such   as UTF-8 [RFC3629]).  A userpart is allowed to contain only code   points that are allowed by the PRECIS IdentifierClass defined inSection 4.2 of [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.   The syntax for a username is defined as follows, using the Augmented   Backus-Naur Form (ABNF) [RFC5234].      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                   ;   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" inSection 2.3 of [RFC4013]   (when corrected per [Err1812]).  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 code points that can be used inSaint-Andre & Melnikov       Standards Track                    [Page 5]

RFC 8265             PRECIS: Usernames and Passwords        October 2017      existing SASL mechanisms and in application protocols that use      SASL, 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.   This specification defines two profiles for usernames: the   UsernameCaseMapped profile performs case mapping, and the   UsernameCasePreserved performs case preservation (see further   discussion underSection 3.2).   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.3.2.  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 their use (or not) of 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, because not doing so can   lead to false accepts 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.  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 [RFC4422],Saint-Andre & Melnikov       Standards Track                    [Page 6]

RFC 8265             PRECIS: Usernames and Passwords        October 2017      SASL mechanisms are not to apply this or any other profile to      authorization identifiers, only to authentication identifiers.   o  Application protocols that use SASL (such as IMAP [RFC3501] and      the Extensible Messaging and Presence Protocol (XMPP) [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 [RFC4422], SASL application protocols      are not to apply this or any other profile to authentication      identifiers, only to authorization identifiers.   o  Application protocols that do not use SASL (such as HTTP      authentication with the HTTP Basic and Digest schemes as specified      in [RFC7617] and [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).   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).3.3.  UsernameCaseMapped Profile3.3.1.  Rules   The following rules are defined for use within the UsernameCaseMapped   profile of the PRECIS IdentifierClass.   1.  Width Mapping Rule: Map fullwidth and halfwidth code points to       their decomposition mappings (see Unicode Standard Annex #11       [UAX11]).   2.  Additional Mapping Rule: There is no additional mapping rule.Saint-Andre & Melnikov       Standards Track                    [Page 7]

RFC 8265 PRECIS: Usernames and Passwords October 2017 3. 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 [Unicode]; see further discussion inSection 3.2. 4. Normalization Rule: Apply Unicode Normalization Form C (NFC) to all strings. 5. Directionality Rule: Apply the "Bidi Rule" defined in [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.3.3.2. Preparation 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). 1. Apply the width mapping rule specified inSection 3.3.1. It is necessary to apply the rule at this point because otherwise the PRECIS "HasCompat" category specified inSection 9.17 of [RFC8264] would forbid fullwidth and halfwidth code points. 2. Ensure that the string consists only of Unicode code points that are explicitly allowed by the PRECIS IdentifierClass defined inSection 4.2 of [RFC8264].3.3.3. Enforcement An entity that performs enforcement according to this profile MUST prepare an input string as described inSection 3.3.2 and MUST also apply the following rules specified inSection 3.3.1 in the order shown: 1. Case Mapping Rule 2. Normalization Rule 3. Directionality Rule 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).Saint-Andre & Melnikov Standards Track [Page 8]

RFC 8265 PRECIS: Usernames and Passwords October 2017 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).3.3.4. Comparison An entity that performs comparison of two strings according to this profile MUST prepare each string as specified inSection 3.3.2 and then MUST enforce the rules specified inSection 3.3.3. 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).3.4. UsernameCasePreserved Profile3.4.1. Rules The following rules are defined for use within the UsernameCasePreserved profile of the PRECIS IdentifierClass. 1. Width Mapping Rule: Map fullwidth and halfwidth code points to their decomposition mappings (see Unicode Standard Annex #11 [UAX11]). 2. Additional Mapping Rule: There is no additional mapping rule. 3. Case Mapping Rule: There is no case mapping rule. 4. Normalization Rule: Apply Unicode Normalization Form C (NFC) to all strings. 5. Directionality Rule: Apply the "Bidi Rule" defined in [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.3.4.2. Preparation An entity that prepares a string for subsequent enforcement according to this profile MUST proceed as follows (applying the steps in the order shown).Saint-Andre & Melnikov Standards Track [Page 9]

RFC 8265 PRECIS: Usernames and Passwords October 2017 1. Apply the width mapping rule specified inSection 3.4.1. It is necessary to apply the rule at this point because otherwise the PRECIS "HasCompat" category specified inSection 9.17 of [RFC8264] would forbid fullwidth and halfwidth code points. 2. Ensure that the string consists only of Unicode code points that are explicitly allowed by the PRECIS IdentifierClass defined inSection 4.2 of [RFC8264].3.4.3. Enforcement An entity that performs enforcement according to this profile MUST prepare a string as described inSection 3.4.2 and MUST also apply the following rules specified inSection 3.4.1 in the order shown: 1. Normalization Rule 2. Directionality Rule 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). 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).3.4.4. Comparison An entity that performs comparison of two strings according to this profile MUST prepare each string as specified inSection 3.4.2 and then MUST enforce the rules specified inSection 3.4.3. 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).Saint-Andre & Melnikov Standards Track [Page 10]

RFC 8265             PRECIS: Usernames and Passwords        October 20173.5.  Application-Layer Constructs   Both the UsernameCaseMapped and UsernameCasePreserved profiles enable   an application protocol, implementation, or deployment to create   application-layer constructs such as a 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).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).      +--------------------------+---------------------------------+      | # | 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)     |      +--------------------------+---------------------------------+                   Table 1: A Sample of Legal Userparts   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 theseSaint-Andre & Melnikov       Standards Track                   [Page 11]

RFC 8265             PRECIS: Usernames and Passwords        October 2017   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 "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.   The following examples illustrate strings that are not valid   userparts (not usernames) because they violate the format defined   above.      +--------------------------+---------------------------------+      | # | 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 INFINITY (U+221E) |      +--------------------------+---------------------------------+       Table 2: A Sample of Strings That Violate the Userpart Rules   Regarding example 8: although this is not a valid userpart, it is a   valid username because it is a space-separated sequence of userparts.   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.   Regarding example 11: symbol characters such as "∞" (INFINITY,   U+221E) are not allowed in the PRECIS IdentifierClass.Saint-Andre & Melnikov       Standards Track                   [Page 12]

RFC 8265             PRECIS: Usernames and Passwords        October 20174.  Passwords4.1.  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 inSection 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" inSection 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 (andRFC 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 applyingSaint-Andre & Melnikov       Standards Track                   [Page 13]

RFC 8265 PRECIS: Usernames and Passwords October 2017 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 inSection 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 inRFC 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 andSaint-Andre & Melnikov Standards Track [Page 14]

RFC 8265             PRECIS: Usernames and Passwords        October 2017       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 inSection 4.2.1 and   then MUST enforce the rules specified inSection 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).   SeeSection 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).Saint-Andre & Melnikov       Standards Track                   [Page 15]

RFC 8265             PRECIS: Usernames and Passwords        October 2017   +------------------------------------+------------------------------+   | # | 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>          |   +------------------------------------+------------------------------+                   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 PRECISSaint-Andre & Melnikov       Standards Track                   [Page 16]

RFC 8265             PRECIS: Usernames and Passwords        October 2017   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.  SeeSection 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 (seeSection 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 PRECISSaint-Andre & Melnikov       Standards Track                   [Page 17]

RFC 8265 PRECIS: Usernames and Passwords October 2017 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" (fromAppendix 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 underSection 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 codeSaint-Andre & Melnikov Standards Track [Page 18]

RFC 8265 PRECIS: Usernames and Passwords October 2017 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" (fromAppendix 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 underSection 9.13 of [RFC8264]. In practice, this change will probably have no effect on comparison, but user-oriented softwareSaint-Andre & Melnikov Standards Track [Page 19]

RFC 8265             PRECIS: Usernames and Passwords        October 2017      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 inRFC 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.Saint-Andre & Melnikov       Standards Track                   [Page 20]

RFC 8265             PRECIS: Usernames and Passwords        October 2017   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 inRFC 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.Saint-Andre & Melnikov       Standards Track                   [Page 21]

RFC 8265 PRECIS: Usernames and Passwords October 20177.4. 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". BecauseRFC 7613 obsoletedRFC 4013, which registered the SASLprep profile of Stringprep, IANA previously marked that profile as "Not Current" and citedRFC 7613 as an additional reference. IANA has modified the profile so that the current document is now cited as the additional reference.8. Security Considerations8.1. 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 inSection 4.2.3. Instead, because the system performs cryptographic calculations to verify the password, it will prepare the password as defined inSection 4.2.1 and enforce the rules as defined inSection 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.Saint-Andre & Melnikov Standards Track [Page 22]

RFC 8265             PRECIS: Usernames and Passwords        October 20179.  References9.1.  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 inRFC2119 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/>.Saint-Andre & Melnikov       Standards Track                   [Page 23]

RFC 8265             PRECIS: Usernames and Passwords        October 20179.2.  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>.Saint-Andre & Melnikov       Standards Track                   [Page 24]

RFC 8265             PRECIS: Usernames and Passwords        October 2017   [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 fromRFC 7613   The following changes were made from [RFC7613].   o  Corrected the order of operations for the UsernameCaseMapped      profile to ensure consistency with [RFC8264].Saint-Andre & Melnikov       Standards Track                   [Page 25]

RFC 8265             PRECIS: Usernames and Passwords        October 2017   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.comSaint-Andre & Melnikov       Standards Track                   [Page 26]

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