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Internet Engineering Task Force (IETF)
Request for Comments: 7613
Obsoletes: 4013
Category: Standards Track
ISSN: 2070-1721
P. Saint-Andre
&yet
A. Melnikov
Isode Ltd
August 2015

Preparation, Enforcement, and Comparison of Internationalized Strings

Representing Usernames and Passwords

Abstract

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

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7613.

Copyright Notice

Copyright © 2015 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................4
   2. Terminology .....................................................5
   3. Usernames .......................................................6
      3.1. Definition .................................................6
      3.2. UsernameCaseMapped Profile .................................7
           3.2.1. Preparation .........................................7
           3.2.2. Enforcement .........................................7
           3.2.3. Comparison ..........................................8
      3.3. UsernameCasePreserved Profile ..............................8
           3.3.1. Preparation .........................................8
           3.3.2. Enforcement .........................................8
           3.3.3. Comparison ..........................................9
      3.4. Case Mapping vs. Case Preservation .........................9
      3.5. Application-Layer Constructs ..............................10
      3.6. Examples ..................................................11
   4. Passwords ......................................................13
      4.1. Definition ................................................13
      4.2. OpaqueString Profile ......................................14
           4.2.1. Preparation ........................................14
           4.2.2. Enforcement ........................................14
           4.2.3. Comparison .........................................15
      4.3. Examples ..................................................15
   5. Use in Application Protocols ...................................16
   6. Migration ......................................................16
      6.1. Usernames .................................................17
      6.2. Passwords .................................................18
   7. IANA Considerations ............................................19
      7.1. UsernameCaseMapped Profile ................................19
      7.2. UsernameCasePreserved Profile .............................20
      7.3. OpaqueString Profile ......................................20
      7.4. Stringprep Profile ........................................21
   8. Security Considerations ........................................21
      8.1. Password/Passphrase Strength ..............................21
      8.2. Identifier Comparison .....................................21
      8.3. Reuse of PRECIS ...........................................21
      8.4. Reuse of Unicode ..........................................22
   9. References .....................................................22
      9.1. Normative References ......................................22
      9.2. Informative References ....................................23
   Appendix A. Differences from RFC 4013 .............................26
   Acknowledgements ..................................................27
   Authors' Addresses ................................................27

1. 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 [HTTP-BASIC-AUTH]) 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 [HTTP-DIGEST-AUTH]).

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

Profiles of the PRECIS framework enable software to handle Unicode characters outside the ASCII range in an automated way, so that such characters are treated carefully and consistently in application protocols. In large measure, these profiles are designed to protect application developers from the potentially negative consequences of supporting the full range of Unicode characters. For instance, in almost all application protocols it would be dangerous to treat the Unicode character SUPERSCRIPT ONE (U+00B9) as equivalent to DIGIT ONE (U+0031), because that would result in false positives during comparison, authentication, and authorization (e.g., an attacker could easy spoof an account "user1@example.com").

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

The methods defined here might be applicable wherever usernames or passwords are used. However, the methods are not intended for use in preparing strings that are not usernames (e.g., 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.

This document obsoletes RFC 4013 (the SASLprep profile of stringprep [RFC3454]) but can be used by technologies other than SASL [RFC4422], such as HTTP authentication as specified in [HTTP-BASIC-AUTH] and [HTTP-DIGEST-AUTH].

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 [XMPP-ADDR], which is intended to obsolete [RFC6122]). Non-coordinated updates to protocol implementations are discouraged because they can have a negative impact on interoperability and security.

2. Terminology

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

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

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

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

3. Usernames

3.1. Definition

This document specifies that a username is a string of Unicode code points [Unicode], encoded using UTF-8 [RFC3629], and structured as an ordered sequence of "userparts". A userpart is allowed to contain only code points that are in turn allowed by the PRECIS IdentifierClass defined in Section 4.2 of [RFC7564], 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*(idbyte)
                   ;
                   ; an "idbyte" is a byte used to represent a
                   ; UTF-8 encoded Unicode code point that can be
                   ; contained in a string that conforms to the
                   ; PRECIS IdentifierClass
                   ;

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

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

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.

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

3.2. UsernameCaseMapped Profile

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

3.2.1. Preparation

An entity that prepares a string according to this profile MUST first map fullwidth and halfwidth characters to their decomposition mappings (see Unicode Standard Annex #11 [UAX11]). This is necessary because the PRECIS "HasCompat" category specified in Section 9.17 of [RFC7564] would otherwise forbid fullwidth and halfwidth characters. After applying this width-mapping rule, the entity then MUST ensure that the string consists only of Unicode code points that conform to the PRECIS IdentifierClass defined in Section 4.2 of [RFC7564]. In addition, the entity then MUST encode the string as UTF-8 [RFC3629].

3.2.2. Enforcement

An entity that performs enforcement according to this profile MUST prepare a string as described in Section 3.2.1 and MUST also apply the rules specified below for the UsernameCaseMapped profile (these rules MUST be applied in the order shown):

  1. Width-Mapping Rule: Applied as part of preparation (see above).
  1. Additional Mapping Rule: There is no additional mapping rule.
  1. Case-Mapping Rule: Uppercase and titlecase characters MUST be mapped to their lowercase equivalents, preferably using Unicode Default Case Folding as defined in the Unicode Standard [Unicode] (at the time of this writing, the algorithm is specified in Chapter 3 of [Unicode7.0], but the chapter number might change in a future version of the Unicode Standard); see further discussion in Section 3.4.
  1. Normalization Rule: Unicode Normalization Form C (NFC) MUST be applied to all characters.
  1. Directionality Rule: Applications MUST apply the "Bidi Rule" defined in [RFC5893] to strings that contain right-to-left characters (i.e., each of the six conditions of the Bidi Rule must be satisfied).

3.2.3. Comparison

An entity that performs comparison of two strings according to this profile MUST prepare each string as specified in Section 3.2.1 and then enforce the rules specified in Section 3.2.2. The two strings are to be considered equivalent if they are an exact octet-for-octet match (sometimes called "bit-string identity").

3.3. UsernameCasePreserved Profile

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

3.3.1. Preparation

An entity that prepares a string according to this profile MUST first map fullwidth and halfwidth characters to their decomposition mappings (see Unicode Standard Annex #11 [UAX11]). This is necessary because the PRECIS "HasCompat" category specified in Section 9.17 of [RFC7564] would otherwise forbid fullwidth and halfwidth characters. After applying this width-mapping rule, the entity then MUST ensure that the string consists only of Unicode code points that conform to the PRECIS IdentifierClass defined in Section 4.2 of [RFC7564]. In addition, the entity then MUST encode the string as UTF-8 [RFC3629].

3.3.2. Enforcement

An entity that performs enforcement according to this profile MUST prepare a string as described in Section 3.3.1 and MUST also apply the rules specified below for the UsernameCasePreserved profile (these rules MUST be applied in the order shown):

  1. Width-Mapping Rule: Applied as part of preparation (see above).
  1. Additional Mapping Rule: There is no additional mapping rule.
  1. Case-Mapping Rule: Uppercase and titlecase characters MUST NOT be mapped to their lowercase equivalents; see further discussion in Section 3.4.
  1. Normalization Rule: Unicode Normalization Form C (NFC) MUST be applied to all characters.
  1. Directionality Rule: Applications MUST apply the "Bidi Rule" defined in [RFC5893] to strings that contain right-to-left characters (i.e., each of the six conditions of the Bidi Rule must be satisfied).

3.3.3. Comparison

An entity that performs comparison of two strings according to this profile MUST prepare each string as specified in Section 3.3.1 and then enforce the rules specified in Section 3.3.2. The two strings are to be considered equivalent if they are an exact octet-for-octet match (sometimes called "bit-string identity").

3.4. Case Mapping vs. Case Preservation

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

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

In particular:

  • SASL mechanisms that follow the recommendations in this document MUST specify whether and when case mapping is to be applied to authentication identifiers. SASL mechanisms SHOULD delay any case mapping to the last possible moment, such as when doing a lookup by username, 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 deployment policy). In keeping with [RFC4422], SASL mechanisms are not to apply this or any other profile to authorization identifiers, only to authentication identifiers.
  • 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 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.
  • Application protocols that do not use SASL (such as HTTP authentication with the HTTP Basic and Digest schemes as specified in [HTTP-BASIC-AUTH] and [HTTP-DIGEST-AUTH]) 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 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).

3.5. 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". Although such a construct is not a profile of the PRECIS IdentifierClass (because U+0020 SPACE is not allowed in the IdentifierClass), it can be created at the application layer because U+0020 SPACE can be used as a separator between instances of the PRECIS IdentifierClass (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 | <fu&#xDF;ball>       | The third character is LATIN    |
      |   |                      | SMALL LETTER SHARP S (U+00DF)   |
      +--------------------------+---------------------------------+
      | 4 | <&#x3C0;>            | A userpart of GREEK SMALL       |
      |   |                      | LETTER PI (U+03C0)              |
      +--------------------------+---------------------------------+
      | 5 | <&#x3A3;>            | A userpart of GREEK CAPITAL     |
      |   |                      | LETTER SIGMA (U+03A3)           |
      +--------------------------+---------------------------------+
      | 6 | <&#x3C3;>            | A userpart of GREEK SMALL       |
      |   |                      | LETTER SIGMA (U+03C3)           |
      +--------------------------+---------------------------------+
      | 7 | <&#x3C2;>            | A userpart of GREEK SMALL       |
      |   |                      | LETTER FINAL SIGMA (U+03C2)     |
      +--------------------------+---------------------------------+

Table 1: A Sample of Legal Userparts

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

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

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

Table 2: A Sample of Strings That Violate the Userpart Rule

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

4. Passwords

4.1. Definition

This document specifies that a password is a string of Unicode code points [Unicode], encoded using UTF-8 [RFC3629], and conformant to the OpaqueString profile (specified below) of the PRECIS FreeformClass defined in Section 4.3 of [RFC7564].

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

      password   = 1*(freebyte)
                   ;
                   ; a "freebyte" is a byte used to represent a
                   ; UTF-8 encoded Unicode code point that can be
                   ; contained in a string that conforms to the
                   ; PRECIS FreeformClass
                   ;

All code points and blocks not explicitly allowed in the PRECIS FreeformClass are disallowed; this includes private use characters, surrogate code points, and the other code points and blocks defined as "Prohibited Output" in Section 2.3 of RFC 4013 (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. 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 characters 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 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 [RFC7564]).

4.2.1. Preparation

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

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 characters MUST NOT be mapped to their decomposition mappings (see Unicode Standard Annex #11 [UAX11]).
  1. Additional Mapping Rule: Any instances of non-ASCII space MUST be mapped to ASCII space (U+0020); a non-ASCII space is any Unicode code point having a Unicode general category of "Zs" (with the exception of U+0020).
  1. Case-Mapping Rule: Uppercase and titlecase characters MUST NOT be mapped to their lowercase equivalents.
  1. Normalization Rule: Unicode Normalization Form C (NFC) MUST be applied to all characters.
  1. 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 range of characters that are allowed in a string and therefore reduce the amount of entropy that is possible in a password. Such rules are intended to minimize the possibility that the same string will be displayed differently on a layout system set for right-to-left display and a layout system set for left-to-right display; however, passwords are typically not displayed at all and are rarely meant to be interoperable across different layout systems in the way that non-secret strings like domain names and usernames are. Furthermore, it is perfectly acceptable for opaque strings other than passwords to be presented differently in different layout systems, as long as the presentation is consistent in any given layout system.

4.2.3. Comparison

An entity that performs comparison of two strings according to this profile MUST prepare each string as specified in Section 4.2.1 and then enforce the rules specified in Section 4.2.2. The two strings are to be considered equivalent if they are an exact octet-for-octet match (sometimes called "bit-string identity").

4.3. Examples

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

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

Table 3: A Sample of Legal Passwords

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

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

Table 4: A String That Violates the Password Rules

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 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 [RFC7564] 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 characters or character ranges, safeguards to mitigate the effects of visually similar characters, etc.), application-layer constructs (see Section 3.5), and related matters.

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

6. Migration

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

6.1. Usernames

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

  • SASLprep specified the use of Unicode Normalization Form KC (NFKC), whereas the UsernameCaseMapped and UsernameCasePreserved profiles employ Unicode Normalization Form C (NFC). In practice, this change is unlikely to cause significant problems, because NFKC provides methods for mapping Unicode code points with compatibility equivalents to those equivalents, whereas the PRECIS IdentifierClass entirely disallows Unicode code points with compatibility equivalents (i.e., during comparison, NFKC is more "aggressive" about finding matches than NFC). A few examples might suffice to indicate the nature of the problem:
  1. LATIN SMALL LETTER LONG S (U+017F) is compatibility equivalent to LATIN SMALL LETTER S (U+0073).
  1. ROMAN NUMERAL FOUR (U+2163) is compatibility equivalent to LATIN CAPITAL LETTER I (U+0049) and LATIN CAPITAL LETTER V (U+0056).
  1. LATIN SMALL LIGATURE FI (U+FB01) is compatibility equivalent to LATIN SMALL LETTER F (U+0066) and 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 (e.g., HENRYIV with the last two characters being LATIN CAPITAL LETTER I (U+0049) and LATIN CAPITAL LETTER V (U+0056) vs. "HENRYIV" with the last character being 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.

  • SASLprep mapped the "characters commonly mapped to nothing" from Appendix B.1 of [RFC3454]) to nothing, whereas the PRECIS IdentifierClass entirely disallows most of these characters, which correspond to the code points from the PRECIS "M" category defined under Section 9.13 of [RFC7564] (with the exception of MONGOLIAN TODO SOFT HYPHEN (U+1806), which was "commonly mapped to nothing" in Unicode 3.2 but at the time of this writing does not have a derived property of Default_Ignorable_Code_Point in Unicode 7.0). For migration purposes, the operator might want to remove from usernames any code points contained in the PRECIS "M" category (e.g., 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.
  • SASLprep allowed uppercase and titlecase characters, whereas the UsernameCaseMapped profile maps uppercase and titlecase characters to their lowercase equivalents (by contrast, the UsernameCasePreserved profile matches SASLprep in this regard). For migration purposes, the operator can 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 RFC 4013 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:

  • SASLprep specified the use of Unicode Normalization Form KC (NFKC), whereas the OpaqueString profile employs Unicode Normalization Form C (NFC). Because NFKC is more aggressive about finding matches than NFC, in practice this change is unlikely to cause significant problems and indeed has the security benefit of probably resulting in fewer false positives when comparing passwords. A few examples might suffice to indicate the nature of the problem:
  1. LATIN SMALL LETTER LONG S (U+017F) is compatibility equivalent to LATIN SMALL LETTER S (U+0073).
  1. ROMAN NUMERAL FOUR (U+2163) is compatibility equivalent to LATIN CAPITAL LETTER I (U+0049) and LATIN CAPITAL LETTER V (U+0056).
  1. LATIN SMALL LIGATURE FI (U+FB01) is compatibility equivalent to LATIN SMALL LETTER F (U+0066) and 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.

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

7. IANA Considerations

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 characters to their
      decomposition mappings.
   
   Additional Mapping Rule:  None.
   
   Case-Mapping Rule:  Map uppercase and titlecase characters to
      lowercase.
   
   Normalization Rule:  NFC.
   Directionality Rule:  The "Bidi Rule" defined in RFC 5893 applies.
   
   Enforcement:  To be defined by security or application protocols that
      use this profile.
   
   Specification:  RFC 7613 (this document), Section 3.2.

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.
   
   Width-Mapping Rule:  Map fullwidth and halfwidth characters to their
      decomposition mappings.
   
   Additional Mapping Rule:  None.
   
   Case-Mapping Rule:  None.
   
   Normalization Rule:  NFC.
   
   Directionality Rule:  The "Bidi Rule" defined in RFC 5893 applies.
   
   Enforcement:  To be defined by security or application protocols that
      use this profile.
   
   Specification:  RFC 7613 (this document), Section 3.3.

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 characters to ASCII
      space.
   
   Case-Mapping Rule:  None.
   
   Normalization Rule:  NFC.
   
   Directionality Rule:  None.
   
   Enforcement:  To be defined by security or application protocols that
      use this profile.
   
   Specification:  RFC 7613 (this document), Section 4.2.

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 this document obsoletes RFC 4013, which registered the SASLprep profile of stringprep, IANA has marked that profile as "Not Current" and cited this document as an 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. Identifier Comparison

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

8.3. Reuse of PRECIS

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

8.4. Reuse of Unicode

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

9. References

9.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://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, <http://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,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc6365>.
   
   [RFC7564]  Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
              Preparation, Enforcement, and Comparison of
              Internationalized Strings in Application Protocols",
              RFC 7564, DOI 10.17487/RFC7564, May 2015,
              <http://www.rfc-editor.org/info/rfc7564>.
   
   [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/>.

[Unicode7.0] 

              The Unicode Consortium, "The Unicode Standard,
              Version 7.0.0", (Mountain View, CA: The Unicode
              Consortium, 2014 ISBN 978-1-936213-09-2),
              <http://www.unicode.org/versions/Unicode7.0.0/>.

9.2. Informative References

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

[HTTP-BASIC-AUTH] 

              Reschke, J., "The 'Basic' HTTP Authentication Scheme",
              Work in Progress, draft-ietf-httpauth-basicauth-update-07,
              February 2015.

[HTTP-DIGEST-AUTH]

Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP

Digest Access Authentication", Work in Progress,

draft-ietf-httpauth-digest-19, April 2015.

   [RFC20]    Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <http://www.rfc-editor.org/info/rfc20>.
   
   [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              DOI 10.17487/RFC3454, December 2002,
              <http://www.rfc-editor.org/info/rfc3454>.
   
   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
              VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501,
              March 2003, <http://www.rfc-editor.org/info/rfc3501>.
   
   [RFC4013]  Zeilenga, K., "SASLprep: Stringprep Profile for User Names
              and Passwords", RFC 4013, DOI 10.17487/RFC4013,
              February 2005, <http://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,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc5802>.
   
   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891,
              DOI 10.17487/RFC5891, August 2010,
              <http://www.rfc-editor.org/info/rfc5891>.
   
   [RFC5893]  Alvestrand, H., Ed., and C. Karp, "Right-to-Left Scripts
              for Internationalized Domain Names for Applications
              (IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010,
              <http://www.rfc-editor.org/info/rfc5893>.
   
   [RFC5894]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Background, Explanation, and
              Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010,
              <http://www.rfc-editor.org/info/rfc5894>.
   
   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
              March 2011, <http://www.rfc-editor.org/info/rfc6120>.
   
   [RFC6122]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Address Format", RFC 6122,
              DOI 10.17487/RFC6122, March 2011,
              <http://www.rfc-editor.org/info/rfc6122>.
   
   [RFC6943]  Thaler, D., Ed., "Issues in Identifier Comparison for
              Security Purposes", RFC 6943, DOI 10.17487/RFC6943,
              May 2013, <http://www.rfc-editor.org/info/rfc6943>.
   
   [RFC7542]  DeKok, A., "The Network Access Identifier", RFC 7542,
              DOI 10.17487/RFC7542, May 2015,
              <http://www.rfc-editor.org/info/rfc7542>.
   
   [UTS39]    Unicode Technical Standard #39, "Unicode Security
              Mechanisms", edited by Mark Davis and Michel Suignard,
              <http://unicode.org/reports/tr39/>.

[XMPP-ADDR] 

              Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Address Format", Work in Progress,
              draft-ietf-xmpp-6122bis-24, June 2015.

Appendix A. Differences from RFC 4013

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

The following substantive modifications were made from RFC 4013.

  • A single SASLprep algorithm was replaced by three separate algorithms: one for usernames with case mapping, one for usernames with case preservation, and one for passwords.
  • The new preparation algorithms use PRECIS instead of a stringprep profile. The new algorithms work independently of Unicode versions.
  • As recommended in the PRECIS framework, changed the Unicode normalization form from NFKC to NFC.
  • Some Unicode code points that were mapped to nothing in RFC 4013 are simply disallowed by PRECIS.

Acknowledgements

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

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

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

Benoit Claise and Stephen Farrell provided helpful input during IESG review.

Thanks to Matt Miller as document shepherd, Marc Blanchet and Yoshiro YONEYA as working group chairs, and Pete Resnick and Barry Leiba as area directors.

Peter Saint-Andre wishes to acknowledge Cisco Systems, Inc., for employing him during his work on earlier draft versions of this document.

Authors' Addresses

Peter Saint-Andre
&yet 

   Email: peter@andyet.com
   URI:   https://andyet.com/
   
   Alexey Melnikov
   Isode Ltd
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX
   United Kingdom
   
   Email: Alexey.Melnikov@isode.com