| PRECIS | P. Saint-Andre |
| Internet-Draft | &yet |
| Obsoletes: 4013 (if approved) | A. Melnikov |
| Intended status: Standards Track | Isode Ltd |
| Expires: June 26, 2015 | December 23, 2014 |
Preparation, Enforcement, and Comparison of Internationalized Strings Representing Usernames and Passwords
draft-ietf-precis-saslprepbis-13
This document describes methods for handling Unicode strings representing usernames and passwords. This document obsoletes RFC 4013.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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Usernames and passwords are widely used for authentication and authorization on the Internet, either directly when provided in plaintext (as in the SASL PLAIN mechanism [RFC4616] or the HTTP Basic scheme [RFC2617] / [I-D.ietf-httpauth-basicauth-update]) or indirectly when provided as the input to a cryptographic algorithm such as a hash function (as in the SASL SCRAM mechanism [RFC5802] or the HTTP Digest scheme [RFC2617] / [I-D.ietf-httpauth-digest]).
To increase the likelihood that the input and comparison of usernames and passwords will work in ways that make sense for typical users throughout the world, this document defines rules for preparing, enforcing, and comparing internationalized strings that represent usernames and passwords. Such strings consist of characters from the Unicode character set [UNICODE], with special attention to characters outside the ASCII range [RFC20]. The rules for handling such strings are specified through profiles of the string classes defined in the PRECIS framework specification [I-D.ietf-precis-framework].
Profiles of the PRECIS framework enable software to handle Unicode characters outside the ASCII range in an automated way, so that such characters are treated carefully and consistently in application protocols. In large measure, these profiles are designed to protect application developers from the potentially negative consequences of supporting the full range of Unicode characters. For instance, in almost all application protocols it would be dangerous to treat the Unicode character SUPERSCRIPT ONE (U+0089) as equivalent to DIGIT ONE (U+0031), since that would result in false positives during comparison, authentication, and authorization (e.g., an attacker could easy spoof an account "user1@example.com").
Whereas a naive use of Unicode would make such attacks trivially easy, the PRECIS profile defined here for usernames generally protects applications from inadvertently causing such problems. (Similar considerations apply to passwords, although here it is desirable to support a wider range of characters so as to maximize entropy during 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., email addresses and LDAP distinguished names), nor in cases where identifiers or secrets are not strings (e.g., keys and certificates) or require specialized handling.
This document obsoletes RFC 4013 (the "SASLprep" profile of stringprep [RFC3454]) but can be used by technologies other than the Simple Authentication and Security Layer (SASL) [RFC4422], such as HTTP authentication [RFC2617] / [I-D.ietf-httpauth-basicauth-update] / [I-D.ietf-httpauth-digest].
Many important terms used in this document are defined in [I-D.ietf-precis-framework], [RFC5890], [RFC6365], and [UNICODE]. The term "non-ASCII space" refers to any Unicode code point having a general category of "Zs", with the exception of U+0020 (here called "ASCII space").
As used here, the term "password" is not literally limited to a word; i.e., a password could be a passphrase consisting of more than one word, perhaps separated by spaces or other such 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 [RFC2617] / [I-D.ietf-httpauth-basicauth-update] / [I-D.ietf-httpauth-digest]), whether or not they use SASL. Note well that the exact form of a username in any particular SASL mechanism or application technology is a matter for implementation and deployment, and that a username does not necessarily map to any particular application identifier (such as the localpart of an email address).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
This document specifies that a username is a string of Unicode code points [UNICODE], encoded using UTF-8 [RFC3629], and structured as an ordered sequence of "userparts" (where the complete username can consist of a single userpart or a space-separated sequence of userparts).
The syntax for a username is defined as follows using the Augmented Backus-Naur Form (ABNF) [RFC5234].
username = userpart *(1*SP userpart)
userpart = 1*(idbyte)
;
; an "idbyte" is a byte used to represent a
; UTF-8 encoded Unicode code point that can be
; contained in a string that conforms to the
; PRECIS "IdentifierClass"
;
All code points and blocks not explicitly allowed in the PRECIS IdentifierClass are disallowed; this includes private use characters, surrogate code points, and the other code points and blocks that were defined as "Prohibited Output" in [RFC4013]. In addition, common constructions such as "user@example.com" are allowed as usernames under this specification, as they were under [RFC4013].
A username MUST NOT be zero bytes in length. This rule is to be enforced after any normalization and mapping of code points.
In protocols that provide usernames as input to a cryptographic algorithm such as a hash function, the client will need to perform proper preparation of the username before applying the algorithm.
This specification defines two profiles for usernames: one that performs case mapping and one that performs case preservation (see further discussion under Section 3.4).
The definition of the UsernameCaseMapped profile of the IdentifierClass is provided in the following sections, including detailed information about preparation, enforcement, and comparison (on the distinction between these actions, refer to [I-D.ietf-precis-framework]).
An entity that prepares a string according to this profile MUST ensure that the string consists only of Unicode code points that conform to the "IdentifierClass" base string class defined in [I-D.ietf-precis-framework]. In addition, the string MUST be encoded as UTF-8 [RFC3629].
An entity that performs enforcement according to this profile MUST prepare a string as described in the previous section and MUST also apply the rules specified below for the UsernameCaseMapped profile (these rules MUST be applied in the order shown).
An entity that performs comparison of two strings according to this profile MUST prepare each string and enforce the rules specified in the previous two sections. The two strings are to be considered equivalent if they are an exact octet-for-octet match (sometimes called "bit-string identity").
The definition of the UsernameCasePreserved profile of the IdentifierClass is provided in the following sections, including detailed information about preparation, enforcement, and comparison (on the distinction between these actions, refer to [I-D.ietf-precis-framework]).
An entity that prepares a string according to this profile MUST ensure that the string consists only of Unicode code points that conform to the "IdentifierClass" base string class defined in [I-D.ietf-precis-framework]. In addition, the string MUST be encoded as UTF-8 [RFC3629].
An entity that performs enforcement according to this profile MUST prepare a string as described in the previous section and MUST also apply the rules specified below for the UsernameCasePreserved profile (these rules MUST be applied in the order shown).
An entity that performs comparison of two strings according to this profile MUST prepare each string and enforce the rules specified in the previous two sections. The two strings are to be considered equivalent if they are an exact octet-for-octet match (sometimes called "bit-string identity").
In order to accomodate the widest range of username constructs in applications, this document defines two username profiles: UsernameCaseMapped and UsernameCasePreserved.
Case mapping is a matter for the application protocol, protocol implementation, or end deployment. In general, this document suggests that it is preferable to apply the UsernameCaseMapped profile and therefore perform case mapping, since not doing so can lead to false positives during authentication and authorization (as described in [RFC6943]) and can result in confusion among end users given the prevalence of case mapping in many existing protocols and applications. However, there can be good reasons to apply the UsernameCasePreserved profile and thus not perform case mapping, such as backward compatibility with deployed infrastructure.
In particular:
If the specification for a SASL mechanism, SASL application protocol, or non-SASL application protocol uses the UsernameCaseMapped profile, it MUST clearly describe whether case mapping is to be applied at the level of the protocol itself, implementations thereof, or service deployments (all of these approaches can be legitimate depending on the application in question).
Both the UsernameCaseMapped and UsernameCasePreserved profiles allow an application protocol, implementation, or deployment to create application-layer constructs such as "user@domain" or "Firstname Middlename Lastname". One example of the former is the Network Access Identifier specified in [I-D.ietf-radext-nai]. (Such constructs are possible because the PRECIS IdentifierClass allows any ASCII7 character, because spaces can be used to separate userpart instances, and because domain names as specified in [RFC5890] and [RFC5892] are a subset of the PRECIS IdentifierClass.)
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).
Table 1: A sample of legal userparts
+--------------------------+---------------------------------+
| # | Userpart | Notes |
+--------------------------+---------------------------------+
| 1 | <juliet@example.com> | The at-sign is allowed in the |
| | | PRECIS IdentifierClass |
+--------------------------+---------------------------------+
| 2 | <fussball> | |
+--------------------------+---------------------------------+
| 3 | <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) |
+--------------------------+---------------------------------+
Several points are worth noting. Regarding examples 2 and 3: although in German the character esszett (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.
Table 2: A sample of strings that violate the userpart rule
+--------------------------+---------------------------------+
| # | Non-Userpart string | Notes |
+--------------------------+---------------------------------+
| 8 | <foo bar> | Space (U+0020) is disallowed in |
| | | the userpart |
+--------------------------+---------------------------------+
| 9 | <> | Zero-length userpart |
+--------------------------+---------------------------------+
| 10| <henryⅣ> | The sixth character is ROMAN |
| | | NUMERAL FOUR (U+2163) |
+--------------------------+---------------------------------+
| 11| <♚> | A localpart of BLACK CHESS KING |
| | | (U+265A) |
+--------------------------+---------------------------------+
Here again, several points are worth noting. Regarding example 10, the Unicode character ROMAN NUMERAL FOUR (U+2163) has a compatibility equivalent of the string formed of LATIN CAPITAL LETTER I (U+0049) and LATIN CAPITAL LETTER V (U+0056), but characters with compatibility equivalents are not allowed in the PRECIS IdentiferClass. Regarding example 11: symbol characters such as BLACK CHESS KING (U+265A) are not allowed in the PRECIS IdentifierClass.
This document specifies that a password is a string of Unicode code points [UNICODE], encoded using UTF-8 [RFC3629], and conformant to OpaqueString profile of the PRECIS FreeformClass specified below.
The syntax for a password is defined as follows using the Augmented Backus-Naur Form (ABNF) [RFC5234].
password = 1*(freebyte)
;
; a "freebyte" is a byte used to represent a
; UTF-8 encoded Unicode code point that can be
; contained in a string that conforms to the
; PRECIS "FreefromClass"
;
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.
A password 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 passwords as input to a cryptographic algorithm such as a hash function, the client will need to perform proper preparation of the password before applying the algorithm, since the password is not available to the server in plaintext form.
The definition of the OpaqueString profile is provided in the following sections, including detailed information about preparation, enforcement, and comparison (on the distinction between these actions, refer to [I-D.ietf-precis-framework]).
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 [I-D.ietf-precis-framework]. In addition, the string MUST be encoded as UTF-8 [RFC3629].
An entity that performs enforcement according to this profile MUST prepare a string as described in the previous section and MUST also apply the rules specified below (these rules MUST be applied in the order shown).
An entity that performs comparison of two strings according to this profile MUST prepare each string and enforce the rules specified in the previous two sections. The two strings are to be considered equivalent if they are an exact octet-for-octet match (sometimes called "bit-string identity").
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).
Table 3: A sample of legal passwords
+------------------------------------+------------------------------+
| # | Password | Notes |
+------------------------------------+------------------------------+
| 12| <correct horse battery staple> | ASCII space is allowed |
+------------------------------------+------------------------------+
| 13| <Correct Horse Battery Staple> | Different from example 12 |
+------------------------------------+------------------------------+
| 14| <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) |
+------------------------------------+------------------------------+
The following examples illustrate strings that are not valid passwords because they violate the format defined above.
Table 4: A sample of strings that violate the password rules
+------------------------------------+------------------------------+
| # | Password | Notes |
+------------------------------------+------------------------------+
| 16| <foo bar> | Non-ASCII space (here, OGHAM |
| | | SPACE MARK, U+1680) is not |
| | | allowed |
+------------------------------------+------------------------------+
| 17| <my cat is a 	by> | Controls are disallowed |
+------------------------------------+------------------------------+
This specification defines only the PRECIS-based rules for handling of strings conforming to the UsernameCaseMapped and UsernameCasePreserved profiles of the PRECIS IdentifierClass, and strings conforming to the OpaqueString profile of the PRECIS FreeformClass. It is the responsibility of an application protocol to specify the protocol slots in which such strings can appear, the entities that are expected to enforce the rules governing such strings, and when in protocol processing or interface handling the rules need to be enforced. See Section 6 of [I-D.ietf-precis-framework] for guidelines about using PRECIS profiles in applications.
Above and beyond the PRECIS-based rules specified here, application protocols can also define application-specific rules governing such strings (rules regarding minimum or maximum length, further restrictions on allowable characters or character ranges, safeguards to mitigate the effects of visually similar characters, etc.), application-layer constructs (see Section 3.5), and related matters.
Some PRECIS profile definitions encourage entities that enforce the rules to be liberal in what they accept. However, for usernames and passwords such a policy can be problematic since it can lead to false positives. An in-depth discussion can be found in "Issues in Identifier Comparison for Security Purposes" [RFC6943].
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.
Deployments that currently use SASLprep for handling usernames might need to scrub existing data when migrating to use of the rules defined in this specification. In particular:
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 usernames, for migration purposes deployments might want to search their database of usernames for Unicode code points with compatibility equivalents and map those code points to their compatibility equivalents.
Depending on local service policy, migration from RFC 4013 to this specification might not involve any scrubbing of data (since passwords might not be stored in the clear anyway); however, service providers need to be aware of possible issues that might arise during migration. In particular:
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.
The IANA shall add the following entries to the PRECIS Profiles Registry.
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.
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].
The security considerations described in [I-D.ietf-precis-framework] apply to the "IdentifierClass" and "FreeformClass" base string classes used in this document for usernames and passwords, respectively.
The security considerations described in [UTS39] apply to the use of Unicode characters in usernames and passwords.
| [I-D.ietf-precis-framework] | Saint-Andre, P. and M. Blanchet, "Precis Framework: Handling Internationalized Strings in Protocols", Internet-Draft draft-ietf-precis-framework-21, December 2014. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC3629] | Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. |
| [RFC5234] | Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. |
| [UNICODE] | The Unicode Consortium, "The Unicode Standard, Version 6.3", 2013. |
This document builds upon the PRECIS framework defined in [I-D.ietf-precis-framework], which differs fundamentally from the stringprep technology [RFC3454] used in SASLprep [RFC4013]. The primary difference is that stringprep profiles allowed all characters except those which were explicitly disallowed, whereas PRECIS profiles disallow all characters except those which are explicitly allowed (this "inclusion model" was originally used for internationalized domain names in [RFC5891]; see [RFC5894] for further discussion). It is important to keep this distinction in mind when comparing the technology defined in this document to SASLprep [RFC4013].
The following substantive modifications were made from RFC 4013.
The following individuals provided helpful feedback on this document: Marc Blanchet, Alan DeKok, Joe Hildebrand, Jeffrey Hutzelman, Simon Josefsson, Jonathan Lennox, James Manger, Matt Miller, Chris Newman, Yutaka OIWA, Pete Resnick, Andrew Sullivan, and Nico Williams. Nico in particular deserves special recognition for providing text that was used in Section 3.4. Thanks also to Yoshiro YONEYA and Takahiro NEMOTO for implementation feedback.
This document borrows some text from [RFC4013] and [RFC6120].
Peter Saint-Andre wishes to acknowledge Cisco Systems, Inc., for employing him during his work on earlier versions of this document.