| PRECIS | P. Saint-Andre |
| Internet-Draft | Cisco Systems, Inc. |
| Obsoletes: 4013 (if approved) | A. Melnikov |
| Intended status: Standards Track | Isode Ltd |
| Expires: February 05, 2014 | August 04, 2013 |
Preparation and Comparison of Internationalized Strings Representing Usernames and Passwords
draft-ietf-precis-saslprepbis-04
This document describes how to handle Unicode strings representing usernames and passwords. This profile is intended to be used by protocols that exchange or otherwise make use of 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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This Internet-Draft will expire on February 05, 2014.
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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]) 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]). 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 and comparing internationalized strings that represent usernames and passwords.
The methods specified in this document define a PRECIS profile as explained in the PRECIS framework specification [I-D.ietf-precis-framework]. This document assumes that all strings are comprised of characters from the Unicode character set [UNICODE], with special attention to characters outside the ASCII range [RFC20]. 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].
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 Username PRECIS profile defined in this document 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.)
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]), 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 either as an ordered sequence of "userparts" (where the complete username can consist of a single userpart or a space-separated sequence of userparts) or as a userpart@domainpart (where the domainpart is an IP literal, an IPv4 address, or a fully-qualified domain name).
The syntax for a username is defined as follows using the Augmented Backus-Naur Form (ABNF) [RFC5234].
username = userpart [1*(1*SP userpart)]
/ userpart '@' domainpart
userpart = 1*(idpoint)
;
; an "idpoint" is a UTF-8 encoded Unicode code point
; that conforms to the PRECIS "IdentifierClass"
;
domainpart = IP-literal / IPv4address / ifqdn
;
; the "IPv4address" and "IP-literal" rules are
; defined in RFC 3986, and the first-match-wins
; (a.k.a. "greedy") algorithm described in RFC 3986
; applies
;
; reuse of the IP-literal rule from RFC 3986 implies
; that IPv6 addresses are enclosed in square brackets
; (i.e., beginning with '[' and ending with ']')
;
ifqdn = 1*1023(domainpoint)
;
; a "domainpoint" is a UTF-8 encoded Unicode code
; point that conforms to RFC 5890
;
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.
Each userpart of a username MUST conform to the definition of the PRECIS IdentifierClass provided in [I-D.ietf-precis-framework], where the width mapping, additional mapping, case mapping, normalization, and directionality rules are as follows.
With regard to directionality, the "Bidi Rule" provided in [RFC5893] applies.
SASL mechanisms that directly re-use this profile 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, username comparisons, or generating a cryptographic salt from a username. In keeping with RFC4422, SASL mechanisms are not to apply this or any other profile to authorization identifiers.
Application protocols that use SASL (such as IMAP [RFC4616] and XMPP [RFC6120]) and that directly re-use this profile MUST specify whether 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. In keeping with RFC4422, SASL application protocols are not to apply this or any other profile to authentication identifiers.
Application protoocls that do not use SASL (such as HTTP authentication with the Basic and Digest schemes [RFC2617]) MUST specify whether and when case mapping is to be applied to authentication identifiers and authorization identifiers. Such application protocols SHOULD delay any case mapping to the last possible moment, such as when doing a lookup by username, username comparisons, or generating a cryptographic salt from a username.
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, since the username is not available to the server in plaintext form.
This document specifies that a password is a string of Unicode code points [UNICODE], encoded using UTF-8 [RFC3629], and conformant to the PRECIS FreeformClass.
The syntax for a password is defined as follows using the Augmented Backus-Naur Form (ABNF) [RFC5234].
password = 1*(freepoint)
;
; a "freepoint" is a UTF-8 encoded
; Unicode code point 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.
A password MUST NOT be zero bytes in length. This rule is to be enforced after any normalization and mapping of code points.
A password MUST conform to the definition of the PRECIS FreeformClass provided in [I-D.ietf-precis-framework], where the width mapping, additional mapping, case mapping, normalization, and directionality rules are as described below.
With regard to directionality, the "Bidi Rule" (defined in [RFC5893]) and similar rules are unnecessary and inapplicable to passwords, since 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. Furthermore, such rules are intended to minimize the possibility that the same string will be displayed differently on a system set for right-to-left display and a system set for left-to-right display; however, passwords are typically not displayed at all and are rarely meant to be interoperable across different systems in the way that non-secret strings like domain names and usernames are.
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 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:
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:
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 [UTR39] apply to the use of Unicode characters in usernames and passwords.
[Note to RFC Editor: please change XXXX to the number issued for this specification.]
The IANA shall add an entry to the PRECIS Usage Registry for reuse of the PRECIS IdentifierClass, as follows:
The IANA shall add an entry to the PRECIS Usage Registry for reuse of the PRECIS FreeformClass, as follows:
| [I-D.ietf-precis-framework] | Saint-Andre, P. and M. Blanchet, "Precis Framework: Handling Internationalized Strings in Protocols", Internet-Draft draft-ietf-precis-framework-09, July 2013. |
| [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 6.1", 2012. |
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, Matt Miller, Chris Newman, Yutaka OIWA, Pete Resnick, Andrew Sullivan, and Nico Williams (Nico in particular provided text that was used in Section 2.2). Thanks also to Yoshiro YONEYA and Takahiro NEMOTO for implementation feedback.
This document borrows some text from [RFC4013] and [RFC6120].