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2 Precis P. Saint-Andre
3 Internet-Draft Cisco Systems, Inc.
4 Obsoletes: 4013 (if approved) A. Melnikov
5 Intended status: Standards Track Isode Ltd
6 Expires: March 27, 2013 September 23, 2012
8 Preparation and Comparison of Internationalized Strings Representing
9 Simple User Names and Passwords
10 draft-melnikov-precis-saslprepbis-04
12 Abstract
14 This document describes how to handle Unicode strings representing
15 simple user names and passwords, primarily for purposes of
16 comparison. This profile is intended to be used by Simple
17 Authentication and Security Layer (SASL) mechanisms (such as PLAIN
18 and SCRAM-SHA-1), as well as other protocols that exchange simple
19 user names or passwords. This document obsoletes RFC 4013.
21 Status of this Memo
23 This Internet-Draft is submitted in full conformance with the
24 provisions of BCP 78 and BCP 79.
26 Internet-Drafts are working documents of the Internet Engineering
27 Task Force (IETF). Note that other groups may also distribute
28 working documents as Internet-Drafts. The list of current Internet-
29 Drafts is at http://datatracker.ietf.org/drafts/current/.
31 Internet-Drafts are draft documents valid for a maximum of six months
32 and may be updated, replaced, or obsoleted by other documents at any
33 time. It is inappropriate to use Internet-Drafts as reference
34 material or to cite them other than as "work in progress."
36 This Internet-Draft will expire on March 27, 2013.
38 Copyright Notice
40 Copyright (c) 2012 IETF Trust and the persons identified as the
41 document authors. All rights reserved.
43 This document is subject to BCP 78 and the IETF Trust's Legal
44 Provisions Relating to IETF Documents
45 (http://trustee.ietf.org/license-info) in effect on the date of
46 publication of this document. Please review these documents
47 carefully, as they describe your rights and restrictions with respect
48 to this document. Code Components extracted from this document must
49 include Simplified BSD License text as described in Section 4.e of
50 the Trust Legal Provisions and are provided without warranty as
51 described in the Simplified BSD License.
53 Table of Contents
55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
56 1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 3
57 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
58 2. Simple User Names . . . . . . . . . . . . . . . . . . . . . . 4
59 2.1. Definition . . . . . . . . . . . . . . . . . . . . . . . . 4
60 2.2. Preparation . . . . . . . . . . . . . . . . . . . . . . . 4
61 3. Passwords . . . . . . . . . . . . . . . . . . . . . . . . . . 5
62 3.1. Definition . . . . . . . . . . . . . . . . . . . . . . . . 5
63 3.2. Preparation . . . . . . . . . . . . . . . . . . . . . . . 5
64 4. Migration . . . . . . . . . . . . . . . . . . . . . . . . . . 6
65 4.1. User Names . . . . . . . . . . . . . . . . . . . . . . . . 6
66 4.2. Passwords . . . . . . . . . . . . . . . . . . . . . . . . 7
67 5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
68 5.1. Password/Passphrase Strength . . . . . . . . . . . . . . . 8
69 5.2. Reuse of PRECIS . . . . . . . . . . . . . . . . . . . . . 8
70 5.3. Reuse of Unicode . . . . . . . . . . . . . . . . . . . . . 9
71 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
72 6.1. Use of NameClass . . . . . . . . . . . . . . . . . . . . . 9
73 6.2. Use of FreeClass . . . . . . . . . . . . . . . . . . . . . 9
74 7. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 9
75 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
76 8.1. Normative References . . . . . . . . . . . . . . . . . . . 10
77 8.2. Informative References . . . . . . . . . . . . . . . . . . 10
78 Appendix A. Differences from RFC 4013 . . . . . . . . . . . . . . 11
79 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 11
80 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
82 1. Introduction
84 1.1. Overview
86 User names and passwords are used pervasively in authentication and
87 authorization on the Internet. To increase the likelihood that the
88 input and comparison of user names and passwords will work in ways
89 that make sense for typical users throughout the world, this document
90 defines rules for preparing and comparing internationalized strings
91 that represent simple user names and passwords.
93 The algorithms defined in this document assume that all strings are
94 comprised of characters from the Unicode character set [UNICODE].
96 The algorithms are designed for use in Simple Authentication and
97 Security Layer (SASL) [RFC4422] mechanisms, such as PLAIN [RFC4616]
98 and SCRAM-SHA-1 [RFC5802]. However, they might be applicable
99 wherever simple user names or passwords are used. This profile is
100 not intended for use in preparing strings that are not simple user
101 names (e.g., email addresses, DNS domain names, LDAP distinguished
102 names), nor in cases where identifiers or secrets are not strings
103 (e.g., keys or certificates) or require different handling (e.g.,
104 case folding).
106 This document builds upon the PRECIS framework defined in
107 [FRAMEWORK], which differs fundamentally from the stringprep
108 technology [RFC3454] used in SASLprep [RFC4013]. The primary
109 difference is that stringprep profiles allowed all characters except
110 those which were explicitly disallowed, whereas PRECIS profiles
111 disallow all characters except those which are explicitly allowed
112 (this "inclusion model" was originally used for internationalized
113 domain names in [RFC5891]; see [RFC5894] for further discussion). It
114 is important to keep this distinction in mind when comparing the
115 technology defined in this document to SASLprep [RFC4013].
117 This document obsoletes RFC 4013.
119 1.2. Terminology
121 Many important terms used in this document are defined in
122 [FRAMEWORK], [RFC4422], [RFC5890], [RFC6365], and [UNICODE]. The
123 term "non-ASCII space" refers to any Unicode code point with a
124 general category of "Zs", with the exception of U+0020 (here called
125 "ASCII space").
127 As used here, the term "password" is not literally limited to a word;
128 i.e., a password could be a passphrase consisting of more than one
129 word, perhaps separated by spaces or other such characters.
131 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
132 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
133 "OPTIONAL" in this document are to be interpreted as described in
134 [RFC2119].
136 2. Simple User Names
138 2.1. Definition
140 Some SASL mechanisms (e.g., CRAM-MD5, DIGEST-MD5, and SCRAM) specify
141 that the authentication identity used in the context of such
142 mechanisms is a "simple user name" (see Section 2 of [RFC4422] as
143 well as [RFC4013]). However, the exact form of a simple user name in
144 any particular mechanism or deployment thereof is a local matter, and
145 a simple user name does not necessarily map to an application
146 identifier such as the localpart of an email address.
148 For purposes of preparation and comparison of authentication
149 identities, this document specifies that a simple user name is a
150 string of Unicode code points [UNICODE], encoded using UTF-8
151 [RFC3629], and structured as an ordered sequence of "simpleparts"
152 (where the complete simple user name can consist of a single
153 simplepart or a space-separated sequence of simpleparts).
155 Therefore the syntax for a simple user name is defined as follows
156 using the Augmented Backus-Naur Form (ABNF) as specified in
157 [RFC5234].
159 simpleusername = simplepart [1*(1*SP simplepart)]
160 simplepart = 1*(namepoint)
161 ;
162 ; a "namepoint" is a UTF-8 encoded
163 ; Unicode code point that conforms to
164 ; the "NameClass" string class defined
165 ; in draft-ietf-precis-framework
166 ;
168 Note well that all code points and blocks not explicitly allowed in
169 the PRECIS NameClass are disallowed; this includes private use
170 characters, surrogate code points, and the other code points and
171 blocks defined as "Prohibited Output" in Section 2.3 of RFC 4013.
173 2.2. Preparation
175 A simple user name MUST NOT be zero bytes in length. This rule is to
176 be enforced after any normalization and mapping of code points.
178 Each simplepart of a simple user name MUST conform to the definition
179 of the PRECIS NameClass provided in [FRAMEWORK], where the
180 normalization, casemapping, and directionality rules are as described
181 below.
183 1. Unicode Normalization Form C (NFC) MUST be applied to all
184 characters.
186 2. Uppercase and titlecase characters MUST be mapped to their
187 lowercase equivalents.
189 3. Additional mappings MAY be applied, such as those defined in
190 [I-D.yoneya-precis-mappings].
192 With regard to directionality, the "Bidi Rule" provided in [RFC5893]
193 applies.
195 3. Passwords
197 3.1. Definition
199 For purposes of preparation and comparison of passwords, this
200 document specifies that a password is a string of Unicode code points
201 [UNICODE], encoded using UTF-8 [RFC3629], and conformant to the
202 PRECIS FreeClass.
204 Therefore the syntax for a password is defined as follows using the
205 Augmented Backus-Naur Form (ABNF) as specified in [RFC5234].
207 password = 1*(freepoint)
208 ;
209 ; a "freepoint" is a UTF-8 encoded
210 ; Unicode code point that conforms to
211 ; the "FreeClass" string class defined
212 ; in draft-ietf-precis-framework
213 ;
215 Note well that all code points and blocks not explicitly allowed in
216 the PRECIS FreeClass are disallowed; this includes private use
217 characters, surrogate code points, and the other code points and
218 blocks defined as "Prohibited Output" in Section 2.3 of RFC 4013.
220 3.2. Preparation
222 A password MUST NOT be zero bytes in length. This rule is to be
223 enforced after any normalization and mapping of code points.
225 A password MUST be treated as follows, where the operations specified
226 MUST be completed in the order shown:
228 1. Apply Unicode Normalization Form C (NFC) to all characters.
230 2. Map any instances of non-ASCII space to ASCII space (U+0020).
232 3. Ensure that the resulting string conforms to the definition of
233 the PRECIS FreeClass.
235 With regard to directionality, the "Bidi Rule" (defined in [RFC5893])
236 and similar rules are unnecessary and inapplicable to passwords,
237 since they can reduce the range of characters that are allowed in a
238 string and therefore reduce the amount of entropy that is possible in
239 a password. Furthermore, such rules are intended to minimize the
240 possibility that the same string will be displayed differently on a
241 system set for right-to-left display and a system set for left-to-
242 right display; however, passwords are typically not displayed at all
243 and are rarely meant to be interoperable across different systems in
244 the way that non-secret strings like domain names and user names are.
246 4. Migration
248 The rules defined in this specification differ slightly from those
249 defined by the SASLprep specification [RFC4013]. The following
250 sections describe these differences, along with their implications
251 for migration, in more detail.
253 4.1. User Names
255 Deployments that currently use SASLprep for handling user names might
256 need to scrub existing data when migrating to use of the rules
257 defined in this specification. In particular:
259 o SASLprep specified the use of Unicode Normalization Form KC
260 (NFKC), whereas this usage of the PRECIS NameClass employs Unicode
261 Normalization Form C (NFC). In practice this change is unlikely
262 to cause significant problems, because NFKC provides methods for
263 mapping Unicode code points with compatibility equivalents to
264 those equivalents, whereas the PRECIS NameClass entirely disallows
265 Unicode code points with compatibility equivalents (i.e., during
266 comparison NFKC is more "aggressive" about finding matches than is
267 NFC). A few examples might suffice to indicate the nature of the
268 problem: (1) U+017F LATIN SMALL LETTER LONG S is compatibility
269 equivalent to U+0073 LATIN SMALL LETTER S (2) U+2163 ROMAN NUMERAL
270 FOUR is compatibility equivalent to U+0049 LATIN CAPITAL LETTER I
271 and U+0056 LATIN CAPITAL LETTER V (3) U+FB01 LATIN SMALL LIGATURE
272 FI is compatibility equivalent to U+0066 LATIN SMALL LETTER F and
273 U+0069 LATIN SMALL LETTER I. Under SASLprep, the use of NFKC also
274 handled the mapping of fullwidth and halfwidth code points to
275 their decomposition equivalents (see
276 [I-D.yoneya-precis-mappings]). Although it is expected that code
277 points with compatibility equivalents are rare in existing user
278 names, for migration purposes deployments might want to search
279 their database of user names for Unicode code points with
280 compatibility equivalents and map those code points to their
281 compatibility equivalents.
283 o SASLprep mapped non-ASCII spaces to ASCII space (U+0020), whereas
284 the PRECIS NameClass entirely disallows non-ASCII spaces. The
285 non-ASCII space characters are U+00A0 NO-BREAK SPACE, U+1680 OGHAM
286 SPACE MARK, U+180E MONGOLIAN VOWEL SEPARATOR, U+2000 EN QUAD
287 through U+200A HAIR SPACE, U+202F NARROW NO-BREAK SPACE, U+205F
288 MEDIUM MATHEMATICAL SPACE, and U+3000 IDEOGRAPHIC SPACE. For
289 migration purposes, deployments might want to convert non-ASCII
290 space characters to ASCII space in simple user names.
292 o SASLprep mapped the "characters commonly mapped to nothing" from
293 Appendix B.1 of [RFC3454]) to nothing, whereas the PRECIS
294 NameClass entirely disallows most of these characters, which
295 correspond to the code points from the "M" category defined under
296 Section 6.13 of [FRAMEWORK] (with the exception of U+1806
297 MONGOLIAN TODO SOFT HYPHEN, which was "commonly mapped to nothing"
298 in Unicode 3.2 but at the time of this writing does not have a
299 derived property of Default_Ignorable_Code_Point in Unicode 6.1).
300 For migration purposes, deployments might want to remove code
301 points contained in the PRECIS "M" category from simple user
302 names.
304 o SASLprep allowed uppercase and titlecase characters, whereas this
305 usage of the PRECIS NameClass maps uppercase and titlecase
306 characters to their lowercase equivalents. For migration
307 purposes, deployments can either convert uppercase and titlecase
308 characters to their lowercase equivalents in simple user names
309 (thus losing the case information) or preserve uppercase and
310 titlecase characters and ignore the case difference when comparing
311 simple user names.
313 4.2. Passwords
315 Depending on local service policy, migration from RFC 4013 to this
316 specification might not involve any scrubbing of data (since
317 passwords might not be stored in the clear anyway); however, service
318 providers need to be aware of possible issues that might arise during
319 migration. In particular:
321 o SASLprep specified the use of Unicode Normalization Form KC
322 (NFKC), whereas this usage of the PRECIS FreeClass employs Unicode
323 Normalization Form C (NFC). Because NFKC is more aggressive about
324 finding matches than NFC, in practice this change is unlikely to
325 cause significant problems and indeed has the security benefit of
326 probably resulting in fewer false positives when comparing
327 passwords. A few examples might suffice to indicate the nature of
328 the problem: (1) U+017F LATIN SMALL LETTER LONG S is compatibility
329 equivalent to U+0073 LATIN SMALL LETTER S (2) U+2163 ROMAN NUMERAL
330 FOUR is compatibility equivalent to U+0049 LATIN CAPITAL LETTER I
331 and U+0056 LATIN CAPITAL LETTER V (3) U+FB01 LATIN SMALL LIGATURE
332 FI is compatibility equivalent to U+0066 LATIN SMALL LETTER F and
333 U+0069 LATIN SMALL LETTER I. Under SASLprep, the use of NFKC also
334 handled the mapping of fullwidth and halfwidth code points to
335 their decomposition equivalents (see
336 [I-D.yoneya-precis-mappings]). Although it is expected that code
337 points with compatibility equivalents are rare in existing
338 passwords, some passwords that matched when SASLprep was used
339 might no longer work when the rules in this specification are
340 applied.
342 o SASLprep mapped the "characters commonly mapped to nothing" from
343 Appendix B.1 of [RFC3454]) to nothing, whereas the PRECIS
344 FreeClass entirely disallows such characters, which correspond to
345 the code points from the "M" category defined under Section 6.13
346 of [FRAMEWORK] (with the exception of U+1806 MONGOLIAN TODO SOFT
347 HYPHEN, which was commonly mapped to nothing in Unicode 3.2 but at
348 the time of this writing is allowed by Unicode 6.1). In practice,
349 this change will probably have no effect on comparison, but user-
350 oriented software might reject such code points instead of
351 ignoring them during password preparation.
353 5. Security Considerations
355 5.1. Password/Passphrase Strength
357 The ability to include a wide range of characters in passwords and
358 passphrases can increase the potential for creating a strong password
359 with high entropy. However, in practice, the ability to include such
360 characters ought to be weighed against the possible need to reproduce
361 them on various devices using various input methods.
363 5.2. Reuse of PRECIS
365 The security considerations described in [FRAMEWORK] apply to the
366 "NameClass" and "FreeClass" base string classes used in this document
367 for simple user names and passwords, respectively.
369 5.3. Reuse of Unicode
371 The security considerations described in [UTR39] apply to the use of
372 Unicode characters in user names and passwords.
374 6. IANA Considerations
376 6.1. Use of NameClass
378 The IANA shall add an entry to the PRECIS Usage Registry for reuse of
379 the PRECIS NameClass in SASL, as follows:
381 Applicability: Usernames in SASL and Kerberos.
382 Base Class: NameClass.
383 Subclass: No.
384 Replaces: The SASLprep profile of Stringprep.
385 Normalization: NFC.
386 Casemapping: Map uppercase and titlecase characters to lowercase.
387 Additional Mappings: None.
388 Directionality: The "Bidi Rule" defined in RFC 5893 applies.
389 Specification: RFC XXXX. [Note to RFC Editor: please change XXXX to
390 the number issued for this specification.]
392 6.2. Use of FreeClass
394 The IANA shall add an entry to the PRECIS Usage Registry for reuse of
395 the PRECIS FreeClass in SASL, as follows:
397 Applicability: Passwords in SASL and Kerberos.
398 Base Class: FreeClass
399 Subclass: No.
400 Replaces: The SASLprep profile of Stringprep.
401 Normalization: NFC.
402 Casemapping: None.
403 Additional Mappings: Map non-ASCII space characters to ASCII space.
404 Directionality: None.
405 Specification: RFC XXXX. [Note to RFC Editor: please change XXXX to
406 the number issued for this specification.]
408 7. Open Issues
410 We need to compare the output obtained when applying the new rules
411 with Unicode 3.2 and Unicode 6.1 data to the output obtained when
412 applying the SASLprep rules with Unicode 3.2 data, then make sure
413 that the PRECIS Working Group and KITTEN Working Group are
414 comfortable with any changes to the Unicode characters that are
415 allowed and disallowed. (See also the migration issues described
416 under Section 4.)
418 8. References
420 8.1. Normative References
422 [FRAMEWORK]
423 Saint-Andre, P. and M. Blanchet, "Precis Framework:
424 Handling Internationalized Strings in Protocols",
425 draft-ietf-precis-framework-05 (work in progress),
426 August 2012.
428 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
429 Requirement Levels", BCP 14, RFC 2119, March 1997.
431 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
432 10646", STD 63, RFC 3629, November 2003.
434 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
435 Specifications: ABNF", STD 68, RFC 5234, January 2008.
437 [UNICODE] The Unicode Consortium, "The Unicode Standard, Version
438 6.1", 2012,
439 .
441 8.2. Informative References
443 [I-D.yoneya-precis-mappings]
444 YONEYA, Y. and T. NEMOTO, "Mapping characters for PRECIS
445 classes", draft-yoneya-precis-mappings-02 (work in
446 progress), July 2012.
448 [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
449 Internationalized Strings ("stringprep")", RFC 3454,
450 December 2002.
452 [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names
453 and Passwords", RFC 4013, February 2005.
455 [RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
456 Authentication and Security Layer (SASL)", RFC 4422,
457 June 2006.
459 [RFC4616] Zeilenga, K., "The PLAIN Simple Authentication and
460 Security Layer (SASL) Mechanism", RFC 4616, August 2006.
462 [RFC5802] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
463 "Salted Challenge Response Authentication Mechanism
464 (SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010.
466 [RFC5890] Klensin, J., "Internationalized Domain Names for
467 Applications (IDNA): Definitions and Document Framework",
468 RFC 5890, August 2010.
470 [RFC5891] Klensin, J., "Internationalized Domain Names in
471 Applications (IDNA): Protocol", RFC 5891, August 2010.
473 [RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
474 Internationalized Domain Names for Applications (IDNA)",
475 RFC 5893, August 2010.
477 [RFC5894] Klensin, J., "Internationalized Domain Names for
478 Applications (IDNA): Background, Explanation, and
479 Rationale", RFC 5894, August 2010.
481 [RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
482 Internationalization in the IETF", BCP 166, RFC 6365,
483 September 2011.
485 [UTR39] The Unicode Consortium, "Unicode Technical Report #39:
486 Unicode Security Mechanisms", August 2010,
487 .
489 Appendix A. Differences from RFC 4013
491 The following substantive modifications were made from RFC 4013.
493 o A single SASLprep algorithm was replaced by two separate
494 algorithms: one for simple user names and another for passwords.
495 o The new preparation algorithms use PRECIS instead of a stringprep
496 profile. The new algorithms work independenctly of Unicode
497 versions.
498 o As recommended in the PRECIS framwork, changed the Unicode
499 normalization form from NFKC to NFC.
500 o Some Unicode code points that were mapped to nothing in RFC 4013
501 are simply disallowed by PRECIS.
503 Appendix B. Acknowledgements
505 Thanks to Yoshiro YONEYA and Takahiro NEMOTO for implementation
506 feedback. Thanks also to Marc Blanchet, Joe Hildebrand, Alan DeKok,
507 Simon Josefsson, Jonathan Lennox, Matt Miller, Pete Resnick, and
508 Andrew Sullivan for their input regarding the text.
510 This document borrows some text from RFC 4013 and RFC 6120.
512 Authors' Addresses
514 Peter Saint-Andre
515 Cisco Systems, Inc.
516 1899 Wynkoop Street, Suite 600
517 Denver, CO 80202
518 USA
520 Phone: +1-303-308-3282
521 Email: psaintan@cisco.com
523 Alexey Melnikov
524 Isode Ltd
525 5 Castle Business Village
526 36 Station Road
527 Hampton, Middlesex TW12 2BX
528 UK
530 Email: Alexey.Melnikov@isode.com