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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: draft-ietf-precis-framework has been published as RFC 7564 -- Possible downref: Non-RFC (?) normative reference: ref. 'UNICODE' == Outdated reference: draft-ietf-precis-mappings has been published as RFC 7790 -- Obsolete informational reference (is this intentional?): RFC 3454 (Obsoleted by RFC 7564) -- Obsolete informational reference (is this intentional?): RFC 4013 (Obsoleted by RFC 7613) Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 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: August 17, 2013 February 13, 2013 8 Preparation and Comparison of Internationalized Strings Representing 9 Simple User Names and Passwords 10 draft-ietf-precis-saslprepbis-00 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 August 17, 2013. 38 Copyright Notice 40 Copyright (c) 2013 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 56 1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 2 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 . . . . . . . . . . . . . . . . . . . . 8 71 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 72 6.1. Use of NameClass . . . . . . . . . . . . . . . . . . . . 9 73 6.2. Use of FreeClass . . . . . . . . . . . . . . . . . . . . 9 74 7. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 10 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 . . . . . . . . . . . . . . . . . . 12 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. (In many 92 authentication technologies passwords are not directly compared 93 because the actual password is used as input to an algorithm such as 94 a hash function; however, non-ASCII code points in the input string 95 still need to be handled correctly.) 96 The algorithms defined in this document assume that all strings are 97 comprised of characters from the Unicode character set [UNICODE]. 99 The algorithms are designed for use in Simple Authentication and 100 Security Layer (SASL) [RFC4422] mechanisms, such as PLAIN [RFC4616] 101 and SCRAM-SHA-1 [RFC5802]. However, they might be applicable 102 wherever simple user names or passwords are used. This profile is 103 not intended for use in preparing strings that are not simple user 104 names (e.g., email addresses, DNS domain names, LDAP distinguished 105 names), nor in cases where identifiers or secrets are not strings 106 (e.g., keys or certificates) or require different handling (e.g., 107 case folding). 109 This document builds upon the PRECIS framework defined in 110 [FRAMEWORK], which differs fundamentally from the stringprep 111 technology [RFC3454] used in SASLprep [RFC4013]. The primary 112 difference is that stringprep profiles allowed all characters except 113 those which were explicitly disallowed, whereas PRECIS profiles 114 disallow all characters except those which are explicitly allowed 115 (this "inclusion model" was originally used for internationalized 116 domain names in [RFC5891]; see [RFC5894] for further discussion). It 117 is important to keep this distinction in mind when comparing the 118 technology defined in this document to SASLprep [RFC4013]. 120 This document obsoletes RFC 4013. 122 1.2. Terminology 124 Many important terms used in this document are defined in 125 [FRAMEWORK], [RFC4422], [RFC5890], [RFC6365], and [UNICODE]. The 126 term "non-ASCII space" refers to any Unicode code point with a 127 general category of "Zs", with the exception of U+0020 (here called 128 "ASCII space"). 130 As used here, the term "password" is not literally limited to a word; 131 i.e., a password could be a passphrase consisting of more than one 132 word, perhaps separated by spaces or other such characters. 134 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 135 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 136 "OPTIONAL" in this document are to be interpreted as described in 137 [RFC2119]. 139 2. Simple User Names 141 2.1. Definition 143 Some SASL mechanisms (e.g., CRAM-MD5, DIGEST-MD5, and SCRAM) specify 144 that the authentication identity used in the context of such 145 mechanisms is a "simple user name" (see Section 2 of [RFC4422] as 146 well as [RFC4013]). However, the exact form of a simple user name in 147 any particular mechanism or deployment thereof is a local matter, and 148 a simple user name does not necessarily map to an application 149 identifier such as the localpart of an email address. 151 For purposes of preparation and comparison of authentication 152 identities, this document specifies that a simple user name is a 153 string of Unicode code points [UNICODE], encoded using UTF-8 154 [RFC3629], and structured as an ordered sequence of "simpleparts" 155 (where the complete simple user name can consist of a single 156 simplepart or a space-separated sequence of simpleparts). 158 Therefore the syntax for a simple user name is defined as follows 159 using the Augmented Backus-Naur Form (ABNF) as specified in 160 [RFC5234]. 162 simpleusername = simplepart [1*(1*SP simplepart)] 163 simplepart = 1*(namepoint) 164 ; 165 ; a "namepoint" is a UTF-8 encoded 166 ; Unicode code point that conforms to 167 ; the "NameClass" string class defined 168 ; in draft-ietf-precis-framework 169 ; 171 Note well that all code points and blocks not explicitly allowed in 172 the PRECIS NameClass are disallowed; this includes private use 173 characters, surrogate code points, and the other code points and 174 blocks defined as "Prohibited Output" in Section 2.3 of RFC 4013. 176 2.2. Preparation 178 A simple user name MUST NOT be zero bytes in length. This rule is to 179 be enforced after any normalization and mapping of code points. 181 Each simplepart of a simple user name MUST conform to the definition 182 of the PRECIS NameClass provided in [FRAMEWORK], where the 183 normalization, casemapping, and directionality rules are as described 184 below. 186 1. Unicode Normalization Form C (NFC) MUST be applied to all 187 characters. 189 2. Uppercase and titlecase characters MUST be mapped to their 190 lowercase equivalents. 192 3. Additional mappings MAY be applied, such as those defined in 193 [I-D.ietf-precis-mappings]. 195 With regard to directionality, the "Bidi Rule" provided in [RFC5893] 196 applies. 198 3. Passwords 200 3.1. Definition 202 For purposes of preparation and comparison of passwords, this 203 document specifies that a password is a string of Unicode code points 204 [UNICODE], encoded using UTF-8 [RFC3629], and conformant to the 205 PRECIS FreeClass. 207 Therefore the syntax for a password is defined as follows using the 208 Augmented Backus-Naur Form (ABNF) as specified in [RFC5234]. 210 password = 1*(freepoint) 211 ; 212 ; a "freepoint" is a UTF-8 encoded 213 ; Unicode code point that conforms to 214 ; the "FreeClass" string class defined 215 ; in draft-ietf-precis-framework 216 ; 218 Note well that all code points and blocks not explicitly allowed in 219 the PRECIS FreeClass are disallowed; this includes private use 220 characters, surrogate code points, and the other code points and 221 blocks defined as "Prohibited Output" in Section 2.3 of RFC 4013. 223 3.2. Preparation 225 A password MUST NOT be zero bytes in length. This rule is to be 226 enforced after any normalization and mapping of code points. 228 A password MUST be treated as follows, where the operations specified 229 MUST be completed in the order shown: 231 1. Apply Unicode Normalization Form C (NFC) to all characters. 233 2. Map any instances of non-ASCII space to ASCII space (U+0020). 235 3. Ensure that the resulting string conforms to the definition of 236 the PRECIS FreeClass. 238 With regard to directionality, the "Bidi Rule" (defined in [RFC5893]) 239 and similar rules are unnecessary and inapplicable to passwords, 240 since they can reduce the range of characters that are allowed in a 241 string and therefore reduce the amount of entropy that is possible in 242 a password. Furthermore, such rules are intended to minimize the 243 possibility that the same string will be displayed differently on a 244 system set for right-to-left display and a system set for left-to- 245 right display; however, passwords are typically not displayed at all 246 and are rarely meant to be interoperable across different systems in 247 the way that non-secret strings like domain names and user names are. 249 4. Migration 251 The rules defined in this specification differ slightly from those 252 defined by the SASLprep specification [RFC4013]. The following 253 sections describe these differences, along with their implications 254 for migration, in more detail. 256 4.1. User Names 258 Deployments that currently use SASLprep for handling user names might 259 need to scrub existing data when migrating to use of the rules 260 defined in this specification. In particular: 262 o SASLprep specified the use of Unicode Normalization Form KC 263 (NFKC), whereas this usage of the PRECIS NameClass employs Unicode 264 Normalization Form C (NFC). In practice this change is unlikely 265 to cause significant problems, because NFKC provides methods for 266 mapping Unicode code points with compatibility equivalents to 267 those equivalents, whereas the PRECIS NameClass entirely disallows 268 Unicode code points with compatibility equivalents (i.e., during 269 comparison NFKC is more "aggressive" about finding matches than is 270 NFC). A few examples might suffice to indicate the nature of the 271 problem: (1) U+017F LATIN SMALL LETTER LONG S is compatibility 272 equivalent to U+0073 LATIN SMALL LETTER S (2) U+2163 ROMAN NUMERAL 273 FOUR is compatibility equivalent to U+0049 LATIN CAPITAL LETTER I 274 and U+0056 LATIN CAPITAL LETTER V (3) U+FB01 LATIN SMALL LIGATURE 275 FI is compatibility equivalent to U+0066 LATIN SMALL LETTER F and 276 U+0069 LATIN SMALL LETTER I. Under SASLprep, the use of NFKC also 277 handled the mapping of fullwidth and halfwidth code points to 278 their decomposition equivalents (see [I-D.ietf-precis-mappings]). 279 Although it is expected that code points with compatibility 280 equivalents are rare in existing user names, for migration 281 purposes deployments might want to search their database of user 282 names for Unicode code points with compatibility equivalents and 283 map those code points to their compatibility equivalents. 285 o SASLprep mapped non-ASCII spaces to ASCII space (U+0020), whereas 286 the PRECIS NameClass entirely disallows non-ASCII spaces. The 287 non-ASCII space characters are U+00A0 NO-BREAK SPACE, U+1680 OGHAM 288 SPACE MARK, U+180E MONGOLIAN VOWEL SEPARATOR, U+2000 EN QUAD 289 through U+200A HAIR SPACE, U+202F NARROW NO-BREAK SPACE, U+205F 290 MEDIUM MATHEMATICAL SPACE, and U+3000 IDEOGRAPHIC SPACE. For 291 migration purposes, deployments might want to convert non-ASCII 292 space characters to ASCII space in simple user names. 294 o SASLprep mapped the "characters commonly mapped to nothing" from 295 Appendix B.1 of [RFC3454]) to nothing, whereas the PRECIS 296 NameClass entirely disallows most of these characters, which 297 correspond to the code points from the "M" category defined under 298 Section 6.13 of [FRAMEWORK] (with the exception of U+1806 299 MONGOLIAN TODO SOFT HYPHEN, which was "commonly mapped to nothing" 300 in Unicode 3.2 but at the time of this writing does not have a 301 derived property of Default_Ignorable_Code_Point in Unicode 6.1). 302 For migration purposes, deployments might want to remove code 303 points contained in the PRECIS "M" category from simple user 304 names. 306 o SASLprep allowed uppercase and titlecase characters, whereas this 307 usage of the PRECIS NameClass maps uppercase and titlecase 308 characters to their lowercase equivalents. For migration 309 purposes, deployments can either convert uppercase and titlecase 310 characters to their lowercase equivalents in simple user names 311 (thus losing the case information) or preserve uppercase and 312 titlecase characters and ignore the case difference when comparing 313 simple user names. 315 4.2. Passwords 317 Depending on local service policy, migration from RFC 4013 to this 318 specification might not involve any scrubbing of data (since 319 passwords might not be stored in the clear anyway); however, service 320 providers need to be aware of possible issues that might arise during 321 migration. In particular: 323 o SASLprep specified the use of Unicode Normalization Form KC 324 (NFKC), whereas this usage of the PRECIS FreeClass employs Unicode 325 Normalization Form C (NFC). Because NFKC is more aggressive about 326 finding matches than NFC, in practice this change is unlikely to 327 cause significant problems and indeed has the security benefit of 328 probably resulting in fewer false positives when comparing 329 passwords. A few examples might suffice to indicate the nature of 330 the problem: (1) U+017F LATIN SMALL LETTER LONG S is compatibility 331 equivalent to U+0073 LATIN SMALL LETTER S (2) U+2163 ROMAN NUMERAL 332 FOUR is compatibility equivalent to U+0049 LATIN CAPITAL LETTER I 333 and U+0056 LATIN CAPITAL LETTER V (3) U+FB01 LATIN SMALL LIGATURE 334 FI is compatibility equivalent to U+0066 LATIN SMALL LETTER F and 335 U+0069 LATIN SMALL LETTER I. Under SASLprep, the use of NFKC also 336 handled the mapping of fullwidth and halfwidth code points to 337 their decomposition equivalents (see [I-D.ietf-precis-mappings]). 338 Although it is expected that code points with compatibility 339 equivalents are rare in existing passwords, some passwords that 340 matched when SASLprep was used might no longer work when the rules 341 in this specification are applied. 343 o SASLprep mapped the "characters commonly mapped to nothing" from 344 Appendix B.1 of [RFC3454]) to nothing, whereas the PRECIS 345 FreeClass entirely disallows such characters, which correspond to 346 the code points from the "M" category defined under Section 6.13 347 of [FRAMEWORK] (with the exception of U+1806 MONGOLIAN TODO SOFT 348 HYPHEN, which was commonly mapped to nothing in Unicode 3.2 but at 349 the time of this writing is allowed by Unicode 6.1). In practice, 350 this change will probably have no effect on comparison, but user- 351 oriented software might reject such code points instead of 352 ignoring them during password preparation. 354 5. Security Considerations 356 5.1. Password/Passphrase Strength 358 The ability to include a wide range of characters in passwords and 359 passphrases can increase the potential for creating a strong password 360 with high entropy. However, in practice, the ability to include such 361 characters ought to be weighed against the possible need to reproduce 362 them on various devices using various input methods. 364 5.2. Reuse of PRECIS 366 The security considerations described in [FRAMEWORK] apply to the 367 "NameClass" and "FreeClass" base string classes used in this document 368 for simple user names and passwords, respectively. 370 5.3. Reuse of Unicode 372 The security considerations described in [UTR39] apply to the use of 373 Unicode characters in user names and passwords. 375 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. 383 Base Class: NameClass. 385 Subclass: No. 387 Replaces: The SASLprep profile of Stringprep. 389 Normalization: NFC. 391 Casemapping: Map uppercase and titlecase characters to lowercase. 393 Additional Mappings: None. 395 Directionality: The "Bidi Rule" defined in RFC 5893 applies. 397 Specification: RFC XXXX. [Note to RFC Editor: please change XXXX to 398 the number issued for this specification.] 400 6.2. Use of FreeClass 402 The IANA shall add an entry to the PRECIS Usage Registry for reuse of 403 the PRECIS FreeClass in SASL, as follows: 405 Applicability: Passwords in SASL and Kerberos. 407 Base Class: FreeClass 409 Subclass: No. 411 Replaces: The SASLprep profile of Stringprep. 413 Normalization: NFC. 415 Casemapping: None. 417 Additional Mappings: Map non-ASCII space characters to ASCII space. 419 Directionality: None. 421 Specification: RFC XXXX. [Note to RFC Editor: please change XXXX to 422 the number issued for this specification.] 424 7. Open Issues 426 We need to compare the output obtained when applying the new rules 427 with Unicode 3.2 and Unicode 6.1 data to the output obtained when 428 applying the SASLprep rules with Unicode 3.2 data, then make sure 429 that the PRECIS Working Group and KITTEN Working Group are 430 comfortable with any changes to the Unicode characters that are 431 allowed and disallowed. (See also the migration issues described 432 under Section 4.) 434 8. References 436 8.1. Normative References 438 [FRAMEWORK] 439 Saint-Andre, P. and M. Blanchet, "Precis Framework: 440 Handling Internationalized Strings in Protocols", draft- 441 ietf-precis-framework-06 (work in progress), September 442 2012. 444 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 445 Requirement Levels", BCP 14, RFC 2119, March 1997. 447 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 448 10646", STD 63, RFC 3629, November 2003. 450 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 451 Specifications: ABNF", STD 68, RFC 5234, January 2008. 453 [UNICODE] The Unicode Consortium, "The Unicode Standard, Version 454 6.1", 2012, 455 . 457 8.2. Informative References 459 [I-D.ietf-precis-mappings] 460 YONEYA, Y. and T. NEMOTO, "Mapping characters for PRECIS 461 classes", draft-ietf-precis-mappings-01 (work in 462 progress), December 2012. 464 [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of 465 Internationalized Strings ("stringprep")", RFC 3454, 466 December 2002. 468 [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names 469 and Passwords", RFC 4013, February 2005. 471 [RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple 472 Authentication and Security Layer (SASL)", RFC 4422, June 473 2006. 475 [RFC4616] Zeilenga, K., "The PLAIN Simple Authentication and 476 Security Layer (SASL) Mechanism", RFC 4616, August 2006. 478 [RFC5802] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams, 479 "Salted Challenge Response Authentication Mechanism 480 (SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010. 482 [RFC5890] Klensin, J., "Internationalized Domain Names for 483 Applications (IDNA): Definitions and Document Framework", 484 RFC 5890, August 2010. 486 [RFC5891] Klensin, J., "Internationalized Domain Names in 487 Applications (IDNA): Protocol", RFC 5891, August 2010. 489 [RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left Scripts for 490 Internationalized Domain Names for Applications (IDNA)", 491 RFC 5893, August 2010. 493 [RFC5894] Klensin, J., "Internationalized Domain Names for 494 Applications (IDNA): Background, Explanation, and 495 Rationale", RFC 5894, August 2010. 497 [RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in 498 Internationalization in the IETF", BCP 166, RFC 6365, 499 September 2011. 501 [UTR39] The Unicode Consortium, "Unicode Technical Report #39: 502 Unicode Security Mechanisms", August 2010, 503 . 505 Appendix A. Differences from RFC 4013 507 The following substantive modifications were made from RFC 4013. 509 o A single SASLprep algorithm was replaced by two separate 510 algorithms: one for simple user names and another for passwords. 512 o The new preparation algorithms use PRECIS instead of a stringprep 513 profile. The new algorithms work independenctly of Unicode 514 versions. 516 o As recommended in the PRECIS framwork, changed the Unicode 517 normalization form from NFKC to NFC. 519 o Some Unicode code points that were mapped to nothing in RFC 4013 520 are simply disallowed by PRECIS. 522 Appendix B. Acknowledgements 524 Thanks to Yoshiro YONEYA and Takahiro NEMOTO for implementation 525 feedback. Thanks also to Marc Blanchet, Joe Hildebrand, Alan DeKok, 526 Simon Josefsson, Jonathan Lennox, Matt Miller, Pete Resnick, and 527 Andrew Sullivan for their input regarding the text. 529 This document borrows some text from RFC 4013 and RFC 6120. 531 Authors' Addresses 533 Peter Saint-Andre 534 Cisco Systems, Inc. 535 1899 Wynkoop Street, Suite 600 536 Denver, CO 80202 537 USA 539 Phone: +1-303-308-3282 540 Email: psaintan@cisco.com 542 Alexey Melnikov 543 Isode Ltd 544 5 Castle Business Village 545 36 Station Road 546 Hampton, Middlesex TW12 2BX 547 UK 549 Email: Alexey.Melnikov@isode.com