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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group M. Blanchet 3 Internet-Draft Viagenie 4 Intended status: Informational A. Sullivan 5 Expires: April 21, 2011 October 18, 2010 7 Stringprep Revision Problem Statement 8 draft-ietf-precis-problem-statement-00.txt 10 Abstract 12 Using Unicode codepoints in protocol strings that expect comparison 13 with other strings [[anchor1: The WG will need to decide whether 14 "other strings" is too broad. In particular, what about protocol 15 slots that can take strings other than plain ASCII? 16 --ajs@shinkuro.com]] requires preparation of the string that contains 17 the Unicode codepoints. Internationalizing Domain Names in 18 Applications (IDNA2003) defined and used Stringprep and Nameprep. 19 Other protocols subsequently defined Stringprep profiles. A new 20 approach different from Stringprep and Nameprep is used for a 21 revision of IDNA2003 (called IDNA2008). Other Stringprep profiles 22 need to be similarly updated or a replacement of Stringprep need to 23 be designed. This document outlines the issues to be faced by those 24 designing a Stringprep replacement. 26 Status of this Memo 28 This Internet-Draft is submitted in full conformance with the 29 provisions of BCP 78 and BCP 79. 31 Internet-Drafts are working documents of the Internet Engineering 32 Task Force (IETF). Note that other groups may also distribute 33 working documents as Internet-Drafts. The list of current Internet- 34 Drafts is at http://datatracker.ietf.org/drafts/current/. 36 Internet-Drafts are draft documents valid for a maximum of six months 37 and may be updated, replaced, or obsoleted by other documents at any 38 time. It is inappropriate to use Internet-Drafts as reference 39 material or to cite them other than as "work in progress." 41 This Internet-Draft will expire on April 21, 2011. 43 Copyright Notice 45 Copyright (c) 2010 IETF Trust and the persons identified as the 46 document authors. All rights reserved. 48 This document is subject to BCP 78 and the IETF Trust's Legal 49 Provisions Relating to IETF Documents 50 (http://trustee.ietf.org/license-info) in effect on the date of 51 publication of this document. Please review these documents 52 carefully, as they describe your rights and restrictions with respect 53 to this document. Code Components extracted from this document must 54 include Simplified BSD License text as described in Section 4.e of 55 the Trust Legal Provisions and are provided without warranty as 56 described in the Simplified BSD License. 58 This document may contain material from IETF Documents or IETF 59 Contributions published or made publicly available before November 60 10, 2008. The person(s) controlling the copyright in some of this 61 material may not have granted the IETF Trust the right to allow 62 modifications of such material outside the IETF Standards Process. 63 Without obtaining an adequate license from the person(s) controlling 64 the copyright in such materials, this document may not be modified 65 outside the IETF Standards Process, and derivative works of it may 66 not be created outside the IETF Standards Process, except to format 67 it for publication as an RFC or to translate it into languages other 68 than English. 70 Table of Contents 72 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 73 2. Usage and Issues of Stringprep . . . . . . . . . . . . . . . . 5 74 2.1. Issues raised during newprep BOF . . . . . . . . . . . . . 5 75 2.2. Specific issues with particular Stringprep profiles . . . 6 76 2.3. Inclusion vs. exclusion of characters . . . . . . . . . . 6 77 2.4. Stringprep and NFKC . . . . . . . . . . . . . . . . . . . 7 78 2.5. Case mapping . . . . . . . . . . . . . . . . . . . . . . . 7 79 2.6. Whether to use ASCII-compatible encoding . . . . . . . . . 7 80 2.7. Issues with delimiters . . . . . . . . . . . . . . . . . . 8 81 3. Considerations for Stringprep replacement . . . . . . . . . . 8 82 4. Security Considerations . . . . . . . . . . . . . . . . . . . 9 83 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 84 6. Discussion home for this draft . . . . . . . . . . . . . . . . 9 85 7. Informative References . . . . . . . . . . . . . . . . . . . . 9 86 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12 88 1. Introduction 90 Internationalizing Domain Names in Applications (IDNA2003) [RFC3490], 91 [RFC3491], [RFC3492], [RFC3454] described a mechanism for encoding 92 UTF-8 labels making up Internationalized Domain Names (IDNs) as 93 standard DNS labels. The labels were processed using a method called 94 Nameprep [RFC3491] and Punycode [RFC3492]. That method was specific 95 to IDNA2003, but is generalized as Stringprep [RFC3454]. The general 96 mechanism can be used to help other protocols with similar needs, but 97 with different constraints than IDNA2003. 99 Stringprep defines a framework within which protocols define their 100 Stringprep profiles. Known IETF specifications using Stringprep are 101 listed below: 102 o The Nameprep profile [RFC3490] for use in Internationalized Domain 103 Names (IDNs); 104 o NFSv4 [RFC3530] and NFSv4.1 [RFC5661]; 105 o The iSCSI profile [RFC3722] for use in Internet Small Computer 106 Systems Interface (iSCSI) Names; 107 o EAP [RFC3748]; 108 o The Nodeprep and Resourceprep profiles [RFC3920] for use in the 109 Extensible Messaging and Presence Protocol (XMPP), and the XMPP to 110 CPIM mapping [RFC3922]; 111 o The Policy MIB profile [RFC4011] for use in the Simple Network 112 Management Protocol (SNMP); 113 o The SASLprep profile [RFC4013] for use in the Simple 114 Authentication and Security Layer (SASL), and SASL itself 115 [RFC4422]; 116 o TLS [RFC4279]; 117 o IMAP4 using SASLprep [RFC4314]; 118 o The trace profile [RFC4505] for use with the SASL ANONYMOUS 119 mechanism; 120 o The LDAP profile [RFC4518] for use with LDAP [RFC4511] and its 121 authentication methods [RFC4513]; 122 o Plain SASL using SASLprep [RFC4616]; 123 o NNTP using SASLprep [RFC4643]; 124 o PKIX subject identification using LDAPprep [RFC4683]; 125 o Internet Application Protocol Collation Registry [RFC4790]; 126 o SMTP Auth using SASLprep [RFC4954]; 127 o POP3 Auth using SASLprep [RFC5034]; 128 o TLS SRP using SASLprep [RFC5054]; 129 o IRI and URI in XMPP [RFC5122]; 130 o PKIX CRL using LDAPprep [RFC5280]; 131 o IAX using Nameprep [RFC5456]; 132 o SASL SCRAM using SASLprep [RFC5802]; 133 o Remote management of Sieve using SASLprep [RFC5804]; 134 o The i;unicode-casemap Unicode Collation [RFC5051]. 136 There turned out to be some difficulties with IDNA2003, documented in 137 [RFC4690]. These difficulties led to a new IDN specification, called 138 IDNA2008 [RFC5890], [RFC5891], [RFC5892], [RFC5893]. Additional 139 background and explanations of the decisions embodied in IDNA2008 is 140 presented in [RFC5894]. One of the effects of IDNA2008 is that 141 Nameprep and Stringprep are not used at all. Instead, an algorithm 142 based on Unicode properties of codepoints is defined. That algorithm 143 generates a stable and complete table of the supported Unicode 144 codepoints. This algorithm is based on an inclusion-based approach, 145 instead of the exclusion-based approach of Stringprep/Nameprep. 147 This document lists the shortcomings and issues found by protocols 148 listed above that defined Stringprep profiles. It also lists some 149 early conclusions and requirements for a potential replacement of 150 Stringprep. 152 2. Usage and Issues of Stringprep 154 2.1. Issues raised during newprep BOF 156 During IETF 77, a BOF discussed the current state of the protocols 157 that have defined Stringprep profiles [NEWPREP]. The main 158 conclusions are : 159 o Stringprep is bound to a specific version of Unicode: 3.2. 160 Stringprep has not been updated to new versions of Unicode. 161 Therefore, the protocols using Stringprep are stuck to Unicode 162 3.2. 163 o The protocols need to be updated to support new versions of 164 Unicode. The protocols would like to not be bound to a specific 165 version of Unicode, but rather have better Unicode agility in the 166 way of IDNA2008. This is important partly because it is usually 167 impossible for an application to require Unicode 3.2; the 168 application gets whatever version of Unicode is available on the 169 host. 170 o The protocols require better bidirectional support (bidi) than 171 currently offered by Stringprep. 172 o If the protocols are updated to use a new version of Stringprep or 173 another framework, then backward compatibility is an important 174 requirement. For example, Stringprep is based on and may use NFKC 175 [UAX15], while IDNA2008 mostly uses NFC [UAX15]. 176 o Protocols use each other; for example, a protocol can use user 177 identifiers that are later passed to SASL, LDAP or another 178 authentication mechanism. Therefore, common set of rules or 179 classes of strings are preferred over specific rules for each 180 protocol. 182 Protocols that use Stringprep profiles use strings for different 183 purposes: 184 o XMPP uses a different Stringprep profile for each part of the XMPP 185 address (JID): a localpart which is similar to a username and used 186 for authentication, a domainpart which is a domain name and a 187 resource part which is less restrictive than the localpart. 188 o iSCSI uses a Stringprep profile for the IQN, which is very similar 189 to (often is) a DNS domain name. 190 o SASL and LDAP uses a Stringprep profile for usernames. 191 o LDAP uses a set of Stringprep profiles. 193 During the newprep BOF, it was the consensus of the attendees that it 194 would be highly preferable to have a replacement of Stringprep, with 195 similar characteristics to IDNA2008. That replacement should be 196 defined so that the protocols could use internationalized strings 197 without a lot of specialized internationalization work, since 198 internationalization expertise is not available in the respective 199 protocols or working groups. 201 2.2. Specific issues with particular Stringprep profiles 203 [[anchor6: This section is where issues raised in the individual 204 profile reviews goes. A review of the WG trac state on 2010-10-06 of 205 the tracker suggests those reviews haven't happened yet. 206 --ajs@shinkuro.com]] 208 2.3. Inclusion vs. exclusion of characters 210 One of the primary changes of IDNA2008 is in the way it approaches 211 Unicode characters. IDNA2003 created an explicit list of excluded or 212 mapped-away characters; anything in Unicode 3.2 that was not so 213 listed could be assumed to be allowed under the protocol. IDNA2008 214 begins instead from the assumption that characters are disallowed, 215 and then relies on Unicode properties to derive whether a given 216 character actually is allowed in the protocol. 218 Moreover, there is more than one class of "allowed in the protocol". 219 While some characters are simply disallowed, some are allowed only in 220 certain contexts. The reasons for the context-dependent rules have 221 to do with the way some characters are used. For instance, the ZERO 222 WIDTH JOINER and ZERO WIDTH NON-JOINER characters (ZWJ, U+200D and 223 ZWNJ, U+200C) are allowed with contextual rules because they are 224 required in some circumstances, yet are considered punctuation by 225 Unicode and would therefore be DISALLOWED under the usual IDNA2008 226 derivation rules. 228 The working group needs to decide whether similar contextual cases 229 need to be supported. 231 2.4. Stringprep and NFKC 233 Stringprep profiles may use normalization. If they do, they use NFKC 234 [UAX15]. It is not clear that NFKC is the right normalization to use 235 in all cases. In [UAX15], there is the following observation 236 regarding Normalization Forms KC and KD: "It is best to think of 237 these Normalization Forms as being like uppercase or lowercase 238 mappings: useful in certain contexts for identifying core meanings, 239 but also performing modifications to the text that may not always be 240 appropriate." For things like the spelling of users' names, then, 241 NKFC may not be the best form to use. At the same time, one of the 242 nice things about NFKC is that it deals with the width of characters 243 that are otherwise similar, by canonicalizing half-width to full- 244 width. This mapping step can be crucial in practice. The WG will 245 need to analyze the different use profiles and consider whether NFKC 246 or NFC is a better normalization for each profile. 248 2.5. Case mapping 250 In IDNA2003, labels are always mapped to lower case before the 251 Punycode transformation. In IDNA2003, there is no mapping at all: 252 input is either a valid U-label or it is not. At the same time, 253 upper-case characters are by definition not valid U-labels, because 254 they fall into the Unstable category (category B) of [RFC5892]. 256 If there are protocols that require upper and lower cases be 257 preserved, then the analogy with IDNA2008 will break down. The 258 working group will need to decide whether there are any cases that 259 require upper case, and what to do about it if so. 261 2.6. Whether to use ASCII-compatible encoding 263 The development of IDNA2008 depended on the notion that there was a 264 narrow repertoire of reasonable traditional labels, and what was 265 necessary was to internationalize that repertoire rather than to 266 incorporate any characters into domain name labels. More exactly, 267 the idea was to internationalize the traditional hostname rules (the 268 "LDH rule". See [RFC4690], section 5.1.). Efforts to 269 internationalize email ([RFC5336]) have started from different 270 assumptions. The email example suggests that in some cases, the 271 right answer might be to internationalize the target protocol rather 272 than to depend on a technology to ensure protocol slots can use only 273 ASCII. The working group will need to determine which approach is 274 correct for the different use-cases. 276 2.7. Issues with delimiters 278 There are two kinds of issues to address with delimiters. First, 279 exactly where a delimiter will appear on the screen when dealing with 280 bidirectional parts of a string can be extremely surprising. In the 281 case of IDNA2008, just what to do in these cases remains a display 282 issue (there is no question about the wire format, because the wire 283 format is an A-label and it is always left to right). 285 Second, there is the question of whether to include different kinds 286 of protocol separators. For instance, FULL STOP, U+002E (.) may not 287 be available on all keyboards. In addition, in some languages there 288 is more than one full stop which are variants of one another. The 289 working group will need to decide how to handle such cases: whether 290 there will be a mapping, some restrictions, or something else. 292 3. Considerations for Stringprep replacement 294 The above suggests the following direction for the working group: 295 o A stringprep replacement should be defined. 296 o The replacement should take an approach similar to IDNA2008, in 297 that it enables Unicode agility. 298 o Protocols share similar characteristics of strings. Therefore, 299 defining i18n preparation algorithms for a (small) set of string 300 classes may be sufficient for most cases and provides the 301 coherence among a set of protocol friends. 302 o The sets of string classes need to be evaluated for the following 303 properties: 304 * the normalization needed (NFC vs NFKC); 305 * whether case-folding, case preservation, and case-insensitive 306 matching is needed; 307 * what restrictions on input are reasonable for the class (i.e. 308 whether there is something like an "LDH rule" for the class), 309 or whether the ASCII-only input in the protocol slot is lightly 310 constrained; 311 * the extent to which bidi considerations are important for the 312 class. 314 Existing deployments already depend on Stringprep profiles. 315 Therefore, the working group will need to consider the effects of any 316 new strategy on existing deployments. By way of comparison, it is 317 worth noting that some characters were acceptable in IDNA labels 318 under IDNA2003, but are not protocol-valid under IDNA2008 (and 319 conversely). Different implementers may make different decisions 320 about what to do in such cases; this could have interoperability 321 effects. The working group will need to trade better support for 322 different linguistic environments against the potential side effects 323 of backward incompatibility. 325 4. Security Considerations 327 This document merely states what problems are to be solved, and does 328 not define a protocol. There are undoubtedly security implications 329 of the particular results that will come from the work to be 330 completed. 332 5. IANA Considerations 334 This document has no actions for IANA. 336 6. Discussion home for this draft 338 This document is intended to define the problem space discussed on 339 the precis@ietf.org mailing list. 341 7. Informative References 343 [NEWPREP] "Newprep BoF Meeting Minutes", March 2010. 345 [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of 346 Internationalized Strings ("stringprep")", RFC 3454, 347 December 2002. 349 [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello, 350 "Internationalizing Domain Names in Applications (IDNA)", 351 RFC 3490, March 2003. 353 [RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep 354 Profile for Internationalized Domain Names (IDN)", 355 RFC 3491, March 2003. 357 [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode 358 for Internationalized Domain Names in Applications 359 (IDNA)", RFC 3492, March 2003. 361 [RFC3530] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R., 362 Beame, C., Eisler, M., and D. Noveck, "Network File System 363 (NFS) version 4 Protocol", RFC 3530, April 2003. 365 [RFC3722] Bakke, M., "String Profile for Internet Small Computer 366 Systems Interface (iSCSI) Names", RFC 3722, April 2004. 368 [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. 369 Levkowetz, "Extensible Authentication Protocol (EAP)", 370 RFC 3748, June 2004. 372 [RFC3920] Saint-Andre, P., Ed., "Extensible Messaging and Presence 373 Protocol (XMPP): Core", RFC 3920, October 2004. 375 [RFC3922] Saint-Andre, P., "Mapping the Extensible Messaging and 376 Presence Protocol (XMPP) to Common Presence and Instant 377 Messaging (CPIM)", RFC 3922, October 2004. 379 [RFC4011] Waldbusser, S., Saperia, J., and T. Hongal, "Policy Based 380 Management MIB", RFC 4011, March 2005. 382 [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names 383 and Passwords", RFC 4013, February 2005. 385 [RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites 386 for Transport Layer Security (TLS)", RFC 4279, 387 December 2005. 389 [RFC4314] Melnikov, A., "IMAP4 Access Control List (ACL) Extension", 390 RFC 4314, December 2005. 392 [RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and 393 Security Layer (SASL)", RFC 4422, June 2006. 395 [RFC4505] Zeilenga, K., "Anonymous Simple Authentication and 396 Security Layer (SASL) Mechanism", RFC 4505, June 2006. 398 [RFC4511] Sermersheim, J., "Lightweight Directory Access Protocol 399 (LDAP): The Protocol", RFC 4511, June 2006. 401 [RFC4513] Harrison, R., "Lightweight Directory Access Protocol 402 (LDAP): Authentication Methods and Security Mechanisms", 403 RFC 4513, June 2006. 405 [RFC4518] Zeilenga, K., "Lightweight Directory Access Protocol 406 (LDAP): Internationalized String Preparation", RFC 4518, 407 June 2006. 409 [RFC4616] Zeilenga, K., "The PLAIN Simple Authentication and 410 Security Layer (SASL) Mechanism", RFC 4616, August 2006. 412 [RFC4643] Vinocur, J. and K. Murchison, "Network News Transfer 413 Protocol (NNTP) Extension for Authentication", RFC 4643, 414 October 2006. 416 [RFC4683] Park, J., Lee, J., Lee, H., Park, S., and T. Polk, 417 "Internet X.509 Public Key Infrastructure Subject 418 Identification Method (SIM)", RFC 4683, October 2006. 420 [RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and 421 Recommendations for Internationalized Domain Names 422 (IDNs)", RFC 4690, September 2006. 424 [RFC4790] Newman, C., Duerst, M., and A. Gulbrandsen, "Internet 425 Application Protocol Collation Registry", RFC 4790, 426 March 2007. 428 [RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension 429 for Authentication", RFC 4954, July 2007. 431 [RFC5034] Siemborski, R. and A. Menon-Sen, "The Post Office Protocol 432 (POP3) Simple Authentication and Security Layer (SASL) 433 Authentication Mechanism", RFC 5034, July 2007. 435 [RFC5051] Crispin, M., "i;unicode-casemap - Simple Unicode Collation 436 Algorithm", RFC 5051, October 2007. 438 [RFC5054] Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin, 439 "Using the Secure Remote Password (SRP) Protocol for TLS 440 Authentication", RFC 5054, November 2007. 442 [RFC5122] Saint-Andre, P., "Internationalized Resource Identifiers 443 (IRIs) and Uniform Resource Identifiers (URIs) for the 444 Extensible Messaging and Presence Protocol (XMPP)", 445 RFC 5122, February 2008. 447 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 448 Housley, R., and W. Polk, "Internet X.509 Public Key 449 Infrastructure Certificate and Certificate Revocation List 450 (CRL) Profile", RFC 5280, May 2008. 452 [RFC5336] Yao, J. and W. Mao, "SMTP Extension for Internationalized 453 Email Addresses", RFC 5336, September 2008. 455 [RFC5456] Spencer, M., Capouch, B., Guy, E., Miller, F., and K. 456 Shumard, "IAX: Inter-Asterisk eXchange Version 2", 457 RFC 5456, February 2010. 459 [RFC5661] Shepler, S., Eisler, M., and D. Noveck, "Network File 460 System (NFS) Version 4 Minor Version 1 Protocol", 461 RFC 5661, January 2010. 463 [RFC5802] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams, 464 "Salted Challenge Response Authentication Mechanism 465 (SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010. 467 [RFC5804] Melnikov, A. and T. Martin, "A Protocol for Remotely 468 Managing Sieve Scripts", RFC 5804, July 2010. 470 [RFC5890] Klensin, J., "Internationalized Domain Names for 471 Applications (IDNA): Definitions and Document Framework", 472 RFC 5890, August 2010. 474 [RFC5891] Klensin, J., "Internationalized Domain Names in 475 Applications (IDNA): Protocol", RFC 5891, August 2010. 477 [RFC5892] Faltstrom, P., "The Unicode Code Points and 478 Internationalized Domain Names for Applications (IDNA)", 479 RFC 5892, August 2010. 481 [RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left Scripts for 482 Internationalized Domain Names for Applications (IDNA)", 483 RFC 5893, August 2010. 485 [RFC5894] Klensin, J., "Internationalized Domain Names for 486 Applications (IDNA): Background, Explanation, and 487 Rationale", RFC 5894, August 2010. 489 [UAX15] "Unicode Standard Annex #15: Unicode Normalization Forms", 490 UAX 15, September 2009. 492 Authors' Addresses 494 Marc Blanchet 495 Viagenie 496 2600 boul. Laurier, suite 625 497 Quebec, QC G1V 4W1 498 Canada 500 Email: Marc.Blanchet@viagenie.ca 501 URI: http://viagenie.ca 503 Andrew Sullivan 504 519 Maitland St. 505 London, ON N6B 2Z5 506 Canada 508 Email: ajs@crankycanuck.ca