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2 Network Working Group M. Blanchet
3 Internet-Draft Viagenie
4 Intended status: Informational A. Sullivan
5 Expires: July 26, 2013 Dyn, Inc.
6 January 22, 2013
8 Stringprep Revision and PRECIS Problem Statement
9 draft-ietf-precis-problem-statement-09.txt
11 Abstract
13 If a protocol expects to compare two strings and is prepared only for
14 those strings to be ASCII, then using Unicode codepoints in those
15 strings requires they be prepared somehow. Internationalizing Domain
16 Names in Applications (here called IDNA2003) defined and used
17 Stringprep and Nameprep. Other protocols subsequently defined
18 Stringprep profiles. A new approach different from Stringprep and
19 Nameprep is used for a revision of IDNA2003 (called IDNA2008). Other
20 Stringprep profiles need to be similarly updated or a replacement of
21 Stringprep needs to be designed. This document outlines the issues
22 to be faced by those designing a Stringprep replacement.
24 Status of this Memo
26 This Internet-Draft is submitted in full conformance with the
27 provisions of BCP 78 and BCP 79.
29 Internet-Drafts are working documents of the Internet Engineering
30 Task Force (IETF). Note that other groups may also distribute
31 working documents as Internet-Drafts. The list of current Internet-
32 Drafts is at http://datatracker.ietf.org/drafts/current/.
34 Internet-Drafts are draft documents valid for a maximum of six months
35 and may be updated, replaced, or obsoleted by other documents at any
36 time. It is inappropriate to use Internet-Drafts as reference
37 material or to cite them other than as "work in progress."
39 This Internet-Draft will expire on July 26, 2013.
41 Copyright Notice
43 Copyright (c) 2013 IETF Trust and the persons identified as the
44 document authors. All rights reserved.
46 This document is subject to BCP 78 and the IETF Trust's Legal
47 Provisions Relating to IETF Documents
48 (http://trustee.ietf.org/license-info) in effect on the date of
49 publication of this document. Please review these documents
50 carefully, as they describe your rights and restrictions with respect
51 to this document. Code Components extracted from this document must
52 include Simplified BSD License text as described in Section 4.e of
53 the Trust Legal Provisions and are provided without warranty as
54 described in the Simplified BSD License.
56 Table of Contents
58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
59 2. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
60 3. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5
61 4. Stringprep Profiles Limitations . . . . . . . . . . . . . . . 6
62 5. Major Topics for Consideration . . . . . . . . . . . . . . . . 7
63 5.1. Comparison . . . . . . . . . . . . . . . . . . . . . . . . 7
64 5.1.1. Types of Identifiers . . . . . . . . . . . . . . . . . 7
65 5.1.2. Effect of comparison . . . . . . . . . . . . . . . . . 8
66 5.2. Dealing with characters . . . . . . . . . . . . . . . . . 8
67 5.2.1. Case folding, case sensitivity, and case
68 preservation . . . . . . . . . . . . . . . . . . . . . 8
69 5.2.2. Stringprep and NFKC . . . . . . . . . . . . . . . . . 8
70 5.2.3. Character mapping . . . . . . . . . . . . . . . . . . 9
71 5.2.4. Prohibited characters . . . . . . . . . . . . . . . . 9
72 5.2.5. Internal structure, delimiters, and special
73 characters . . . . . . . . . . . . . . . . . . . . . . 9
74 5.2.6. Restrictions because of glyph similarity . . . . . . . 10
75 5.3. Where the data comes from and where it goes . . . . . . . 10
76 5.3.1. User input and the source of protocol elements . . . . 10
77 5.3.2. User output . . . . . . . . . . . . . . . . . . . . . 11
78 5.3.3. Operations . . . . . . . . . . . . . . . . . . . . . . 11
79 6. Considerations for Stringprep replacement . . . . . . . . . . 12
80 7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
81 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
82 9. Discussion home for this draft . . . . . . . . . . . . . . . . 13
83 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
84 11. Informative References . . . . . . . . . . . . . . . . . . . . 14
85 Appendix A. Classification of Stringprep Profiles . . . . . . . . 18
86 Appendix B. Evaluation of Stringprep Profiles . . . . . . . . . . 18
87 B.1. iSCSI Stringprep Profile: RFC3722 (and RFC3721,
88 RFC3720) . . . . . . . . . . . . . . . . . . . . . . . . . 18
89 B.2. SMTP/POP3/ManageSieve Stringprep Profiles:
90 RFC4954,RFC5034,RFC 5804 . . . . . . . . . . . . . . . . . 20
91 B.3. IMAP Stringprep Profiles: RFC5738, RFC4314: Usernames . . 22
92 B.4. IMAP Stringprep Profiles: RFC5738: Passwords . . . . . . . 23
93 B.5. Anonymous SASL Stringprep Profiles: RFC4505 . . . . . . . 24
94 B.6. XMPP Stringprep Profiles: RFC3920 Nodeprep . . . . . . . . 26
95 B.7. XMPP Stringprep Profiles: RFC3920 Resourceprep . . . . . . 27
96 B.8. EAP Stringprep Profiles: RFC3748 . . . . . . . . . . . . . 28
97 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28
99 1. Introduction
101 Internationalizing Domain Names in Applications (here called
102 IDNA2003) [RFC3490], [RFC3491], [RFC3492], [RFC3454] describes a
103 mechanism for encoding Unicode labels making up Internationalized
104 Domain Names (IDNs) as standard DNS labels. The labels were
105 processed using a method called Nameprep [RFC3491] and Punycode
106 [RFC3492]. That method was specific to IDNA2003, but is generalized
107 as Stringprep [RFC3454]. The general mechanism is used by other
108 protocols with similar needs, but with different constraints than
109 IDNA2003.
111 Stringprep defines a framework within which protocols define their
112 Stringprep profiles. Some known IETF specifications using Stringprep
113 are listed below:
114 o The Nameprep profile [RFC3490] for use in Internationalized Domain
115 Names (IDNs);
116 o IAX using Nameprep [RFC5456];
117 o NFSv4 [RFC3530] and NFSv4.1 [RFC5661];
118 o The iSCSI profile [RFC3722] for use in Internet Small Computer
119 Systems Interface (iSCSI) Names;
120 o EAP [RFC3748];
121 o The Nodeprep and Resourceprep profiles [RFC3920] for use in the
122 Extensible Messaging and Presence Protocol (XMPP), and the XMPP to
123 CPIM mapping [RFC3922] (the latter of these relies on the former);
124 o IRI and URI in XMPP [RFC5122];
125 o The Policy MIB profile [RFC4011] for use in the Simple Network
126 Management Protocol (SNMP);
127 o TLS [RFC4279];
128 o The LDAP profile [RFC4518] for use with LDAP [RFC4511] and its
129 authentication methods [RFC4513];
130 o PKIX subject identification using LDAPprep [RFC4683];
131 o PKIX CRL using LDAPprep [RFC5280];
132 o The SASLprep profile [RFC4013] for use in the Simple
133 Authentication and Security Layer (SASL), and SASL itself
134 [RFC4422];
135 o Plain SASL using SASLprep [RFC4616];
136 o SMTP Auth using SASLprep [RFC4954];
137 o POP3 Auth using SASLprep [RFC5034];
138 o TLS SRP using SASLprep [RFC5054];
139 o SASL SCRAM using SASLprep [RFC5802];
140 o Remote management of Sieve using SASLprep [RFC5804];
141 o NNTP using SASLprep [RFC4643];
142 o IMAP4 using SASLprep [RFC4314];
143 o The trace profile [RFC4505] for use with the SASL ANONYMOUS
144 mechanism;
146 o Internet Application Protocol Collation Registry [RFC4790];
147 o The unicode-casemap Unicode Collation [RFC5051].
149 However, a review (see [ietf78precis]) of these protocol
150 specifications found that they are very similar and can be grouped
151 into a short number of classes. Moreover, many reuse the same
152 Stringprep profile, such as the SASL one.
154 IDNA2003 was replaced because of some limitations described in
155 [RFC4690]. The new IDN specification, called IDNA2008 [RFC5890],
156 [RFC5891], [RFC5892], [RFC5893] was designed based on the
157 considerations found in [RFC5894]. One of the effects of IDNA2008 is
158 that Nameprep and Stringprep are not used at all. Instead, an
159 algorithm based on Unicode properties of codepoints is defined. That
160 algorithm generates a stable and complete table of the supported
161 Unicode codepoints for each Unicode version. This algorithm uses an
162 inclusion-based approach, instead of the exclusion-based approach of
163 Stringprep/Nameprep. That is, IDNA2003 created an explicit list of
164 excluded or mapped-away characters; anything in Unicode 3.2 that was
165 not so listed could be assumed to be allowed under the protocol.
166 IDNA2008 begins instead from the assumption that code points are
167 disallowed, and then relies on Unicode properties to derive whether a
168 given code point actually is allowed in the protocol.
170 This document lists the shortcomings and issues found by protocols
171 listed above that defined Stringprep profiles. It also lists the
172 requirements for any potential replacement of Stringprep.
174 2. Keywords
176 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
177 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
178 document are to be interpreted as described in [RFC2119].
180 This document uses various internationalization terms, which are
181 defined and discussed in [RFC6365].
183 Additionally, this document defines the following keyword:
184 o PRECIS: Preparation and Comparison of Internationalized Strings
186 3. Conventions
188 A single Unicode code point in this memo is denoted by "U+" followed
189 by four to six hexadecimal digits, as used in [Unicode61], Appendix
190 A.
192 4. Stringprep Profiles Limitations
194 During IETF 77 (March 2010), a BOF discussed the current state of the
195 protocols that have defined Stringprep profiles [NEWPREP]. The main
196 conclusions from that discussion were as follows:
197 o Stringprep is bound to version 3.2 of Unicode. Stringprep has not
198 been updated to new versions of Unicode. Therefore, the protocols
199 using Stringprep are stuck at Unicode 3.2, and their
200 specifications need to be updated to support new versions of
201 Unicode.
202 o The protocols would like to not be bound to a specific version of
203 Unicode, but rather have better Unicode version agility in the way
204 of IDNA2008. This is important partly because it is usually
205 impossible for an application to require Unicode 3.2; the
206 application gets whatever version of Unicode is available on the
207 host.
208 o The protocols require better bidirectional support (bidi) than
209 currently offered by Stringprep.
210 o If the protocols are updated to use a new version of Stringprep or
211 another framework, then backward compatibility is an important
212 requirement. For example, Stringprep normalization is based on
213 and profiles may use Unicode Normalization Form KC (NFKC) [UAX15],
214 while IDNA2008 mostly uses Unicode Normalization Form C (NFC)
215 [UAX15].
216 o Identifiers are passed between protocols. For example, the same
217 username string of codepoints may be passed between SASL, XMPP,
218 LDAP and EAP. Therefore, common set of rules or classes of
219 strings are preferred over specific rules for each protocol.
220 Without real planning in advance, many Stringprep profiles reuse
221 other profiles, so this goal was accomplished by accident with
222 Stringprep.
224 Protocols that use Stringprep profiles use strings for different
225 purposes:
226 o XMPP uses a different Stringprep profile for each part of the XMPP
227 address (JID): a localpart which is similar to a username and used
228 for authentication, a domainpart which is a domain name, and a
229 resource part which is less restrictive than the localpart.
230 o iSCSI uses a Stringprep profile for the names of protocol
231 participants (called initiators and targets). The IQN format of
232 iSCSI names contains a reversed DNS domain name.
233 o SASL and LDAP uses a Stringprep profile for usernames.
234 o LDAP uses a set of Stringprep profiles.
236 The apparent judgement of the BOF attendees [NEWPREP] was that it
237 would be highly desirable to have a replacement of Stringprep, with
238 similar characteristics to IDNA2008. That replacement should be
239 defined so that the protocols could use internationalized strings
240 without a lot of specialized internationalization work, since
241 internationalization expertise is not available in the respective
242 protocols or working groups. Accordingly, the IESG formed the PRECIS
243 working group to undertake the task.
245 Notwithstanding the desire evident in [NEWPREP] and the chartering of
246 a working group, IDNA2008 may be a poor model for what other
247 protocols ought to do, because it is designed to support an old
248 protocol that is designed to operate on the scale of the entire
249 Internet. Moreover, IDNA2008 is intended to be deployed without any
250 change to the base DNS protocol. Other protocols may aim at
251 deployment in more local environments, or may have protocol version
252 negotiation built in.
254 5. Major Topics for Consideration
256 This section provides an overview of major topics that a Stringprep
257 replacement needs to address. The headings correspond roughly with
258 categories under which known Stringprep-using protocol RFCs have been
259 evaluated. For the details of those evaluations, see Appendix A.
261 5.1. Comparison
263 5.1.1. Types of Identifiers
265 Following [I-D.iab-identifier-comparison], it is possible to organize
266 identifiers into three classes in respect of how they may be compared
267 with one another:
269 Absolute Identifiers Identifiers that can be compared byte-by-byte
270 for equality.
271 Definite Identifiers Identifiers that have a well-defined comparison
272 algorithm on which all parties agree.
273 Indefinite Identifiers Identifiers that have no single comparison
274 algorithm on which all parties agree.
276 Definite Identifiers include cases like the comparison of Unicode
277 code points in different encodings: they do not match byte for byte,
278 but can all be converted to a single encoding which then does match
279 byte for byte. Indefinite Identifiers are sometimes algorithmically
280 comparable by well-specified subsets of parties. For more discussion
281 of these categories, see [I-D.iab-identifier-comparison].
283 The section on treating the existing known cases, Appendix A uses the
284 categories above.
286 5.1.2. Effect of comparison
288 The three classes of comparison style outlined in Section 5.1.1 may
289 have different effects when applied. It is necessary to evaluate the
290 effects if a comparison results in a false positive, and what the
291 effects are if a comparison results in a false negative, especially
292 in terms of the consequences to security and usability.
294 5.2. Dealing with characters
296 This section outlines a range of issues having to do with characters
297 in the target protocols, and outlines the ways in which IDNA2008
298 might be a good analogy to other protocols, and ways in which it
299 might be a poor one.
301 5.2.1. Case folding, case sensitivity, and case preservation
303 In IDNA2003, labels are always mapped to lower case before the
304 Punycode transformation. In IDNA2008, there is no mapping at all:
305 input is either a valid U-label or it is not. At the same time,
306 upper-case characters are by definition not valid U-labels, because
307 they fall into the Unstable category (category B) of [RFC5892].
309 If there are protocols that require upper and lower cases be
310 preserved, then the analogy with IDNA2008 will break down.
311 Accordingly, existing protocols are to be evaluated according to the
312 following criteria:
314 1. Does the protocol use case folding? For all blocks of code
315 points, or just for certain subsets?
316 2. Is the system or protocol case sensitive?
317 3. Does the system or protocol preserve case?
319 5.2.2. Stringprep and NFKC
321 Stringprep profiles may use normalization. If they do, they use NFKC
322 [UAX15] (most profiles do). It is not clear that NFKC is the right
323 normalization to use in all cases. In [UAX15], there is the
324 following observation regarding Normalization Forms KC and KD: "It is
325 best to think of these Normalization Forms as being like uppercase or
326 lowercase mappings: useful in certain contexts for identifying core
327 meanings, but also performing modifications to the text that may not
328 always be appropriate." In general, it can be said that NFKC is more
329 aggressive about finding matches between codepoints than NFC. For
330 things like the spelling of users' names, then, NFKC may not be the
331 best form to use. At the same time, one of the nice things about
332 NFKC is that it deals with the width of characters that are otherwise
333 similar, by canonicalizing half-width to full-width. This mapping
334 step can be crucial in practice. A replacement for Stringprep
335 depends on analyzing the different use profiles and considering
336 whether NFKC or NFC is a better normalization for each profile.
338 For the purposes of evaluating an existing example of Stringprep use,
339 it is helpful to know whether it uses no normalization, NFKC, or NFC.
341 5.2.3. Character mapping
343 Along with the case mapping issues raised in Section 5.2.1, there is
344 the question of whether some characters are mapped either to other
345 characters or to nothing during Stringprep. [RFC3454], Section 3,
346 outlines a number of characters that are mapped to nothing, and also
347 permits Stringprep profiles to define their own mappings.
349 5.2.4. Prohibited characters
351 Along with case folding and other character mappings, many protocols
352 have characters that are simply disallowed. For example, control
353 characters and special characters such as "@" or "/" may be
354 prohibited in a protocol.
356 One of the primary changes of IDNA2008 is in the way it approaches
357 Unicode code points, using the new inclusion-based approach (see
358 Section 1).
360 Because of the default assumption in IDNA2008 that a code point is
361 not allowed by the protocol, it has more than one class of "allowed
362 by the protocol"; this is unlike IDNA2003. While some code points
363 are disallowed outright, some are allowed only in certain contexts.
364 The reasons for the context-dependent rules have to do with the way
365 some characters are used. For instance, the ZERO WIDTH JOINER and
366 ZERO WIDTH NON-JOINER (ZWJ, U+200D and ZWNJ, U+200C) are allowed with
367 contextual rules because they are required in some circumstances, yet
368 are considered punctuation by Unicode and would therefore be
369 DISALLOWED under the usual IDNA2008 derivation rules. The goal of
370 IDNA2008 is to provide the widest repertoire of code points possible
371 and consistent with the traditional DNS "LDH" (letters, digits,
372 hyphen; see [RFC0952]) rule, trusting to the operators of individual
373 zones to make sensible (and usually more restrictive) policies for
374 their zones.
376 5.2.5. Internal structure, delimiters, and special characters
378 IDNA2008 has a special problem with delimiters, because the delimiter
379 "character" in the DNS wire format is not really part of the data.
380 In DNS, labels are not separated exactly; instead, a label carries
381 with it an indicator that says how long the label is. When the label
382 is presented in presentation format as part of a fully qualified
383 domain name, the label separator FULL STOP, U+002E (.) is used to
384 break up the labels. But because that label separator does not
385 travel with the wire format of the domain name, there is no way to
386 encode a different, "internationalized" separator in IDNA2008.
388 Other protocols may include characters with similar special meaning
389 within the protocol. Common characters for these purposes include
390 FULL STOP, U+002E (.); COMMERCIAL AT, U+0040 (@); HYPHEN-MINUS,
391 U+002D (-); SOLIDUS, U+002F (/); and LOW LINE, U+005F (_). The mere
392 inclusion of such a character in the protocol is not enough for it to
393 be considered similar to another protocol using the same character;
394 instead, handling of the character must be taken into consideration
395 as well.
397 An important issue to tackle here is whether it is valuable to map to
398 or from these special characters as part of the Stringprep
399 replacement. In some locales, the analogue to FULL STOP, U+002E is
400 some other character, and users may expect to be able to substitute
401 their normal stop for FULL STOP, U+002E. At the same time, there are
402 predictability arguments in favour of treating identifiers with FULL
403 STOP, U+002E in them just the way they are treated under IDNA2008.
405 5.2.6. Restrictions because of glyph similarity
407 Homoglyphs are similarly (or identically) rendered glyphs of
408 different codepoints. For DNS names, homoglyphs may enable phishing.
409 If a protocol requires some visual comparison by end-users, then the
410 issue of homoglyphs is to be considered. In the DNS context, theses
411 issues are documented in [RFC5894] and [RFC4690]. IDNA2008 does not,
412 however, have a mechanism to deal with them, trusting to DNS zone
413 operators to enact sensible policies for the subset of Unicode they
414 wish to support, given their user community. A similar policy/
415 protocol split may not be desirable in every protocol.
417 5.3. Where the data comes from and where it goes
419 5.3.1. User input and the source of protocol elements
421 Some protocol elements are provided by users, and others are not.
422 Those that are not may presumably be subject to greater restrictions,
423 whereas those that users provide likely need to permit the broadest
424 range of code points. The following questions are helpful:
426 1. Do users input the strings directly?
427 2. If so, how? (keyboard, stylus, voice, copy-paste, etc.)
428 3. Where do we place the dividing line between user interface and
429 protocol? (see [RFC5895])
431 5.3.2. User output
433 Just as only some protocol elements are expected to be entered
434 directly by users, only some protocol elements are intended to be
435 consumed directly by users. It is important to know how users are
436 expected to be able to consume the protocol elements, because
437 different environments present different challenges. An element that
438 is only ever delivered as part of a vCard remains in machine-readable
439 format, so the problem of visual confusion is not a great one. Is
440 the protocol element published as part of a vCard, a web directory,
441 on a business card, or on "the side of a bus"? Do users use the
442 protocol element as an identifier (which means that they might enter
443 it again in some other context)? (See also Section 5.2.6.)
445 5.3.3. Operations
447 Some strings are useful as part of the protocol but are not used as
448 input to other operations (for instance, purely informative or
449 descriptive text). Other strings are used directly as input to other
450 operations (such as cryptographic hash functions), or are used
451 together with other strings to (such as concatenating a string with
452 some others to form a unique identifier).
454 5.3.3.1. String classes
456 Strings often have a similar function in different protocols. For
457 instance, many different protocols contain user identifiers or
458 passwords. A single profile for all such uses might be desirable.
460 Often, a string in a protocol is effectively a protocol element from
461 another protocol. For instance, different systems might use the same
462 credentials database for authentication.
464 5.3.3.2. Community Considerations
466 A Stringprep replacement that does anything more than just update
467 Stringprep to the latest version of Unicode will probably entail some
468 changes. It is important to identify the willingness of the
469 protocol-using community to accept backwards-incompatible changes.
470 By the same token, it is important to evaluate the desire of the
471 community for features not available under Stringprep.
473 5.3.3.3. Unicode Incompatible Changes
475 IDNA2008 uses an algorithm to derive the validity of a Unicode code
476 point for use under IDNA2008. It does this by using the properties
477 of each code point to test its validity.
479 This approach depends crucially on the idea that code points, once
480 valid for a protocol profile, will not later be made invalid. That
481 is not a guarantee currently provided by Unicode. Properties of code
482 points may change between versions of Unicode. Rarely, such a change
483 could cause a given code point to become invalid under a protocol
484 profile, even though the code point would be valid with an earlier
485 version of Unicode. This is not merely a theoretical possibility,
486 because it has occurred ([RFC6452]).
488 Accordingly, as in IDNA2008, a Stringprep replacement that intends to
489 be Unicode version agnostic will need to work out a mechanism to
490 address cases where incompatible changes occur because of new Unicode
491 versions.
493 6. Considerations for Stringprep replacement
495 The above suggests the following guidance:
496 o A Stringprep replacement should be defined.
497 o The replacement should take an approach similar to IDNA2008, (e.g.
498 by using codepoint properties instead of codepoint whitelisting)
499 in that it enables better Unicode agility.
500 o Protocols share similar characteristics of strings. Therefore,
501 defining internationalization preparation algorithms for the
502 smallest set of string classes may be sufficient for most cases,
503 providing coherence among a set of related protocols or protocols
504 where identifiers are exchanged.
505 o The sets of string classes need to be evaluated according to the
506 considerations that make up the headings in Section 5
507 o It is reasonable to limit scope to Unicode code points, and rule
508 the mapping of data from other character encodings outside the
509 scope of this effort.
510 o The replacement ought at least to provide guidance to applications
511 using the replacement on how to handle protocol incompatibilities
512 resulting from changes to Unicode. In an ideal world, the
513 Stringprep replacement would handle the changes automatically, but
514 it appears that such automatic handling would require magic and
515 cannot be expected.
516 o Compatibility within each protocol between a technique that is
517 Stringprep-based and the technique's replacement has to be
518 considered very carefully.
520 Existing deployments already depend on Stringprep profiles.
521 Therefore, a replacement must consider the effects of any new
522 strategy on existing deployments. By way of comparison, it is worth
523 noting that some characters were acceptable in IDNA labels under
524 IDNA2003, but are not protocol-valid under IDNA2008 (and conversely);
525 disagreement about what to do during the transition has resulted in
526 different approaches to mapping. Different implementers may make
527 different decisions about what to do in such cases; this could have
528 interoperability effects. It is necessary to trade better support
529 for different linguistic environments against the potential side
530 effects of backward incompatibility.
532 7. Security Considerations
534 This document merely states what problems are to be solved, and does
535 not define a protocol. There are undoubtedly security implications
536 of the particular results that will come from the work to be
537 completed. Moreover, the Stringprep Security Considerations
538 [RFC3454] Section applies. See also the analysis in the subsections
539 of Appendix B, below.
541 8. IANA Considerations
543 This document has no actions for IANA.
545 9. Discussion home for this draft
547 Note: RFC-Editor, please remove this section before publication.
549 This document is intended to define the problem space discussed on
550 the precis@ietf.org mailing list.
552 10. Acknowledgements
554 This document is the product of the PRECIS IETF Working Group, and
555 participants in that Working Group were helpful in addressing issues
556 with the text.
558 Specific contributions came from David Black, Alan DeKok, Simon
559 Josefsson, Bill McQuillan, Alexey Melnikov, Peter Saint-Andre, Dave
560 Thaler, and Yoshiro Yoneya.
562 Dave Thaler provided the "buckets" insight in Section 5.1.1, central
563 to the organization of the problem.
565 Evaluations of Stringprep profiles that are included in Appendix B
566 were done by: David Black, Alexey Melnikov, Peter Saint-Andre, Dave
567 Thaler.
569 11. Informative References
571 [I-D.iab-identifier-comparison]
572 Thaler, D., "Issues in Identifier Comparison for Security
573 Purposes", draft-iab-identifier-comparison-07 (work in
574 progress), August 2012.
576 [NEWPREP] "Newprep BoF Meeting Minutes", March 2010.
578 [RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
579 host table specification", RFC 952, October 1985.
581 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
582 Requirement Levels", BCP 14, RFC 2119, March 1997.
584 [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
585 Internationalized Strings ("stringprep")", RFC 3454,
586 December 2002.
588 [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
589 "Internationalizing Domain Names in Applications (IDNA)",
590 RFC 3490, March 2003.
592 [RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
593 Profile for Internationalized Domain Names (IDN)",
594 RFC 3491, March 2003.
596 [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
597 for Internationalized Domain Names in Applications
598 (IDNA)", RFC 3492, March 2003.
600 [RFC3530] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
601 Beame, C., Eisler, M., and D. Noveck, "Network File System
602 (NFS) version 4 Protocol", RFC 3530, April 2003.
604 [RFC3722] Bakke, M., "String Profile for Internet Small Computer
605 Systems Interface (iSCSI) Names", RFC 3722, April 2004.
607 [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
608 Levkowetz, "Extensible Authentication Protocol (EAP)",
609 RFC 3748, June 2004.
611 [RFC3920] Saint-Andre, P., Ed., "Extensible Messaging and Presence
612 Protocol (XMPP): Core", RFC 3920, October 2004.
614 [RFC3922] Saint-Andre, P., "Mapping the Extensible Messaging and
615 Presence Protocol (XMPP) to Common Presence and Instant
616 Messaging (CPIM)", RFC 3922, October 2004.
618 [RFC4011] Waldbusser, S., Saperia, J., and T. Hongal, "Policy Based
619 Management MIB", RFC 4011, March 2005.
621 [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names
622 and Passwords", RFC 4013, February 2005.
624 [RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
625 for Transport Layer Security (TLS)", RFC 4279,
626 December 2005.
628 [RFC4314] Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
629 RFC 4314, December 2005.
631 [RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
632 Security Layer (SASL)", RFC 4422, June 2006.
634 [RFC4505] Zeilenga, K., "Anonymous Simple Authentication and
635 Security Layer (SASL) Mechanism", RFC 4505, June 2006.
637 [RFC4511] Sermersheim, J., "Lightweight Directory Access Protocol
638 (LDAP): The Protocol", RFC 4511, June 2006.
640 [RFC4513] Harrison, R., "Lightweight Directory Access Protocol
641 (LDAP): Authentication Methods and Security Mechanisms",
642 RFC 4513, June 2006.
644 [RFC4518] Zeilenga, K., "Lightweight Directory Access Protocol
645 (LDAP): Internationalized String Preparation", RFC 4518,
646 June 2006.
648 [RFC4616] Zeilenga, K., "The PLAIN Simple Authentication and
649 Security Layer (SASL) Mechanism", RFC 4616, August 2006.
651 [RFC4643] Vinocur, J. and K. Murchison, "Network News Transfer
652 Protocol (NNTP) Extension for Authentication", RFC 4643,
653 October 2006.
655 [RFC4683] Park, J., Lee, J., Lee, H., Park, S., and T. Polk,
656 "Internet X.509 Public Key Infrastructure Subject
657 Identification Method (SIM)", RFC 4683, October 2006.
659 [RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
660 Recommendations for Internationalized Domain Names
661 (IDNs)", RFC 4690, September 2006.
663 [RFC4790] Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
664 Application Protocol Collation Registry", RFC 4790,
665 March 2007.
667 [RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension
668 for Authentication", RFC 4954, July 2007.
670 [RFC5034] Siemborski, R. and A. Menon-Sen, "The Post Office Protocol
671 (POP3) Simple Authentication and Security Layer (SASL)
672 Authentication Mechanism", RFC 5034, July 2007.
674 [RFC5051] Crispin, M., "i;unicode-casemap - Simple Unicode Collation
675 Algorithm", RFC 5051, October 2007.
677 [RFC5054] Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
678 "Using the Secure Remote Password (SRP) Protocol for TLS
679 Authentication", RFC 5054, November 2007.
681 [RFC5122] Saint-Andre, P., "Internationalized Resource Identifiers
682 (IRIs) and Uniform Resource Identifiers (URIs) for the
683 Extensible Messaging and Presence Protocol (XMPP)",
684 RFC 5122, February 2008.
686 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
687 Housley, R., and W. Polk, "Internet X.509 Public Key
688 Infrastructure Certificate and Certificate Revocation List
689 (CRL) Profile", RFC 5280, May 2008.
691 [RFC5456] Spencer, M., Capouch, B., Guy, E., Miller, F., and K.
692 Shumard, "IAX: Inter-Asterisk eXchange Version 2",
693 RFC 5456, February 2010.
695 [RFC5661] Shepler, S., Eisler, M., and D. Noveck, "Network File
696 System (NFS) Version 4 Minor Version 1 Protocol",
697 RFC 5661, January 2010.
699 [RFC5802] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
700 "Salted Challenge Response Authentication Mechanism
701 (SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010.
703 [RFC5804] Melnikov, A. and T. Martin, "A Protocol for Remotely
704 Managing Sieve Scripts", RFC 5804, July 2010.
706 [RFC5890] Klensin, J., "Internationalized Domain Names for
707 Applications (IDNA): Definitions and Document Framework",
708 RFC 5890, August 2010.
710 [RFC5891] Klensin, J., "Internationalized Domain Names in
711 Applications (IDNA): Protocol", RFC 5891, August 2010.
713 [RFC5892] Faltstrom, P., "The Unicode Code Points and
714 Internationalized Domain Names for Applications (IDNA)",
715 RFC 5892, August 2010.
717 [RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
718 Internationalized Domain Names for Applications (IDNA)",
719 RFC 5893, August 2010.
721 [RFC5894] Klensin, J., "Internationalized Domain Names for
722 Applications (IDNA): Background, Explanation, and
723 Rationale", RFC 5894, August 2010.
725 [RFC5895] Resnick, P. and P. Hoffman, "Mapping Characters for
726 Internationalized Domain Names in Applications (IDNA)
727 2008", RFC 5895, September 2010.
729 [RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
730 Internationalization in the IETF", BCP 166, RFC 6365,
731 September 2011.
733 [RFC6452] Faltstrom, P. and P. Hoffman, "The Unicode Code Points and
734 Internationalized Domain Names for Applications (IDNA) -
735 Unicode 6.0", RFC 6452, November 2011.
737 [UAX15] "Unicode Standard Annex #15: Unicode Normalization Forms",
738 UAX 15, September 2009.
740 [Unicode61]
741 The Unicode Consortium. The Unicode Standard, Version
742 6.1, defined by:, "The Unicode Standard -- Version 6.1",
743 (Mountain View, CA: The Unicode Consortium, 2012. ISBN
744 978-1-936213-02-3), September 2009,
745 .
747 [ietf78precis]
748 Blanchet, M., "PRECIS Framework", Proceedings of the
749 Seventy-Eighth Internet Engineering Task
750 Force https://www.ietf.org/proceedings/78/, July 2010,
751 .
753 Appendix A. Classification of Stringprep Profiles
755 A number of the known cases of Stringprep use were evaluated during
756 the preparation of this document. The known cases are here described
757 in two ways. The types of identifiers the protocol uses is first
758 called out in the ID type column (from Section 5.1.1), using the
759 short forms "a" for Absolute, "d" for Definite, and "i" for
760 Indefinite. Next, there is a column that contains an "i" if the
761 protocol string comes from user input, an "o" if the protocol string
762 becomes user-facing output, "b" if both are true, and "n" if neither
763 is true.
765 +------+--------+-------+
766 | RFC | IDtype | User? |
767 +------+--------+-------+
768 | 3722 | a | b |
769 | 3748 | - | - |
770 | 3920 | a,d | b |
771 | 4505 | a | i |
772 | 4314 | a,d | b |
773 | 4954 | a,d | b |
774 | 5034 | a,d | b |
775 | 5804 | a,d | b |
776 +------+--------+-------+
778 Table 1
780 Appendix B. Evaluation of Stringprep Profiles
782 This section is a summary of evaluation of Stringprep profiles that
783 was done to get a good understanding of the usage of Stringprep.
784 This summary is by no means normative nor the actual evaluations
785 themselves. A template was used for reviewers to get a coherent view
786 of all evaluations.
788 B.1. iSCSI Stringprep Profile: RFC3722 (and RFC3721, RFC3720)
790 Description: An iSCSI session consists of an initiator (i.e., host
791 or server that uses storage) communicating with a target (i.e., a
792 storage array or other system that provides storage). Both the
793 iSCSI initiator and target are named by iSCSI Names. The iSCSI
794 Stringprep profile is used for iSCSI names.
795 How it is used: iSCSI initiators and targets (see above). They can
796 also be used to identify SCSI ports (these are software entities
797 in the iSCSI protocol, not hardware ports), and iSCSI logical
798 units (storage volumes), although both are unusual in practice.
800 What entities create these identifiers? Generally a Human user (1)
801 configures an Automated system (2) that generates the names.
802 Advance configuration of the system is required due to the
803 embedded use of external unique identifier (from the DNS or IEEE).
804 How is the string input in the system? Keyboard and copy-paste are
805 common. Copy-paste is common because iSCSI names are long enough
806 to be problematic for humans to remember, causing use of email,
807 sneaker-net, text files, etc. to avoid mistype mistakes.
808 Where do we place the dividing line between user interface and
809 protocol? The iSCSI protocol requires that all internationalization
810 string preparation occur in the user interface. The iSCSI
811 protocol treats iSCSI names as opaque identifiers that are
812 compared byte-by-byte for equality. iSCSI names are generally not
813 checked for correct formatting by the protocol.
814 What entities enforce the rules? There are no iSCSI-specific
815 enforcement entities, although the use of unique identifier
816 information in the names relies on DNS registrars and the IEEE
817 Registration Authority.
818 Comparison Byte-by-byte
819 Case Folding, Sensitivity, Preservation Case folding is required for
820 the code blocks specified in RFC 3454, Table B.2. The overall
821 iSCSI naming system (UI + protocol) is case-insensitive.
822 What is the impact if the comparison results in a false positive?
823 Potential access to the wrong storage. - If the initiator has no
824 access to the wrong storage, an authentication failure is the
825 probable result. - If the initiator has access to the wrong
826 storage, the resulting mis-identification could result in use of
827 the wrong data and possible corruption of stored data.
828 What is the impact if the comparison results in a false negative?
829 Denial of authorized storage access.
830 What are the security impacts? iSCSI names may be used as the
831 authentication identities for storage systems. Comparison
832 problems could result in authentication problems, although note
833 that authentication failure ameliorates some of the false positive
834 cases.
835 Normalization NFKC, as specified by RFC 3454.
836 Mapping Yes, as specified by table B.1 in RFC 3454
837 Disallowed Characters Only the following characters are allowed: -
838 ASCII dash, dot, colon - ASCII lower case letters and digits -
839 Unicode lower case characters as specified by RFC 3454 All other
840 characters are disallowed.
841 Which other strings or identifiers are these most similar to? None -
842 iSCSI names are unique to iSCSI.
843 Are these strings or identifiers sometimes the same as strings or
844 identifiers from other protocols? No
845 Does the identifier have internal structure that needs to be
846 respected? Yes - ASCII dot, dash and colon are used for internal
847 name structure. These are not reserved characters in that they
848 can occur in the name in locations other than those used for
849 structuring purposes (e.g., only the first occurrence of a colon
850 character is structural, others are not).
851 How are users exposed to these strings? How are they published?
852 iSCSI names appear in server and storage system configuration
853 interfaces. They also appear in system logs.
854 Is the string / identifier used as input to other operations?
855 Effectively, no. The rarely used port and logical unit names
856 involve concatenation, which effectively extends a unique iSCSI
857 Name for a target to uniquely identify something within that
858 target.
859 How much tolerance for change from existing Stringprep approach?
860 Good tolerance; the community would prefer that
861 internationalization experts solve internationalization problems.
862 How strong a desire for change (e.g., for Unicode agility)? Unicode
863 agility is desired in principle as long as nothing significant
864 breaks.
866 B.2. SMTP/POP3/ManageSieve Stringprep Profiles: RFC4954,RFC5034,RFC
867 5804
869 Description: Authorization identity (user identifier) exchanged
870 during SASL authentication: AUTH (SMTP/POP3) or AUTHENTICATE
871 (ManageSieve) command.
872 How It's Used: Used for proxy authorization, e.g. to [lawfully]
873 impersonate a particular user after a privileged authentication
874 Who Generates It: Typically generated by email system administrators
875 using some tools/conventions, sometimes from some backend
876 database. - In some setups human users can register own usernames
877 (e.g. webmail self registration)
878 User Input Methods: - Typed by user / selected from a list - Copy-
879 and-paste - Perhaps voice input - Can also be specified in
880 configuration files or on a command line
881 Enforcement: - Rules enforced by server / add-on service (e.g.,
882 gateway service) on registration of account
883 Comparison Method: "Type 1" (byte-for-byte) or "type 2" (compare by
884 a common algorithm that everyone agrees on (e.g., normalize and
885 then compare the result byte-by-byte))
886 Case Folding, Sensitivity, Preservation: Most likely case sensitive.
887 Exact requirements on case-sensitivity/case-preservation depend on
888 a specific implementation, e.g. an implementation might treat all
889 user identifiers as case insensitive (or case insensitive for US-
890 ASCII subset only).
892 Impact of Comparison: False positives: - an unauthorized user is
893 allowed email service access (login) False negatives: - an
894 authorized user is denied email service access
895 Normalization: NFKC (as per RFC 4013)
896 Mapping: (see Section 2 of RFC 4013 for the full list): Non ASCII
897 spaces are mapped to space, etc.
898 Disallowed Characters: (see Section 2 of RFC 4013 for the full
899 list): Unicode Control characters, etc.
900 String Classes: - simple username. See Section 2 of RFC 4013 for
901 details on restrictions. Note that some implementations allow
902 spaces in these. While implementations are not required to use a
903 specific format, an authorization identity frequently has the same
904 format as an email address (and EAI email address in the future),
905 or as a left hand side of an email address. Note: whatever is
906 recommended for SMTP/POP/ManageSieve authorization identity should
907 also be used for IMAP authorization identities, as IMAP/POP3/SMTP/
908 ManageSieve are frequently implemented together.
909 Internal Structure: None
910 User Output: Unlikely, but possible. For example, if it is the same
911 as an email address.
912 Operations: - Sometimes concatenated with other data and then used
913 as input to a cryptographic hash function
914 How much tolerance for change from existing Stringprep approach? Not
915 sure.
916 Background information: In RFC 5034, when describing the POP3 AUTH
917 command: The authorization identity generated by the SASL exchange
918 is a simple username, and SHOULD use the SASLprep profile (see
919 RFC4013) of the StringPrep algorithm (see RFC3454) to prepare
920 these names for matching. If preparation of the authorization
921 identity fails or results in an empty string (unless it was
922 transmitted as the empty string), the server MUST fail the
923 authentication. In RFC 4954, when describing the SMTP AUTH
924 command: The authorization identity generated by this SASL
925 exchange is a "simple username" (in the sense defined in
926 SASLprep), and both client and server SHOULD (*) use the SASLprep
927 profile of the StringPrep algorithm to prepare these names for
928 transmission or comparison. If preparation of the authorization
929 identity fails or results in an empty string (unless it was
930 transmitted as the empty string), the server MUST fail the
931 authentication. (*) Note: Future revision of this specification
932 may change this requirement to MUST. Currently, the SHOULD is
933 used in order to avoid breaking the majority of existing
934 implementations. In RFC 5804, when describing the ManageSieve
935 AUTHENTICATE command: The authorization identity generated by this
936 SASL exchange is a "simple username" (in the sense defined in
937 SASLprep), and both client and server MUST use the SASLprep
938 profile of the StringPrep algorithm to prepare these names for
939 transmission or comparison. If preparation of the authorization
940 identity fails or results in an empty string (unless it was
941 transmitted as the empty string), the server MUST fail the
942 authentication.
944 B.3. IMAP Stringprep Profiles: RFC5738, RFC4314: Usernames
946 Evaluation Note These documents have 2 types of strings (usernames
947 and passwords), so there are two separate templates.
948 Description: "username" parameter to the IMAP LOGIN command,
949 identifiers in IMAP ACL commands. Note that any valid username is
950 also an IMAP ACL identifier, but IMAP ACL identifiers can include
951 other things like name of group of users.
952 How It's Used: Used for authentication (Usernames), or in IMAP
953 Access Control Lists (Usernames or Group names)
954 Who Generates It: - Typically generated by email system
955 administrators using some tools/conventions, sometimes from some
956 backend database. - In some setups human users can register own
957 usernames (e.g. webmail self registration)
958 User Input Methods: - Typed by user / selected from a list - Copy-
959 and-paste - Perhaps voice input - Can also be specified in
960 configuration files or on a command line
961 Enforcement: - Rules enforced by server / add-on service (e.g.,
962 gateway service) on registration of account
963 Comparison Method: Type 1" (byte-for-byte) or "type 2" (compare by a
964 common algorithm that everyone agrees on (e.g., normalize and then
965 compare the result byte-by-byte))
966 Case Folding, Sensitivity, Preservation: - Most likely case
967 sensitive. Exact requirements on case-sensitivity/
968 case-preservation depend on a specific implementation, e.g. an
969 implementation might treat all user identifiers as case
970 insensitive (or case insensitive for US-ASCII subset only).
971 Impact of Comparison: False positives: - an unauthorized user is
972 allowed IMAP access (login) - improperly grant privileges (e.g.,
973 access to a specific mailbox, ability to manage ACLs for a
974 mailbox) False negatives: - an authorized user is denied IMAP
975 access - unable to use granted privileges (e.g., access to a
976 specific mailbox, ability to manage ACLs for a mailbox)
977 Normalization: NFKC (as per RFC 4013)
978 Mapping: (see Section 2 of RFC 4013 for the full list): non ASCII
979 spaces are mapped to space
980 Disallowed Characters: (see Section 2 of RFC 4013 for the full
981 list): Unicode Control characters, etc.
982 String Classes: - simple username. See Section 2 of RFC 4013 for
983 details on restrictions. Note that some implementations allow
984 spaces in these. While IMAP implementations are not required to
985 use a specific format, an IMAP username frequently has the same
986 format as an email address (and EAI email address in the future),
987 or as a left hand side of an email address. Note: whatever is
988 recommended for IMAP username should also be used for ManageSieve,
989 POP3 and SMTP authorization identities, as IMAP/POP3/SMTP/
990 ManageSieve are frequently implemented together.
991 Internal Structure: None
992 User Output: Unlikely, but possible. For example, if it is the same
993 as an email address. - access control lists (e.g. in IMAP ACL
994 extension), both when managing membership and listing membership
995 of existing access control lists. - often show up as mailbox names
996 (under Other Users IMAP namespace)
997 Operations: - Sometimes concatenated with other data and then used
998 as input to a cryptographic hash function
999 How much tolerance for change from existing Stringprep approach? Not
1000 sure. Non-ASCII IMAP usernames are currently prohibited by IMAP
1001 (RFC 3501). However they are allowed when used in IMAP ACL
1002 extension.
1004 B.4. IMAP Stringprep Profiles: RFC5738: Passwords
1006 Description: "Password" parameter to the IMAP LOGIN command
1007 How It's Used: Used for authentication (Passwords)
1008 Who Generates It: Either generated by email system administrators
1009 using some tools/conventions, or specified by the human user.
1010 User Input Methods: - Typed by user - Copy-and-paste - Perhaps voice
1011 input - Can also be specified in configuration files or on a
1012 command line
1013 Enforcement: Rules enforced by server / add-on service (e.g.,
1014 gateway service or backend databse) on registration of account
1015 Comparison Method: "Type 1" (byte-for-byte)
1016 Case Folding, Sensitivity, Preservation: Most likely case sensitive.
1017 Impact of Comparison: False positives: - an unauthorized user is
1018 allowed IMAP access (login) False negatives: - an authorized user
1019 is denied IMAP access
1020 Normalization: NFKC (as per RFC 4013)
1021 Mapping: (see Section 2 of RFC 4013 for the full list): non ASCII
1022 spaces are mapped to space
1023 Disallowed Characters: (see Section 2 of RFC 4013 for the full
1024 list): Unicode Control characters, etc.
1025 String Classes: Currently defined as "simple username" (see Section
1026 2 of RFC 4013 for details on restrictions.), however this is
1027 likely to be a different class from usernames. Note that some
1028 implementations allow spaces in these. Password in all email
1029 related protocols should be treated in the same way. Same
1030 passwords are frequently shared with web, IM, etc. applications.
1031 Internal Structure: None
1032 User Output: - text of email messages (e.g. in "you forgot your
1033 password" email messages) - web page / directory - side of the bus
1034 / in ads -- possible
1035 Operations: Sometimes concatenated with other data and then used as
1036 input to a cryptographic hash function. Frequently stored as is,
1037 or hashed.
1038 How much tolerance for change from existing Stringprep approach? Not
1039 sure. Non-ASCII IMAP passwords are currently prohibited by IMAP
1040 (RFC 3501), however they are likely to be in widespread use.
1041 Background information: RFC 5738 (IMAP INTERNATIONALIZATION): 5.
1042 UTF8=USER Capability If the "UTF8=USER" capability is advertised,
1043 that indicates the server accepts UTF-8 user names and passwords
1044 and applies SASLprep RFC4013 to both arguments of the LOGIN
1045 command. The server MUST reject UTF-8 that fails to comply with
1046 the formal syntax in RFC 3629 RFC3629 or if it encounters Unicode
1047 characters listed in Section 2.3 of SASLprep RFC 4013 RFC4013.
1048 RFC 4314 (IMAP4 Access Control List (ACL) Extension): 3. Access
1049 control management commands and responses Servers, when processing
1050 a command that has an identifier as a parameter (i.e., any of
1051 SETACL, DELETEACL, and LISTRIGHTS commands), SHOULD first prepare
1052 the received identifier using "SASLprep" profile SASLprep of the
1053 "Stringprep" algorithm Stringprep. If the preparation of the
1054 identifier fails or results in an empty string, the server MUST
1055 refuse to perform the command with a BAD response. Note that
1056 Section 6 recommends additional identifier's verification steps.
1057 and in Section 6: This document relies on SASLprep to describe
1058 steps required to perform identifier canonicalization
1059 (preparation). The preparation algorithm in SASLprep was
1060 specifically designed such that its output is canonical, and it is
1061 well-formed. However, due to an anomaly PR29 in the specification
1062 of Unicode normalization, canonical equivalence is not guaranteed
1063 for a select few character sequences. Identifiers prepared with
1064 SASLprep can be stored and returned by an ACL server. The anomaly
1065 affects ACL manipulation and evaluation of identifiers containing
1066 the selected character sequences. These sequences, however, do
1067 not appear in well-formed text. In order to address this problem,
1068 an ACL server MAY reject identifiers containing sequences
1069 described in PR29 by sending the tagged BAD response. This is in
1070 addition to the requirement to reject identifiers that fail
1071 SASLprep preparation as described in Section 3.
1073 B.5. Anonymous SASL Stringprep Profiles: RFC4505
1075 Description: RFC 4505 defines a "trace" field:
1077 Comparison: this field is not intended for comparison (only used for
1078 logging)
1079 Case folding; case sensitivity, preserve case: No case folding/case
1080 sensitive
1081 Do users input the strings directly? Yes. Possibly entered in
1082 configuration UIs, or on a command line. Can also be stored in
1083 configuration files. The value can also be automatically
1084 generated by clients (e.g. a fixed string is used, or a user's
1085 email address).
1086 How users input strings? Keyboard/voice, stylus (pick from a list).
1087 Copy-paste - possibly.
1088 Normalization: None
1089 Disallowed Characters Control characters are disallowed. (See
1090 Section 3 of RFC 4505)
1091 Which other strings or identifiers are these most similar to? RFC
1092 4505 says that the trace "should take one of two forms: an
1093 Internet email address, or an opaque string that does not contain
1094 the '@' U+0040) character and that can be interpreted by the
1095 system administrator of the client's domain." In practice, this
1096 is a freeform text, so it belongs to a different class from "email
1097 address" or "username".
1098 Are these strings or identifiers sometimes the same as strings or
1099 identifiers from other protocols (e.g., does an IM system sometimes
1100 use the same credentials database for authentication as an email
1101 system)? Yes: see above. However there is no strong need to keep
1102 them consistent in the future.
1103 How are users exposed to these strings, how are they published? No.
1104 However, The value can be seen in server logs
1105 Impacts of false positives and false negatives: False positive: a
1106 user can be confused with another user. False negative: two
1107 distinct users are treated as the same user. But note that the
1108 trace field is not authenticated, so it can be easily falsified.
1109 Tolerance of changes in the community The community would be
1110 flexible.
1111 Delimiters No internal structure, but see comments above about
1112 frequent use of email addresses.
1113 Background information: The Anonymous Mechanism The mechanism
1114 consists of a single message from the client to the server. The
1115 client may include in this message trace information in the form
1116 of a string of UTF-8-encoded Unicode characters prepared in
1117 accordance with StringPrep and the "trace" Stringprep profile
1118 defined in Section 3 of this document. The trace information,
1119 which has no semantical value, should take one of two forms: an
1120 Internet email address, or an opaque string that does not contain
1121 the '@' (U+0040) character and that can be interpreted by the
1122 system administrator of the client's domain. For privacy reasons,
1123 an Internet email address or other information identifying the
1124 user should only be used with permission from the user. 3. The
1125 "trace" Profile of "Stringprep" This section defines the "trace"
1126 profile of StringPrep. This profile is designed for use with the
1127 SASL ANONYMOUS Mechanism. Specifically, the client is to prepare
1128 the message production in accordance with this profile. The
1129 character repertoire of this profile is Unicode 3.2. No mapping
1130 is required by this profile. No Unicode normalization is required
1131 by this profile. The list of unassigned code points for this
1132 profile is that provided in Appendix A of StringPrep. Unassigned
1133 code points are not prohibited. Characters from the following
1134 tables of StringPrep are prohibited: - C.2.1 (ASCII control
1135 characters) - C.2.2 (Non-ASCII control characters) - C.3 (Private
1136 use characters) - C.4 (Non-character code points) - C.5 (Surrogate
1137 codes) - C.6 (Inappropriate for plain text) - C.8 (Change display
1138 properties are deprecated) - C.9 (Tagging characters) No
1139 additional characters are prohibited. This profile requires
1140 bidirectional character checking per Section 6 of StringPrep.
1142 B.6. XMPP Stringprep Profiles: RFC3920 Nodeprep
1144 Description: Localpart of JabberID ("JID"), as in:
1145 localpart@domainpart/resourcepart
1146 How It's Used: - Usernames (e.g., stpeter@jabber.org) - Chatroom
1147 names (e.g., precis@jabber.ietf.org) - Publish-subscribe nodes -
1148 Bot names
1149 Who Generates It: - Typically, end users via an XMPP client -
1150 Sometimes created in an automated fashion
1151 User Input Methods: - Typed by user - Copy-and-paste - Perhaps voice
1152 input - Clicking a URI/IRI
1153 Enforcement: - Rules enforced by server / add-on service (e.g.,
1154 chatroom service) on registration of account, creation of room,
1155 etc.
1156 Comparison Method: "Type 2" (common algorithm)
1157 Case Folding, Sensitivity, Preservation: - Strings are always folded
1158 to lowercase - Case is not preserved
1159 Impact of Comparison: False positives: - unable to authenticate at
1160 server (or authenticate to wrong account) - add wrong person to
1161 buddy list - join the wrong chatroom - improperly grant privileges
1162 (e.g., chatroom admin) - subscribe to wrong pubsub node - interact
1163 with wrong bot - allow communication with blocked entity False
1164 negatives: - unable to authenticate - unable to add someone to
1165 buddy list - unable to join desired chatroom - unable to use
1166 granted privileges (e.g., chatroom admin) - unable to subscribe to
1167 desired pubsub node - unable to interact with desired bot -
1168 disallow communication with unblocked entity
1170 Normalization: NFKC
1171 Mapping: Spaces are mapped to nothing
1172 Disallowed Characters: ",&,',/,:,<,>,@
1173 String Classes: - Often similar to generic username - Often similar
1174 to localpart of email address - Sometimes same as localpart of
1175 email address
1176 Internal Structure: None
1177 User Output: - vCard - email signature - web page / directory - text
1178 of message (e.g., in a chatroom)
1179 Operations: - Sometimes concatenated with other data and then used
1180 as input to a cryptographic hash function
1182 B.7. XMPP Stringprep Profiles: RFC3920 Resourceprep
1184 Description: - Resourcepart of JabberID ("JID"), as in:
1185 localpart@domainpart/resourcepart - Typically free-form text
1186 How It's Used: - Device / session names (e.g.,
1187 stpeter@jabber.org/Home) - Nicknames (e.g.,
1188 precis@jabber.ietf.org/StPeter)
1189 Who Generates It: - Often human users via an XMPP client - Often
1190 generated in an automated fashion by client or server
1191 User Input Methods: - Typed by user - Copy-and-paste - Perhaps voice
1192 input - Clicking a URI/IRI
1193 Enforcement: - Rules enforced by server / add-on service (e.g.,
1194 chatroom service) on account login, joining a chatroom, etc.
1195 Comparison Method: "Type 2" (byte-for-byte)
1196 Case Folding, Sensitivity, Preservation: - Strings are never folded
1197 - Case is preserved
1198 Impact of Comparison: False positives: - interact with wrong device
1199 (e.g., for file transfer or voice call) - interact with wrong
1200 chatroom participant - improperly grant privileges (e.g., chatroom
1201 moderator) - allow communication with blocked entity False
1202 negatives: - unable to choose desired chatroom nick - unable to
1203 use granted privileges (e.g., chatroom moderator) - disallow
1204 communication with unblocked entity
1205 Normalization: NFKC
1206 Mapping: Spaces are mapped to nothing
1207 Disallowed Characters: None
1208 String Classes: Basically a free-form identifier
1209 Internal Structure: None
1210 User Output: - text of message (e.g., in a chatroom) - device names
1211 often not exposed to human users
1212 Operations: Sometimes concatenated with other data and then used as
1213 input to a cryptographic hash function
1215 B.8. EAP Stringprep Profiles: RFC3748
1217 Description: RFC 3748 section 5 references Stringprep, but the WG
1218 did not agree with the text (was added by IESG) and there are no
1219 known implementations that use Stringprep. The main problem with
1220 that text is that the use of strings is a per-method concept, not
1221 a generic EAP concept and so RFC 3748 itself does not really use
1222 Stringprep, but individual EAP methods could. As such, the
1223 answers to the template questions are mostly not applicable, but a
1224 few answers are universal across methods. The list of IANA
1225 registered EAP methods is at http://www.iana.org/assignments/
1226 eap-numbers/eap-numbers.xml#eap-numbers-3
1227 Comparison Methods: n/a (per-method)
1228 Case Folding, Case Sensitivity, Case Preservation: n/a (per-method)
1229 Impact of comparison: A false positive results in unauthorized
1230 network access (and possibly theft of service if some else is
1231 billed). A false negative results in lack of authorized network
1232 access (no connectivity).
1233 User input: n/a (per-method)
1234 Normalization: n/a (per-method)
1235 Mapping: n/a (per-method)
1236 Disallowed characters: n/a (per-method)
1237 String classes: Although some EAP methods may use a syntax similar
1238 to other types of identifiers, EAP mandates that the actual values
1239 must not be assumed to be identifiers usable with anything else.
1240 Internal structure: n/a (per-method)
1241 User output: Identifiers are never human displayed except perhaps as
1242 they're typed by a human.
1243 Operations: n/a (per-method)
1244 Community considerations: There is no resistance to change for the
1245 base EAP protocol (as noted, the WG didn't want the existing
1246 text). However actual use of Stringprep, if any, within specific
1247 EAP methods may have resistance. It is currently unknown whether
1248 any EAP methods use Stringprep.
1250 Authors' Addresses
1252 Marc Blanchet
1253 Viagenie
1254 246 Aberdeen
1255 Quebec, QC G1R 2E1
1256 Canada
1258 Email: Marc.Blanchet@viagenie.ca
1259 URI: http://viagenie.ca
1260 Andrew Sullivan
1261 Dyn, Inc.
1262 150 Dow St
1263 Manchester, NH 03101
1264 U.S.A.
1266 Email: asullivan@dyn.com