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2 Network Working Group J. Snell
3 Internet-Draft June 22, 2010
4 Updates: 4287 (if approved)
5 Intended status: Informational
6 Expires: December 24, 2010
8 The Atom "deleted-entry" Element
9 draft-snell-atompub-tombstones-10.txt
11 Abstract
13 This specification adds mechanisms to the Atom Syndication Format
14 which publishers of Atom Feed and Entry documents can use to
15 explicitly identify Atom entries that have been removed.
17 Status of this Memo
19 This Internet-Draft is submitted to IETF in full conformance with the
20 provisions of BCP 78 and BCP 79.
22 Internet-Drafts are working documents of the Internet Engineering
23 Task Force (IETF). Note that other groups may also distribute
24 working documents as Internet-Drafts. The list of current Internet-
25 Drafts is at http://datatracker.ietf.org/drafts/current/.
27 Internet-Drafts are draft documents valid for a maximum of six months
28 and may be updated, replaced, or obsoleted by other documents at any
29 time. It is inappropriate to use Internet-Drafts as reference
30 material or to cite them other than as "work in progress."
32 This Internet-Draft will expire on December 24, 2010.
34 Copyright Notice
36 Copyright (c) 2010 IETF Trust and the persons identified as the
37 document authors. All rights reserved.
39 This document is subject to BCP 78 and the IETF Trust's Legal
40 Provisions Relating to IETF Documents
41 (http://trustee.ietf.org/license-info) in effect on the date of
42 publication of this document. Please review these documents
43 carefully, as they describe your rights and restrictions with respect
44 to this document. Code Components extracted from this document must
45 include Simplified BSD License text as described in Section 4.e of
46 the Trust Legal Provisions and are provided without warranty as
47 described in the Simplified BSD License.
49 Table of Contents
51 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
52 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
53 3. The at:deleted-entry element . . . . . . . . . . . . . . . . . 3
54 4. Deleted Entry Document . . . . . . . . . . . . . . . . . . . . 5
55 5. Digital Signatures . . . . . . . . . . . . . . . . . . . . . . 6
56 6. Encryption . . . . . . . . . . . . . . . . . . . . . . . . . . 7
57 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
58 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
59 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
60 10. Normative References . . . . . . . . . . . . . . . . . . . . . 9
61 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
63 1. Introduction
65 This specification adds mechanisms to the Atom Syndication Format
66 which publishers of Atom Feed and Entry documents can use to
67 explicitly identify Atom entries that have been removed.
69 2. Notational Conventions
71 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
72 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
73 document are to be interpreted as described in BCP 14, [RFC2119]
75 This specification uses XML Namespaces [W3C.REC-xml-names-19990114]
76 to uniquely identify XML element names. It uses the following
77 namespace prefix for the indicated namespace URI;
79 "at": "http://purl.org/atompub/tombstones/1.0"
81 3. The at:deleted-entry element
83 The at:deleted-entry element represents an Atom Entry that has been
84 removed.
86 deletedEntry = element at:deleted-entry {
87 atomCommonAttributes,
88 attribute ref { atomUri },
89 attribute when { atomDateConstruct },
90 ( element at:by { atomPersonConstruct}?,
91 & element at:comment {atomTextConstruct}?,
92 & element atom:link*,
93 & element atom:source?,
94 & extensionElement* )
95 }
97 The at:deleted-entry element MUST contain a ref attribute whose value
98 specifies the value of the atom:id of the entry that has been
99 removed.
101 The at:deleted-entry element MUST contain a when attribute whose
102 value is an [RFC3339] "date-time" specifying the instant the entry
103 was removed. An uppercase "T" character MUST be used to separate
104 date and time, and an uppercase "Z" character MUST be present in the
105 absence of a numeric time zone offset
107 The at:deleted-entry element MAY contain one at:by element used to
108 identify the entity that removed the entry. The at:by element is an
109 Atom Person Construct as defined by Section 3.2 of [RFC4287].
111 The at:deleted-entry element MAY contain one at:comment element whose
112 value provides additional, language-sensitive information about the
113 deletion operation. The atom:comment element is an Atom Text
114 Construct as defined by Section 3.1 of [RFC4287].
116 The at:deleted-entry element MAY contain any number of atom:link
117 elements as specified by Section 4.2.7 of [RFC4287].
119 The at:deleted-entry element MAY contain one atom:source element as
120 defined by Section 4.2.11 of [RFC4287]. Within the context of an at:
121 deleted-entry element, the atom:source element is intended to allow
122 the aggregation of at:deleted-entry element from different feeds
123 while retaining information about an at:deleted-entry's source feed.
124 When an at:deleted-entry element appears in a Feed document other
125 than it's source Feed or when an at:deleted-entry element that has a
126 source Feed document is used in the context of a Deleted Entry
127 Document, it MUST contain an atom:source element.
129 An Atom feed MAY contain any number of at:deleted-entry elements, but
130 MUST NOT contain more than one with the same combination of ref and
131 when attribute values.
133
135 ...
136
137
141
142
145
146 John Doe
147 jdoe@example.org
148
149 Removed comment spam
150
151 ...
152
154 An Atom feed MAY contain atom:entry elements and at:deleted-entry
155 elements sharing the same atom:id value. Atom processors SHOULD
156 ignore any at:deleted-entry elements sharing an atom:id value with an
157 atom:entry whose atom:updated element specifies a date and time more
158 recent than or equal to the at:deleted-entry element's when value.
160 Implementors should note that the at:deleted-entry element is
161 informative in nature only and may be ignored by Atom processors.
162 The presence of an at:deleted-entry element does not guarantee that
163 the atom:entry to which it is referring will no longer be available.
165 Elements and attributes from other XML vocabularies MAY be used
166 within an at:deleted-entry element following the same model defined
167 by Section 6 of [RFC4287]. Processors encountering such markup MUST
168 NOT stop processing or signal an error. It might be the case that
169 the Processor is able to process the foreign markup correctly and
170 does so. When unknown markup is encountered as a child of at:
171 deleted-entry, Processors MAY bypass the markup and any textual
172 content and MUST NOT change their behavior as a result of the
173 markup's presence.
175 This specification allows the use of IRIs [RFC3987] in precisely the
176 same manner specified in Section 2 of [RFC4287].
178 Any element defined by this specification MAY have an xml:base
179 attribute [W3C.REC-xmlbase-20010627]. When xml:base is used, it
180 serves the function described in section 5.1.1 of [RFC3986],
181 establishing the base URI (or IRI) for resolving any relative
182 references found within the effective scope of the xml:base
183 attribute.
185 Any element defined by this specification MAY have an xml:lang
186 attribute, whose content indicates the natural language for the
187 element and its descendents. Requirements regarding the content and
188 interpretation of xml:lang are specified in XML 1.0
189 [W3C.REC-xml-20040204], Section 2.12.
191 4. Deleted Entry Document
193 A "Deleted Entry Document" represents exactly one at:deleted-entry
194 element outside the context of an Atom feed. It's root is the at:
195 deleted-entry element.
197 namespace at = "http://purl.org/atompub/tombstones/1.0"
198 start = at:deleted-entry
200 Deleted Entry Documents are specified in terms of the XML Information
201 Set, serialized as XML 1.0 [W3C.REC-xml-20040204] and identified with
202 the "application/atomdeleted+xml" media type. Deleted Entry
203 Documents MUST be well-formed XML. This specification does not
204 define a DTD for Deleted Entry Documents, and hence does not require
205 them to be valid (in the sense used by XML).
207 5. Digital Signatures
209 The at:deleted-entry element MAY have an Enveloped Signature, as
210 described by XML-Signature and Syntax Processing
211 [W3C.REC-xmldsig-core-20020212].
213 Processors MUST NOT reject an at:deleted-entry containing such a
214 signature because they are not capable of verifying it; they MUST
215 continue processing and MAY inform the user of their failure to
216 validate the signature.
218 In other words, the presence of an element with the namespace URI
219 "http://www.w3.org/2000/09/xmldsig#" and a local name of "Signature"
220 as a child of the document element MUST NOT cause an Processor to
221 fail merely because of its presence.
223 Section 6.5.1 of [W3C.REC-xmldsig-core-20020212] requires support for
224 Canonical XML [W3C.REC-xml-c14n-20010315]. However, many
225 implementers do not use it because signed XML documents enclosed in
226 other XML documents have their signatures broken. Thus, Processors
227 that verify signed at:deleted-entry elements MUST be able to
228 canonicalize with the exclusive XML canonicalization method
229 identified by the URI "http://www.w3.org/2001/10/xml-exc-c14n#", as
230 specified in Exclusive XML Canonicalization
231 [W3C.REC-xml-exc-c14n-20020718].
233 Intermediaries such as aggregators may need to add an atom:source
234 element to an at:deleted-entry that does not contain its own atom:
235 source element. If such an entry is signed, the addition will break
236 the signature. Thus, a publisher of individually-signed at:deleted-
237 entry's should strongly consider adding an atom:source element to
238 those elements before signing them. Implementers should also be
239 aware of the issues concerning the use of markup in the "xml:"
240 namespace as it interacts with canonicalization.
242 Section 4.4.2 of [W3C.REC-xmldsig-core-20020212] requires support for
243 DSA signatures and recommends support for RSA signatures. However,
244 because of the much greater popularity in the market of RSA versus
245 DSA, Atom Processors that verify signed Atom Documents MUST be able
246 to verify RSA signatures, but do not need be able to verify DSA
247 signatures. Due to security issues that can arise if the keying
248 material for message authentication code (MAC) authentication is not
249 handled properly, Atom Documents SHOULD NOT use MACs for signatures.
251 6. Encryption
253 The root of a Deleted Entry Document (the at:deleted-entry element)
254 MAY be encrypted, using the mechanisms described by XML Encryption
255 Syntax and Processing [W3C.REC-xmlenc-core-20021210].
257 Section 5.1 of [W3C.REC-xmlenc-core-20021210] requires support of
258 TripleDES, AES-128, and AES-256. Processors that decrypt Deleted
259 Entry Documents MUST be able to decrypt with AES-128 in Cipher Block
260 Chaining (CBC) mode.
262 Encryption based on [W3C.REC-xmlenc-core-20021210] does not ensure
263 integrity of the original document. There are known cryptographic
264 attacks where someone who cannot decrypt a message can still change
265 bits in a way where part or all the decrypted message makes sense but
266 has a different meaning. Thus, Processors that decrypt Deleted Entry
267 Documents SHOULD check the integrity of the decrypted document by
268 verifying the hash in the signature (if any) in the document, or by
269 verifying a hash of the document within the document (if any).
271 When a Deleted Entry Document is to be both signed and encrypted, it
272 is generally a good idea to first sign the document, then encrypt the
273 signed document. This provides integrity to the base document while
274 encrypting all the information, including the identity of the entity
275 that signed the document. Note that, if MACs are used for
276 authentication, the order MUST be that the document is signed and
277 then encrypted, and not the other way around.
279 7. Security Considerations
281 As specified in [RFC4287], Atom processors should be aware of the
282 potential for spoofing attacks where an attacker publishes atom:entry
283 or atom:deleted-entry elements using the same atom:id values as
284 entries from other Atom feeds. An attacker may attempt to trick an
285 application into believing that a given entry has either been removed
286 from or added to a feed. To mitigate this issue, Atom processors are
287 advised to ignore at:deleted-entry elements referencing entries that
288 have not previously appeared within the containing Feed document and
289 should take steps to verify the origin of the Atom feed before
290 considering the entries to be removed.
292 The at:deleted-entry element can be encrypted and signed using
293 [W3C.REC-xmlenc-core-20021210] and [W3C.REC-xmldsig-core-20020212],
294 respectively, and are subject to the security considerations implied
295 by their use.
297 Digital signatures provide authentication, message integrity, and
298 non-repudiation with proof of origin. Encryption provides data
299 confidentiality.
301 An application supporting the use of digitally signed atom:entry and
302 at:deleted-entry elements should be aware of the potential issues
303 that could arise if a at:deleted-entry element indicating the
304 deletion of an atom:entry element has been signed using a different
305 key than what was used to sign the atom:entry, or when an unsigned
306 at:deleted-entry is used to indicate the deletion of a signed atom:
307 entry. Either case can potentially indicate a form of spoofing
308 attack. Processors must take steps to verify the validity of the at:
309 deleted-entry element.
311 8. IANA Considerations
313 A Deleted Entry Document, when serialized as XML 1.0, can be
314 identified with the following media type:
316 MIME media type name: application
317 MIME subtype name: atomdeleted+xml
318 Mandatory parameters: None.
319 Optional parameters:
320 "charset": This parameter has semantics identical to the charset
321 parameter of the "application/xml" media type as specified in
322 [RFC3023].
323 Encoding considerations: Identical to those of "application/xml" as
324 described in [RFC3023], Section 3.2.
325 Security considerations: As defined in this specification.
326 In addition, as this media type uses the "+xml" convention, it
327 shares the same security considerations as described in [RFC3023],
328 Section 10.
329 Interoperability considerations: There are no known interoperability
330 issues.
331 Published specification: This specification.
332 Applications that use this media type: No known applications
333 currently use this media type.
335 Additional information:
337 Magic number(s): As specified for "application/xml" in [RFC3023],
338 Section 3.2.
339 File extension: .atomdeleted
340 Fragment identifiers: As specified for "application/xml" in
341 [RFC3023], Section 5.
342 Base URI: As specified in [RFC3023], Section 6.
343 Macintosh File Type code: TEXT
344 Person and email address to contact for further information:
345 James M Snell
346 Intended usage: COMMON
347 Author/Change controller: IESG
349 9. Acknowledgements
351 The author gratefully acknowledges the feedback from the members of
352 the Atom Publishing Format and Protocol working group during the
353 development of this specification.
355 10. Normative References
357 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
358 Requirement Levels", BCP 14, RFC 2119, March 1997.
360 [RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media
361 Types", RFC 3023, January 2001.
363 [RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the
364 Internet: Timestamps", RFC 3339, July 2002.
366 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
367 Resource Identifier (URI): Generic Syntax", STD 66,
368 RFC 3986, January 2005.
370 [RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
371 Identifiers (IRIs)", RFC 3987, January 2005.
373 [RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
374 Syndication Format", RFC 4287, December 2005.
376 [W3C.REC-xml-20040204]
377 Bray, T., Maler, E., Sperberg-McQueen, C., Paoli, J., and
378 F. Yergeau, "Extensible Markup Language (XML) 1.0 (Third
379 Edition)", World Wide Web Consortium FirstEdition REC-xml-
380 20040204, February 2004,
381 .
383 [W3C.REC-xml-c14n-20010315]
384 Boyer, J., "Canonical XML Version 1.0", World Wide Web
385 Consortium Recommendation REC-xml-c14n-20010315,
386 March 2001,
387 .
389 [W3C.REC-xml-exc-c14n-20020718]
390 Reagle, J., Boyer, J., and D. 3rd, "Exclusive XML
391 Canonicalization Version 1.0", World Wide Web Consortium
392 Recommendation REC-xml-exc-c14n-20020718, July 2002,
393 .
395 [W3C.REC-xml-names-19990114]
396 Hollander, D., Bray, T., and A. Layman, "Namespaces in
397 XML", World Wide Web Consortium FirstEdition REC-xml-
398 names-19990114, January 1999,
399 .
401 [W3C.REC-xmlbase-20010627]
402 Marsh, J., "XML Base", World Wide Web Consortium
403 FirstEdition REC-xmlbase-20010627, June 2001,
404 .
406 [W3C.REC-xmldsig-core-20020212]
407 Solo, D., Eastlake, D., and J. Reagle, "XML-Signature
408 Syntax and Processing", World Wide Web Consortium
409 FirstEdition REC-xmldsig-core-20020212, February 2002,
410 .
412 [W3C.REC-xmlenc-core-20021210]
413 Eastlake, D. and J. Reagle, "XML Encryption Syntax and
414 Processing", World Wide Web Consortium Recommendation REC-
415 xmlenc-core-20021210, December 2002,
416 .
418 Author's Address
420 James M Snell
422 Phone:
423 Email: jasnell@us.ibm.com
424 URI: http://ibm.com