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2 Network Working Group J. Snell
3 Internet-Draft July 16, 2012
4 Intended status: Standards Track
5 Expires: January 17, 2013
7 The Atom "deleted-entry" Element
8 draft-snell-atompub-tombstones-17
10 Abstract
12 This specification adds mechanisms to the Atom Syndication Format
13 which publishers of Atom Feed and Entry documents can use to
14 explicitly identify Atom entries that have been removed.
16 Status of this Memo
18 This Internet-Draft is submitted to IETF in full conformance with the
19 provisions of BCP 78 and BCP 79.
21 Internet-Drafts are working documents of the Internet Engineering
22 Task Force (IETF). Note that other groups may also distribute
23 working documents as Internet-Drafts. The list of current Internet-
24 Drafts is at http://datatracker.ietf.org/drafts/current/.
26 Internet-Drafts are draft documents valid for a maximum of six months
27 and may be updated, replaced, or obsoleted by other documents at any
28 time. It is inappropriate to use Internet-Drafts as reference
29 material or to cite them other than as "work in progress."
31 This Internet-Draft will expire on January 17, 2013.
33 Copyright Notice
35 Copyright (c) 2012 IETF Trust and the persons identified as the
36 document authors. All rights reserved.
38 This document is subject to BCP 78 and the IETF Trust's Legal
39 Provisions Relating to IETF Documents
40 (http://trustee.ietf.org/license-info) in effect on the date of
41 publication of this document. Please review these documents
42 carefully, as they describe your rights and restrictions with respect
43 to this document. Code Components extracted from this document must
44 include Simplified BSD License text as described in Section 4.e of
45 the Trust Legal Provisions and are provided without warranty as
46 described in the Simplified BSD License.
48 Table of Contents
50 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
51 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
52 3. The at:deleted-entry element . . . . . . . . . . . . . . . . . 3
53 4. Deleted Entry Document . . . . . . . . . . . . . . . . . . . . 6
54 5. Digital Signatures . . . . . . . . . . . . . . . . . . . . . . 6
55 6. Encryption . . . . . . . . . . . . . . . . . . . . . . . . . . 7
56 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
57 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
58 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
59 10. Normative References . . . . . . . . . . . . . . . . . . . . . 9
60 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
62 1. Introduction
64 This specification adds mechanisms to the Atom Syndication Format
65 which publishers of Atom Feed and Entry documents can use to
66 explicitly identify Atom entries that have been removed.
68 2. Notational Conventions
70 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
71 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
72 document are to be interpreted as described in [RFC2119].
74 This specification uses XML Namespaces [W3C.REC-xml-names-19990114]
75 to uniquely identify XML element names. It uses the following
76 namespace prefix for the indicated namespace URI;
78 "at": "http://purl.org/atompub/tombstones/1.0"
80 3. The at:deleted-entry element
82 The at:deleted-entry element represents an Atom Entry that has been
83 removed.
85 deletedEntry =
86 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 { atomLink }*
93 & element atom:source { atomSource }?
94 & anyElement* )
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. In such cases, the
156 implication is that the particular atom:entry has either been
157 published to the feed and then subsequently removed; or that a
158 previously removed entry has been republished to the feed following a
159 previous deletion. To determine which condition applies, the
160 processor needs to compare the value of the at:deleted-entry
161 element's when attribute to the value of the corresponding atom:entry
162 element's atom:updated value:
163 o If the when attribute specifies a value equal to or more recent
164 than that specified by the atom:updated element, the atom:entry is
165 assumed to have been published then subsequently removed.
166 Processors SHOULD ignore the older atom:entry element.
167 o If the when attribute specifies a value older than that specified
168 by the atom:updated element, the atom:entry is assumed to have
169 been republished to the feed following a prior removal.
170 Processors SHOULD ignore the older at:deleted-entry element.
172 Publishers of feeds containing at:deleted-entry elements should note
173 that the at:deleted-entry element is advisory in nature only and may
174 be ignored by Atom processors. The presence of an at:deleted-entry
175 element does not guarantee that the atom:entry to which it is
176 referring will no longer be available. For example, if an entry was
177 published to a feed document that was published and processed
178 yesterday by an aggregator application, then is subsequently deleted
179 today, with a corresponding at:deleted-entry element added to the
180 feed as a signal that the entry was deleted, there is no guarantee
181 that the aggregator application will pay any attention to the at:
182 deleted-entry element during subsequent processing operations.
184 Elements and attributes from any XML vocabulary MAY be used within an
185 at:deleted-entry element. Processors encountering such markup MUST
186 NOT stop processing or signal an error. It might be the case that
187 the Processor is able to process the foreign markup correctly and
188 does so. When unknown markup is encountered as a child of at:
189 deleted-entry, Processors MAY bypass the markup and any textual
190 content and MUST NOT change their behavior as a result of the
191 markup's presence.
193 This specification allows the use of IRIs [RFC3987] in precisely the
194 same manner specified in Section 2 of [RFC4287].
196 Any element defined by this specification MAY have an xml:base
197 attribute [W3C.REC-xmlbase-20010627]. When xml:base is used, it
198 serves the function described in section 5.1.1 of [RFC3986],
199 establishing the base URI (or IRI) for resolving any relative
200 references found within the effective scope of the xml:base
201 attribute.
203 Any element defined by this specification MAY have an xml:lang
204 attribute, whose content indicates the natural language for the
205 element and its descendents. Requirements regarding the content and
206 interpretation of xml:lang are specified in XML 1.0
207 [W3C.REC-xml-20040204], Section 2.12.
209 4. Deleted Entry Document
211 A "Deleted Entry Document" represents exactly one at:deleted-entry
212 element outside the context of an Atom feed. It's root is the at:
213 deleted-entry element.
215 namespace at = "http://purl.org/atompub/tombstones/1.0"
216 start = at:deleted-entry
218 Deleted Entry Documents are specified in terms of the XML Information
219 Set, serialized as XML 1.0 [W3C.REC-xml-20040204] and identified with
220 the "application/atomdeleted+xml" media type. Deleted Entry
221 Documents MUST be well-formed XML. This specification does not
222 define a DTD for Deleted Entry Documents, and hence does not require
223 them to be valid (in the sense used by XML).
225 5. Digital Signatures
227 The at:deleted-entry element MAY have an Enveloped Signature, as
228 described by XML-Signature and Syntax Processing
229 [W3C.REC-xmldsig-core-20020212].
231 Processors MUST NOT reject an at:deleted-entry containing such a
232 signature because they are not capable of verifying it; they MUST
233 continue processing and MAY inform the user of their failure to
234 validate the signature.
236 In other words, the presence of an element with the namespace URI
237 "http://www.w3.org/2000/09/xmldsig#" and a local name of "Signature"
238 as a child of the document element MUST NOT cause an Processor to
239 fail merely because of its presence.
241 Section 6.5.1 of [W3C.REC-xmldsig-core-20020212] requires support for
242 Canonical XML [W3C.REC-xml-c14n-20010315]. However, many
243 implementers do not use it because signed XML documents enclosed in
244 other XML documents have their signatures broken. Thus, Processors
245 that verify signed at:deleted-entry elements MUST be able to
246 canonicalize with the exclusive XML canonicalization method
247 identified by the URI "http://www.w3.org/2001/10/xml-exc-c14n#", as
248 specified in Exclusive XML Canonicalization
249 [W3C.REC-xml-exc-c14n-20020718].
251 Intermediaries such as aggregators may need to add an atom:source
252 element to an at:deleted-entry that does not contain its own atom:
253 source element. If such an entry is signed, the addition will break
254 the signature. Thus, a publisher of individually-signed at:deleted-
255 entry's should strongly consider adding an atom:source element to
256 those elements before signing them. Implementers should also be
257 aware of the issues concerning the use of markup in the "xml:"
258 namespace as it interacts with canonicalization.
260 Section 4.4.2 of [W3C.REC-xmldsig-core-20020212] requires support for
261 DSA signatures and recommends support for RSA signatures. However,
262 because of the much greater popularity in the market of RSA versus
263 DSA, Atom Processors that verify signed Atom Documents MUST be able
264 to verify RSA signatures, but do not need be able to verify DSA
265 signatures. Due to security issues that can arise if the keying
266 material for message authentication code (MAC) authentication is not
267 handled properly, Atom Documents SHOULD NOT use MACs for signatures.
269 6. Encryption
271 The root of a Deleted Entry Document (the at:deleted-entry element)
272 MAY be encrypted, using the mechanisms described by XML Encryption
273 Syntax and Processing [W3C.REC-xmlenc-core-20021210].
275 Section 5.1 of [W3C.REC-xmlenc-core-20021210] requires support of
276 TripleDES, AES-128, and AES-256. Processors that decrypt Deleted
277 Entry Documents MUST be able to decrypt with AES-128 in Cipher Block
278 Chaining (CBC) mode.
280 Encryption based on [W3C.REC-xmlenc-core-20021210] does not ensure
281 integrity of the original document. There are known cryptographic
282 attacks where someone who cannot decrypt a message can still change
283 bits in a way where part or all the decrypted message makes sense but
284 has a different meaning. Thus, Processors that decrypt Deleted Entry
285 Documents SHOULD check the integrity of the decrypted document by
286 verifying the hash in the signature (if any) in the document, or by
287 verifying a hash of the document within the document (if any).
289 When a Deleted Entry Document is to be both signed and encrypted, it
290 is generally a good idea to first sign the document, then encrypt the
291 signed document. This provides integrity to the base document while
292 encrypting all the information, including the identity of the entity
293 that signed the document. Note that, if MACs are used for
294 authentication, the order MUST be that the document is signed and
295 then encrypted, and not the other way around. Further, if MACs are
296 used along with a symmetric encryption algorithm, the same key SHOULD
297 NOT be used in the generation of the MAC and the encryption.
299 7. Security Considerations
301 As specified in [RFC4287], Atom processors should be aware of the
302 potential for spoofing attacks where an attacker publishes atom:entry
303 or atom:deleted-entry elements using the same atom:id values as
304 entries from other Atom feeds. An attacker may attempt to trick an
305 application into believing that a given entry has either been removed
306 from or added to a feed. To mitigate this issue, Atom processors are
307 advised to ignore at:deleted-entry elements referencing entries that
308 have not previously appeared within the containing Feed document and
309 should take steps to verify the origin of the Atom feed before
310 considering the entries to be removed.
312 The at:deleted-entry element can be encrypted and signed using
313 [W3C.REC-xmlenc-core-20021210] and [W3C.REC-xmldsig-core-20020212],
314 respectively, and are subject to the security considerations implied
315 by their use.
317 Digital signatures provide authentication and message integrity with
318 proof of origin. Encryption provides data confidentiality.
320 An application supporting the use of digitally signed atom:entry and
321 at:deleted-entry elements should be aware of the potential issues
322 that could arise if a at:deleted-entry element indicating the
323 deletion of an atom:entry element has been signed using a different
324 key than what was used to sign the atom:entry, or when an unsigned
325 at:deleted-entry is used to indicate the deletion of a signed atom:
326 entry. Either case can potentially indicate a form of spoofing
327 attack. Processors must take steps to verify the validity of the at:
328 deleted-entry element.
330 8. IANA Considerations
332 A Deleted Entry Document, when serialized as XML 1.0, can be
333 identified with the following media type:
335 Type name: application
336 Subtype name: atomdeleted+xml
337 Required parameters: None
338 Optional parameters: "charset" : This parameter has semantics
339 identical to the charset parameter of the "application/xml" media
340 type as specified in [RFC3023].
341 Encoding considerations: Identical to those of "application/xml"
342 as described in [RFC3023], Section 3.2.
343 Security considerations: As defined in this specification. In
344 addition, as this media type uses the "+xml" convention, it shares
345 the same security considerations as described in [RFC3023],
346 Section 10.
347 Interoperability considerations: There are no known
348 interoperability issues.
349 Published specification: This specification.
350 Applications that use this media type: Undefined. As an extension
351 to the Atom Syndication Format ([RFC4287]), this specification may
352 be used within any application that uses the Atom Format.
353 Additional information:
354 Magic number(s): As specified for "application/xml" in
355 [RFC3023], Section 3.2
356 File extension(s): .atomdeleted
357 Macintosh file type code(s): TEXT
358 Person & email address to contact for further information: James M
359 Snell
360 Intended usage: COMMON
361 Restrictions on usage: None.
362 Author: James M Snell
363 Change controller: IESG
365 9. Acknowledgements
367 The author gratefully acknowledges the feedback from the members of
368 the Atom Publishing Format and Protocol working group during the
369 development of this specification.
371 10. Normative References
373 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
374 Requirement Levels", BCP 14, RFC 2119, March 1997.
376 [RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media
377 Types", RFC 3023, January 2001.
379 [RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the
380 Internet: Timestamps", RFC 3339, July 2002.
382 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
383 Resource Identifier (URI): Generic Syntax", STD 66,
384 RFC 3986, January 2005.
386 [RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
387 Identifiers (IRIs)", RFC 3987, January 2005.
389 [RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
390 Syndication Format", RFC 4287, December 2005.
392 [W3C.REC-xml-20040204]
393 Yergeau, F., Maler, E., Bray, T., Sperberg-McQueen, C.,
394 and J. Paoli, "Extensible Markup Language (XML) 1.0 (Third
395 Edition)", World Wide Web Consortium FirstEdition REC-xml-
396 20040204, February 2004,
397 .
399 [W3C.REC-xml-c14n-20010315]
400 Boyer, J., "Canonical XML Version 1.0", World Wide Web
401 Consortium Recommendation REC-xml-c14n-20010315,
402 March 2001,
403 .
405 [W3C.REC-xml-exc-c14n-20020718]
406 Reagle, J., 3rd, D., and J. Boyer, "Exclusive XML
407 Canonicalization Version 1.0", World Wide Web Consortium
408 Recommendation REC-xml-exc-c14n-20020718, July 2002,
409 .
411 [W3C.REC-xml-names-19990114]
412 Hollander, D., Bray, T., and A. Layman, "Namespaces in
413 XML", World Wide Web Consortium FirstEdition REC-xml-
414 names-19990114, January 1999,
415 .
417 [W3C.REC-xmlbase-20010627]
418 Marsh, J., "XML Base", World Wide Web Consortium
419 FirstEdition REC-xmlbase-20010627, June 2001,
420 .
422 [W3C.REC-xmldsig-core-20020212]
423 Solo, D., Reagle, J., and D. Eastlake, "XML-Signature
424 Syntax and Processing", World Wide Web Consortium
425 FirstEdition REC-xmldsig-core-20020212, February 2002,
426 .
428 [W3C.REC-xmlenc-core-20021210]
429 Eastlake, D. and J. Reagle, "XML Encryption Syntax and
430 Processing", World Wide Web Consortium Recommendation REC-
431 xmlenc-core-20021210, December 2002,
432 .
434 Author's Address
436 James M Snell
438 Email: jasnell@us.ibm.com
439 URI: http://ibm.com