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Snell 3 Internet-Draft January 24, 2012 4 Intended status: Informational 5 Expires: July 27, 2012 7 The Atom "deleted-entry" Element 8 draft-snell-atompub-tombstones-14.txt 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 July 27, 2012. 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 . . . . . . . . . . . . . . . . . . . . 5 54 5. Digital Signatures . . . . . . . . . . . . . . . . . . . . . . 6 55 6. Encryption . . . . . . . . . . . . . . . . . . . . . . . . . . 7 56 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 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. 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 any XML vocabulary MAY be used within an 166 at:deleted-entry element. Processors encountering such markup MUST 167 NOT stop processing or signal an error. It might be the case that 168 the Processor is able to process the foreign markup correctly and 169 does so. When unknown markup is encountered as a child of at: 170 deleted-entry, Processors MAY bypass the markup and any textual 171 content and MUST NOT change their behavior as a result of the 172 markup's presence. 174 This specification allows the use of IRIs [RFC3987] in precisely the 175 same manner specified in Section 2 of [RFC4287]. 177 Any element defined by this specification MAY have an xml:base 178 attribute [W3C.REC-xmlbase-20010627]. When xml:base is used, it 179 serves the function described in section 5.1.1 of [RFC3986], 180 establishing the base URI (or IRI) for resolving any relative 181 references found within the effective scope of the xml:base 182 attribute. 184 Any element defined by this specification MAY have an xml:lang 185 attribute, whose content indicates the natural language for the 186 element and its descendents. Requirements regarding the content and 187 interpretation of xml:lang are specified in XML 1.0 188 [W3C.REC-xml-20040204], Section 2.12. 190 4. Deleted Entry Document 192 A "Deleted Entry Document" represents exactly one at:deleted-entry 193 element outside the context of an Atom feed. It's root is the at: 194 deleted-entry element. 196 namespace at = "http://purl.org/atompub/tombstones/1.0" 197 start = at:deleted-entry 199 Deleted Entry Documents are specified in terms of the XML Information 200 Set, serialized as XML 1.0 [W3C.REC-xml-20040204] and identified with 201 the "application/atomdeleted+xml" media type. Deleted Entry 202 Documents MUST be well-formed XML. This specification does not 203 define a DTD for Deleted Entry Documents, and hence does not require 204 them to be valid (in the sense used by XML). 206 5. Digital Signatures 208 The at:deleted-entry element MAY have an Enveloped Signature, as 209 described by XML-Signature and Syntax Processing 210 [W3C.REC-xmldsig-core-20020212]. 212 Processors MUST NOT reject an at:deleted-entry containing such a 213 signature because they are not capable of verifying it; they MUST 214 continue processing and MAY inform the user of their failure to 215 validate the signature. 217 In other words, the presence of an element with the namespace URI 218 "http://www.w3.org/2000/09/xmldsig#" and a local name of "Signature" 219 as a child of the document element MUST NOT cause an Processor to 220 fail merely because of its presence. 222 Section 6.5.1 of [W3C.REC-xmldsig-core-20020212] requires support for 223 Canonical XML [W3C.REC-xml-c14n-20010315]. However, many 224 implementers do not use it because signed XML documents enclosed in 225 other XML documents have their signatures broken. Thus, Processors 226 that verify signed at:deleted-entry elements MUST be able to 227 canonicalize with the exclusive XML canonicalization method 228 identified by the URI "http://www.w3.org/2001/10/xml-exc-c14n#", as 229 specified in Exclusive XML Canonicalization 230 [W3C.REC-xml-exc-c14n-20020718]. 232 Intermediaries such as aggregators may need to add an atom:source 233 element to an at:deleted-entry that does not contain its own atom: 234 source element. If such an entry is signed, the addition will break 235 the signature. Thus, a publisher of individually-signed at:deleted- 236 entry's should strongly consider adding an atom:source element to 237 those elements before signing them. Implementers should also be 238 aware of the issues concerning the use of markup in the "xml:" 239 namespace as it interacts with canonicalization. 241 Section 4.4.2 of [W3C.REC-xmldsig-core-20020212] requires support for 242 DSA signatures and recommends support for RSA signatures. However, 243 because of the much greater popularity in the market of RSA versus 244 DSA, Atom Processors that verify signed Atom Documents MUST be able 245 to verify RSA signatures, but do not need be able to verify DSA 246 signatures. Due to security issues that can arise if the keying 247 material for message authentication code (MAC) authentication is not 248 handled properly, Atom Documents SHOULD NOT use MACs for signatures. 250 6. Encryption 252 The root of a Deleted Entry Document (the at:deleted-entry element) 253 MAY be encrypted, using the mechanisms described by XML Encryption 254 Syntax and Processing [W3C.REC-xmlenc-core-20021210]. 256 Section 5.1 of [W3C.REC-xmlenc-core-20021210] requires support of 257 TripleDES, AES-128, and AES-256. Processors that decrypt Deleted 258 Entry Documents MUST be able to decrypt with AES-128 in Cipher Block 259 Chaining (CBC) mode. 261 Encryption based on [W3C.REC-xmlenc-core-20021210] does not ensure 262 integrity of the original document. There are known cryptographic 263 attacks where someone who cannot decrypt a message can still change 264 bits in a way where part or all the decrypted message makes sense but 265 has a different meaning. Thus, Processors that decrypt Deleted Entry 266 Documents SHOULD check the integrity of the decrypted document by 267 verifying the hash in the signature (if any) in the document, or by 268 verifying a hash of the document within the document (if any). 270 When a Deleted Entry Document is to be both signed and encrypted, it 271 is generally a good idea to first sign the document, then encrypt the 272 signed document. This provides integrity to the base document while 273 encrypting all the information, including the identity of the entity 274 that signed the document. Note that, if MACs are used for 275 authentication, the order MUST be that the document is signed and 276 then encrypted, and not the other way around. 278 7. Security Considerations 280 As specified in [RFC4287], Atom processors should be aware of the 281 potential for spoofing attacks where an attacker publishes atom:entry 282 or atom:deleted-entry elements using the same atom:id values as 283 entries from other Atom feeds. An attacker may attempt to trick an 284 application into believing that a given entry has either been removed 285 from or added to a feed. To mitigate this issue, Atom processors are 286 advised to ignore at:deleted-entry elements referencing entries that 287 have not previously appeared within the containing Feed document and 288 should take steps to verify the origin of the Atom feed before 289 considering the entries to be removed. 291 The at:deleted-entry element can be encrypted and signed using 292 [W3C.REC-xmlenc-core-20021210] and [W3C.REC-xmldsig-core-20020212], 293 respectively, and are subject to the security considerations implied 294 by their use. 296 Digital signatures provide authentication, message integrity, and 297 non-repudiation with proof of origin. Encryption provides data 298 confidentiality. 300 An application supporting the use of digitally signed atom:entry and 301 at:deleted-entry elements should be aware of the potential issues 302 that could arise if a at:deleted-entry element indicating the 303 deletion of an atom:entry element has been signed using a different 304 key than what was used to sign the atom:entry, or when an unsigned 305 at:deleted-entry is used to indicate the deletion of a signed atom: 306 entry. Either case can potentially indicate a form of spoofing 307 attack. Processors must take steps to verify the validity of the at: 308 deleted-entry element. 310 8. IANA Considerations 312 A Deleted Entry Document, when serialized as XML 1.0, can be 313 identified with the following media type: 315 Type name: application 316 Subtype name: atomdeleted+xml 317 Required parameters: None 318 Optional parameters: "charset" : This parameter has semantics 319 identical to the charset parameter of the "application/xml" media 320 type as specified in [RFC3023]. 321 Encoding considerations: Identical to those of "application/xml" 322 as described in [RFC3023], Section 3.2. 323 Security considerations: As defined in this specification. In 324 addition, as this media type uses the "+xml" convention, it shares 325 the same security considerations as described in [RFC3023], 326 Section 10. 327 Interoperability considerations: There are no known 328 interoperability issues. 329 Published specification: This specification. 330 Applications that use this media type: Undefined. As an extension 331 to the Atom Syndication Format ([RFC4287]), this specification may 332 be used within any application that uses the Atom Format. 333 Additional information: 334 Magic number(s): As specified for "application/xml" in 335 [RFC3023], Section 3.2 336 File extension(s): .atomdeleted 337 Macintosh file type code(s): TEXT 338 Person & email address to contact for further information: James M 339 Snell 340 Intended usage: COMMON 341 Restrictions on usage: None. 343 Author: James M Snell 344 Change controller: IESG 346 9. Acknowledgements 348 The author gratefully acknowledges the feedback from the members of 349 the Atom Publishing Format and Protocol working group during the 350 development of this specification. 352 10. Normative References 354 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 355 Requirement Levels", BCP 14, RFC 2119, March 1997. 357 [RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media 358 Types", RFC 3023, January 2001. 360 [RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the 361 Internet: Timestamps", RFC 3339, July 2002. 363 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 364 Resource Identifier (URI): Generic Syntax", STD 66, 365 RFC 3986, January 2005. 367 [RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource 368 Identifiers (IRIs)", RFC 3987, January 2005. 370 [RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom 371 Syndication Format", RFC 4287, December 2005. 373 [W3C.REC-xml-20040204] 374 Yergeau, F., Maler, E., Sperberg-McQueen, C., Paoli, J., 375 and T. Bray, "Extensible Markup Language (XML) 1.0 (Third 376 Edition)", World Wide Web Consortium FirstEdition REC-xml- 377 20040204, February 2004, 378 . 380 [W3C.REC-xml-c14n-20010315] 381 Boyer, J., "Canonical XML Version 1.0", World Wide Web 382 Consortium Recommendation REC-xml-c14n-20010315, 383 March 2001, 384 . 386 [W3C.REC-xml-exc-c14n-20020718] 387 Reagle, J., 3rd, D., and J. Boyer, "Exclusive XML 388 Canonicalization Version 1.0", World Wide Web Consortium 389 Recommendation REC-xml-exc-c14n-20020718, July 2002, 390 . 392 [W3C.REC-xml-names-19990114] 393 Hollander, D., Bray, T., and A. Layman, "Namespaces in 394 XML", World Wide Web Consortium FirstEdition REC-xml- 395 names-19990114, January 1999, 396 . 398 [W3C.REC-xmlbase-20010627] 399 Marsh, J., "XML Base", World Wide Web Consortium 400 FirstEdition REC-xmlbase-20010627, June 2001, 401 . 403 [W3C.REC-xmldsig-core-20020212] 404 Solo, D., Reagle, J., and D. Eastlake, "XML-Signature 405 Syntax and Processing", World Wide Web Consortium 406 FirstEdition REC-xmldsig-core-20020212, February 2002, 407 . 409 [W3C.REC-xmlenc-core-20021210] 410 Eastlake, D. and J. Reagle, "XML Encryption Syntax and 411 Processing", World Wide Web Consortium Recommendation REC- 412 xmlenc-core-20021210, December 2002, 413 . 415 Author's Address 417 James M Snell 419 Phone: 420 Email: jasnell@us.ibm.com 421 URI: http://ibm.com