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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 RFC 6721

Network Working Group                                           J. Snell
Internet-Draft                                          December 6, 2010
Updates: 4287 (if approved)
Intended status: Informational
Expires: June 9, 2011

                    The Atom "deleted-entry" Element


   This specification adds mechanisms to the Atom Syndication Format
   which publishers of Atom Feed and Entry documents can use to
   explicitly identify Atom entries that have been removed.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 9, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  3
   3.  The at:deleted-entry element . . . . . . . . . . . . . . . . .  3
   4.  Deleted Entry Document . . . . . . . . . . . . . . . . . . . .  5
   5.  Digital Signatures . . . . . . . . . . . . . . . . . . . . . .  6
   6.  Encryption . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   10. Normative References . . . . . . . . . . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10

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1.  Introduction

   This specification adds mechanisms to the Atom Syndication Format
   which publishers of Atom Feed and Entry documents can use to
   explicitly identify Atom entries that have been removed.

2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, [RFC2119]

   This specification uses XML Namespaces [W3C.REC-xml-names-19990114]
   to uniquely identify XML element names.  It uses the following
   namespace prefix for the indicated namespace URI;

    "at": "http://purl.org/atompub/tombstones/1.0"

3.  The at:deleted-entry element

   The at:deleted-entry element represents an Atom Entry that has been

     deletedEntry = element at:deleted-entry {
       attribute ref { atomUri },
       attribute when { atomDateConstruct },
       ( element at:by { atomPersonConstruct }?
       & element at:comment { atomTextConstruct }?
       & element atom:link*
       & element atom:source?
       & anyElement* )

   The at:deleted-entry element MUST contain a ref attribute whose value
   specifies the value of the atom:id of the entry that has been

   The at:deleted-entry element MUST contain a when attribute whose
   value is an [RFC3339] "date-time" specifying the instant the entry
   was removed.  An uppercase "T" character MUST be used to separate
   date and time, and an uppercase "Z" character MUST be present in the
   absence of a numeric time zone offset

   The at:deleted-entry element MAY contain one at:by element used to
   identify the entity that removed the entry.  The at:by element is an

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   Atom Person Construct as defined by Section 3.2 of [RFC4287].

   The at:deleted-entry element MAY contain one at:comment element whose
   value provides additional, language-sensitive information about the
   deletion operation.  The atom:comment element is an Atom Text
   Construct as defined by Section 3.1 of [RFC4287].

   The at:deleted-entry element MAY contain any number of atom:link
   elements as specified by Section 4.2.7 of [RFC4287].

   The at:deleted-entry element MAY contain one atom:source element as
   defined by Section 4.2.11 of [RFC4287].  Within the context of an at:
   deleted-entry element, the atom:source element is intended to allow
   the aggregation of at:deleted-entry element from different feeds
   while retaining information about an at:deleted-entry's source feed.
   When an at:deleted-entry element appears in a Feed document other
   than it's source Feed or when an at:deleted-entry element that has a
   source Feed document is used in the context of a Deleted Entry
   Document, it MUST contain an atom:source element.

   An Atom feed MAY contain any number of at:deleted-entry elements, but
   MUST NOT contain more than one with the same combination of ref and
   when attribute values.

     <feed xmlns="http://www.w3.org/2005/Atom"
        <!-- Minimal deleted-entry -->

        <!-- Extended deleted-entry -->
            <name>John Doe</name>
          <at:comment>Removed comment spam</at:comment>

   An Atom feed MAY contain atom:entry elements and at:deleted-entry
   elements sharing the same atom:id value.  Atom processors SHOULD
   ignore any at:deleted-entry elements sharing an atom:id value with an

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   atom:entry whose atom:updated element specifies a date and time more
   recent than or equal to the at:deleted-entry element's when value.

   Implementors should note that the at:deleted-entry element is
   informative in nature only and may be ignored by Atom processors.
   The presence of an at:deleted-entry element does not guarantee that
   the atom:entry to which it is referring will no longer be available.

   Elements and attributes from any XML vocabulary MAY be used within an
   at:deleted-entry element.  Processors encountering such markup MUST
   NOT stop processing or signal an error.  It might be the case that
   the Processor is able to process the foreign markup correctly and
   does so.  When unknown markup is encountered as a child of at:
   deleted-entry, Processors MAY bypass the markup and any textual
   content and MUST NOT change their behavior as a result of the
   markup's presence.

   This specification allows the use of IRIs [RFC3987] in precisely the
   same manner specified in Section 2 of [RFC4287].

   Any element defined by this specification MAY have an xml:base
   attribute [W3C.REC-xmlbase-20010627].  When xml:base is used, it
   serves the function described in section 5.1.1 of [RFC3986],
   establishing the base URI (or IRI) for resolving any relative
   references found within the effective scope of the xml:base

   Any element defined by this specification MAY have an xml:lang
   attribute, whose content indicates the natural language for the
   element and its descendents.  Requirements regarding the content and
   interpretation of xml:lang are specified in XML 1.0
   [W3C.REC-xml-20040204], Section 2.12.

4.  Deleted Entry Document

   A "Deleted Entry Document" represents exactly one at:deleted-entry
   element outside the context of an Atom feed.  It's root is the at:
   deleted-entry element.

   namespace at = "http://purl.org/atompub/tombstones/1.0"
   start = at:deleted-entry

   Deleted Entry Documents are specified in terms of the XML Information
   Set, serialized as XML 1.0 [W3C.REC-xml-20040204] and identified with
   the "application/atomdeleted+xml" media type.  Deleted Entry
   Documents MUST be well-formed XML.  This specification does not
   define a DTD for Deleted Entry Documents, and hence does not require

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   them to be valid (in the sense used by XML).

5.  Digital Signatures

   The at:deleted-entry element MAY have an Enveloped Signature, as
   described by XML-Signature and Syntax Processing

   Processors MUST NOT reject an at:deleted-entry containing such a
   signature because they are not capable of verifying it; they MUST
   continue processing and MAY inform the user of their failure to
   validate the signature.

   In other words, the presence of an element with the namespace URI
   "http://www.w3.org/2000/09/xmldsig#" and a local name of "Signature"
   as a child of the document element MUST NOT cause an Processor to
   fail merely because of its presence.

   Section 6.5.1 of [W3C.REC-xmldsig-core-20020212] requires support for
   Canonical XML [W3C.REC-xml-c14n-20010315].  However, many
   implementers do not use it because signed XML documents enclosed in
   other XML documents have their signatures broken.  Thus, Processors
   that verify signed at:deleted-entry elements MUST be able to
   canonicalize with the exclusive XML canonicalization method
   identified by the URI "http://www.w3.org/2001/10/xml-exc-c14n#", as
   specified in Exclusive XML Canonicalization

   Intermediaries such as aggregators may need to add an atom:source
   element to an at:deleted-entry that does not contain its own atom:
   source element.  If such an entry is signed, the addition will break
   the signature.  Thus, a publisher of individually-signed at:deleted-
   entry's should strongly consider adding an atom:source element to
   those elements before signing them.  Implementers should also be
   aware of the issues concerning the use of markup in the "xml:"
   namespace as it interacts with canonicalization.

   Section 4.4.2 of [W3C.REC-xmldsig-core-20020212] requires support for
   DSA signatures and recommends support for RSA signatures.  However,
   because of the much greater popularity in the market of RSA versus
   DSA, Atom Processors that verify signed Atom Documents MUST be able
   to verify RSA signatures, but do not need be able to verify DSA
   signatures.  Due to security issues that can arise if the keying
   material for message authentication code (MAC) authentication is not
   handled properly, Atom Documents SHOULD NOT use MACs for signatures.

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6.  Encryption

   The root of a Deleted Entry Document (the at:deleted-entry element)
   MAY be encrypted, using the mechanisms described by XML Encryption
   Syntax and Processing [W3C.REC-xmlenc-core-20021210].

   Section 5.1 of [W3C.REC-xmlenc-core-20021210] requires support of
   TripleDES, AES-128, and AES-256.  Processors that decrypt Deleted
   Entry Documents MUST be able to decrypt with AES-128 in Cipher Block
   Chaining (CBC) mode.

   Encryption based on [W3C.REC-xmlenc-core-20021210] does not ensure
   integrity of the original document.  There are known cryptographic
   attacks where someone who cannot decrypt a message can still change
   bits in a way where part or all the decrypted message makes sense but
   has a different meaning.  Thus, Processors that decrypt Deleted Entry
   Documents SHOULD check the integrity of the decrypted document by
   verifying the hash in the signature (if any) in the document, or by
   verifying a hash of the document within the document (if any).

   When a Deleted Entry Document is to be both signed and encrypted, it
   is generally a good idea to first sign the document, then encrypt the
   signed document.  This provides integrity to the base document while
   encrypting all the information, including the identity of the entity
   that signed the document.  Note that, if MACs are used for
   authentication, the order MUST be that the document is signed and
   then encrypted, and not the other way around.

7.  Security Considerations

   As specified in [RFC4287], Atom processors should be aware of the
   potential for spoofing attacks where an attacker publishes atom:entry
   or atom:deleted-entry elements using the same atom:id values as
   entries from other Atom feeds.  An attacker may attempt to trick an
   application into believing that a given entry has either been removed
   from or added to a feed.  To mitigate this issue, Atom processors are
   advised to ignore at:deleted-entry elements referencing entries that
   have not previously appeared within the containing Feed document and
   should take steps to verify the origin of the Atom feed before
   considering the entries to be removed.

   The at:deleted-entry element can be encrypted and signed using
   [W3C.REC-xmlenc-core-20021210] and [W3C.REC-xmldsig-core-20020212],
   respectively, and are subject to the security considerations implied
   by their use.

   Digital signatures provide authentication, message integrity, and

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   non-repudiation with proof of origin.  Encryption provides data

   An application supporting the use of digitally signed atom:entry and
   at:deleted-entry elements should be aware of the potential issues
   that could arise if a at:deleted-entry element indicating the
   deletion of an atom:entry element has been signed using a different
   key than what was used to sign the atom:entry, or when an unsigned
   at:deleted-entry is used to indicate the deletion of a signed atom:
   entry.  Either case can potentially indicate a form of spoofing
   attack.  Processors must take steps to verify the validity of the at:
   deleted-entry element.

8.  IANA Considerations

   A Deleted Entry Document, when serialized as XML 1.0, can be
   identified with the following media type:

   Type name:  application
   Subtype name:  atomdeleted+xml
   Required parameters:  None.
   Optional parameters:
     "charset":  This parameter has semantics identical to the charset
       parameter of the "application/xml" media type as specified in
   Encoding considerations:  Identical to those of "application/xml" as
     described in [RFC3023], Section 3.2.
   Security considerations:  As defined in this specification.
     In addition, as this media type uses the "+xml" convention, it
     shares the same security considerations as described in [RFC3023],
     Section 10.
   Interoperability considerations:  There are no known interoperability
   Published specification:  This specification.
   Applications that use this media type:  No known applications
     currently use this media type.
   Additional information:
     Magic number(s):  As specified for "application/xml" in [RFC3023],
       Section 3.2.
     File extension:  .atomdeleted
     Macintosh File Type code:  TEXT
   Person and email address to contact for further information:
     James M Snell <jasnell@us.ibm.com>
   Intended usage:  COMMON
   Author/Change controller:  IESG

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9.  Acknowledgements

   The author gratefully acknowledges the feedback from the members of
   the Atom Publishing Format and Protocol working group during the
   development of this specification.

10.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3023]  Murata, M., St. Laurent, S., and D. Kohn, "XML Media
              Types", RFC 3023, January 2001.

   [RFC3339]  Klyne, G., Ed. and C. Newman, "Date and Time on the
              Internet: Timestamps", RFC 3339, July 2002.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

   [RFC4287]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
              Syndication Format", RFC 4287, December 2005.

              Maler, E., Paoli, J., Yergeau, F., Sperberg-McQueen, C.,
              and T. Bray, "Extensible Markup Language (XML) 1.0 (Third
              Edition)", World Wide Web Consortium FirstEdition REC-xml-
              20040204, February 2004,

              Boyer, J., "Canonical XML Version 1.0", World Wide Web
              Consortium Recommendation REC-xml-c14n-20010315,
              March 2001,

              Reagle, J., Boyer, J., and D. 3rd, "Exclusive XML
              Canonicalization Version 1.0", World Wide Web Consortium
              Recommendation REC-xml-exc-c14n-20020718, July 2002,


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              Hollander, D., Layman, A., and T. Bray, "Namespaces in
              XML", World Wide Web Consortium FirstEdition REC-xml-
              names-19990114, January 1999,

              Marsh, J., "XML Base", World Wide Web Consortium
              FirstEdition REC-xmlbase-20010627, June 2001,

              Solo, D., Reagle, J., and D. Eastlake, "XML-Signature
              Syntax and Processing", World Wide Web Consortium
              FirstEdition REC-xmldsig-core-20020212, February 2002,

              Eastlake, D. and J. Reagle, "XML Encryption Syntax and
              Processing", World Wide Web Consortium Recommendation REC-
              xmlenc-core-20021210, December 2002,

Author's Address

   James M Snell

   Email: jasnell@us.ibm.com
   URI:   http://ibm.com

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