RFC6721: The Atom "deleted-entry" Element

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Related keywords:  (Atom Feed) (Entry Documents)

Internet Engineering Task Force (IETF)                          J. Snell
Request for Comments: 6721                                September 2012
Category: Standards Track
ISSN: 2070-1721

                    The Atom "deleted-entry" Element


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

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2012 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   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 ....................................................2
   2. Notational Conventions ..........................................2
   3. The at:deleted-entry Element ....................................2
   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

1.  Introduction

   Atom [RFC4287] is an XML-based document format that describes lists
   of related information known as "feeds".  Feeds are composed of a
   number of items known as "entries", each with an extensible set of
   attached metadata.  The primary use case that Atom addresses is the
   syndication of Web content, such as weblogs and news headlines to Web
   sites as well as directly to user agents.

   In the base Atom format, when an entry is removed from a feed but a
   consumer has already received and processed that entry, perhaps
   adding it to a local cache or display, there is no mechanism for
   determining that the entry has been removed.  This specification adds
   a mechanism to the Atom Syndication Format that publishers of Atom
   Feed and Entry documents can use to explicitly identify Atom entries
   that have been removed.  Atom consumers can use that information to
   adjust such things as their document cache and user interfaces.

2.  Notational Conventions

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

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     deletedEntry =
       element at:deleted-entry {
         attribute ref { atomUri },
         attribute when { atomDateConstruct },
         ( element at:by { atomPersonConstruct }?
         & element at:comment { atomTextConstruct }?
         & element atom:link { atomLink }*
         & element atom:source { atomSource }?
         & 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
   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 elements 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 its 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.

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     <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.  In such cases, the
   implication is that the particular atom:entry has either been
   published to the feed and then subsequently removed, or that a
   previously removed entry has been republished to the feed following a
   previous deletion.  To determine which condition applies, the
   Processor needs to compare the value of the at:deleted-entry
   element's when attribute to the value of the corresponding atom:entry
   element's atom:updated value:

   o  If the when attribute specifies a value equal to or more recent
      than that specified by the atom:updated element, the atom:entry is
      assumed to have been published and then subsequently removed.
      Processors SHOULD ignore the older atom:entry element.

   o  If the when attribute specifies a value older than that specified
      by the atom:updated element, the atom:entry is assumed to have
      been republished to the feed following a prior removal.
      Processors SHOULD ignore the older at:deleted-entry element.

   Publishers of feeds containing at:deleted-entry elements should note
   that the at:deleted-entry element is advisory in nature only, and it
   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.  For example, if an entry was
   published to a feed document that was published and processed
   yesterday by an aggregator application, and then is subsequently

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   deleted today with a corresponding at:deleted-entry element added to
   the feed as a signal that the entry was deleted, there is no
   guarantee that the aggregator application will pay any attention to
   the at:deleted-entry element during subsequent processing operations.

   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 but MUST NOT change their behavior as a result of the
   markup's presence.

   This specification allows the use of Internationalized Resource
   Identifiers (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.  Its 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
   them to be valid (in the sense used by XML).

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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 a 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
   Digital Signature Algorithm (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; they do not
   need be able to verify DSA signatures.  Due to security issues that
   can arise if the keying material for the message authentication code
   (MAC) is not handled properly, Atom Documents SHOULD NOT use MACs for

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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 in which someone who cannot decrypt a message can still
   change bits in a way in which 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 and 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.  Further, if MACs are
   used along with a symmetric encryption algorithm, the same key SHOULD
   NOT be used in the generation of the MAC and the encryption.

7.  Security Considerations

   As specified in [RFC4287], Atom Processors should be aware of the
   potential for spoofing attacks in which 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 is subject to the security considerations implied
   by their use.

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   Digital signatures provide authentication and message integrity with
   proof of origin.  Encryption provides data confidentiality.

   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 an at:deleted-entry element that indicates the
   deletion of an atom:entry element has been signed using a different
   key than what was used to sign the atom:entry, or if 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 [RFC3023].

      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 issues.

      Published specification: This specification.

      Applications that use this media type: Undefined.  As an extension
      to the Atom Syndication Format ([RFC4287]), this specification may
      be used within any application that uses the Atom Format.

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      Additional information:

         Magic number(s): As specified for "application/xml" in
         [RFC3023], Section 3.2

         File extension(s): .atomdeleted

         Macintosh file type code(s): TEXT

      Person & email address to contact for further information: James M
      Snell <jasnell@us.ibm.com>

      Intended usage: COMMON

      Restrictions on usage: None.

      Author: James M Snell <jasnell@us.ibm.com>

      Change controller: IESG

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.

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              Yergeau, F., Maler, E., Sperberg-McQueen, C., Paoli, J.,
              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., 3rd, D., and J. Boyer, "Exclusive XML
              Canonicalization Version 1.0", World Wide Web Consortium
              Recommendation REC-xml-exc-c14n-20020718, July 2002,

              Hollander, D., Bray, T., and A. Layman, "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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