The Atom "deleted-entry" Element
draft-snell-atompub-tombstones-16

Abstract

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.

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This Internet-Draft will expire on December 31, 2012.

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.

Table of Contents

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

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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 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 removed.

 
  deletedEntry =
    element at:deleted-entry {
      atomCommonAttributes,
      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 removed.

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 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"
        xmlns:at="http://purl.org/atompub/tombstones/1.0"> 
     ...
     <!-- Minimal deleted-entry --> 
     <at:deleted-entry 
       ref="tag:example.org,2005:/entries/1" 
       when="2005-11-29T12:11:12Z"/> 
     
     <!-- Extended deleted-entry --> 
     <at:deleted-entry 
       ref="tag:example.org,2005:/entries/2"
       when="2005-11-29T12:11:12Z"> 
       <at:by> 
         <name>John Doe</name> 
         <email>jdoe@example.org</email> 
       </at:by> 
       <at:comment>Removed comment spam</at:comment> 
     </at:deleted-entry> 
     ...
  </feed> 

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 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. If an at:deleted-entry element and an atom:entry element sharing the same atom:id value exist in the same feed and the at:deleted-entry element's when value specifies a more recent date than that specified by the atom:entry element's atom:updated element, the implication is that the entry has been deleted since the time it was published and the presence of the at:deleted-entry represents a retraction of the older entry.

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 attribute.

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 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 [W3C.REC-xmldsig-core-20020212].

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 [W3C.REC-xml-exc-c14n-20020718].

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.

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. 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 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 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 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 [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.
• 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. References

Author's Address