XML Encryption Draft Requirements

1. Introduction

The XML 1.0 Recommendation [XML] describes the syntax of a class of data objects called XML documents. There is interest in a specification of XML syntax and processing for encrypting digital content, including portions of XML documents and protocol messages. This documents provides requirements for such a specification.

2. Design Principles and Scope

This section describes requirements over intended result, how these motivations are realized are addressed in subsequent sections.

1. The XML Encryption specification must describe how to use XML to represent a digitally encrypted Web resource (including XML itself). {prop1,prop2}
   1. The specification must provide for the encryption a part or totality of an XML document
      1. Granularity of encryption is limited to an element (including start/end tags) or element content (between the start/end tags). {prop2, WS}

         Otherwise, sensitive attribute values are secured by transforming the original document into a element only version. {List: Hallam-Baker} (However, requiring applications to do this in a "non-standard" way is costly, {List: Simon} and can make the data useless to intermediate processors which is counter to the purpose of partial encryption. {List: Reagle}).

         The Working Group (WG) solicits comment on this requirement from the broader community. After much discussion the WG has decided to  proceed under the requirement of element encryption while remaining open to further comment, experimentation and specification of attribute encryption proposals.

   2. The specification must provide for the separation of encryption information from encrypted data, and support reference mechanisms for addressing encryption information from encrypted data sections and vice-versa. {HP: R3.7, prop2}
   3. The specification must provide for recursive encryption (capable of encrypting XML with portions already encrypted). {prop1,prop2}
2. The mechanisms of encryption must be simple: describe how to encrypt/decrypt digital content, XML documents, and portions thereof. {Reagle}
   1. Only enough information necessary for decryption need be provided. {Reagle}
   2. The specification will not address the confidence or trust applications place in the provision of a key
   3. The specification will not address authentication. {List: Reagle, WS}
   4. The specification will not address authorization and access control. {List: Reagle, Simon, Kudoh, WS}
3. The Working Group (WG) must use pre-existing specifications unless it can explicitly justify the need for a new one. {Reagle}For example, its should use Information Set as a data model for XML instances and Canonical XML for canonicalization.
4. The specification must define a minimal set of algorithms and key structures necessary for interoperability purposes. {Reagle}
5. The specification should strive to limit optionality and maximize extensibility such that all of the specification can be quickly implemented
6. Whenever possible, any encryption resource or algorithm is a first class object (which can also be encrypted or signed), and identified by a URI. {prop1,prop2

3. Requirements

1. Encryption Data Model and Syntax

1. The XML data model used by XML Encryption in identifying or representing data that has been processed must be predicated on:
   1. a simple enumerated subset of InfoSet items (e.g., element, attribute, etc.) and properties {e.g., child, parent, localname, prefix, etc.) {WS}

      The WG is still working on this issue in the context of our XML processing model and its relationship to tree and event based parsers.

2. XML Encryption can be applied to any Web resource -- including non-XML content. {prop1,prop2} Also, see Requirements: Objects.
   1. A non-XML object when encrypted is encoded in an instance of XML; when decrypted it must revert to its original media type. {TimBL}

2. Objects

1. Binary data must be encoded as Base64 when represented in XML. {FTF1}
2. The specification must not define packaging representations of non XML data (e.g., MIME-objects) other than the encrypted and encoded information appearing within the EncryptedData element.
3. The specification must not define a packaging format that describes the relationships between encrypted objects. For instance, the specification will not specify how an application can designate that a set of encrypted objects are actually encryptions over different representations (encodings, compression, etc.) of the same object. {prop3: open issue 2, resolved at FTF1}

3. Processing

1. Parsing  {WS}
   1. XML Encryption applications must be XML-namespaces [XML-namespaces] aware.
   2. XML Encryption applications must be XML Schema [XML-schema] aware in that they create XML encryption instances conforming to the schema definition. {Reagle}
   3. Implementation of the specification should have a minimal affect upon XML parser and schema implementations. While {WS:Simon} demonstrated that XML-encryption functionality did not necessarily require any changes to particular DOM and XML parser implementations, it may be necessary for implementors to tweak their parsers as needed.
2. XML Instance Validity {WS}
   1. Encrypted instances must be well-formed but need not be valid (i.e. applications that encrypt the element structure are purposefully hiding that structure.)
   2. Instance authors that want to validate encrypted instances must do one of the follow:
      1. Write the original schema so as to validate resulting instances given the change in its structure and inclusion of element types from the XML Encryption namespace.
      2. Provide a post-encryption schema for validating encrypted instances.
      3. Only encrypt PCDATA text of element content and place its decryption and key information in an external document. (This requires granular detached/external encryption.)
3. The processing model must be described using Information Set terminology and implementations can be based on application specific logic (e.g., XPath and DOM are not required).  {List: Ferguson, FTF1}

   The WG is still working on understanding its processing model requirements.

4. The referencing model must be based on XML Signature's Reference Processing Model [XMLDSIG] with the following two qualifications:
   1. As recommended by [XMLDSIG], where a referencing mechanism supports transforms (e.g., KeyRetreivalMethod and Ciphertext URI) any fragment processing should be specified as part of the transform.
   2. Where a referencing mechanism (e.g., DataReference and KeyReference) does not support Transforms, applications should support same-document XPointers '#xpointer(/)' and '#xpointer(id("ID"))'.
5. Transforms  {WS}
   1. Encryption Transforms: The specification must not enable the specification of additional transforms as  part of encrypting and decrypting data; transforms on data being encrypted/decrypted must be done by the application. For example, compression could be done by compressing the content and rapping that data in an XML compression syntax and then encrypted. {FTF1}
6. Encryption and Signatures
   1. The specification must address how to use XML Signature with XML Encryption such that multiple parties may selectively encrypt and sign portions of documents that might already be signed and encrypted.
      1. The interaction of encryption and signing is an application issue and out of scope of the specification. {List: Ashwood, Hirsch, FTF1}
      2. Applications have the following options:
         1. When data is encrypted, so is its Signature; consequently those Signature you can see can be validated. (However, this is not always easily accomplished with detached Signatures.){List: Finney}
         2. Employ the "decrypt-except" signature transform, being developed as a separate specification. It works as follows: during signature transform processing, if you encounter a decrypt transform, decrypt all encrypted content in the document except for those excepted by an enumerated set of references. {List: Maruyama, FTF1}
7. The encryption and XML processing should be
   1. Fast {List: Ferguson}
   2. Memory efficient {List: Ferguson}
   3. Work with tree and event based parsers {List: Ferguson}

4. Algorithms and Structures

1. The solution must work with arbitrary encryption algorithms, including symmetric and asymmetric keys schemes as well as dynamic negotiation of keying material. {prop1,prop2}
2. The specification must specify or reference one mandatory to implement algorithm for only the most common application scenarios.
   1. Stream Encryption Algorithms {FTF1}
      1. none
   2. Block Encryption Algorithms {FTF1}
      1. AES with CMS keylength is required to implement
      2. 3DES is required to implement -- this may be relaxed when AES as matures.
      3. AES at other keylengths is optional to implement.
   3. Chaining Modes {FTF1}
      1. CBC (Cipher Block Chaining) with PKCS#5 padding is optional to implement.
   4. Key Transport {FTF1}
      1. RSA-OEAP used with AES is required to implement.
      2. RSA-v1.5 used with 3DES is required to implement -- this may be relaxed as AES matures.
   5. Key Agreement {FTF1}
      1. Diffie-Hellman is optional to implement
   6. Symmetric Key Wrap {FTF1}
      1. AES KeyWrap is mandatory -- when it's completely specified.
      2. CMS-KeyWrap Triple-DES and RC2 is required.
   7. Message Authentication {FTF1}
      1. AES/3DES with SHA1 is optional to implement.
      2. XML Signature [XMLDSIG] is optional to implement.
   8. Canonicalization {FTF1}
      1. Canonical XML is required to implement.

         The WG is still working on understanding its processing model requirements.

   9. Compression {FTF1}
      1. none
   10. Initialization Vector {FTF1}
       1. ?
3. Key Structures
   1. Scope: the only defined key structures must be those required by the mandatory and recommended algorithms. {Reagle}
   2. The specification will not address how to specify the intended recipient of keying information beyond an optional "hint" attribute. {prop3: open issue 1, FTF1}
   3. EnryptedKey element should be a child of a KeyInfo element. {prop3, FTF1}
   4. Definitions:

      The WG is still considering its requirements with respect to the use of dsig:KeyInfo. Which element is  root, do we use schema class extention or open content models? {List: Reagle}

5. Security

1. The specification must address security issues arising where the following occur: {prop3: issue 5} in software and hardware implementations {List: Lambert
   1. An attacker may know the original structure of the plain-text via its schema. {List: Wiley}
   2. An attacker may know the length and redundancy of the plain-text data. {List: Finney}
2. As already stated, the specification should provide for the optional creation of a checksum over the data encrypted in the ciphertext. (This enables an application to verify the success of the decryption process instead of continuing to process data with the wrong key.) {List: Lambert, FTF1}
3. As already stated, the specification should warn users about weaknesses resulting from the order of signing and encryption operations.
4. The specification should warn application designers and users about revealing information about the encrypted data
   1. in a signature over that data prior to encrypted.
   2. via any semantics inferred from a URI.
5. Signature order: The specification's Security Considerations should address weaknesses resulting from:
   1. sign before you encrypt:{List: Finney} This can be problematic as, "the only way to achieve secure channels is to encrypt first, then MAC. Though signature is different from MAC, but we should keep in mind that digital signature is an extension of MAC."  {List: Wang}
   2. encrypt before you sign: {List: Wang} This can be problematic as, "it was shown that by choosing your RSA key pair, and keeping the factors of N, you could create many different statements easily, with very real possibilities for fraud. {List: Ashwood}

6. Misc

 

7. Coordination

The XML Encryption specification should meet the requirements of (so as to support) or work with the following applications:

• XW3C XML Signature {Reagle}
• W3C XML Protocols {Reagle}
• Oasis XML-Based Security Services TC (SSTC) {Reagle}
• Synchronized Multimedia Integration Language. {List: Simon}

To ensure the above requirements are adequately addressed, the XML Encryption specification must be reviewed by a designated member of the following communities:

• XML Signature WG
• XML Protocol
• XML Schema WG
• XML Core WG
• Internationalization IG

8 Intellectual Property

1. The specification should be free of encumbering technologies: requiring no licensing fees for implementation and use. {List: Ferguson}

   "Members of the XML Encryption Working Group and any other Working Group constituted within the XML Encryption Activity are expected to disclose any intellectual property they have in this area. Any intellectual property essential to implement specifications produced by this Activity must be at least available for licensing on a royalty-free basis. At the suggestion of the Working Group, and at the discretion of the Director of W3C, technologies may be accepted if they are licensed on reasonable, non-discriminatory terms." XML Encryption Charter.

4. References

C2000

Crypto 2000 XML Encryption BoF. Santa Barbara, CA. August 24 .

DOM

Document Object Model Core, Level 3. Arnaud Le Hors. W3C Working Draft.
http://www.w3.org/TR/DOM-Level-3-Core/core.html

FTF1

XML Encryption Face-to-Face. Boston, MA. March 2001

HP

Requirements and Goals for the Design of an 'XML Encryption Standard'. Gerald Huck and Arne Priewe. November 2000.

InfoSet

XML Information Set, W3C Working Draft. John Cowan.
http://www.w3.org/TR/xml-infoset.

List

XML Encryption List (an unmoderated and unchartered public list).

MyProof

MyProof Position Paper On XML Encryption

prop1

XML Encryption strawman proposal. Ed Simon and Brian LaMacchia. Aug 09 2000.

prop2

Another proposal of XML Encryption. Takeshi Imamura. Aug 14 2000.

prop3

XML Encryption Syntax and Processing. Dillaway, Fox, Imamura, LaMacchia, Maruyama, Schaad, Simon. December 2000.

WS

W3C XML Encryption Workshop [minutes]. SanFrancisco. November 2, 2000.

XML

Extensible Markup Language (XML) 1.0 Recommendation. T. Bray, J. Paoli, C. M. Sperberg-McQueen. February 1998.

http://www.w3.org/TR/1998/REC-xml-19980210

XML-C14N

Canonical XML. Working Draft. J. Boyer. January 2001.

http://www.w3.org/TR/2001/PR-xml-c14n-20010119

XML-ns

Namespaces in XML Recommendation. T. Bray, D. Hollander, A. Layman. January 1999.

http://www.w3.org/TR/1999/REC-xml-names-19990114

XML-schema

XML Schema Part 1: Structures Working Draft. D. Beech, M. Maloney, N. Mendelshohn. October 2000.

http://www.w3.org/TR/2000/CR-xmlschema-1-20001024/
XML Schema Part 2: Datatypes Working Draft. P. Biron, A. Malhotra. October 2000.

http://www.w3.org/TR/2000/CR-xmlschema-2-20001024/

XMLDSIG

XML-Signature Syntax and Processing. Working Draft. D. Eastlake, J. Reagle, and D. Solo.
http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/

XSet

Full Fidelity Information Set Representation. Jonathan Borden. XML-Dev

http://lists.xml.org/archives/xml-dev/200008/msg00239.html

URI

RFC2396. Uniform Resource Identifiers (URI): Generic Syntax. T. Berners-Lee, R. Fielding, L. Masinter. August 1998
http://www.ietf.org/rfc/rfc2396.txt