XML Encryption Requirements

This document lists the design principles, scope, and requirements for the XML Encryption. It includes requirements as they relate to the encryption syntax, data model, format, cryptographic processing, and external requirements and coordination.

1. Introduction

The XML 1.0 Recommendation [XML] specifies the syntax of a class of resources called XML documents. This specification provides requirements for a XML syntax and processing for encrypting digital content, including portions of XML documents and protocol messages.

2. Design Principles and Scope

This section describes high level principles of design and definition of scope. They are an expression of intent/motivation. How these motivations are realized are addressed in subsequent sections.

The XML Encryption specification must describe how to use XML to represent a digitally encrypted Web resource (including XML itself). {prop1, prop2}. The XML representation of the encrypted resource must be a first class object (i.e., referenceable and consequently describable, signable, etc.) and represented by a distinct element type.
1. The specification must provide for the encryption of a part or totality of an XML document
  1. Granularity of encryption in an XML document is limited to an element (including start/end tags) or element content (between the start/end tags). {prop2, WS, FTF1}
    After much discussion about the requirements, complexities, and alternatives of attribute encryption {List: Hallam-Baker, Simon, Reagle} 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 or alternatives that satisfy the requirement to encrypt sensitive attribute values.
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 allow for the super-encryption of data (i.e., encrypting XML in which some elements are already encrypted). {prop1, prop2} Super-encrypted data must use the same syntax and semantics as any other encrypted data.
The specification must provide a mechanism for conveying encryption key information to a recipient. The structure must be flexible so as to meet a variety of application requirements including:
- Carrying an encrypted key value that is encrypted to the recipient with an asymmetric or symmetric cipher.
- Providing a name or URI reference to a known key
It must be possible (though it is not necessary) to include key information as part of an XML encrypted data representation or referenced externally. Additionally, keys must be able to (though it is not necessary) to identify the data that they encrypt.

The mechanisms of encryption must be simple: describe how to encrypt/decrypt digital content, XML documents, and portions thereof. {Reagle}
1. Only information necessary for decryption need be provided. {Reagle}.The specification must permit the efficient encoding of encrypted data and related information when parties have pre-agreed upon the encryption approach and keying material. Hence, the specification must not mandate the presence of any attributes describing how the data is encrypted.
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}
The Working Group (WG) must use pre-existing specifications unless it can explicitly justify the need for a new one. {Reagle} For example, it should use DOM or Information Set as a data model for XML instances and Canonical XML for canonicalization unless a compelling argument for an alternative can be made.
The specification must define a minimal (extensible) set of algorithms and key structures necessary for interoperability purposes. {Reagle}
The specification should strive to limit optionality and maximize extensibility such that all of the specification can be quickly implemented
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

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 the data model (e.g., element, attribute, etc.) and properties {e.g., child, parent, localname, prefix, etc.) {WS}
  The WG is still testing this requirement in the context of implementing using tree and event based parsers.
XML Encryption can be applied to any Web resource -- including non-XML content. {prop1, prop2} Also, see Requirements: Objects.
1. When a non-XML object (i.e., external data) is encrypted, the information necessary to aid the recipient in decrypting the object is captured in an instance of XML (i.e. the encryption method, keying information, etc.). It is an application decision whether to include the encrypted object cipher data within this XML, as a base64 encoded CDATA, or to simply reference the external cipher data octet sequence. In either case, the decrypted data must revert to the media type of the original object. {TimBL, Dillaway}

2. Objects

It must be possible to indicate the original type (e.g., XML CDATA, image/gif) of the encrypted data to aid the decryptor in processing it. For non-XML data, existing MIME type definitions [MIME] should be used.
Binary data must be encoded as Base64 when represented in XML. {FTF1}
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 XML Encryption defined syntax.
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

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 encryption schema definitions. {Reagle}
3. Implementation of the specification should work with existing XML parser and schema implementations. However, alterations to particular DOM and/or XML parser implementations may prove beneficial in terms of simplifying application development or improving runtime efficiency. These details are outside the scope of the XML Encryption specification.
XML Instance Validity {WS}
1. Encrypted instances must be well-formed but need not be valid against their original definition (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 following:
  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.)
The processing model must be described using XML, DOM, or Information Set terminology and implementations can be based on application specific logic (e.g., XPath and DOM are not required to implement). {List: Ferguson, FTF1}
The WG is still working on understanding its processing model requirements in the light of implementation experience.
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 any fragment processing should be specified as part of the transform.
2. Where a referencing mechanism does not support Transforms, applications should support same-document XPointers '#xpointer(/)' and '#xpointer(id("ID"))'.
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 wrapping that data in an XML compression syntax and then encrypting it. {FTF1}
Encryption and Signatures
1. The specification must recommend approaches for use of XML Signature with XML Encryption such that multiple parties may selectively encrypt and sign portions of documents that might already be signed and encrypted. Recipients should be able to easily determine whether or not to decrypt data prior to signature validation.
  1. 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" [XML-DSIG-Decrypt] signature transform. 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}.
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

The solution must work with arbitrary encryption algorithms, including symmetric and asymmetric keys schemes as well as dynamic negotiation of keying material. {prop1, prop2}
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-OAEP 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-3DES is required.
7. Message Integrity
  1. AES/3DES with SHA1 is optional to implement.
8. Message Authentication {FTF1}
  1. XML Signature [XMLDSIG] is recommended to implement.
9. Canonicalization {FTF1}
  1. Canonical XML is optional to implement.
10. Compression {FTF1}
  1. none
Key Structures
1. Scope: the only defined key structures must be those required by the mandatory and recommended algorithms. {Reagle}
2. The specification should not address how to specify the intended recipient of keying information beyond an optional "hint" attribute. {prop3: open issue 1, FTF1}
3. The specification should leverage the XML Signature specification's syntax for keying information (dsig:KeyInfo element) to the maximum extent possible.{prop3, FTF1}

5. Security

The XML Encryption specification must include a discussion of potential vulnerabilities and recommended practices when using the defined processing model in a larger application context. While it is impossible to predict all the ways an XML Encryption standard may be used, the discussion should alert users to ways in which potentially subtle weaknesses might be introduced.

At a minimum, the following types of vulnerabilities must be addressed.

Security issues arising from known plain-text and data length information
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}
Processing of invalid decrypted data if an integrity checking mechanism is not used in conjunction with encryption. {List: Lambert, FTF1}
Potential weaknesses resulting from combining signing and encryption operations.
1. sign before you encrypt: the signature may reveal information about the data that has now been encrypted unless proper precautions are taken (such as properly adding an encrypted random string to the plaintext before hashing). {List: Finney}
2. encrypt before you sign: Users might mistakenly sign encrypted data under a semantic (e.g., asserts or agrees to) associated with the data's decrypted form. [XMLDSIG: Only What is "Seen" Should be Signed]. Additionally, there may be multiple {data,key} pairs that result in the same encrypted data, therefore special care must be taken in the selection of the encryption function or in the signature process to mitigate the possibility of signature repudiation (e.g., "I didn't say this, I signed a different message encrypted under a different key.") {List: Wang, Ashwood}.
The specification should warn application designers and users about revealing information about the encrypted data
1. via any semantics inferred from a URI.

6. 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

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. January 2001.
http://www.w3.org/TR/DOM-Level-3-Core/core.html
FTF1: XML Encryption Face-to-Face. Boston, MA. March 2000
HP: Requirements and Goals for the Design of an 'XML Encryption Standard'. Gerald Huck and Arne Priewe. November 2000.
InfoSet: XML Information Set, W3C Proposed Recommendation. John Cowan. August 2001.; http://www.w3.org/TR/2001/PR-xml-infoset-20010810/
List: XML Encryption List (an unmoderated and unchartered public list).
MIME: RFC2046. MIME Part Two: Media Types November 1996.; http://rfc.net/rfc2046.html
MyProof: MyProof Position Paper On XML Encryption. Steve Wiley.
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. W3C Recommendation. J. Boyer. March 2001.; http://www.w3.org/TR/2001/REC-xml-c14n-20010315
http://www.ietf.org/rfc/rfc3076.txt
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 W3C Recommendation. D. Beech, M. Maloney, N. Mendelsohn, H. Thompson. May 2001.; http://www.w3.org/TR/2001/REC-xmlschema-1-20010502/
XML Schema Part 2: Datatypes W3C Recommendation. P. Biron, A. Malhotra. May 2001.; http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/
XMLDSIG: XML-Signature Syntax and Processing. W3C Proposed Recommendation. D. Eastlake, J. Reagle, and D. Solo. August 2001.; http://www.w3.org/TR/2001/PR-xmldsig-core-20010820/
XML-DSIG-Decrypt: Decryption Transform for XML Signature. W3C Working Draft. T. Imamura and H. Maruyama.; http://www.w3.org/TR/2001/WD-xmlenc-decrypt-20010626

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