Decryption Transform for XML Signature

W3C Candidate Recommendation XX Foo 2002

This version:: n/a; $Revision: hacked to bits$ on $Date: today$ GMT by $Author: merlin$
Latest version:: http://www.w3.org/TR/xmlenc-decrypt
Previous version:: http://www.w3.org/TR/2001/WD-xmlenc-decrypt-20011018
Editors: Takeshi Imamura <imamu@jp.ibm.com>; Hiroshi Maruyama <maruyama@jp.ibm.com>
Contributors: See References.

Abstract

This document specifies an XML Signature "decryption transform" that enables XML Signature applications to distinguish between those XML Encryption structures that were encrypted before signing (and must not be decrypted) and those that were encrypted after signing (and must be decrypted) for the signature to validate.

Status of this document

This specification from the XML Encryption Working Group (Activity) is a Candidate Recommendation of the W3C. None of the last call issues on the XML Encryption specifications concerned this specification. Furthermore, the WG considers this specification to be stable and invites implementation feedback during this period.

The exit criteria for this phase is at least two interoperable implementations of this transform with acceptable performance. The interoperability of this specification will be demonstrated as an algorithm in the XML Encryption Syntax and Processing Interoperability Report. We expect to meet all requirements of that report within the two month Candidate Recommendation period (closing April 25). Specific areas where we would appreciate further experience are:

Do implementations achieve satisfactory performance?
Does the specification satisfy application scenario requirements for encrypting and signing portions of XML?

Publication of this document does not imply endorsement by the W3C membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite a W3C Working Draft as anything other than a "work in progress." A list of current W3C working drafts can be found at http://www.w3.org/TR/.

Please send comments to the editors (<imamu@jp.ibm.com>, <maruyama@jp.ibm.com>) and cc: the list xml-encryption@w3.org (publicly archived).

Patent disclosures relevant to this specification may be found on the Working Group's patent disclosure page in conformance with W3C policy.

1 Introduction

1.1 Purpose

It has been noted by David Solo in [Solo] that both signature [XML-Signature] and encryption [XML-Encryption] operations may be performed on an XML document at any time and in any order, especially in scenarios such as workflow. For example, Alice wishes to order and pay for a book from Bob using the mutually trusted payment system ZipPay. Bob creates an order form including the book title, price and his account info. He wants to sign all of this information, but will subsequently encrypt his account info for ZipPay only. He sends this to Alice who affirms the book title and price, signs the form and presents the twice-signed order with her own payment information to ZipPay. To validate both signatures ZipPay will have to know that the cipher data version of the encrypted information is necessary for validating Alice's signature, but the plain data form is necessary for validating Bob's signature. (See "Sign What You See" (section 5.2) for more on signing encrypted data.)

Since encryption operations applied to part of the signed content after a signature operation cause a signature not to be verifiable, it is necessary to decrypt the portions encrypted after signing before the signature is verified. The "decryption transform" proposed in this document provides a mechanism; decrypting only signed-then-encrypted portions (and ignoring encrypted-then-signed ones). A signer can insert this transform in a transform sequence (e.g., before Canonical XML [XML-C14N] or XPath [XPath]) if there is a possibility that someone will encrypt portions of the signature.

The transform defined in this document is intended to propose a resolution to the decryption/verification ordering issue within signed resources. It is out of scope of this document to deal with the cases where the ordering can be derived from the context. For example, when a ds:DigestValue element or a (part of) ds:SignedInfo element is encrypted, the ordering is obvious (without decryption, signature verification is not possible) and there is no need to introduce a new transform.

1.2 Editorial Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [Keywords].

This document makes use of the XML Encryption [XML-Encryption] and XML Signature [XML-Signature] namespaces, and defines it own, with the following prefixes:

While applications MUST support XML and XML namespaces, the use of our "xenc", "ds", and "dcrpt" XML namespace prefixes is OPTIONAL; we use this facility to provide compact and readable exposition.

2 Decryption Transform

This transform requires an XPath node-set [XPath] for input. If an octet stream is given as input, it must be converted to a node-set as described in The Reference Processing Model (section 4.3.3.2) of the XML Signature specification [XML-Signature]. The transform decrypts all the xenc:EncryptedData elements [XML-Encryption] except for those specified by dcrpt:Except elements. dcrpt:Except is defined below via XML Schema [XML-Schema] and appears as direct child elements of the ds:Transform element. The output of the transform is a node-set or octet stream, depending on the type of the encrypted data.

The type of an xenc:EncryptedData element is determined by its Type attribute. Two type URIs are always considered to indicate that an xenc:EncryptedData contains XML data:

Other XML type URIs can be specified using the dcrpt:XMLType element. This element may appear zero or more times as a direct child of the ds:Transform element. The dcrpt:XMLType element MUST contain either a Prefix attribute or a URI attribute; it MUST NOT contain both. If the Type attribute of any xenc:EncryptedData starts with a value provided by a dcrypt:XMLType Prefix attribute, or exactly matches a value provided by a dcrypt:XMLType URI attribute, then the plaintext content of that encrypted data is considered to be XML and processed accordingly, as defined below.

Elements to be excepted from the decryption process are identified by dcrpt:Except elements. The REQUIRED URI attribute value of the dcrpt:Except element MUST be a non-empty same-document URI reference [URI] (i.e., a number sign ('#') character followed by an XPointer expression (as profiled by [XML-Signature, Section 4.3.3.2])) and identify one or more xenc:EncryptedData elements within the input to this transform. For example, the URI #id-1 would except an xenc:EncryptedData with an Id attribute having the value id-1. Similarly, the URI #xpointer(/Document/Chapter/SignedData/xenc:EncryptedData[1]) would except the first xenc:EncryptedData child of the selected SignedData element. It is important to be aware that XPointers are evaluated with respect to the full input document structure, irrespective of whether a subset has been supplied as input to this transform.

2.1 Processing Rules

This section describes the processing rules of the transform. The rules are written as three functions; the inputs and outputs of the transform are the inputs and outputs of the decryptIncludedNodes() function, which itself calls the decryptXML() or decryptOctets() function.

The transform operates over a node-set X, and its parsing context , which consists of the following items:

The decryptIncludedNodes() function also takes a set of dcrpt:Except elements that identify xenc:EncryptedData elements that should not be processed by this transform, and a set of dcrpt:XMLType elements identifying additional xenc:EncryptedData Type URIs that should be considered to indicate XML content. When dereferencing dcrpt:Except URIs, the application MUST behave as if the root document node of the input node set is used to initialize the [XPointer] evaluation context, even if this node is not part of the node set. Unlike [XML-Signature], the URI may be evaluated against a different document from the signature document. If the input is a different document then, as per [XPointer], use of the here() function is an error.

2.1.1 Functions

(In decryptXML(), all of the steps except the actual decryption are necessary because XPath does not permit one to remove and then replace a node. Consequently, we must serialize (1), wrap (2), reparse (4), and trim the node set (5).)

If any of the above steps in decryptXML() or decryptOctets() fails for whatever reasons (e.g., the decryption key cannot be located, parsing fails, etc.), the function also fails.

2.1.2 Restrictions and Limitations

These restrictions are necessary to ensure that the transform is performed correctly.

3 Transform Creation (Non-Normative)

It is out of scope of this document how to create a ds:Transform element and where to insert it in a transform sequence. In this section, we just show a way to create the element as an advisory.

4 Example

Suppose the following XML document is to be signed. Note that the part of this document ([12]) is already encrypted prior to signature. In addition, the signer anticipates that some parts of this document, for example, the cardinfo element ([07-11]) will be encrypted after signing.

In order to let the recipient know the proper order of decryption and signature verification, the signer includes the decryption transform ([06-08] below) in the signature. Assuming that an additional encryption is done on the cardinfo element ([22]), the recipient would see the following encrypt-sign-encrypt document:

The recipient should first look at the Signature element ([01-26]) for verification. It refers to the order element ([16-24]) with two transforms: decryption ([06-08]) and canonicalization ([09]). The decryption transform instructs the signature verifier to decrypt all the encrypted data except for the one specified in the Except element ([07]). After decrypting the EncryptedData in line [22], the order element is canonicalized and signature-verified.

5 Security Considerations

5.1 Signatures Over Encrypted Data May Reveal Information

When this algorithm is used to facilitate subsequent encryption of data already signed, the digest value of the signed resource still appears in clear text in a ds:Reference element. As noted by Hal Finney in [Finney], such a signature may reveal information (via the digest value) over encrypted data that increases the encryption's vulnerability to plain-text-guessing attacks. This consideration is out of scope of this document and (if relevant) should be addressed by applications. For example, as proposed by Amir Herzberg in [Herzberg], one may include a random 'salt' in a resource being signed to increase its entropy.

Another approach is that when a signature referent is encrypted, one may also encrypt the signature (or at least the ds:DigestValue elements). As noted by Joseph Reagle in [Reagle], this latter solution works only if signature and encryption are well known by each other. For example, the signature may not be known of because it is detached. Or, it may be already encrypted! Consider, Alice Encrypts element A and the Signature over the parent of A. Bob Encrypts element B (sibling of A) but not the Signature since he doesn't know about it. Alice then decrypts A and it's Signature, which may provide information to a subsequent plain text attack on the encrypted B.

5.2 "Sign What You See"

This specification serves scenarios in which a person might sign encrypted data. Because XML Signature [XML-Signature] has only a simple semantic whereby a key is associated with some data -- and nothing more -- the signing of encrypted data is a legitimate process. For example, someone might run a content-neutral time stamp service that will sign any data sent to it with its time-stamping key under the semantic, "I received this on $date $time." However, applications often explicitly or implicitly associate more substantive semantics (e.g., authorizes, agrees, authors) with a signature. No one should be asked to apply a signature and its semantic to data he or she did not see. Just as the principles of Only What is 'Seen' Should be Signed and 'See' What is Signed are important for understanding the import of an XML Signature, they are doubly important when semantics are associated with that signature: one MUST NOT infer that a signature over encrypted data is also a signature over its plain text form, nor that the meaning of that signature over the encrypted data also applies to the plain text. If one wishes to sign the plain text form of data which is later encrypted, use the transform specified in this document!

6 Appendices

6.A Text Wrapping

This appendix specifies the process for wrapping text in a certain parsing context, which is performed in decryptXML() (section 2.1.1). The process is a part of the process proposed by Richard Tobin in [Tobin] for constructing the infoset [XML-Infoset] of an external entity.

The process consists of the following steps:

Emit an XML declaration with the version number "1.0".
If the parsing context contains any general entities, then emit a document type declaration that provides entity declarations declaring the entities.
Emit a dummy element start-tag with namespace declaration attributes declaring all the namespaces in the parsing context.
Emit the text.
Emit a dummy element end-tag.

In the above steps, the document type declaration and dummy element tags MUST be encoded in UTF-8.

Consider the following document containing an EncryptedData element:

<!DOCTYPE Document [
  <!ENTITY dsig "http://www.w3.org/2000/09/xmldsig#">
]>
<Document xmlns="http://example.org/">
  <foo:Body xmlns:foo="http://example.org/foo">
    <EncryptedData xmlns="http://www.w3.org/2001/04/xmlenc#"
                   Type="http://www.w3.org/2001/04/xmlenc#Element">
      ...
    </EncryptedData> 
  </foo:Body>
</Document>

If the EncryptedData element is fed through the decryption transform and is decrypted to the text "<One><foo:Two/></One>", then the text will be wrapped as follows:

<?xml version="1.0"?>
<!DOCTYPE dummy [
  <!ENTITY dsig "http://www.w3.org/2000/09/xmldsig#">
]>
<dummy xmlns="http://example.org/"
       xmlns:foo="http://example.org/foo"><One><foo:Two/></One></dummy>