Decryption Transform for XML Signature

W3C Working Draft 18 October 2001

This version:
Latest version:
Previous version:
Takeshi Imamura <imamu@jp.ibm.com>
Hiroshi Maruyama <maruyama@jp.ibm.com>
See References.


This document specifies the "decryption transform", which enables XML Signatures verification even if both signature and encryption operations are performed on an XML document.

Status of this document

This is the Last Call for the "Decryption Transform for XML Signature" Working Draft from the XML Encryption Working Group (Activity Statement). This version represents a consensus towards satisfying the requirement of partially signing and encrypting XML documents. The last call period is 3 weeks, ending on 9 November 2001.

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(archives )

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

Table of Contents

  1. Introduction
    1. Purpose
    2. Editorial Conventions
  2. Decryption Transform
    1. Processing Rules
  3. Transform Creation (Non-Normative)
  4. Example
  5. Security Considerations
    1. Signatures Over Encrypted Data May Reveal Information
    2. Signed What you See
  6. References

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. Two 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 6) 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

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 "enc", "ds", and "dcrpt" XML namespace prefixes is OPTIONAL; we use this facility to provide compact and readable exposition.

2 Decryption Transform


This transform takes as a parameter a set of references to encrypted portions that are not to be decrypted by the transform. These references are expressed by the dcrpt:Except elements that appear as direct child elements of the ds:Transform element. The value of the URI attribute of the dcrpt:Except element MUST be a non-empty same-document URI reference [URI] (i.e., a number sign ('#') character followed by a fragment identifier) and identify an enc:EncryptedData element.

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 of the XML Signature specification [XML-Signature]. The transform decrypts all the enc:EncryptedData elements (as defined in the forthcoming XML Encryption standard [XML-Encryption]) except for those specified by dcrpt:Except elements. The output of the transform is also a node-set.

  Schema Definition:

  <?xml version="1.0" encoding="utf-8"?>
  <!DOCTYPE schema  PUBLIC "-//W3C//DTD XMLSchema 200102//EN"
     <!ENTITY % p ''>
     <!ENTITY % s ''>
  <schema xmlns='http://www.w3.org/2001/XMLSchema' version='0.1'

  <element name="Except" type="dt:ExceptType"/>
  <complexType name='ExceptType'>
    <attribute name='Id' type='ID' use='optional'/>
    <attribute name='URI' type='anyURI' use='optional'/>

2.1 Processing Rules

This section describes the processing rules of the transform. The rules are written as two functions, whereas the inputs and outputs of the transform are the inputs and outputs of the noDecryptNodes() function, which itself calls Decypt(). The processing rules and functions are conceptual only and do not mandate any particular implementation.

[Definition: Let X be a node-set. Let e be the first element node in X. A parsing context of X consists of the following items:

N = noDecryptNodes(X, R)
where X is a node-set and R is a set of dcrpt:Except elements specified as a parameter of the transform.
This function evaluates the URI attributes of all the dcrpt:Except elements in R in the context of X. N is a set of enc:EncryptedData element nodes in X referenced by the elements.

This transform performs the following steps:

  1. Let X be a node-set given as input. Let R be a set of dcrpt:Except elements given as a parameter of the transform.
  2. Within X, for every e, an element node with the type enc:EncryptedData, such that e is not a member of noDecryptNodes(X, R).
    1. Let C be a parsing context of X.
    2. Let Y be decrypt(X, e, C). If this function succeeds, replace X with Y. Otherwise, the implementation MAY signal a failure of the transform. Alternatively, it MAY also continue processing without changing X (although it should take an appropriate means to avoid an infinite loop).

    If such e cannot be selected, the algorithm terminates and the result of the transformation is X.

[Definition: A node-set is said to be single-rooted if and only if all of its member nodes are either (1) the first node in the node-set in the document order, (2) a descendant node of the first node, or (3) an attribute node or a namespace node of another node in this node-set.]

During the above steps, X MUST always be a single-rooted node-set. If X is not single-rooted, this transform MUST fail.

Y = decrypt(X, e, C)
where X is a node-set, e is an element node with the type enc:EncryptedData in X, and C is a parsing context of X.
Y is a node-set obtained by the following steps:
  1. Convert X to an octet stream as described in The Reference Processing Model of the XML Signature specification [XML-Signature].
  2. Wrap the resulting octet stream with the octets representation of dummy tags (i.e., <dummy> and </dummy>) as described in [Tobin], and if needed, prepend the octets representing an XML declaration and a document type declaration. In order to parse the octet stream in the context of C, all the namespace declarations in C MUST be added to the dummy element. Also all the entity declarations in C MUST be added to the document type declaration.
  3. Decrypt the element corresponding to e (which may require parsing) and replace it with the resulting octet stream according to the XML Encryption [XML-Encryption].
  4. Parse the decrypted octet stream as described in The Reference Processing Model of the XML Signature specification, resulting in a node-set.
  5. Y is the node-set obtained by removing the root node, the dummy element node, and its associated set of attribute and namespace nodes from the node-set obtained in Step 4.
If any of the above steps fails for whatever reasons (e.g., the decryption key cannot be located, parsing in Step 4 fails, etc.), this function also fails.

Note: Condition places certain restrictions on the use of this transform. First, the input to the transform MUST be single-rooted. Second, if the first node of the inpassumeut is an element node with the type enc:EncryptedData, the decrypted octet stream MUST be of type http://www.w3.org/2001/04/xmlenc#Element. These restrictions are necessary to ensure that the decrypted octet stream is parsed correctly in a given parsing context.

Note: This transform does not deal with any detached enc:EncryptedKey elements. When an enc:EncryptedData element is decrypted, some enc:EncryptedKey elements detached from the enc:EncryptedData element have to be removed if the enc:EncryptedKey elements are in the scope of a signature being validated. However, it is unclear how the transform should deal with the enc:EncryptedKey elements, and hence it is not recommended in this document to detach enc:EncryptedKey elements from an enc:EncryptedData element or to include detached enc:EncryptedKey elements in the scope of a signature.

4 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 advisory.

A ds:Transform element can be created by the following steps:

  1. Apply all the transforms being placed before this transform to a data object being signed.
  2. If the transform just before this transform outputs an octet stream, convert it to a node-set as described in The Reference Processing Model of the XML Signature specification [XML-Signature].
  3. For each node in the node-set, if the node is an element node with the type enc:EncryptedData, create an dcrpt:Except element referencing the node.
  4. Create a ds:Transform element, including the algorithm identifier of this transform and all the dcrpt:Except elements created in Step 3.

5 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.

   [01] <order Id="order">
   [02]   <item>
   [03]     <title>XML and Java</title>
   [04]     <price>100.0</price>
   [05]     <quantity>1</quantity>
   [06]   </item>
   [07]   <cardinfo>
   [08]     <name>Your Name</name>
   [09]     <expiration>04/2002</expiration>
   [10]     <number>5283 8304 6232 0010</number>
   [11]   </cardinfo>
   [12]   <EncryptedData Id="enc1" xmlns="http://www.w3.org/2001/04/xmlenc#">...</EncryptedData>
   [13] </order>

In order to let the recipient know the proper order of decryption and signature verification, the signer include 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:

   [01] <Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
   [02]   <SignedInfo>
   [03]     ...
   [04]     <Reference URI="#order">
   [05]       <Transforms>
   [06]         <Transform Algorithm="http://www.w3.org/2001/04/decrypt#">
   [07]           <Except URI="#enc1" xmlns="http://www.w3.org/2001/04/decrypt#"/>
   [08]         </Transform>
   [09]         <Transform 
   [10]       </Transforms>
   [11]       ...
   [12]     </Reference>
   [13]   </SignedInfo>
   [14]   <SignatureValue>...</SignatureValue>
   [15]   <Object>
   [16]     <order Id="order">
   [17]       <item>
   [18]         <title>XML and Java</title>
   [19]         <price>100.0</price>
   [20]         <quantity>1</quantity>
   [21]       </item>
   [22]       <EncryptedData Id="enc2" xmlns="http://www.w3.org/2001/04/xmlenc#">...</EncryptedData>
   [23]       <EncryptedData Id="enc1" xmlns="http://www.w3.org/2001/04/xmlenc#">...</EncryptedData>
   [24]     </order>
   [25]   </Object>
   [26] </Signature>

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 C14N ([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.

6 Security Considerations

6.1 Signatures Over Encrypted Data May Reveal Information

When this algorithm is used to permit 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 vulnerabaility 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 it's 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.

6.2 "Sign What You See"

This specification serves scenarios in which a person might sign encrypted data. Because [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 timestamping 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!

7 References

Hal Finney. Re: Combining signing and encrypting, XML Encryption mailing list, 2000.
Amir Herzberg. Signing encrypted data, XML Encryption mailing list, 2001.
Joseph Reagle. Re: Signing and Encryption, XML Encryption mailing list, 2001.
David Solo. Combining signing and encrypting, XML Encryption mailing list, 2000.
Richard Tobin. Infoset for external entities, XML Core mailing list [W3C Member Only], 2000.
T. Berners-Lee, R. Fielding, and L. Masinter. Uniform Resource Identifiers (URI): Generic Syntax, RFC 2396, 1998.
John Boyer. Canonical XML Version 1.0, W3C Rec., 2001.
Donald Eastlake and Joseph Reagle. XML Encryption Syntax and Processing, W3C Working Draft, 2001.
Donald Eastlake, Joseph Reagle, and David Solo. XML-Signature Syntax and Processing, W3C Proposed Rec., 2001.
James Clark and Steve DeRose. XML Path Language (XPath) Version 1.0, W3C Rec., 1999.