Decryption Transform for XML Signature (original) (raw)

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 document is the W3C Decryption Transform for XML Signature Recommendation (REC). This document has been reviewed by W3C Members and other interested parties and has been endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited as a normative reference from another document. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web.

This specification was produced by the W3C XML Encryption Working Group (Activity) which believes the specification is sufficient for the creation of independent interoperable implementations as demonstrated in the Interoperablity Report.

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

Pleae report errors in this document to xml-encryption@w3.org (public archive).

The list of known errors in this specification is available at http://www.w3.org/Encryption/2002/12-xmlenc-decrypt-errata.

The English version of this specification is the only normative version. Information about translations of this document (if any) is available http://www.w3.org/Encryption/2002/12-xmlenc-translations.

A list of current W3C Recommendations and other technical documents can be found at http://www.w3.org/TR/.

Introduction
Decryption Transform Syntax
XML-Mode Decryption Transform
Binary-Mode Decryption Transform
1. Processing Rules
2. Example
Security Considerations
1. Signatures Over Encrypted Data May Reveal Information
2. "Sign What You See"
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. 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:DigestValueelement 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", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [Keywords].

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

xmlns:xenc="http://www.w3.org/2001/04/xmlenc#" xmlns:ds="http://www.w3.org/2000/09/xmldsig#" xmlns:dcrpt="http://www.w3.org/2002/07/decrypt#"

While implementations 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. Additionally, the entity &xenc; is borrowed from [XML-Encryption] to provide short-hand identifiers for URIs defined in that specification. For example"&xenc;Element" corresponds to "http://www.w3.org/2001/04/xmlenc#Element".

1.3 Acknlowledgments

The contributions of the following Working Group participants to this specification are gratefully acknowledged:

Hal Finney, PGP Security
Amir Herzberg, NewGenPay
Takeshi Imamura, IBM
Ari Kermaier, Phaos
Hiroshi Maruyama, IBM
Joseph Reagle, W3C
David Solo, Citigroup

2 Decryption Transform Syntax

This transform supports two modes of operation. In XML mode the data is encrypted XML and the result of decryption is a node set. In binary mode the data is an encrypted octet sequence and the result of decryption is an octet sequence. In both modes, xenc:EncryptedData elements in the input node-set can be excluded from processing usingdcrpt:Except elements. dcrpt:Except is defined below via XML Schema [XML-Schema] and appears as direct child elements of the ds:Transform element.

The REQUIRED URI attribute value of thedcrpt:Except element identifies xenc:EncryptedDataelements within the input to the transform. The value MUST be a non-empty same-document [URI] reference (i.e., a number sign '#' character) followed by an XPointer expression [XPointer] as profiled by The Reference Processing Model [XML-Signature, section 4.3.3.2].

Schema Definition:

<!ENTITY % p ''>
<!ENTITY % s ''>

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

3 XML Mode Decryption Transform

Identifier:

http://www.w3.org/2002/07/decrypt#XML

This mode of the 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 except for those specified by dcrpt:Except elements. The output of the transform is a node-set.

3.1 Processing Rules

This section describes the processing rules of the transform. The rules are written as two functions; the inputs and outputs of the transform are those of the decryptXML()function, which itself calls the decryptNodeSet() function.

O = decryptXML(N, E)

where N is a node-set and E is a set of exception URIs held by URI attributes ofdcrpt:Except elements. O is a node-set, computed as follows:

Let Y, a set of replacement node-sets, be decryptNodeSet(N, E).
Canonicalize-with-Replacement: convert N to an octet stream B, (which MUST be well-formed (see section 3.4.1)), using [XML-C14N] as described in The Reference Processing Model [XML-Signature, section 4.3.3.2]; but, in place of any decrypted xenc:EncryptedData element_d_ and its descendants, process the replacement node-set_Od_, from Y. During this step, canonicalization of the replacement node-set MUST be augmented (see section 3.4.2) as follows:
- A namespace declaration xmlns="" MUST be emitted with every apex element that has no namespace node declaring a value for the default namespace as described in Serializing XML [XML-Encryption, section 4.3.3].
- If a node-set is replacing an element from N whose parent element is not in N, then its apex elements MUST inherit attributes associated with the XML namespace from the parent element., such as xml:base,xml:lang and xml:space.
  B may not be in canonical form.
Let O, the output of this function, be a node-set converted from B as described in The Reference Processing Model [XML-Signature, section 4.3.3.2].
- If parsing of B fails, then the implementation MUST signal a failure of the transform.
- Note that even if there are no decryptedxenc:EncryptedData elements, then N is still canonicalized and parsed.

Y = decryptNodeSet(N, E)

where N is a node-set and E is a set of exception URIs held by URI attributes ofdcrpt:Except elements. Y is a set of node-sets, computed as follows:

Let D be a node-set containing all element nodes in_N_ with the type xenc:EncryptedData that are not identified by any exception URI in E.
- When dereferencing an exception URI in the context of the original input node set, the implementation MUST behave as if the document node of the input node-set is used to initialize the XPointer evaluation context [XPointer], even if the node is not part of the node-set. Unlike XML Signature [XML-Signature], the exception URI may be evaluated against a different document than the "root node of the XML document containing the URI attribute." If the input is a different document then, as per XPointer [XPointer], use of the here() function is an error.
- When dereferencing an exception URI in the context of a replacement node-set, bare name [XPointer] exception URIs are used to locate xenc:EncryptedData elements with matchingId attributes. Implementors MAY attempt to resolve full XPointers into replacement node-sets using appropriate techniques to take into account the location of the replacement node-set in the input document, see References Using Non-barename XPointers (section 3.4.5).
- If an exception URI fails to dereference any nodes, then the resulting error MUST be ignored; it may be the result of part of the input document being encrypted.
Let Y be {}, an empty set.
For each xenc:EncryptedData element d from_D_:
1. Decrypt d, without regard for which, if any, of its descendants are in N, and process it in accordance with the value of its Type attribute, resulting in a node-set Od.
  - For example, processing of anxenc:EncryptedData element with theType attribute whose value is &xenc;Element or &xenc;Content is specified in A Decrypt Implementation (section 4.3.1) of [XML-Encryption], and the result is a node-set.
  - If the Type attribute is absent, is not known to the decryptor, or the result of its processing is not a node-set, then the implementation MUST signal a failure of the transform.
  - If decryption of any xenc:EncryptedData element fails, then the implementation MUST signal a failure of the transform.
2. Replace Y withY ∪ {Od}.
3. If O d containsxenc:EncryptedData that are not in E, replace Y with_Y_ ∪ decryptNodeSet(Od, E). This recursively decrypts super-encrypted data within the replacement node-set.

3.2 Transform Creation (Non-Normative)

This specification does not mandate a mechanism for creating ads:Transform element in a [XML-Signature] transform sequence. However, the following is one (non-normative) approach:

Apply all the transforms being placed before this transform to a data object being signed.
If the transform just before this transform outputs an octet stream, convert it to a node-set as described in The Reference Processing Model [XML-Signature, section 4.3.3.2].
For each node in the node-set, if the node is an element node with the type xenc:EncryptedData, create an dcrpt:Except element referencing the node.
Create a ds:Transform element, including the algorithm identifier of this transform and all the dcrpt:Except elements created in Step 3.

3.3 Example

Suppose that a part of the following XML document ([02-14]) is to be signed. Note that a few parts of the document ([05,11,12]) are already encrypted prior to signature. Also suppose that the signer anticipates that additional parts of the document will be encrypted after signing.

[01] [02] [03] [04] ... [05] <xenc:EncryptedData Id="#secret-1" .../> [06] [07] [08] ... [09] [10] [11] <xenc:EncryptedData .../> [12] <xenc:EncryptedData .../> [13] [14] [15]

In order to let the recipient know the proper order of decryption and signature verification, the signer includes the decryption transform ([a19-a22]) in the signature. The dcrpt:Exceptelements ([a20,a21]) identify parts of the document that are already encrypted.

[a01] [a02] [a03] [a04] ... [a05] <xenc:EncryptedData Id="#secret-1" .../> [a06] [a07] [a08] ... [a09] [a10] [a11] <xenc:EncryptedData .../> [a12] <xenc:EncryptedData .../> [a13] [a14] [a15] <dsig:Signature ...> [a16] ... [a17] <dsig:Reference URI="#tbs"> [a18] dsig:Transforms [a19] <dsig:Transform Algorithm="" title="undefined" rel="noopener noreferrer">http://www.w3.org/2002/07/decrypt#XML"> [a20] <dcrpt:Except URI="#secret-1"/> [a21] <dcrpt:Except URI="#xpointer(id('tbs')/Secrets/*)"/> [a22] [a23] [a24] ... [a25] [a26] ... [a27] [a28]

Consider that this document is subsequently encrypted by various processes, resulting in the following:

[b01] [b02] [b03] <xenc:EncryptedData Id="part-1" Type="&enc;Element" .../> [b04] <xenc:EncryptedData Id="part-2" Type="&enc;Element" .../> [b05] [b06] <xenc:EncryptedData .../> [b07] <xenc:EncryptedData .../> [b08] [b09] [b10] <dsig:Signature ...> [b11] ... [b12] <dsig:Reference URI="#tbs"> [b13] dsig:Transforms [b14] <dsig:Transform Algorithm="" title="undefined" rel="noopener noreferrer">http://www.w3.org/2002/07/decrypt#XML"> [b15] <dcrpt:Except URI="#secret-1"/> [b16] <dcrpt:Except URI="#xpointer(id('tbs')/Secrets/*)"/> [b17] [b18] [b19] ... [b20] [b21] ... [b22] [b23]

Execution of the decryption transform will proceed as follows:

The input to the transform, N, is a node-set containing theToBeSigned element and its children, less comments ([b02-b09]). The parameter to the transform, E, is a set containing the two exception URIs ([b15,b16]).
The first exception URI does not resolve in this document; the second resolves to the two children of the Secrets element ([b06,b07]).
As a result, D for N is a node-set consisting of the two xenc:EncryptedData elements, dpart-1 ([d03]) and dpart-2 ([d04]). Each of these is decrypted, resulting in the following node-sets for Opart-1 and_Opart-2_:
[c01]
[c02] ...
[c03] <xenc:EncryptedData Id="#secret-1" .../>
[c04]
[d01]
[d02] <xenc:EncryptedData Id="#data-2" Type="&enc;Element" .../>
[d03]
After this decryption stage, two new xenc:EncryptedData elements ([c03] and [d02]) have been revealed. However, the first matches an exception URI with a bare name and so is not considered further; hence, D for Opart-1 is empty while D for Opart-2 contains just the xenc:EncryptedData element ddata-2 ([d02]). This is decrypted again, resulting in the following node-set Odata-2:
[e01] ...
No new xenc:EncryptedData element are revealed, so_D_ for Odata-2 is empty and processing falls through to canonicalization.
The canonicalization-with-replace operation canonicalizes the node-set_N_; but, in place of any xenc:EncryptedData elements that were decrypted, it canonicalizes the replacement node-sets. Similarly, it also replaces any decrypted xenc:EncryptedData elements in the replacement node-sets. Further, canonicalization of any replacement node-sets is augmented such that xmlns="" is emitted on any apex elements that have no namespace node declaring a value for the default namespace. The resulting canonicalized data are the following:
[f01]
[f02]
[f03]
[f04] ...
[f05] <xenc:EncryptedData Id="#secret-1" .../>
[f06]
[f07]
[f08] ...
[f09]
[f10]
[f11] <xenc:EncryptedData .../>
[f12] <xenc:EncryptedData .../>
[f13]
[f14]
[f15]
This octet stream is then parsed and returned as the output of the transform.

3.4 Restrictions and Limitations

3.4.1 The Constraint of Well-Formed Data

As specified in step 2 of the decryptXML() function, the octet stream resulting from canonicalization-with-replacement MUST be well-formed. Typically this will be characterized by a single-rooted input node-set, where 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 the node-set. Additionally, if the input node-set has, at its top level, an xenc:EncryptedData element being decrypted, then this SHOULD correspond to an encrypted single-rooted node-set. However, this need not be the case: after decryption, multiple top-level nodes may be well-formed if they consist of white space, comments, processing instructions and a single element. No special processing is required to test for this condition because ill-formed data will result in a parsing error.

3.4.2 Inheriting Attributes from the XML Namespace

As specified in step 2 of the decryptXML() function, the canonicalization with replacement step requires XML namespace attribute inheritance. One of the features of the Canonical XML [XML-C14N] algorithm, which is automatically applied by the decryption transform, is that all ancestral attributes from the XML namespace (e.g., xml:lang) are inherited by any element whose parent node is not in the canonicalized node-set. The inheritance in step 2 ensures these attributes are preserved during decryption and replacement. For example, consider the following signed document:

[01] [02] [03] ... [04] [05] <dsig:Signature ...> [06] ... [07] <dsig:Reference URI="#tbs"> [08] dsig:Transforms [09] <dsig:Transform Algorithm="http://www.w3.org/2001/04/decrypt#XML" /> [10] [11] ... [12] [13] ... [14] [15]

The canonical form of the ToBeSigned element (the target of the #tbs reference, [02-04]) is the following ([a01-a03]):

[a01] [a02] ... [a03]

Consider, however, if this top-level signed element is subsequently encrypted using an XML serialization algorithm that does not include inherited attributes from the XML namespace ([b02-b04]):

[b01] [b02] <xenc:EncryptedData Id="tbs" ...> [b03] ... [b04] [b05] <dsig:Signature ...> [b06] ...

Upon decryption, the document that would be parsed to form the replacement node set would be:

[c01] [c02] ... [c03]

Because this fragment is no longer in its original ancestral context, the canonical form of the resulting ToBeSigned element ([d01-d03]) would not match that which was originally signed and the signature verification operation would fail.

[d01] [d02] ... [d03]

As shown, this failure often occurs when a directly-signed element was encrypted. The remedy is to augment the internal canonicalization of the canonicalization-with-replacement step of decryptXML(): node-sets that are replacing elements whose parent node is not part of the original signed node-set are canonicalized with attributes from the XML namespace that would have been inherited by the unencrypted element in its original document.

While this change is made to maintain the validity of signatures using [XML-C14N], it does not interfere with the validity of signatures using [XML-exc-C14N]. This transform, and the inclusion of attributes from the XML namespace (i.e., 'xml:'), is performed during signature validation and generation. Consequently, the exclusively canonicalized form of the element will maintain these 'xml:' attributes — even if the exclusively canonicalized form of the element would not have had them without this transform.

3.4.3 References and Structural Changes

URIs with a full XPointer or child sequence (whether in exceptions, encrypted data or elsewhere) may fail to resolve if encryption results in a structural change to part of the document relied upon by the reference. For example, the URI #xpointer(/ToBeSigned/*[3]) will no longer function if the first two children of the ToBeSigned element are encrypted together. Care SHOULD be taken when employing such references in association with the decryption transform.

3.4.4 References and Super Encryption

Super-encryption may cause problems if a super-encryptedxenc:EncryptedData element uses same-document references, or if an exceptional super-encrypted xenc:EncryptedData element is referenced by a non-bare name XPointer URI. Super-encryption of signed data under these conditions is NOT RECOMMENDED: super-encryption is precluded, or such references should not be used.

As an example of where super-encryption over same-document references may cause problems, consider the following signed document ([02-05]):

[01] [02] [03] ... [04] ... [05] [06] <dsig:Signature ...> [07] ... [08] <dsig:Reference URI="#tbs"> [09] dsig:Transforms [10] <dsig:Transform Algorithm="http://www.w3.org/2002/07/decrypt#XML" /> [11] [12] ... [13] [14] ... [15] [16] ... [17]

If the Data element ([03]) is subsequently encrypted, along with other data elsewhere in the document, the newxenc:EncryptedData element could use a same-document retrieval method to identify shared keying information ([a06]):

[a01] [a02] [a03] <xenc:EncryptedData ...> [a04] ... [a05] <dsig:KeyInfo ...> [a06] <dsig:RetrievalMethod URI="#key-info" Type="http://www.w3.org/2001/04/xmlenc#EncryptedKey" .../> [a07] [a08] ... [a09] [a10] ... [a11] [a12] <xenc:EncryptedKey Id="key-info" .../> [a13] <dsig:Signature ...> [a14] ... [a15] <dsig:Reference URI="#tbs"> [a16] dsig:Transforms [a17] <dsig:Transform Algorithm="" title="undefined" rel="noopener noreferrer">http://www.w3.org/2002/07/decrypt#XML"/> [a18] [a19] ... [a20] [a21] ... [a22] [a23]

If this new xenc:EncryptedData is subsequently super-encrypted ([b02]), the decryption transform will fail: When the inner retrieval method is processed, it will be dereferenced within the context of a new decrypted document that does not contain the referenced keying information.

[b01] [b02] <xenc:EncryptedData Id="tbs" .../> [b03] <xenc:EncryptedKey Id="key-info" .../> [b04] <dsig:Signature ...> [b05] ... [b06] <dsig:Reference URI="#tbs"> [b07] dsig:Transforms [b08] <dsig:Transform Algorithm="http://www.w3.org/2002/07/decrypt#XML" /> [b09] [b10] ... [b11] [b12] ... [b13] [b14]

Upon decryption, the document that would be parsed to form the replacement node set would be:

[d01] [d02] <xenc:EncryptedData ...> [d03] ... [d04] <dsig:KeyInfo ...> [d05] <dsig:RetrievalMethod URI="#key-info" Type="http://www.w3.org/2001/04/xmlenc#EncryptedKey" .../> [d06] [d07] ... [d08] [d09] ... [d10]

3.4.5 References Using Non-barename XPointers

As an example of where non-barename XPointers may fail, consider the following signed document ([02-07]) which uses a full XPointer ([13]) to identify data that was already encrypted when the signature was generated ([05]), and so should not be processed by the decryption transform:

[01] [02] [03] ... [04] [05] <xenc:EncryptedData Id="#secret-1" .../> [06] [07] [08] <dsig:Signature ...> [09] ... [10] <dsig:Reference URI="#tbs"> [11] dsig:Transforms [12] <dsig:Transform Algorithm="" title="undefined" rel="noopener noreferrer">http://www.w3.org/2002/07/decrypt#XML"> [13] <dcrpt:Except URI="#xpointer(/Document/ToBeSigned/Secrets/*)"/> [14] [15] [16] ... [17] [18] ... [19] [20]

If the Secrets element is subsequently encrypted, as shown in the following example ([a04]); i.e., the existingxenc:EncryptedData is super-encrypted, then the XPointer exception URI will no longer resolve. As a result, the decryption transform will attempt to perform super-decryption of the innerxenc:EncryptedData element and processing will fail.

[a01] [a02] [a03] ... [a04] <xenc:EncryptedData Id="#secrets" .../> [a05] [a06] <dsig:Signature ...> [a07] ... [a08] <dsig:Reference URI="#tbs"> [a09] dsig:Transforms [a10] <dsig:Transform Algorithm="" title="undefined" rel="noopener noreferrer">http://www.w3.org/2002/07/decrypt#XML"> [a11] <dcrpt:Except URI="#xpointer(/Document/ToBeSigned/Secrets/*)"/> [a12] [a13] [a14] ... [a15] [a16] ... [a17] [a18]

The reason that the full XPointer cannot be processed is that the document which results from decrypting the outer xenc:EncryptedDataelement will have the following form ([b01-b05]):

[b01] [b02] [b03] <xenc:EncryptedData Id="#secret-1" .../> [b04] [b05]

3.4.6 Interactions with Other Filters

The XML signature reference processing model allows transforms to remove parts of a node-set undergoing transformation. It is RECOMMENDED that any such transforms appear before the decryption transform. Otherwise, encrypted data that appear in parts of the document that are to be removed and cannot be processed by the recipient will cause the decryption transform to fail.

For example, consider the following document which contains parts that are to be signed for receipt by different individuals. The XPath transformancestor-or-self::*[@For="job"] selects only the subset[02-04]. If this XPath transform occurs after the decryption transform, and another part of the document contains encrypted data (e.g.,[07]), whether created before or after the signature, then the decryption transform may fail to decrypt them and processing will fail. If the XPath transform occurs first, then the encrypted data will not be considered by the decryption transform.

[01] [02] [03] ... [04] [05] [06] ... [07] <xenc:EncryptedData .../> [08] [09]

3.4.7 EncryptedKey is Out of Scope

This transform does not include any xenc:EncryptedKeyelements within its scope of specifically indicating elements, and their exceptions, that should be decrypted. An xenc:EncryptedKeyelement that exists as a descendent of xenc:EncryptedDataelement might be decrypted and will be removed from the original document as part of processing its ancestor xenc:EncryptedData element with the transform. However, a lone xenc:EncryptedKey element will be processed like any other data: a signature is presumed to be over that actual element and not its decrypted form. Consequently, we RECOMMEND thatxenc:EncryptedKey elements always be children of anxenc:EncryptedData element's ds:KeyInfo element when they fall within the scope of a signature.

4 Binary Mode Decryption Transform

Identifier:

http://www.w3.org/2002/07/decrypt#Binary

4.1 Processing Rules

The binary mode of operation is intended for use when generating a signature over binary data that are to be encrypted for transmission to the recipient. Use of this mode of the transform allows a signature to be computed over the plaintext form of the data, rather than the opaque ciphertext. This further allows the ciphertext to be stored elsewhere, identified by a cipher reference, without the need for the signature to take this into account.

This section describes the processing rules of the binary mode of this transform. The inputs and outputs of the transform are those of the decryptBinary() function.

O = decryptBinary(N, E)

where N is a node-set and E is a set of exception URIs held by URI attributes ofdcrpt:Except elements. O is an octet stream, computed as follows:

Let D be a node-set containing all element nodes in_N_ with the type xenc:EncryptedData that are not identified by any exception URI in E.
- When dereferencing an exception URI, the implementation MUST behave as if the document node of the input node-set is used to initialize the XPointer evaluation context [XPointer], even if the node is not part of the node-set. Unlike XML Signature [XML-Signature], the exception URI may be evaluated against a different document from the signature document. If the input is a different document then, as per XPointer [XPointer], use of the here() function is an error.
- If an exception URI fails to dereference any nodes, then the resulting error MUST be ignored; it may be the result of part of the input document being encrypted.
For each xenc:EncryptedData element d from_D_, decrypt d, without regard for which, if any, of its descendants are in N, and without consideration of its Type attribute, resulting in an octet-stream_Od_.
Let O, the output of this transform, be the concatenation of the octet streams Od, ordered in the document order of d.
If there are no EncryptedData elements in_D_, then the result is a zero-length octet stream.

4.2 Example

Consider the following example signed document:

... ... ... ... ...

Much of the encrypted data and signature are elided; the implication of the comment in the encrypted data is that the encrypted content is a binary image.

Execution of the decryption transform will proceed as follows:

The input to the transform, N, is a node set containing theEncryptedData element and its children, less comments. The parameter to the transform, E, is empty.
As a result, D is a node set consisting of the oneEncryptedData element, dimage. This is decrypted, resulting in an octet string Oimage containing the plaintext of the binary image.
There are no other data to decrypt, so the result of this transform is the plaintext obtained in the previous step. This will be used directly as input to the digest algorithm.

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:DigestValueelements). As noted by Joseph Reagle in [Reagle], this latter solution works only if signature and encryption are known to 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 its 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 datedate datetime." 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!