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This document specifies the "decryption transform", which enables XML Signatures verification even if both signature and encryption operations are performed on an XML document.
This is the first draft of the "Decryption Transform for XML Signature" from the XML Encryption Working Group (Activity). Comments and implementation experience of this proposal are solicited.
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 policy.
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, suppose a scenario that Alice has Bob send money to her by postal transfer. Alice draws up a document including a statement, "Bob pays $100 to Alice", and the number of her bank account and sends it to Bob. The number is encrypted with a bank's public key for guarding her privacy. Then, Bob includes the number of his bank account in the document, signs it for proving that he approves the statement, and sends it to the bank. The number is also encrypted. As a result, encryption, signature, and encryption are performed in this order on the document the bank receives.
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.
This document makes use of the XML Encryption [XML-Encryption] and XML Signature [XML-Signature] namespaces with the following prefixes:
xmlns:enc="http://www.w3.org/2001/04/xmlenc#" xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
The XML Encryption namespace is also used as the prefix for an
algorithm identifier defined in this document. While applications
MUST support XML and XML namespaces, the use of our
"enc" and "ds" XML namespace prefixes is
OPTIONAL; we use this facility to provide compact and readable
exposition.
This transform takes as a parameter a list of references to
encrypted portions that are not to be decrypted by this transform.
These references are expressed by enc:DataRef elements
that appear as the direct child elements of the
ds:Transform 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
4.3.3.2 The Reference Processing Model of the XML Signature
specification [XML-Signature]. This
transform decrypts all the enc:EncryptedData elements
(as defined in the forthcoming XML Encryption standard [XML-Encryption]) except for those
specified by enc:DataRef elements. The output of this
transform is a node-set.
In order to help an unambiguous definition of the exact semantics of this transform, we define the following two functions:
enc:EncryptedData in X.<dummy>
and </dummy>) as described in [Tobin].
enc:DataReference elements in R in the context of the
node-set X. N is a set of nodes referenced by
R.This transform performs the following steps:
enc:DataReference elements given as the
parameter of this transform.EncryptedData and whose namespace URI is the
one defined in XML Encryption [XML-Encryption], such that e is
not a member of noDecryptNodes(X,R). If such e cannot
be selected, the algorithm terminates and the result of the
transformation is X.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 signature
being validated. However, it is unclear how this 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 signature.
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:
enc:EncryptedData, create an
enc:DataReference element referencing the node.ds:Transform element, including the
algorithm identifier of this transform and all the
enc:DataRef elements created in Step 3.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/xmlenc#decryption">
[07] <DataReference URI="#enc1"
xmlns="http://www.w3.org/2001/04/xmlenc#"/>
[08] </Transform>
[09] <Transform Algorithm="http://www.w3.org/TR/2000/CR-xml-c14n-20001026"/>
[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 DataRef element
([07]). After decrypting the
EncryptedData in line [22], the
order element is canonicalized and signature-verified.
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.