Enveloped Signature Transform - Security

The Enveloped Signature Transform is commonly used in XML Signature when the parent element is to be signed. The problem is that the Signature element would be within the information to be signed, so it must be removed before doing the validation. You actually can remove the Signature element by using the XPath Filter Transform described previously, so this Transform is primarily provided for convenience.

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Note - The Enveloped Signature Transform is the only Transform (other than the canonicalization Transforms defined by CanonicalizationMethod) required by the XML Specification. The XPath Filter Transform is recommended, but it is not required. You will find in practice that most, if not all, current XML Signature toolkits support the XPath Filter Transform.

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Using the Enveloped Signature Transform is the same as using the XPath Filter Transform with the following expression in Listing 4.22:

Listing 4.22 The XPath Expression Equivalent to the Enveloped Signature Transform

<XPath>
  count(ancestor-or-self::dsig:Signature |
  here()/ancestor::dsig:Signature[1]) >
  count(ancestor-or-self::dsig:Signature)
</XPath>

This XPath expression can a bit difficult to deal with, so having this Transform is convenient. If you are signing a parent element, you should use it. For example, say you want to sign the Order XML document in Listing 4.23 and want the Signature to be the last element within the Order:

Listing 4.23 An <Order> Element Where <Signature> Needs to be the Last Element

<Order>
  <LineItems>
    <LineItem sku="00001">Soap on a Rope</LineItem>
    <LineItem sku="00002">Cinnamon Shampoo</LineItem>
  </LineItems>
  <Signature> ... </Signature>
</Order>

To accomplish this, you must use the Enveloped Signature Transform (or XPath Filtering Transform). The Signature element would then look something like Listing 4.24.

Listing 4.24 The <Signature> Element After the Enveloped Signature Transform

<Signature>
 <SignedInfo>
  <CanonicalizationMethod ... />
  <SignatureMethod ... />
  <Reference URI="">
   <Transforms>
    <Transform
     Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
   </Transforms>
   <DigestMethod ... />
   <DigestValue>...</DigestValue>
  </Reference>
 </SignedInfo>
 <SignatureValue>...</SignatureValue>
</Signature>

In this example, the Reference URI points to the document itself (URI=""), and the Transform algorithm is the Enveloped Signature Transform. If the target of the Signature is not the root of the document, you could use the "same document reference" type for the URI (for example, URI="#someElementID") or an XPath Filtering Transform just prior to the Enveloped Signature Transform to reduce the resulting XML fragment to just the nodes you plan to sign.

Similar to the XPath Filtering Transform, the Enveloped Signature Transform takes in and outputs an XML nodeset. The only difference is that an Enveloped Signature Transform must be applied to a nodeset from within its parent document.

It is good practice to sign what the signer (a human being) actually sees, especially when the meaning of the signature represents the signer's intent. The visual aspects—even seemingly insignificant aspects such as a word being bold—can influence the meaning of a document. If an XML Stylesheet (XSL) is used, you have two choices for how to digitally sign it. One option is to have two Reference URIs: one pointing to the base XML document and the other to the XSL. Another is to use the XSLT Transform. The XSLT Transform simply applies the specific XSL expressions that are included under the Stylesheet element. To refer to an external stylesheet, you can use the xsl:include or xsl:import XSL expressions.

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Note - The XSLT Transform requires octets as input, and the output is an octet stream. Also, because different XSLT processors do not necessarily return consistent results, it is a good idea to use a Canonicalization Transform after an XSLT Transform.

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