Safeguarding the Identity and Integrity of XML Messages - XML Signature Validation
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The process to validate an XML Signature is similar, except that it occurs in reverse. It is composed of two major processes like Signature generation: Reference validation and Signature validation. The goal of Reference validation is to ensure that the resource being pointed to by each Reference has not been changed. The goal of Signature validation is to ensure that the entire SignedInfo block has not been changed. Only if both of these processes succeed is integrity confirmed for the entire Signature.

Reference Validation

For Reference validation, you need to validate that the resources pointed to by the Reference elements have not changed. The first step is to canonicalize the SignedInfo element based on the CanonicalizationMethod element. Then the following steps are completed for each Reference element in the SignedInfo.

1. Get the data that is pointed to in the Reference. This will either be from the URI attribute, or as we mentioned above, it may be supplied by the application calling the XML Signature processing engine.

2. Apply any Transforms to the data returned. For example, if the Reference URI points to an XML document, it is likely that a Canonicalization Transform will be specified in the Reference element.

3. Create a hash of the data using the DigestMethod specified in the Reference.

4. Compare the resulting hash with the DigestValue in the Reference. If there is any difference, the validation fails.

Signature Validation

If Reference validation is successful, the XML Processing engine can proceed to Signature validation. The objective of the Signature validation step is to confirm that SignedInfo has not been changed (integrity) and, as in any digital signature verification, that the appropriate key has signed this information (that is, non-repudiation). Here are the steps for Signature validation:

1. Obtain the key for verification from the KeyInfo block, or in some other manner. Note that it is critical to determine trust for this key—ensuring that it is certainly bound to the expected identity. You learn more about this topic in Chapter 9, "Trust, Access Control, and Rights for Web Services."

2. Using the output of the SignedInfo canonicalization, create a hash of the SignedInfo.

3. Using the verification key, decrypt the SignedInfo element. Compare the hash from step 2 to the result of this verification. If they do not match, Signature validation has failed.

This chapter is from Securing Web Services Security with WS-Security, by Jothy Rosenberg and David Remy (Sams, 2004, ISBN: 0672326515). Check it out at your favorite bookstore today. Buy this book now.

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