Editing XML Signature (section)

== Validation and security considerations ==
When validating an XML Signature, a procedure called '''Core Validation''' is followed.

# '''Reference Validation:''' Each <code>Reference</code>'s digest is verified by retrieving the corresponding resource and applying any transforms and then the specified digest method to it. The result is compared to the recorded <code>DigestValue</code>; if they do not match, validation fails.
# '''Signature Validation:''' The <code>SignedInfo</code> element is serialized using the canonicalization method specified in <code>CanonicalizationMethod</code>, the key data is retrieved using <code>KeyInfo</code> or by other means, and the signature is verified using the method specified in <code>SignatureMethod</code>.

This procedure establishes whether the resources were really signed by the alleged party. However, because of the extensibility of the canonicalization and transform methods, the verifying party must also make sure that what was actually signed or digested is really what was present in the original data, in other words, that the algorithms used there can be trusted not to change the meaning of the signed data.

Because the signed document's structure can be tampered with leading to "signature wrapping" attacks, the validation process should also cover XML document structure. Signed element and signature element should be selected using absolute [[XPath]] expression, not <code>getElementByName</code> methods.<ref name="pk"/>