Possibly Incorrect XML Signature for SAML Responses

[virtualstaticvoid]

I've picked up an issue when validating SAML responses when using SHA256 for the signing algorithm.

In the SamlIdp::Controller#encode_SAMLResponse method, the code to produce the identifier is string interpolated as <ds:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig##{algorithm_name}"> but results in a signature failure when validating SAML response.

I'm using the libsaml gem for my SP, which in-turn uses the xmldsig gem for verifying the response XML.
While debugging I found the Xmldsig::Reference#digest_method method, which looks for http://www.w3.org/2000/09/xmldsig#sha1 and http://www.w3.org/2001/04/xmlenc#sha256 when resolving the Ruby class to use.

From what I can understand from the XML Encryption Syntax and Processing W3C specification, the identifiers for each digest algorithm change according to the algorithm used.

E.g.

• SHA1 = http://www.w3.org/2000/09/xmldsig#sha1
• SHA256 = http://www.w3.org/2001/04/xmlenc#sha256
• SHA512 = http://www.w3.org/2001/04/xmlenc#sha512

Also, I couldn't find any identifiers which had "rsa-" prefix to the "shaXXX" part. I.e. http://www.w3.org/2000/09/xmldsig#rsa-shaXXX. This is correct as per Algorithm Identifiers and Implementation Requirements.

I'm no expert on the subject, so maybe someone who knows can comment on this.