Incorrect Symmetric Key Derivation Length validation

There is an error in validating Signature Key Lengths against policy when derived keys are used with the Symmetric binding. For the "Basic256" policy for example, symmetric key lengths must be 256 bits, but derived keys for signature purposes can be 192 bits. The problem is that the SignatureVerifier class in Santuario asks for a secret key using XMLSecurityConstants.Sym_Sig, and eventually AbstractInboundSecurityToken ends up creating a new AlgorithmSuiteSecurityEvent using this. We need some way to distinguish between the two cases for validating against the AlgorithmSuite policy.