Must be clear by what we mean when a signature is "valid" 
  - Signature Validation
 
  - Does the 
SignatureValue matches the result of processing SignedInfo
    with  CanonicalizationMethod and SignatureMethod as
    specified in §6.2? 
  - Reference Validation
 
  - Does the DigestValue of the derferenced URI matches the 
DigetsValue in SignedInfo? 
  - Trust/Application Validation
 
  - Does the application trust the signed assertions? (Was the key strong enough, is it from
    a trusted party, how old is the signature ...?)