10.3. Artifact Authentication Response
If the OP received artifact request, then the following should be returned:
As specified in Section 4.1.2.
Value: The Artifact value received in the request. If the Artifact was not found at the OP, it MUST be a string "INVALID".
Value: Comma-separated list of fields in this response.
Value: Base 64 encoded signature calculated as specified in Section 6.
Value: A handle for an association between the Relying Party and the OP that SHOULD be used to sign the response.
No other parameters should be returned.