verify.py

        sequence = DerSequence()
        sequence.decode(_digest_algorithm_identifier)
        _digest_algorithm_identifier = sequence[0]

        object_id = DerObjectId()
        object_id.decode(_digest_algorithm_identifier)
        digest_algorithm_identifier = object_id.value
        if digest_algorithm_identifier != '2.16.840.1.101.3.4.2.1':
            raise Exception('invalid digest algorithm identifier')

        _null = sequence[1]
        null = DerNull()
        null.decode(_null)