Special signing state where the the data for an attribute is hashed
        before signing
        :return: state to be used when signing

def signingPayloadState(self, identifier=None):
        """
        Special signing state where the the data for an attribute is hashed
        before signing
        :return: state to be used when signing
        """
        if self.operation.get(TXN_TYPE) == ATTRIB:
            d = deepcopy(super().signingPayloadState(identifier=identifier))
            op = d[OPERATION]
            keyName = {RAW, ENC, HASH}.intersection(set(op.keys())).pop()
            op[keyName] = sha256(op[keyName].encode()).hexdigest()
            return d
        return super().signingPayloadState(identifier=identifier)