S(X1,S(initial)) . . . ) to obtain a signature S[XN+1,S[N]], where N is an integer and S(initial) is any arbitrary initial value, provide the data element XN+1 and the signature S[XN+1,S[N]] to the second resource, and store the signature S[XN+1,S[N]] as the new value for SQ in said memory; and a port adapted to be coupled to the second resource and through which the data element XN+1 and signatur