In general, a MAC is a function f 206 which is a mapping from a message space M 201 to a tag space Z. The function is parameterised by a key k 207 from a key space K (not shown), i.e. the exact mapping is determined by a second input parameter 207 to f, called the secret key k, such that for any k??K, m??M, ??? f(m;k)=z, where z??Z. For example, given a value of k, the message m 202 is mapped to t