f is such that G(x) is able to be calculated from e and u using a function H(u-1,e)=G(x); and (iv) calculating said value y proceeding from a value q selected to be a divisor of p-1 according to the rule ar+bSA +c=0 (mod q) where (a,b,c) is a permutation of the values e',Y,l; where e' is an integer and is derived from e and SA is a secret value; (d) transmitting the signature e,y from A to B; (e)