void silc_mp_modinv(SilcMPInt *inv, SilcMPInt *a, SilcMPInt *n);

    Find multiplicative inverse using Euclid's extended algorithm.
    Computes inverse such that a * inv mod n = 1, where 0 < a < n.
    Algorithm goes like this:

    g(0) = n    v(0) = 0
    g(1) = a    v(1) = 1

    y = g(i-1) / g(i)
    g(i+1) = g(i-1) - y * g(i) = g(i)-1 mod g(i)
    v(i+1) = v(i-1) - y * v(i)

    do until g(i) = 0, then inverse = v(i-1). If inverse is negative then n,
    is added to inverse making it positive again. (Sometimes the algorithm
    has a variable u defined too and it behaves just like v, except that
    initalize values are swapped (i.e. u(0) = 1, u(1) = 0). However, u is
    not needed by the algorithm so it does not have to be included.)