expmod() p/c

proc expmod(a, n, m)
  res := 1;
  repeat
    if n is odd
      n := n - 1;
      res := (res * a) mod m;
    else
      n := n / 2;
      a := (a * a) mod m;
    end if;
  until n = 0;
  return res;
end proc;