And there are obvious bugs in that code, for example: pos_a = key[pos_b] % key_size; probably should have been: pos_a = key[pos_b % key_size] % key_size

And there are obvious bugs in that code, for example: pos_a = key[pos_b] % key_size; probably should have been: pos_a = key[pos_b % key_size] % key_size; And so on...