Your b is a character array. You are going to have problems multiplying character arrays or using xor on character arrays.
Are you wanting to do the xor at the level of the formal additions, like
a(1,1) * b(1) XOR a(2,1) * b(2) XOR a(3,1) * b(3) XOR a(4,1) * b(4)
or are you wanting to use xor even at the level of the multiplications, like 3 * b(1) meaning b(1) XOR b(1) XOR b(1) -- since, after all, multiplication is defined by repeated addition and you want your addition to be done as XOR?
