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copied!<p>At some point you will of course have to check your code against test vectors. But there are quite a few tests that you can perform without having to know or compute any test vectors at all. </p> <p>First the multiplication in GF(2^128) is commutative. Hence you can just compute BLK_MUL(val_1, val_2, out1) and BLK_MUL(val_2, val_1, out2) with any input and you should get the same result. Since your code uses val_1 and val_2 differently this is already quite a good test.</p> <p>Then you can use that multiplication is distributive, I.e. you can test that (x+y)*z = (x*z)+(y*z), (where the addition in GF(2^128) is computed by xoring corresponding bytes of the two values together).</p> <p>Finally, once you have implemented the whole field GF(2^128) you can also exploit that its order is 2^128-1. I.e. if you start with a value x then square it 128 times, then you should get x back.</p> <hr> <p>A few additional comments:</p> <p>The advantage of using equations for testing (over only using test vectors) is that you can easily run a large number of tests. Because it is rather easy to add tests this way I frequently do some simple tests with sparse inputs (e.g. just single bits set in the input) first. If something is wrong then this helps to identify the bugs fast. </p> <p>Your current code uses temporary variables for the result. This is indeed a good idea, since it ensures copy safety. I think a good unit test should also cover this case. I.e. you might want to compute the same result twice: once with input and output pointing to different memory location, and once with the output being the same memory as the input.</p> <p>Furthermore, at least one of the other answers talks about optimizations. I think if you refactor the code then you should look for meaningful components to reuse, rather than blindly looking for look-a-like code snippets. Since GF(2^128) is a field, of course addition and multiplication in the field are meaningful components. Another meaningful component is the multiplication by the polynomial x (which is something that is quite frequently used in crypto). </p>

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