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We describe a new technique for evaluating polynomials over binary finite fields. This is useful in the context of anti-DPA countermeasures when an S-box is expressed as a polynomial over a binary finite field. For $$n$$n-bit S-boxes, our new technique has heuristic complexity $${\fancyscript{O}}(2^{n/2}/\sqrt{n})$$O(2n/2/n) instead of $${\fancyscript{O}}(2^{n/2})$$O(2n/2) proven complexity for the Parity-Split method. We also prove a lower bound of $${{\varOmega }}(2^{n/2}/\sqrt{n})$$Ω(2n/2/n) on the complexity of any method to evaluate $$n$$n-bit S-boxes; this shows that our method is asymptotically optimal. Here, complexity refers to the number of non-linear multiplications required to evaluate the polynomial corresponding to an S-box. In practice, we can evaluate any 8-bit S-box in 10 non-linear multiplications instead of 16 in the Roy–Vivek paper from CHES 2013, and the DES S-boxes in 4 non-linear multiplications instead of 7. We also evaluate any 4-bit S-box in 2 non-linear multiplications instead of 3. Hence our method achieves optimal complexity for the PRESENT S-box.

Fast evaluation of polynomials over binary finite fields and application to side-channel countermeasures