EnRUPT – The Simpler The Better

The updated specification paper includes EnRUPT64 since its automated cryptanalysis has been completed and revealed no significant differences comparing to the originally proposed EnRUPT32. Both variants have roughly the same security properties. Unfortunately, there is no way to keep them simple yet compatible with each other. They can only be used separately.

While originally EnRUPT specification only proposed a variant operating on 32-bit words, it was designed to support variable word length. The difference in word length only affects the rotation operation. It is w/4 (a quarter of the word length), not simply 8. Other word sizes have not been fully analysed yet and may require a different number of rounds to be secure as it is affected nonlinearly. While larger word sizes can be made secure, 16-bit words demonstrate significant weakness and cannot be used efficiently.

Performance of EnRUPT64 is much better than EnRUPT32 on 64-bit processors but much worse on 32-bit processors (about 2 times slower than EnRUPT32). Both perform equally well on 8-bit microprocessors except for small 64-bit blocks [minimum block size of EnRUPT64 is 128 bits], while EnRUPT64 is slightly faster on 16-bit microprocessors due to the absence of rotations.