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S-boxes are often the only nonlinear components in modern block ciphers. They are commonly selected to comply with very specific criteria in order to make a cipher secure against, for example, linear and differential attacks. An M × N S-box can be thought of as a lookup table that relates an M -bit input value to an N -bit output value, or as a set of N boolean functions of M variables (Schneier, 1996).Although cipher specifications generally describe S-boxes using their lookup tables, they can also be described as boolean functions or logic gate circuits. sboxgates, which is presented here, finds equivalent logic gate circuits for S-boxes, given their lookup table specification. Generated circuits are output in a human-readable XML format. The software can convert the output files into C or CUDA (a parallel computing platform for Nvidia GPUs) source code. The generated circuits can also be converted to the DOT graph description language for visualization with Graphviz (Ellson et al., 2002).

The SoDark cipher is used to protect transmitted frames in the second and third generation automatic link establishment (ALE) standards for high frequency (HF) radios. The cipher is primarily meant to prevent unauthorized linking and attacks on the availability of HF radio networks. This paper represents the first known security analysis of the cipher used by the second generation ALE protocol—the de facto world standard—and presents a related-tweak attack on the full eight round version of the algorithm. Under certain conditions, collisions of intermediate states several rounds into the cipher can be detected from the ciphertext with high probability. This enables testing against the intermediate states using only parts of the key. The best attack is a chosen-ciphertext attack which can recover the secret key in about an hour with 100% probability, using 29 chosen ciphertexts.

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