Rotational cryptanalysis

In cryptography, rotational cryptanalysis is a generic cryptanalytic attack against algorithms that rely on three operations: modular addition, rotation and XOR — ARX for short. Algorithms relying on these operations are popular because they are relatively cheap in both hardware and software and run in constant time, making them safe from timing attacks in common implementations.

The basic idea of rotational cryptanalysis is that both the bit rotation and XOR operations preserve correlations between bit-rotated pairs of inputs, and that addition of bit-rotated inputs also preserves bit rotations to a substantial degree. Rotational pairs of inputs can thus be used to "see through" the cipher's cascaded ARX operations to a greater degree than might be expected. A follow-up attack from the same authors and Christian Rechberger breaks collision resistance of up to 53 of 72 rounds in Skein-256, and 57 of 72 rounds in Skein-512. It also affects the Threefish cipher.