Fast Hashing and Rotation-Symmetric Functions

EÆcient hashing is a centerpiece of modern Cryptography. The progress in computing technology enables us to use 64-bit machines with the promise of 128bit machines in the near future. To exploit fully the technology for fast hashing, we need to be able to design cryptographically strong Boolean functions in many variables which can be evaluated faster using partial evaluations from the previous rounds. We introduce a new class of Boolean functions whose evaluation is especially eÆcient and we call them rotation symmetric. Basic cryptographic properties of rotation-symmetric functions are investigated in a broader context of symmetric functions. An algorithm for the design of rotation-symmetric functions is given and two classes of functions are examined. These classes are important from a practical point of view as their forms are short. We show that shortening of rotation-symmetric functions paradoxically leads to more expensive evaluation process.