In a shameless attempt to move the discussion of RC2 into a more technical arena, here are some interesting questions to explore about RC2. --Bob Key expansion - How can you tell whether the permutation is based on some sequence of digits from PI? - What are the diffusion and avalanche properties of this permutation? - What are the linear characteristics of this permutation? - What are the properties of the compression function that maps 16 bits (bytes X and Y) to 8 bits (byte Z) via Z = P[X + Y]? - How does the length of the key influence the mixing of bits during each pass of the expansion algorithm? - Is this a non-linear feedback shift register over the field GF(256)? - If the first pass of expansion is viewed as a hash function that produces 40 or 128 bits out, what are its properties? Round Functions - What are the diffusion and avalanche properties of the two round functions? - What are the linear approximations and how good are they? - What characteristics can be preserved by the round function that performs rotations? - With what probability? - Does the amount of rotation influence the security? - What characteristics can be preserved by the round function that performs the data dependent selection of the expanded key? - With what probability? - Are there any "weak" keys? - Will the expansion algorithm produce them?