3 ms·
Great to know! Upvoted. Unfortunately, it's a random mapping, not necessarily a random permutation. An ideal block cipher would be modeled as a random permut
by KMag 11y ago
Great to know! Upvoted.
Unfortunately, it's a random mapping, not necessarily a random permutation. An ideal block cipher would be modeled as a random permutation. Though, in this particular case the domain and range are the same, so unless I'm missing something, the expected number of fixed points comes out to 1 by the same reasoning.
- nhaehnle 11y agoIndeed, the expected number of fix points is the same for both permutations and mappings. Quite curious when you think about it, because the distribution of fix points is obviously different! (Consider the case n = 2 for the simplest example: there are mappings with exactly 1 fix point, but every permutation has either 0 or 2 fix points). Note though that a correct block cipher is necessarily a permutation, because it's invertible (by definition, a permutation is just an invertible mapping with domain and range equal). A hash function on the other hand needn't be a permutation even when you restrict the domain to inputs of the same bit length as the hash output.