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 Apr 6 comment Question about "toy: streamcipher I guess the dark box represents CB Apr 4 comment How to prove a symmetric encryption scheme provides perfect secrecy? No, the attacker is assumed to know everything about the process, just not the key that was used. Mar 10 comment reconstructing data on a noisy channel? If the cipher is any good, it's indistinguishable from a PRP. If there's an approach better than trying all possibilities I think it would suggest a weakness in the algorithm. Mar 8 comment How to calculate min entropy of the output of PRNG? See this answer to a related question: crypto.stackexchange.com/a/10408/4241 Mar 8 comment How to calculate min entropy of the output of PRNG? I don't see how this can work. You can't even distinguish a good PRNG from a true random source, so an entropy estimate has to grow without bound as more output is seen. Mar 8 comment How to calculate min entropy of the output of PRNG? Are you trying to measure it by observing output or by analyzing the PRNG? Feb 24 comment Does a stream cipher provide perfect secrecy? What exactly does feeding it random bits mean? This is too vague to answer, though things that aren't OTP are not OTP. Feb 3 comment Determining Involutary Keys in Substitution Ciphers For small values of n, that's easiest. For larger, I think you can use the factorization of n. Feb 3 revised Determining Involutary Keys in Substitution Ciphers expand on multiple solution case Feb 2 answered Determining Involutary Keys in Substitution Ciphers Jan 24 awarded Quorum Jan 11 comment Is finding collisions in a part-hash not often enough a bad problem? Come to think of it, I should say that this hypothetical "truncate and xor" function would exhibit collisions as long as you generated proper random inputs of say 64 bits. The collision condition is that they match in the first 32 bits. A test like for (int x=0; x<=MAX_INT; ++x) {ComputeHash(x);} is not good enough to trigger it. Jan 11 comment Is finding collisions in a part-hash not often enough a bad problem? @IamNick It would not be appropriate to model that hash as a pseudo-random function. Jan 11 comment Is finding collisions in a part-hash not often enough a bad problem? @Thomas That's what I was thinking as well. Any deviations from uniform distribution should result in more collisions. It would require some unusual interactions between the "random" test data and the hash function to make collisions less likely than the PRF model predicts. It seems more likely that there's a bug in the collision counting code. Nov 11 comment Why can't I reverse a hash to a possible input? Finding inputs that satisfy a boolean equation is not easy, and is generally known as en.wikipedia.org/wiki/Boolean_satisfiability_problem Oct 30 answered How to convert RSA public keys to one key to use for text hashing? Oct 9 comment How does perfect hash function works? It depends what you mean by "key". But as the Wikipedia article notes, if you use a perfect hash function in your hash table you don't have to worry about collisions. So a key into another data structure is a common use case. Oct 9 comment How does perfect hash function works? "Perfect" means the hash can't assign two pigeons to the same hole, so you need at least as many holes as pigeons. And we knew how many pigeons we had when we designed the hash, so we don't have more holes than pigeons. That is, you can't give me a pigeon that wasn't in the original set and ask me where to put him. As @otus says, the hash is an algorithm, so you can run it on your parrot; but whatever hole it tells you will already have a pigeon in it. Oct 8 comment Hypothetical encryption technique, is it secure? What are the advantages of this over using any existing encryption algorithm? Certainly not efficiency. Sep 4 answered Possibility of the encrypted text being the same as the supplied plain text!