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 Nov 9 answered Can e = 2 be used in unpadded RSA? Nov 9 comment What is the difference between “securely realizes” and “securely implements”? The quoted part is exactly what it is in the contxt of MPC. It is two different points of view of (mostly) the same thing. Internal vs external. Nov 9 comment Why do cryptographic hashes need to be fixed length? Fixed length is no special property from the cryptographically secure part. It comes from the more general specification of hash functions. In fact, it is the defining property of hash functions: "A hash function is any function that can be used to map data of arbitrary size to data of fixed size" (Wikipedia) Nov 9 comment Three party coin flipping protocol with only 2 active participants This is unachievable. From Carol's point of view: She isn't involved in the protocol execution (except possibily some initial input). She doesn't trust Alice and Bob (they could in fact be just one party). And she has to trust the result of the coin toss somehow, while having no influence or ability to verify anything. Without further specification this is impossible. Oct 28 comment ElGamal Generator g problem If ElGamal would be defined today, it would require a Schnorr group, with "find $h$, s.t. $h^r \neq 1$". In the original paper it actually was a generator, if I recall correctly. Anyway: $1$ as $g$ is always a bad choice, because every public key will be $1$ then. And not just if you choose the elements order (or a multiple of it) as private exponent. Oct 28 comment How to toggle a bit homomorphically ? OR is not an addition. The addition is XOR. In order to ensure that true and true returns true, you need to have something like $A + B + AB$. Which also contains a multiplication. NOT gates are easy: Just use XOR and the constant 1 Oct 23 comment How bad is it to use the identity function as hash for ECDSA? "...that your customer is stupid???" Request on mailing list doesn't imply customer. And then no one said stupid, which sounds quite harsh. But as a hint to the sad truth about cryptography: Most people don't understand it, and unless studying crypto extensively, they also have no clue what damage can be done by easy beginner's mistakes. There are countless self-proclaimed "experts", which come up with new schemes and basically re-invent things, which have been dismissed for 30 years, maybe from a different point of view, but with the same weaknesses. Oct 23 comment Is there an additively homomorphic encryption scheme that supports calculating a square root on the ciphertext? There is a substantial difference in using square roots over reals (with approximations, etc.) and square roots in finite rings and fields. $2^{1/2}$ mod $7$ equals both $3$ and $4$. It has nothing to do with $\sqrt{2}\approx 1.414$ Oct 23 comment Is there a format preserving cryptographically secure hash? This is not about the construction of FPE. It is about creating a cryptographic hash function (and utilizing FPE to do so). Cryptographically secure hash functions don't protect against brute force (of small message spaces) at all. It is about collision resistance and preimage resistance. If short message spaces are a problem, you should use a construction with a key anyway (e.g. MAC). Oct 19 comment Can it ever be impossible to invert a PRNG? The entire argument for "impossible to uniquely invert" is not relevant for the cryptographic strength with emphasis on "uniquely": If an attacker can exclude only a portion of the possible states, he might already be able to predict the next bit with a non-negligible probability, breaking the algorithm in the cryptographic sense. Finding out the exact internal state is not required for that. As a rule of thumb: CSPRNGs and PRNGs are very different things, and the similar name is very misleading. Don't mix them up, it doesn't work. Oct 19 comment Deciphering “easy” ciphers without hints Well, without the actual challenge, it's quite difficult to point in the right direction. If the only "flashy" value is 81, and 3,9,27 are just average like the rest, then it is unlikely that 3,9 and 27 are correct key sizes (Factors of the correct key size don't show deviating values, just the multiples). A Vigenere cipher with a key of length 81 sounds unlikely, but not impossible. But anyway, you have found some characteristic, that means you're on the right track. Oct 17 awarded Yearling Oct 16 comment Number of qubits and breaking hashes Snake oil with traces of antibiotics doesn't make it the new wonder-drug. There is yet to find any problem, where D-Wave is actually faster than state-of-the-art classical algorithms. Oct 16 comment Deciphering “easy” ciphers without hints I'd suggest trying to figure out the block size first. With the aurocorrelation method you can figure that out without knowing anything about the alphabet, as long as it is not uniform distributed. Oct 15 answered Deciphering “easy” ciphers without hints Oct 13 comment Very short signatures? (eg: 48bits?) I shiver, when I read the combination of "rsa" and the number "256". Pretty much anything is better than that, since such a number can be factored within seconds most likely. Oct 13 answered How hard to solve the given mod problem Sep 14 comment Distribution based integer factorization The reason for that pyramid is most likely, that you choose primes with a fixed number of bits. A more common assumption is that \$p