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 Feb 28 comment ElGamal signature without calculating the inverse Nice scheme! For the curious: You are right, it is not secure. Let $(\gamma,\delta)$ be a valid signature on the message $x$. Then $(\gamma^2,\gamma \delta)$ will be a valid signature on the message $\gamma x/2$. (This works as long as either $\gamma$ or $x$ is even.) Therefore, the scheme is not secure against existential forgery. Your answer still seems like a good response to the textbook question, though. Feb 27 comment Would this simple encrypted chat program be feasible using One Time Pads? @zuallauz, it doesn't matter if your algorithm is secure in theory against computationally unbounded adversaries if the implementation has a buffer overflow vulnerability that lets anyone penetrate it. That's why I say that in practice this is an information security problem, not just a crypto-mathematics problem. Feb 24 comment PRP, PRF and modular arithmetic Please edit your question to give a precise definition of what you mean by an "arithmetic function". Is AES an "arithmetic function"? How about cubing modulo a RSA modulus? Also, please elaborate on what you mean by "simulate". Finally, I suggest you add some context and motivation: why are you asking? what is the real goal? what research have you done to try to solve this on your own? Feb 24 comment PRP, PRF and modular arithmetic I've upvoted it because I think it is correct and points the original poster to the key technical term, from which they can find more. You can look up what $k$-wise independent hash functions are pretty easily; I expect they're probably in Wikipedia and in good textbooks on derandomization/randomized algorithms. Feb 24 comment $f : \mathbb{Z}_n \rightarrow \mathbb{Z}^\times_n$? Please clarify your question. As it stands, your parenthesis does not correspond to any notion of inversion that I am familiar with. Also please define your notation, e.g., the $\mathbb{Z}_n^\times$ notation. And why is this a cryptography question? Please add some context or motivation, and tell us what you've tried to do on your understand to figure out the answer for yourself. Have you reviewed standard material on modular arithmetic? Feb 24 comment Seed / reseed DRBG too often? rzetterberg, for most applications (if you can reseed), the advice in your library documentation is undoubtedly good advice. But for many DRBG's, that advice is probably more cautious than is really necessary. Therefore, if there's some reason why you can't re-seed, don't take that advice as gospel: do a risk analysis to see if it's really necessary. Feb 21 comment Homomorphic crypto allowing anonymous yes/no votes? Are you aware that there is tons of work on secure voting schemes? Probably hundreds of papers. Have you done a search on this site and a literature search in the literature? Are you familiar with E2E (end-to-end) cryptographic voting systems? For instance, Helios and VoteBox? That would be a good starting point for you. Feb 21 comment Compared to GCM or XTS modes, how secure is H xor R1, E (R2, R1, Message) for confidentiality and integrity? @poncho, I took a second look, and you are right. I updated my message accordingly. The bottom line is still the same, though: I can't see any reason to use this scheme. Feb 21 revised Compared to GCM or XTS modes, how secure is H xor R1, E (R2, R1, Message) for confidentiality and integrity? added 1561 characters in body Feb 21 comment Practical usage of S/KEY You know that S/KEY and OTP have serious problems if they are used on their own (e.g., over an insecure channel, or if the user does not carefully check who they are authenticating to every time), right? That might be why you haven't heard of much deployed use of it. Feb 21 comment Seed / reseed DRBG too often? Actually, the premise that you need to reseed periodically ain't necessarily so. Some documents might suggest that reseeding is mandatory, but if you choose a good DBRG, it's not strictly necessary. The decision of whether to reseed should be based upon a thoughtful risk analysis, not just a blind rule-based "the standard says I have to reseed so I guess I have to". Feb 21 comment Is there a “brainwallet” for GPG keys? @RobKohr, you might personally think that password quality not a problem.... but in practice, it is a problem. A 5-word passphrase might have entropy as low as 50 bits or lower. That's not enough to resist off-line attack and would have security problems if you faced a serious motivated adversary. Feb 21 answered Compared to GCM or XTS modes, how secure is H xor R1, E (R2, R1, Message) for confidentiality and integrity? Feb 18 comment Proving that a function is not a OWF (One-way-function) @Pinocchio, yes, for a random $x$. Yes, my answer does not depend upon how $f$ is constructed; it applies regardless. Feb 18 answered Proving that a function is not a OWF (One-way-function) Feb 18 comment Proof of work for standard computers Two candidates come to mind. (1) Find an input to scrypt that makes the first 20 bits of its output all zeros. Verification is now pretty cheap. (2) Use timelock puzzles. They admit a very large ratio between the time to solve the puzzle vs the time to construct the puzzle (or to verify the solution). Feb 16 comment Known vulnerabilities in (EC-)KCDSA jimmy, Have you done a literature search? That's the obvious first step. Feb 16 comment ML/NN Cryptanalysis My advice would be to stick to one question per question. Here you have 3 separate question, all concatenated into the same question box. That isn't a good fit for this site; a good question should have a single answer. (Actually, the 3rd one -- "any other tricks..." -- is also not a good fit, as it is too unfocused.) Also, we expect you to do some research on your own and tell us what you've tried; I don't see that in the question at present. Finally, I suggest starting from the goal (drive in a nail) and ask for a solution, rather than starting with a hammer and looking for nails. Feb 13 answered OTP sequence uniqueness Feb 11 answered How can we sign a contract digitally between two parties?