Henrick Hellström

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 Feb3 comment Can passwords be stored securely so that a similarity comparison can be made? No, it would just require storing $h = Hash(f(PW))$. If $PW_0$ and $PW_1$ are similar, then $h_0 = h_1$. Feb3 comment Can passwords be stored securely so that a similarity comparison can be made? One answer would be that the entered passwords are always passed through a function $f$ such that $f(PW_0) = f(PW_1)$ iff the passwords are "similar". One trivial example would be converting all entered passwords to all lower case before further processing. Feb2 comment What is difference between PRG, PRF, and PRP It would be a stretch, so not quite. A PRF has to be indistinguishable from a random function. A PRP might be, but doesn't have to be, a PRF in this sense. However, the security proof for e.g. CTR mode is based on the premise that the block cipher (a PRP) might be modeled as a PRF, as long as the key stream is constrained to the square root of the cardinality of the total set of possible blocks. Feb2 comment Proof for exponentiation in modular arithemtic This is a proof by induction. We know it is true for $e = 1$. Using the formula in the answer, we can then prove it is true for $e = 2$ as well, and then for $e = 3$ etc. No matter how large $e$ gets, we might prove that the formula is true for $e + 1$ as well. Hence, by induction, it is true for all integer values of $e$. Feb2 comment What is difference between PRG, PRF, and PRP The difference between a PRF and a PRP is that the PRP is a bijective function and the PRF is not. There are no other differences, but of course this difference has various implications for their respective applications. Jan28 comment Secret sharing - no dealer, modifiable, verifiable If robustness isn't a requirement, also look at Pedersen's original scheme link.springer.com/chapter/10.1007%2F3-540-46416-6_47 Jan28 comment Secret sharing - no dealer, modifiable, verifiable This sounds a lot like the Joint-Uncond-Secure-RSS scheme of link.springer.com/chapter/10.1007%2F3-540-68339-9_31. The trick is to let each participant be a dealer, or, put differently, build the scheme around participants who generate their shares randomly. Jan27 comment How can I find the prime numbers used in RSA? The approach is just basic algebra. If the factors are close to the floor $x$ of the square root, the equation $r + (x-p)(q-x) - x(q+p-2x)$ will be dominated by the first and third term and the second factor of the third term will consequently be a function of the magnitude of $r$. Jan27 comment How cryptographically secure was the original WW2 Enigma machine, from a modern viewpoint? AES is believed to be (IND-CPA) secure even if the plain text is chosen by the attacker, so why do you include the restriction of no operator errors? Jan14 comment Do I need to prepare plain text before encryption? I see no obvious harm in anyone following your recommendation, but it is completely pointless as far as cryptographic security goes. Jan13 comment Do I need to prepare plain text before encryption? Compression does not increase the total entropy of the plain text, but it does increase the average entropy per bit, by, well, compressing it. Theoretically, it might increase security in some sense, but only in so far that the compression algorithm effectively hides the length of the original plain text. Jan6 comment Shared secret: Generating Random Permutation To avoid unnecessary confusion: I noted in my comment that a threshold scheme would introduce a risk of collusion and didn't "propose" such a scheme, but only stated a problem. Jan6 comment Shared secret: Generating Random Permutation How are you going to enforce such a requirement? Suppose Alice, Bob and Carol are playing. Alice has already folded but remain online. Bob has gone all-in but realizes he is about to loose just before the last card is to be revealed. Bob privately asks Alice to pull the plug. Alice pulls the plug. Jan6 comment Shared secret: Generating Random Permutation Please note that any mental poker protocol would leave you with a problem of how to fold, if there are more than two players. If all players have to be present to decrypt the next card, one player might simply go offline and make it impossible for the remaining players to complete the game. If you solve that by using threshold secret sharing, you introduce a possibility of collusion. Dec30 comment Random Number Generation with a Entropy pool versus Seed Your first paragraph is a bit fuzzy. If you need a cryptographically secure pseudo random number generator, it is because you need cryptographically secure pseudo random numbers, and whatever that reason might be, it is cryptographic by definition. If not, you might need a PRNG, but not necessarily a CSPRNG, and should probably ask elsewhere. Dec28 comment Can an RSA public key be generated without ever knowing the factors? crypto.stackexchange.com/questions/9191/… Dec23 comment Single Party Encryption, Multi Party Decryption Benaloh's scheme seems to be exactly what I am asking for, yes. On the negative side, though, the scheme uses commitments that induce an overhead for the mixes by a factor greater than 100 (with the parameter values recommended in the paper). It is obvious that there would be a risk for integrity if only two commitments were used by each mix, but would it lead to a risk of compromised anonymity? Dec23 comment Single Party Encryption, Multi Party Decryption I am missing your thoughts on how, exactly, this might be applied to a mixnet scheme. Dec22 comment Single Party Encryption, Multi Party Decryption Clarification: My question concerns mixnets, so the requirement that decryption is done in sequence and deniably seems essential. If you thought of this and came to a different conclusion, please expand your answer. Dec22 comment Single Party Encryption, Multi Party Decryption That paper seems to deal with a different scenario, where one user distributes a private RSA key to multiple devices. Hence, it violates the first requirement, in so far that there has to be a trusted dealer, and that dealer will (at least at some point in time) know the private key (share) of all of the other decryptors. Furthermore, the paper describes a method for the decryptors to work in parallel to decrypt a message, while the second and fourth requirements in my question entail that decryption must be done in sequence.