Ilmari Karonen
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 Jan 7 comment Permutation of keys that guarantees different hashes @petermlm: Yes, but if $H(x) = H(y)$ for all $x$ and $y$ (that's what $\forall$ means), then the output of $H$ is constant. Jan 7 comment What is the difference between MACTripleDES and TripleDES? That would be a valid question on Stack Overflow, but not here. (Or you could just look at the documentation, and specifically the ComputeHash(Byte[]) method). Also, you cannot "encrypt" or "decrypt" anything with a MAC; that's not what they're for. Jan 7 comment What is the difference between MACTripleDES and TripleDES? You mean this thing? If so, this question on SO might be helpful. Jan 7 comment Permutation of keys that guarantees different hashes You might want to clarify your question. The only functions that satisfy $H(x) = H(y)\ \forall x \ne y$ are constant ones, and those obviously cannot satisfy the second criterion. So, as written, the answer is trivially "no." Jan 6 comment Hash function that allows to decide if A > B if you only have hash(A) and hash(B)? Related, not quite duplicate: crypto.stackexchange.com/questions/8160/… Jan 6 comment Homemade Randomized RSA You can use the public key to implement an encryption oracle, but that's not the only thing you can do with it. So, you know $e$ (since it's public), and have a guess for $m$. What can you do with those? Will it help you answer @Henrick's question? Jan 3 comment Is there an asymmetric encryption algorithm where the public key cannot be derived from the private key Jan 3 comment Is there an asymmetric encryption algorithm where the public key cannot be derived from the private key Hmm, yes, that might work. You'd presumably need a non-standard way of generating $e$ and $d$ (since simply setting e.g. $e = 65537$ is obviously a non-starter here), but just picking a random $e$ coprime to $\lambda(n)$ might work. I've never seen any actual security analysis for this RSA variant, though; in particular, I wonder just what kind of message padding it would need to be secure both as an encryption scheme and as an authentication scheme at the same time. Jan 3 comment Is there an asymmetric encryption algorithm where the public key cannot be derived from the private key Doesn't knowing both the encryption and the decryption exponent allow factoring the modulus? Jan 3 comment What is the correct definition of the blowfish F-function? It seems to me that you've misinterpreted the pseudocode here while translating it to C: the code is clearly using x + y mod 2**32 to mean "add x and y modulo 2**32", or, in other words, "add x and y using 32-bit integer arithmetic" -- not "reduce y modulo 2**32 and add the result to x" like your C code reads. Jan 3 comment Uniformly distributed secure floating point numbers in [0,1) @user: I did. There does not appear to be any basis for that assertion, as the code given in the answer actually gives exactly the same results as casting to float and dividing. Jan 3 comment Why does DES implement so much Cross Wiring? @RichieFrame: Your comment seem to directly contradict the answer given by Yehuda Lindell below. If you don't agree with the answer, or think there's more to this, you might want to consider adding an answer of your own. Jan 2 comment What is the difference between key size and block size (for AES) I'm not 100% sure what you're trying to ask here, but I am pretty sure that, whatever it is, it's probably too broad to be effectively answerable here. You might want to start by picking up an introductory book on cryptography, which should explain how block ciphers like AES work, and how they can and should be used. Dec 30 comment Is there such a thing as a hash function with a fixed size input? Technically, any standard hash function can be made into one that takes a fixed-size input, simply by restricting its inputs to a specific size. Dec 30 comment Encrypting passwords Generally, encrypting passwords is not a good idea. Do you need to store the passwords to automatically authenticate the user to some external service (i.e. are you implementing something like a "password wallet"), or are you just using the passwords to grant the user access to your app? If the latter, you shouldn't be encrypting the passwords at all, but rather hashing them; if the former, can you switch to some other (e.g. public key based) authentication method? Dec 29 comment RSA by hand - did I do something wrong? (c = m on encryption) Dec 28 comment What algorithm to use to rotate values in a predefined manner and be able to decrypt them back to the original? My suggestion above would be to use the block cipher directly to encrypt individual blocks; your crypto library probably exposes that as "ECB mode", which takes no IV / nonce, and treats the plaintext / ciphertext as a series of individual cipher blocks to be encrypted / decrypted separately. (ECB mode is not a general-purpose semantically secure encryption mode by itself, but the raw access it provides to the block cipher is useful e.g. for implementing other modes, or for custom cryptographic schemes like this.) Dec 28 comment Shamir's Secret Scheme : Knowing the threshold Not as far as I can see. But maybe we're thinking of different scenarios. I was talking about adding one extra share to a set of $k$ shares, i.e. going from a polynomial of order (at most) $k-1$ to one of order (at most) $k$. By construction, these polynomials intersect at the $k$ original shares; if they intersect anywhere else, they must be identical. Dec 28 comment Shamir's Secret Scheme : Knowing the threshold Take the interpolated polynomial for the $k$ shares, and evaluate it at the $x$-coordinate of the new share. There's a $1/q$ chance that the result will match the $y$-coordinate of the share, in which case adding the extra share will not alter the polynomial. Dec 28 comment Information theoretical security of an inefficient Shamir based access structure OK, I think I can follow your notation now. You might want to edit that information into the question itself, though.