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 Apr 12 comment Non-repudiation in classical cryptosystem Have both sides use hash based signatures to sign their messages? Apr 12 comment Disk encryption using AES-CBC: is reliance on previous block data a downside? @My1: the problem with GCM (or any mode that expands the ciphertext) is that the disk provides (say) 4k sectors, and the OS using the disk expects 4k sectors, and so there's no additional space for nonces, IVs or tags. This (plus the possibility that someone might write to the disk sector a related plaintext) is the problem that makes disk encryption difficult; if we could expand the disk sector by 256 bits, then it's an easy problem with off-the-shelf answers... Apr 12 comment Secret construction In other words, you're looking for a compression function that'll work for random input??? Apr 12 comment Secret construction I believe just concatinating the random strings (with delimiters) would satisfy all the security requirements you have listed. Apr 12 comment Calculate if a digital signature is valid However, breaking RSA doesn't require us to recover the value of $sk$; recovering $m$ given $m^e$ would suffice to break both RSA public key encryption and RSA signatures (and there's no known proof that performing $e$-th roots (for odd $e$) can allow us to compute $sk$) Apr 11 comment Calculate if a digital signature is valid @Erwin: a logarithm would be "given $a, b, n$ compute $x$ such that $a^x = b \pmod n$. Instead, the RSA problem is the $e$-th root; that is, given $b, e, n$ compute $x$ such that $x^e = b \pmod n$ Apr 11 reviewed Edit Calculate if a digital signature is valid Apr 11 revised Calculate if a digital signature is valid improved formatting; eliminated homomorphic-signature tag (unrelated to the question) Apr 11 comment Calculate if a digital signature is valid While the question was about signatures methods in general, the actual example given is a toy implementation of RSA. Perhaps if you reviewed the crypto.stackexchange.com/questions/9896/… answer, it may address some of your questions (unless you need something more basic; don't worry, we won't be offended by basic questions...) Apr 11 comment Calculate if a digital signature is valid Possible duplicate of How does RSA signature verification work? Apr 11 comment Do I need to use null bytes and \x80 in hash length extension attack? @MaartenBodewes: actually, there are 6 hashes in the SHA-2 family, the length attacks on SHA-256 and SHA-512 (and SHA-1) is identical (except for the differences in the padding); SHA-224 is a bit more tricky (you have to reconstruct the missing 32 bits; doable if you can make a hash query of an extended message); SHA-384, SHA-512/256 SHA-512/224 is essentially invulnerable. Apr 11 answered Do I need to use null bytes and \x80 in hash length extension attack? Apr 10 comment Large Volumes of Asymmetric Encryption I would agree with SEJPM; it would be good if you would explain exactly what your threat model is; what operations do you need to allow various people to perform, and what do you wish them not be able to do. Apr 10 answered Shamir's Secret Sharing vs. Asmuth-Bloom scheme Apr 9 comment Space needed to store an RSA private key Actually, if you use a fixed algorithm to generate the key (say, a specific algorithm from FIPS 186-3), you could just store the seed and the key size. Yes, this would make the process of recovering p, q, d rather expensive; however it makes the stored private key quite small... Apr 9 awarded Enlightened Apr 9 awarded Nice Answer Apr 8 reviewed Edit Is it possible to utilize an AES-128 encryption hardware unit for AES-256? Apr 8 revised Is it possible to utilize an AES-128 encryption hardware unit for AES-256? fixed some typos Apr 8 comment Is it possible to utilize an AES-128 encryption hardware unit for AES-256? @SEJPM: I've seen other AES hardware implementations that use a pre-expanded key schedule, so the answer is "maybe"