CodesInChaos
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 Nov10 comment what is SALSA20 Nonce and its requirements Nov10 answered what is SALSA20 Nonce and its requirements Nov7 comment What is the value of Q such that Q|P-1 where P is a prime number? Assuming this is about generating a finite field with a subgroup of order Q, we usually choose Q first, then choose a r such that P=Q*r+1 with P of appropriate size. Repeat with different Q or r until P is prime. That way you don't need to do any factoring. For Diffie-Hellman we often choose r=2 and vary Q, for signatures fixing Q and varying r can be preferable. Nov7 revised Crypto-Compression Algorithms? added 518 characters in body Nov5 comment What is more secure, a ciphertext encrypted with AES-256 or encrypted with 3 different AES-128 keys? Your title says 3 AES-128 keys, the body says 2. Which one are you talking about? Nov4 comment Square roots, prime factorization Since the OP has now solved the problem, I encourage you to turn the hint into a proper answer that describes the whole solution. Nov4 comment Can DH_anon really be exploited by an attacker? A client that allows anon DH is still vulnerable to MitM with anon DH even if the server disabled it. (unless perhaps it does something unusual like client certificates or other cryptographic authentication) Nov4 answered Can DH_anon really be exploited by an attacker? Nov4 comment RC4 encryption/ decryption with hashing Why new code based on RC4 in 2014? There is a lot of work on getting rid of RC4 because it's potentially weak. Nov3 reviewed Approve Showing that $2^{N-1}\equiv1\pmod N$ when $N=2^p-1$ for prime $p$ Nov3 awarded Popular Question Oct31 comment RSA Decryption given n, e, and phi(n) 1) If there are errors in your first answer, edit it to correct it, instead of posting a new one. 2) I recommend including the essential parts of your code in the answer. An answer in stackexchange should be able to stand on its own, without external resources that might disappear over time. Oct30 comment RSA Decryption given n, e, and phi(n) Use a big integer library, most programming languages either contain one (at least C# and Python do) or have a readily available third party implementation. (unfortunately the wolfram alpha link above doesn't work, it truncated phi) Oct30 comment RSA Decryption given n, e, and phi(n) 1) Compute d as the Modular multiplicative inverse of e, i.e. $d=e^-1 \bmod \phi(n)$ using extended euclidean 2) compute $m=c^d \bmod n$ Oct30 revised Stream Cipher : Never use stream cipher key more than once added 6 characters in body; edited tags Oct30 comment Why develop new stream ciphers when block ciphers can be trivally made into streams? Comparing dissimilar ciphers like AES and RC4 is pretty difficult. On large modern CPUs AES will be much faster. Oct30 comment What are these twist attacks with cost $2^{58.4}$ on NIST P-224 curve, and when do they apply? Bernstein likes doing ECDH using x-only ladders without first verifying if the given x value is on the curve or the twist. For example the Curve25519 implementation in NaCl works like that. Oct30 comment Why is it hard to calculate Logarithm on Diffie Hellman? You compute $g^x \bmod p$ via square-and-multiply in time $(\log(p))^3$, which is feasible for a 1000 bit $p$. The naive algorithms you describe are far too expensive. Oct30 comment Are there any digital signature algorithms in common use that result in 32-byte signatures? There is a less common scheme called BLS, which produces 32 byte signatures at a 128 bit security level. Oct30 comment Are there any digital signature algorithms in common use that result in 32-byte signatures? Modifying an ECC signing lib, especially if it's already Schnorr based like Ed25519, to use hashed Schnorr at a size of 3x security level isn't that hard. Not sure how this interacts with multi-target attacks though.