CodesInChaos
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 59m comment Is there a cryptographic function or system in which it becomes HARDER to break as time passes? @E.Rose Post quantum crypto, like McEliece, NTRU, hash signatures. There is some work on moving these from theoretical schemes to something used in practice. 2d comment What is the private key in RSA? Personally I wouldn't call $d$ the private key, because it can't decrypt on its own. I'd just refer to it as the private exponent. Apr 25 comment What implementation properties of a hash function prevent safe truncation? I feel comfortable truncating SHA2 as long as the truncated size is sufficient for my desired security level. NIST does so as well (consider SHA-512/256 as an example) Apr 24 comment RSA when n= 2360221 and encryption = 5 find decryption key d, when M =1234 1) Factor n 2) Follow the wikipedia article on RSA to compute d. | hint: $2360221 = 1117 \cdot 2113$ Apr 23 comment How to vary bit length of prime numbers in RSA by using MATLAB? You probably need to use some kind of BigInteger type. And use a combined ModPow operation. Apr 23 comment What is the fastest block cipher in the (Intel) world? ChaCha isn't really a fair comparison, since it's only an unkeyed permutation, not a block cipher. Adding a key schedule costs some performance. I think @orlp got down to 2-3 cpb using SSE2 to implement a Blake2s derivative in OCB mode (10 rounds, corresponding to 20 ChaCha rounds). Apr 22 comment What is the fastest block cipher in the (Intel) world? Depends on the CPU, using AES-NI instructions AES is very fast (especially AES-128). Apr 20 comment Is the 'nonce' in bitcoin really a nonce? Even with a stream cipher, the nonce doesn't need to be unique. It's the (key, nonce) tuple that's unique. Apr 19 comment Exploiting XOR one-time-pads with dictionary length that isn't a power of 2 Computer based implementations almost always operate on sequences of bits/bytes which obviously are powers-of-two. Classical implementations often use modular addition. Apr 19 comment Exploiting XOR one-time-pads with dictionary length that isn't a power of 2 How would you even implement xor over an alphabet that isn't a power-of-two? Apr 19 comment How to crack unknown 8-bit encryption algorithm? What do you mean by "8-bit encryption"? The block size? Apr 18 comment Meaning of “random n/2 bit prime length prime p” You need to distinguish between the modulus itself and its size. Apr 17 comment can pdfcrack software crack a 128 bit encrypted pdf file Depends on the password. If it's a good password it should never finish. Apr 16 comment (How) Can we use HOTP for file encryption? 1) It only claims HOTP, not TOTP support. 2) It might hash several consecutive OTPs to form a high entropy secret. If you repeat that process with many starting points and encrypt the same master secret with all of these possible one-time-keys, (essentially forming 1 of n secret sharing) you could get something workable. Apr 14 comment How to reverse this hash function? @RaghuDixit I'm no expert, but wouldn't standard approaches like Gaussian elimination work? Just make sure that you compute the scalar operations in the finite field and not over integers/reals. In particular make sure to use modular multiplicative inverses to compute divisions. Apr 13 comment Is OCB the state of the art in authenticated encryption modes of operation for block ciphers? To improve the security of OCB you could use a naturally tweakable block-cipher and/or wider blocks. Threefish would be an obvious choice. But of course it can't take advantage of AES-NI. Apr 13 comment Is OCB the state of the art in authenticated encryption modes of operation for block ciphers? A recent paper generated such authenticated modes automatically. But those that matched OCB's features were trivially equivalent to OCB. Automated Analysis and Synthesis of Authenticated Encryption Schemes - Viet Tung Hoang and Jonathan Katz and Alex J. Malozemoff Apr 13 comment Calculating characters from the sum of ASCII pairs Attacking this is very similar to attacking a many-time-pad. One advanced attack example: crypto.stackexchange.com/questions/59/… Apr 13 comment Does “double RC4” exhibit the same weaknesses as standard RC4? @MaartenBodewes I'm too lazy to figure out the details. And poncho is our RC4 expert here. Apr 12 comment Does “double RC4” exhibit the same weaknesses as standard RC4? The only weakness in RC4-drop-n (for sufficiently large n), I know of, are the n-gram biases. Those should become much smaller when combining two key-streams, so the amount of data you can produce before it becomes distinguishable should become much bigger.