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bio website github.com/CodesInChaos
location Frankfurt, Germany
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visits member for 3 years, 5 months
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Nov
25
comment OTP from Sony BIOS password recover
With a 64 bit modulus you can simply factor via trial division at cost $2^{32}$.
Nov
25
comment Are there any digital signature algorithms in common use that result in 32-byte signatures?
My mistake, I wanted to say "at a 128 bit security level". 64 bits of security as you suggest is rather weak.
Nov
25
comment Are there any digital signature algorithms in common use that result in 32-byte signatures?
If the order has 128 bits, a normal EC schnorr signature takes 64 bytes and a hashed schnorr signature still costs 48.
Nov
24
comment Recovering El Gamal secret key from signatures
When the nonce is reused you can simply solve those two equations for the private key.
Nov
20
comment Are all binary-additive stream ciphers reciprocal?
Note that this isn't the case for CFB mode, which is a xor based stream cipher but doesn't fulfill your particular definition.
Nov
19
comment special public keys and modulo n
You need distinct primes. Also if you're using CRT you'll need to change your code a bit to take advantage of the multi prime speedup.
Nov
19
comment Modes of encryption for hard drives?
CTR is not a good idea since an attacker who sees two versions of a block can attack it with the usual many-time-pad attacks. This can happen due to SSD wear leveling.
Nov
19
comment Modes of encryption for hard drives?
XTS is one of the most popular modes. See Disk encryption theory on wikipedia.
Nov
19
comment DHE key exchange. Is it really secure?
You're underestimating what "very big" means. No conventional computer will ever be able to count to $2^{256}$ and that doesn't even consider storing that much.
Nov
18
comment Is there a way to systematically calculate the public exponent $e$ in RSA?
If the modulus is so large that you don't find an e via trival division in less than a minute, it's so big that you won't be able to do the rest of RSA. For example no four-digit number is co-prime to 3, 5, 7, 11 and 13.
Nov
18
comment Is there a way to systematically calculate the public exponent $e$ in RSA?
You can factor phi and choose a prime that isn't such a factor. Slow for properly sized numbers, trivial for something like 40. But even with trial division using 3, 5, 7 etc. you will find a divisor within a minute as long as n is small. Inverting e will certainly take longer than finding e.
Nov
18
comment How many characters per block in an El Gamal ECC cryptosystem?
1) Why would you use ElGamal over ECIES? It's already dubious choice for short messages, but using ElGamal in an ECB like mode on long messages is plain insane. 2) AFAIK ElGamal needs padding to be secure, so you can't fill the whole block with actual data.
Nov
12
comment Are modified implementations of cryptographic algorithms a good idea?
1) The DES link goes to DualEC as well. 2) I wouldn't say that the DualEC backdoor was revealed by snowden, only confirmed.
Nov
11
comment Attacks of the MAC construction $\mathcal{H}(m||k)$ for common hashes $\mathcal{H}$?
"and that it is thus pointless to have the key wider than half the hash's size" I disagree with that. Larger keys can prevent multi-target attacks against the key.
Nov
10
comment what is SALSA20 Nonce and its requirements
Related question Why does the crypto_box functionality in NaCl library exposes the nonce to the programmer
Nov
7
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.
Nov
5
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?
Nov
4
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.
Nov
4
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)
Nov
4
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.