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Apr
14
comment Does AES-128 have the same strength as AES-256 with a padded key?
if you use a good expansion (e.g. SHA256(key)), AES-256 will be stronger against cryptoanalysis since it has 14 rounds instead of just 10. For the same reason AES-256 has lower performance than AES-128.
Apr
12
comment Given a message and signature, find a public key that makes the signature valid (ECDSA)
@DrLecter Can't you simply use the same algorithm, but using $M_2$ as input? I expect to get out some public key, but a public key where the corresponding private key isn't known by the attacker.
Apr
12
comment Given a message and signature, find a public key that makes the signature valid (ECDSA)
See ECDSA Signatures allow recovery of the public key on bitcointalk.org
Apr
12
comment Given a message and signature, find a public key that makes the signature valid (ECDSA)
Check out 4.1.6 Public Key Recovery Operation in Certicom - SEC 1: Elliptic Curve Cryptography
Apr
12
revised Difference between plain AES CTR and “CCM-style” CTR
edited tags
Apr
11
awarded  Deputy
Apr
10
comment Difference between plain AES CTR and “CCM-style” CTR
Unfortunately there isn't one standard CTR mode, different protocols use different ways of combining the counter with the IV and incrementing it.
Apr
10
comment Leak-proof protocol: is such a thing possible?
Another covert channel: timing
Apr
10
comment cryptography and use of python programming language
There are some nice libraries for python, such as SAGE or Charm and it's a high level language so it's good for producing working programs quickly. On the other hand it sucks for high performance and side channel resistant crypto implementations. So it's nice for research but not for production crypto.
Apr
10
comment Why do we encrypt-then mac but sign-then-encrypt?
We should sign followed by authenticated encryption.
Apr
8
comment Multi cipher CTR
@user1028028 Why would you want to do that? It's secure as far as I can tell if all blockciphers are secure, but it's very silly.
Apr
8
comment Convert SpookyHash to semi-secure 192bit hash
You can't really trade security against brute-force vs. performance that way. You can reduce the resistance to future cryptoanalysis for a moderate gain in performance by taking a standard crypto hash and reducing the number of rounds to slightly above the best current result.
Apr
7
comment Convert SpookyHash to semi-secure 192bit hash
Just use a proper hash instead. You don't know what you're doing, and even hashes designed by experts need a lot of review before you should use them.
Apr
7
comment Game with symmetric key
Bob can force a draw by sending the same value as Alice.
Apr
7
comment RSA: Letting $p$ and $q$ have different bit-size
This problem is pretty close to multi-prime RSA. In multi-prime RSA the factors are smaller as well, multi-prime RSA results in a performance gain using CRT, yours degrades performance.
Apr
7
comment RSA: Letting $p$ and $q$ have different bit-size
The cost of some factoring methods depends on the size of the modulus (GNFS) other methods depend on the size of the smallest prime (ECM). You need to ensure that the size of the factors is large enough so that the cost of ECM exceeds the cost of GNFS.
Apr
7
revised RSA: Letting $p$ and $q$ have different bit-size
edited tags
Apr
4
comment What are alternatives to number theory based crypto?
Are you talking about asymmetric encryption, or symmetric encryption? For symmetric operations almost nobody number theory based crypto.
Mar
31
revised How is the curve equation used in ECC?
added 4 characters in body; edited tags; edited title
Mar
30
comment Using the same private key for two ECC key pairs
For such an application any sane cryptographer would store a master-key and derive the individual keys with some kind of hash (I recommend HKDF-Expand).