8,789 reputation
12041
bio website github.com/CodesInChaos
location Frankfurt, Germany
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visits member for 2 years, 9 months
seen 5 hours ago

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
8
comment What benefit is there to using AES over my custom cipher for secure storage?
Why would you use any encryption weaker than AES?
Apr
8
comment How exactly does the OpenSSL TLS heartbeat (Heartbleed) exploit work?
While the bug is in a crypto library, it's not a cryptography problem. It's a general security issue and thus fits security.SE better.
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).
Mar
30
comment Using the same private key for two ECC key pairs
Why would you want to do that? I can't think of any situation where this is useful.
Mar
30
revised Why the need to hash before signing small data?
edited tags
Mar
28
comment Why does Fortuna RNG use double SHA-256?
It's easier to fix a dubious property of a building block than it is to argue why you're absolutely sure that it's not a vulnerability. Another advantage of this approach is that it makes it easier to prove the security of a higher level construction. So even if they believe that length extensions are a problem many cryptographers prefer the safe choice of using a stronger hash.
Mar
28
answered Why does Fortuna RNG use double SHA-256?
Mar
28
comment Why does Fortuna RNG use double SHA-256?
I believe you will find the answer in Practical Cryptography Chapter 6.3.1 Length Extensions
Mar
28
comment Is it secure to choose d in a RSA key pair?
If you can't use a different algorithm with faster signing/decryption, you could use multi-prime RSA and CRT. It's not a big speedup, but still nice if performance is really important.
Mar
28
comment Does repeated hashing create a PRF?
Your suggestion is trivially distinguishable from random data: just look at two consecutive blocks and check if the latter is the hash of the former. If you replace the hash by a keyed hash (like HMAC) this becomes the hash based equivalent of OFB mode, which is secure.
Mar
27
comment Is Curve25519-java secure?
I just looked at the NXT coin crypto implementation. They generate the nonce by reducing a 256 bit value modulo the order. Such a reduction causes a bias, which can be exploited for some curves (Bleicherbacher described an attack on DSA based on this). Ed25519 reduces a 512 bit number instead of a 256 bit number to avoid these biases. But NXT coin got lucky that the order of Curve25519 is very close to a power-of-two decreasing the bias, so as far as I can tell it can't actually be exploited.