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Apr
22
comment Burn after read algorythm. Software verification on compromised environment. Software smart card
Intel seems to be working on something that could enable such functionality in future CPUs. (But Intel itself could still subvert it and it obviously can't prevent the analog attack)
Apr
22
comment Is less security required for a short stream cipher than for the AES enciphering of very long messages?
What exactly is your question?
Apr
22
comment Is less security required for a short stream cipher than for the AES enciphering of very long messages?
Why would you use a system that doesn't fulfill strong security notions when they are easily available and still very fast? Figuring out if certain weaknesses apply to your use case is hard and error prone. Pretty much the only exception are hand ciphers where you need to be able to encrypt/decrypt a message without a computer.
Apr
22
comment Which algorithms are used to factorize large integers?
Your definition of L doesn't match wikipedia. You put the $^{1-a}$ at the end of the expression, wikipedia put it on $\log \log n$.
Apr
22
comment Stateless hash based public key cryptography?
[tahoe-dev] Hash based Signatures for Tahoe LAFS. Seems to contain a working implementation of one of those unlimited tree signature schemes. If you like Merkle Trees you should talk to the TahoeLAFS people.
Apr
21
comment Security of KDF1 and KDF2 (hash based KDF's)
This should work as long as the message being hashed has constant length (or at least is prefix free). Else length extensions might bite you.
Apr
21
comment Simple digital signature example with number
Which part did you correct? The only difference I see is that you're using mod n as an operator and mike uses congruence mod n.
Apr
20
comment Diffusion in Shamir's secret sharing scheme
You should insert the questions you're responding to as quotes into your answer. It's a bit hard to follow in its current form.
Apr
20
comment Is there a reason that the N value in scrypt is limited to powers of two?
((UInt64)X[16] * N) >> 32 has similar properties and is relatively cheap on many CPUs. For example on x86 the high word of a 32x32 multiplication ends up in an second register as byproduct of the standard multiplication and thus is easy to compute.
Apr
17
revised Do Chosen-Chiphertext attacks only apply to public-key cryptosystems?
added 42 characters in body; edited title
Apr
16
comment How can I convert numbers into prime numbers?
@gurghet With a PRNG I mean a cryptographically secure stream cipher which obviously has keys of at least 128 bits. So I don't know where your 32 bit key space comes from.
Apr
16
comment How can I convert numbers into prime numbers?
Sounds needlessly complicated. I see no advantage over the straight forward approach.
Apr
16
comment How can I convert numbers into prime numbers?
Hashing to primes is easy. Just seed a PRNG with the hash of the input and use it to generate a prime with any standard algorithm.
Apr
15
comment Are there any elliptic curve asymmetric encryption algorithms?
@hunter Why? What do you gain from encrypting the message itself with ECC instead of hybrid encryption? ECIES has a standard asymmetric encryption API. There are a few situations where RSA works but ECIES does not, but they're relatively exotic.
Apr
15
comment What is a 'secret key factory'? What precisely is it doing?
Your example "128 bit key" has only 64 bits once you convert it from hex to bytes.
Apr
15
revised What is a 'secret key factory'? What precisely is it doing?
added 17 characters in body
Apr
14
comment Does AES-128 have the same strength as AES-256 with a padded key?
While simple padding might not cause any immediate weaknesses, I strongly recommend using random keys uniformly distributed over all 256 bits. That's the way AES has been analyzed, that's what's modeled by a PRP (pseudo-random-permutation) helping with security proofs and it avoids concerns over the weak schedule. Use a proper PRF or hash to expand the key not such an ad-hoc scheme.
Apr
14
comment Does AES-128 have the same strength as AES-256 with a padded key?
I wouldn't be confident in too simply paddings since the AES key schedule is relatively weak. Each half of the 256 bit key gets mixed in less often compared with the 128 bit key since AES-256 only as 1.4 times as many rounds, not twice as many. The additional rounds might compensate for the weaker key schedule in padded AES-256 compared with AES-128, but it certainly feels risky. It might be better to simply concatenate the 128 bit key to itself which at least mixes in the key more often.
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.