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Jan
30
comment Derive both MAC and AES keys from same PBKDF2?
I just remembered that I actually implemented HKDF in C# a couple of months ago: gist.github.com/CodesInChaos/8710228
Jan
30
comment Best way to get 32 bytes from PBKDF2
One simple workaround is deriving a 20 byte master key with expensive PBKDF2, and then applying PBKDF2 with 1 iteration and long output to it. As another simple technique applicable when you only need 32-64 bytes, you can simply hash the output of PBKDF2 once with SHA256 or SHA512 to expand it.
Jan
30
comment Best way to get 32 bytes from PBKDF2
I prefer something similar to the former, but following the HKDF-Expand spec. Second one is no better than calling PBKDF2.GetBytes(32) directly.
Jan
30
revised Block cipher with key longer than block size
added 251 characters in body
Jan
30
comment Block cipher with key longer than block size
@sashank Apart from using "size" instead of "cardinality" this answer looks correct to me. It only argues that there are multiple keys which encrypt one particular $x$ to the same value. It doesn't argue that there are equivalent keys, which encrypt all $x$ to the same value. This fits the question, which is about that particular property, not about equivalent keys, like the question you linked.
Jan
30
awarded  Cleanup
Jan
30
revised Block cipher with key longer than block size
rolled back to a previous revision
Jan
29
comment Derive both MAC and AES keys from same PBKDF2?
Implementing HKDF-Expand on top of HMAC is pretty easy.
Jan
29
reviewed Close ElGamal encryption with private key
Jan
29
comment Derive both MAC and AES keys from same PBKDF2?
It can, but when you do it might be possible for the attacker to compute it more cheaply than for the defender, since the defender needs the whole output, whereas the attacker might be able to confirm it with only part of the output. This shouldn't happen with well designed password based key derivation functions, but PBKDF2 is not one of those.
Jan
29
comment Derive both MAC and AES keys from same PBKDF2?
You should avoid deriving more than the natural output size direclty PBKDF2. With PBKDF2-HMAC-SHA512 that's enough for two keys, with *-SHA1 it isn't. If you need more key material, use HKDF-Expand on the output of PBKDF2.
Jan
29
comment Usage of Fermat primes in RSA
@rath We're talking about primes with this format. There are infinitely many Fermat numbers, but only these few are prime
Jan
29
comment Does re-iterate AES128 with different keys gives any benefict?
It's weaker than AES-256 against generic attacks. There is a meet in the middle attack which can break it with cost 2^128 (not sure how applicable the attack is with realistic cost models). See Attacking 2DES efficiently. Search for DES meet-in-the-middle for several related question.
Jan
29
answered Usage of Fermat primes in RSA
Jan
29
comment How to specify last $t$ bits are only sent when a signature is sent?
If you want shorter signatures you should consider BLS
Jan
29
revised What is the padding scheme for the original Tiger hash?
added 1805 characters in body
Jan
29
answered What is the padding scheme for the original Tiger hash?
Jan
29
comment How can i predict the next number of a long sequence of seemingly random numbers?
@figlesquidge They only have a reduction to the primitive which isn't really a meaningful proof. You still can't prove that SalsaCore is a secure PRF or that AES is a secure PRP. In general with symmetric encryption primitives don't have a meaningful proof, but modes of operation do.
Jan
29
comment Why not the one-time pad with pseudo-number generator
1) Any decent stream cipher will only repeat after more than 2^60 output bytes, so this limitation is not of practical concern 2) The number of different streams and the length of one stream is unrelated. Typical stream ciphers will have at least 2^128 different streams. 3) It's far more important to use a good stream cipher than combining multiple. Very little reason to modify the constants yourself, just choose a random key and IV as seed.
Jan
28
comment How is CipherCloud doing homomorphic encryption?
Not even CipherCloud itself claims to use homomorphic encryption. That's just a misunderstanding the OP had.