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526
bio website touset.org
location San Francisco, CA
age 30
visits member for 2 years, 4 months
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Cyclist. Rubyist.


Jan
4
comment Why are there no MACs inspired by block cipher modes other than CBC and CFB?
Your clarification only seems to have furthered the misunderstanding.
Dec
2
comment Is double hashing collision resistant?
What specific threat do you expect to mitigate by hashing twice?
Dec
2
comment Definition of “pepper” in hash functions
The short of it is that there is no widely-agreed upon use of the word "pepper" when it comes to cryptography.
Nov
14
comment CCA security of a system that splits messages and encrypts each packet
Secure against what?
Nov
10
comment Opinion on my idea for storing credit cards securely
PCI mandates that you never, under any circumstances, store a card's security code.
Nov
3
comment How good is middle cube method with jumbled digits?
Speaking of seed-dependent algorithms, Knuth's attempt at a randomized-algorithm "super-random" number generator is an enlightening read.
Oct
31
comment Type of cipher used where ciphertext has no discernible pattern, spaces. Type?
Any modern cipher operates on bytes, and not characters. You are likely either working with a classical cipher (in which case you have a wide range of cryptanalysis options) or an ASCII encoding of a modern cipher, in which case you're likely out of luck (the output of a modern cipher should be indistinguishable from random data; the ability to distinguish it would be considered a serious break in the algorithm).
Oct
30
comment AES— Brute force attack versus Known plain text attack
To summaize, if you call $F$ "half-encrypt" and $F^{-1}$ "half-decrypt" and take some message for which you know the corresponding ciphertext, you half-encrypt that message with all possible $k_1$ and half-decrypt the ciphertext with all possible $k_2$. There will be some pair of outputs that are identical (e.g., you've "met in the middle"), and for that pair you know the $k_1$, $k_2$ that produced them.
Oct
30
comment AES— Brute force attack versus Known plain text attack
This is a known-plaintext attack, and as demonstrated it is significantly faster than the $2^{128}$ steps (worst-case) it would take to brute force a 128-bit key.
Oct
29
comment Encryption algorithm designed to be easy to decrypt by machine but impractical to decrypt by hand
At that point, why not just used fixed-key AES?
Oct
29
comment How can a good pseudo-random number generator be made?
My recommendation would be to look at the papers behind existing stream ciphers (e.g., Salsa20), and see what they do.
Oct
23
comment How do you test randomness?
You mention that you can only prove with high probability that it wasn't produced randomly or pseudorandomly, but that's not true at all against an intelligent adversary. Any statistical test you perform can be bypassed.
Oct
23
comment Operation which needs much computing power to be created, but just a little to be solved?
Ah, sorry! I had completely inverted his question in my head — he wants the inverse of a trapdoor function. In a sense, he's looking for problems in NP: hard to compute, easy to verify.
Oct
23
comment Operation which needs much computing power to be created, but just a little to be solved?
That's pretty much the basis of much of cryptography.
Oct
23
comment Advantages of combined PRNGs
For what it's worth, combining multiple sources of (potentially-questionable) entropy is exactly what Fortuna is designed to do.
Oct
22
comment What is the use of Mersenne Primes in cryptography
@poncho has a great point. Mersennes can be useful when you need a non-secret prime number that is fast to compute modulo on.
Oct
22
comment Encrypt a file for random access but only after an initial read of the whole file
@CodesInChaos I retract my original statement — you are of course correct; this has the exact properties asked for by the OP.
Oct
22
comment Encrypt a file for random access but only after an initial read of the whole file
That doesn't seem to have the properties that he's asking for.
Oct
21
comment Encrypt a file for random access but only after an initial read of the whole file
You're thinking of an all-or-nothing transform, but something like that fundamentally won't work with random access.
Oct
15
comment What is a tweakable block cipher?
Out of curiosity, what does it mean to say that "changing a key can be expensive"? What does Threefish do differently to AES that makes changing keys "cheap"?