2,953 reputation
526
bio website touset.org
location San Francisco, CA
age 31
visits member for 2 years, 5 months
seen 9 hours ago

Cyclist. Rubyist.


Jan
15
comment Hash which can be used to verify one of multiple inputs?
I'd love to see if this can be done.
Jan
14
comment What is a tweakable block cipher?
Gotcha, thanks! I wasn't considering key scheduling.
Jan
10
comment Can you help me with this Random Number Generator function?
I believe he wants successive calls to the generator to produce the demonstrated outputs.
Jan
9
comment Which block cipher mode does the experts use?
The only real downside to CTR is that accidental IV reuse is absolutely catastrophic.
Jan
7
comment Share encrypted files without giving master key
The problem with this approach, obviously, is that once the files are encrypted it's impossible to re-derive the key. So the keys (or the content hashes) must be stored. Unless you're already storing content-hashes, you might as well just be using a random value. This can be faster (no need to hash potentially large files) and more secure in some use-cases (the same file could be encrypted using different keys for different parties).
Jan
7
comment What is the keyspace of ROT-13?
I tend to think differently on this: ROT13 isn't a cipher, in my opinion, precisely because it is unkeyed. Therefore, the question "what is the size of the keyspace of ROT13" is ill-posed, precisely because it has no key (it quite literally has no key parameter in its function signature). I would argue that ROT13 is an encoding, like Base64 or hex. If you don't consider either of those to be a keyed cipher, it makes little sense to me to consider ROT13 to be one. Of course, you can generalize ROT13 to "ROT" encryption, at which point it becomes keyed with a keyspace size of 26.
Jan
4
comment How does ECDH arrive on a shared secret?
Wait, so if Alice has to calculate $g^a$ by dotting $g$ with itself $a$ times... why can't an attacker just do the same? Dot $g$ with itself, check if the result matches, if not repeat? I assume this is because one can calculate $g^a$ for large enough $a$ much faster than linearly?
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.
Jan
3
answered what is it called when encrypt('a') + encrypt('a') == encrypt('aa')?
Dec
22
awarded  Nice Question
Dec
15
awarded  Nice Answer
Dec
5
answered Is the salt used with PBKDF2 secret?
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
revised AES— Brute force attack versus Known plain text attack
added 54 characters in body