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visits member for 2 years, 9 months
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I am an undergraduate computer science and mathematics student in New Zealand. My fields of interest are computer graphics, in particular the physics of light transport, and to some extent cryptography, as well as programming and software development in general.


Mar
12
comment Would a symmetric cipher with a keylength a big as the data length be information theoretically secure?
I could be way off, though, because I just confused myself. Anyway good question, looking forward to the answers
Mar
12
comment Would a symmetric cipher with a keylength a big as the data length be information theoretically secure?
I don't have a proof but my guts tell me the answer is "no", because it does not seem esufficient that the family of functions $f_k$ used by the generalization of the OTP be all bijective, they also have to satisfy the criteria that $\{ f_k(x) \mid k \in \Sigma \}$ be a permutation of $\Sigma$ for all $x \in \Sigma$ (where $\Sigma$ is the relevant alphabet, e.g. the set of all 256-bit strings). XOR, modular addition and other simple constructs trivially satisfy this through symmetry, but AFAIK it is unknown whether there exist $k_1$, $k_2$ such that $AES_{k_1}(x) = AES_{k_2}(x)$ for some $x$.
Mar
11
comment How to best mix two arbitrary/random n-bit words?
Arbitrary or random? The two are not the same... but if you have two uniform random variables, combining them via any bijective operation produces an equally uniform variable (that could range from a simple XOR to running a cascade of five block ciphers on the variable - the result is the same). So I'm not too sure what you mean, could you clarify?
Mar
10
awarded  Civic Duty
Mar
10
comment Why the same characteristics cannot be used to recover all FEAL4 keys
What are the differentials at the start of round 2 compared to the differentials you started off with? Note the remark about the first subkey being (temporarily) out of reach due to the key whitening step.
Mar
5
comment Long-term data protection, storage of old encrypted traffic and quantum cryptocalipse
I wouldn't be too worried about the "quantum cryptapocalypse", to be honest. Supposedly D-Wave has 512-qubit computers and nobody (there or elsewhere) has even tried to apply the technology to factor a small but not stupidly small semiprime (say, 50-60 bits) as a definitive proof of concept? Come on. I'm not a quantum theoreticist but something smells fishy here. Their excuse of "gearing their work towards optimization problems" is unconvincing, a quantum factorization algorithm that demonstrably works on big inputs would be earth-shattering, much more so than any traveling salesman stuff.
Feb
25
comment Detecting steganography in the stream of short messages
If both the carrier (main stream of bits) and the encryption covering the "steganography" are cryptographically secure, I'm pretty sure you have no way to detect the existence of those messages. In general what happens here is the warden either blocks or corrupts the channel, since the supposed existence of a hidden encrypted stream is impossible to prove or disprove from the data alone. Could you be more specific about the scenario you are describing, because it seems to me that a generic answer won't be very fulfilling?
Feb
15
comment The difference between these 4 breaking Cipher techniques?
@Reid Done, thanks :)
Feb
15
revised The difference between these 4 breaking Cipher techniques?
rearranged attack models
Feb
10
comment Is there a standard way to extend the output of a hash?
What does OP use for secret? A hash function isn't a PRF. What billpg wants is a construction which takes the fixed-length output of the hash function and stretches it without any secret data (for instance, the variable-length processing Skein uses, which basically runs the Threefish cipher in counter mode using the hash output as a key). One could always use a constant value for secret, but then it's hardly standard.
Feb
10
comment What does the notation $1^n$ stand for?
possible duplicate of What does the expression $1^n$ mean as a function argument? and also of What do $0^n$ and $1^n$ mean in cryptography?
Feb
8
answered Hashing a uuidv4 + time to create a unique id
Feb
7
comment Encrypting a payload for transmission over HTTP. AES256 with PBKDF2
@ndsc Take a look here: security.stackexchange.com/questions/3959/…. Thomas Pornin recommended 20000 rounds as a minimum estimate and that was in 2011, computational power has increased quite a bit since then. PBKDF2 will eventually fall behind on the power curve since it is easily parallelizable, but until bcrypt, scrypt and the likes get better support and become a bit more mainstream it's the most reliable. You have nothing to worry about, though, it was just a side note that your iteration count could be a bit higher, not a weakness.
Feb
7
comment Encrypting a payload for transmission over HTTP. AES256 with PBKDF2
@ndsc Btw 20000 PBKDF2 iterations are a bit on the low side these days. Recommend at least 50k and 100k if possible.
Feb
2
comment The difference between these 4 breaking Cipher techniques?
@hakoja Good point about CPA, forgot about that.
Feb
2
revised The difference between these 4 breaking Cipher techniques?
deleted 221 characters in body
Feb
2
answered The difference between these 4 breaking Cipher techniques?
Jan
27
comment How cryptographically secure was the original WW2 Enigma machine, from a modern viewpoint?
Does the question also assume that the workings of the Enigma machine (all versions) are made available to the cryptanalyst, in accordance with Kerckhoffs's principle?
Jan
27
comment How long will my encryption remain private?
@Christian The point I am making is the fact that the bitcoin protocol makes it harder and harder to mine blocks up to a point where it is no longer possible is a form of planned obsolescence, because it is quite possible that in the future, finding even a partial preimage of SHA-256 will become easier than brute force and the protocol will be broken (a rather major problem for a currency). I'm not an expert in bitcoin and do not claim to be, and was only replying to the question about bitcoin being about finding collisions.
Jan
27
comment How long will my encryption remain private?
@Christian Yes, I should have said (and was adding) that only the mining part has planned obsolescence. The transaction part of bitcoin is another story.