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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.


Jun
12
answered Given a private RSA key, how do we get the public key?
Jun
12
comment Algorithm: How to use x and y mouse movement co-ordinates to generate random data?
@zuallauz Cryptographic hash functions take data which isn't necessarily uniformly distributed (not the case here, as you've observed) but still contain entropy, and convert them into a uniformly distributed bitstring with entropy approximately equal to the input (up to the hash function's output size). So it'll take those mouse coordinates and process them into a form fit for use as cryptographic key material (careful not to overestimate entropy)
Jun
11
comment How long to bruteforce a RSA key
I think "for a couple of millenia" is putting it very lightly. Fortunately better algorithms than enumerating primes exist.
Jun
10
comment Is it possible to anonymize web traffic so that the IP Address cannot be determined while still being able to determine distinct IPs?
Can you please specify who you want to be able (and unable) to perform tasks 1 and 2? Should an attacker be able to determine if two requests came from the same IP, and do you want to be able to map request to IP address (and only you), or nobody at all?
Jun
10
comment Have these compositions of block ciphers the same security?
@user7060 Under the assumption that DES is a strong block cipher, yes. It is theoretically sound. But there may be unforeseen weaknesses in DES which make this kind of composition dangerous. And I was just emphasizing that if you are going to be swapping encryption and decryption permutations, you need to do so consistently everywhere, obviously.
Jun
10
reviewed Approve suggested edit on “proof of access” schemes
Jun
10
comment Have these compositions of block ciphers the same security?
@user7060 What I'm saying is that an for an ideal keyed pseudorandom permutation $E_k$, using $E^{-1}_k(X)$ in place of $E_k(X)$ and vice versa makes no difference, thus in the standard model it does not matter if you call the encryption function or the decryption function (with the same key) as long as all three keys are independent. And I mean swapping the two, not just substituting them at will. For instance, $E_k(E_k(X))$ and $E^{-1}_k(E^{-1}_k(X))$ are both good but $E_k(E^{-1}_k(X))$ is a pretty bad cipher...
Jun
10
comment Have these compositions of block ciphers the same security?
The inverse of a block cipher is a block cipher of equivalent strength (it's perfectly OK to use the decryption algorithm to encrypt and the encryption algorithm to decrypt from a security point of view, should you want to do so for some reason) so unless your cipher has weaknesses with respect to related keys, they should all be the same, afaik. I dunno about DES though.
Jun
8
comment Questions about the ideal cipher model
Also, cryptography is usually designed bottom-up. Encryption schemes generally assume ideal underlying primitives will lead to all security properties being achieved, in other words, schemes and protocols are firmly grounded in theory (within the framework of some cryptographic model, here the ideal cipher model). It is when you instantiate those protocols with actual, imperfect block ciphers/hash functions that you leave the realm of theoretical cryptography and hope your primitives are "ideal enough" to be secure with respect to your adversary.
Jun
8
comment Questions about the ideal cipher model
An ideal encryption scheme is an encryption scheme which is assumed to meet the security properties the scheme strives to fulfill. This is less concrete than an imaginary elf performing tasks, and depends on what the encryption scheme is. Let's take the CTR mode of operation, for instance, which is built on top of a block cipher. A security property of this scheme could be "given any reasonable number of plaintext/ciphertext pairs encrypted with the same key, an observer cannot recover the key" or something along those lines.
Jun
8
comment How well analyzed are giant block length ciphers?
Apparently encryption primitives derived from PRP's must use padding as they have a fixed block size (??). Stopped reading there. Snake oil and the author is clueless.
Jun
8
revised What does “message schedule” mean in SHA-256?
fixed parameters to SHA256
Jun
8
answered What does “message schedule” mean in SHA-256?
Jun
7
reviewed Approve suggested edit on Is this OTP scheme safe?
Jun
7
comment Is this OTP scheme safe?
Are you going for plausible deniability? You might want to clean up some of the notation, it's rather confusing at the moment. $K_3$ and $K_4$ are apparently ciphertext and $K_1 \oplus K_2$ may as well be merged into a single variable $K$ as they cannot be separated.
Jun
7
comment What does “running in polynomial time” really mean?
Yes, well it's more general than that, it says that the adversary is assumed to only be able to run polynomial time algorithms, because his computational abilities are bounded (in an asymptotic sense). So if he cannot find such an algorithm to solve problem P, problem P is computationally "secure" (informally) against this adversary. Sure, the adversary can use an exponential time algorithm to try to solve P, but in this security model it is assumed the algorithm will not terminate in a feasible amount of time, and hence is as good as no algorithm. It is mostly terminology.
Jun
7
comment What does “running in polynomial time” really mean?
I think one part of this question (why "efficient" == "polynomial time") might be better suited for CSTheory. However, I think you have the concepts backwards. Consider the task of finding the correct $k$-bit key among all $2^k$ possibilities (brute force). Without additional information provided by cryptanalysis, the best way is to check every key, which takes $O(2^k)$ time. This is not polynomial time but exponential time, and is therefore asymptotically out of reach for a polynomial time adversary. The point of cryptography is to make sure the best attack algorithm is not polynomial time.
Jun
7
comment Is triple des similiar to RSA in that they message size is limited to the key size?
Which applies to both AES and 3DES, so you're missing the "unlike AES" part.
Jun
6
comment Composition of block ciphers and 3DES
@fgrieu Ah, I now understand. Thank you, indeed that is unrealistic.
Jun
6
comment Composition of block ciphers and 3DES
@fgrieu There seems to be a slight miscommunication. I was talking about your statement that "the key has twice as much entropy". The effective key space of $AES'$ is not 256 bits (but still at least 128 bits).