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visits member for 2 years, 10 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.


8h
comment java.util.Random and Dice Rolls
@GuutBoy He wants to recover the state of an instance of the Java Random class given its outputs (as dice rolls) efficiently.
Nov
18
comment special public keys and modulo n
Are you talking about textbook RSA (no padding, just interpret the message as a number and raise it to an exponent) or real world RSA (with padding)?
Nov
9
comment Algorithm of Big Integer class Multiplication in visual studio
This is like 100% off-topic. Please try on stackoverflow.
Nov
3
comment Security of RSA with $\gcd(pq, (p-1)(q-1))\ne 1$
@fgrieu No, because we have a multiple of $p$: the modulus $n$ itself. So we get that particular prime factor "for free" by adding $n$ to the product of prime powers.
Nov
2
comment Security of RSA with $\gcd(pq, (p-1)(q-1))\ne 1$
@fgrieu As I suspected (it was fairly obvious in retrospect) we also need $k$ to be non-smooth: the trick is that $n$ automatically gives us the factor of $p$, which gets us most of the way to $q - 1$ in the context of the Pollard $p - 1$ algorithm. This doesn't really change much, though, it just means that the key generation procedure needs to be adjusted to avoid $k$ being smooth, it is still open whether we can exploit the length difference between $p$ and $q$.
Nov
1
comment Security of RSA with $\gcd(pq, (p-1)(q-1))\ne 1$
@fgrieu Thanks! Hopefully there is a way to improve on the number theory to provide a faster algorithm to find $k$ than brute force.
Nov
1
comment Security of RSA with $\gcd(pq, (p-1)(q-1))\ne 1$
@fgrieu Hmm, the square root approach seems more direct, looks like I missed the obvious! But in any case, yes, it means that subject to the restrictions the modulus has to be pretty unbalanced to offer any security, but I don't have anything better than that so far :/
Oct
30
comment Password generation/storage scheme
Related: crypto.stackexchange.com/questions/9035/… ... and I guess basically every question containing the words "password generation", as this is hardly a novel concept
Oct
29
comment Encryption algorithm designed to be easy to decrypt by machine but impractical to decrypt by hand
@owlstead Yes, as long as you add any kind of nondeterminism in the scheme, that will be enough to defeat precomputation and lookup for a human without access to a computer! (admittedly the question didn't specify the exact threat model, so this may be overkill, but since we're talking about giving the computer an advantage we can be as overkill as we want :))
Oct
29
comment Encryption algorithm designed to be easy to decrypt by machine but impractical to decrypt by hand
Looks like you're referring to a generic feedback scheme. CBC and CFB mode are two such schemes for block ciphers (en.wikipedia.org/wiki/Block_cipher_mode_of_operation)
Oct
29
comment Encryption algorithm designed to be easy to decrypt by machine but impractical to decrypt by hand
Yes, until someone precomputes the value of every letter when encrypted with that exponent and modulus, then it suffices to look up the ciphertext in a small table. To counter this, you could pack multiple letters in a single plaintext, or simply change exponents and modulus often.
Oct
14
comment Adding tweak to a block cipher
(besides, the Skein designers had incentive to keep the Threefish key schedule as simple as possible - it needs to be run for every compression function invocation of the hash function, and the Threefish permutation itself is already not that expensive relative to its key schedule)
Oct
14
comment Adding tweak to a block cipher
@LightBit Correlating "simple" with "weak" is a mistake unless you have references to back up the claim for the Threefish key schedule being vulnerable to attacks. I think this is what owlstead was referring to.
Oct
9
comment Breaking RSA moduli
You will have to define "break" and "RSA moduli" - do you mean "factor" and do "RSA moduli" need to resist factorization, or do you mean any semiprime?
Oct
6
comment Is there a string that's hash is equal to itself?
@fgrieu Indeed :) but it's pretty clear which answers I was referring to. +1'd your answer
Oct
6
comment Is there a string that's hash is equal to itself?
Please note both answers below are either misleading or do not quite follow. I do not have time to write an answer now, but someone else surely will. In the meantime, feel free to look at these couple of near-duplicates: crypto.stackexchange.com/questions/9910/…, crypto.stackexchange.com/questions/971/… (they don't contain an answer to your question, but provide good insights into how to answer this kind of question). Also, keyword "fixed point".
Oct
1
comment Small Encryption Exponent
@fgrieu If this is indeed the expected resolution path, it will be somewhat difficult to produce a solution before the deadline at this point :-)
Oct
1
comment Small Encryption Exponent
To elaborate a bit on poncho's comment, most real world RSA encryption schemes use small encryption exponents for performance, usually 3, 17 or 65537 - so $e = 7$ may not be of any help!
Sep
21
comment How much would it cost in U.S. dollars to brute force a 256 bit key in a year?
@AwalGarg Bruce can do it by simply guessing the key on the first try.
Sep
3
comment AES-ECB as an authentication mechanism
What is "Y" here? The ciphertext?