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1h
comment RSA signature scheme
Is the above supposed to be a signature scheme? If so, how would someone verify it? What would need to be in the public key? Hint: $n$ and $e$ are not sufficient.
1h
comment Direct sum of Binary numbers In Mixcolumns
So, what is $02 \cdot 10000100$? By the above logic, it is $00001000 \oplus 00011011 = 00010011$ (where we xor in 00011011 because the msbit of 10000100 is a 1), and so it is exactly the answer you're looking for
1d
comment A simple attack on DESX in time $2^{120}$
possible duplicate of DES-X , computation load and storage
1d
comment rsa public and private keys
Actually, we know algorithms that will generate provable primes of the same size, and with about the same efficiency (e.g. Shawe-Taylor)
1d
comment Why is the IV passed in the clear when it can be easily encrypted?
This really isn't why we believe it's safe to send the IV in the clear. It's not because the key protects us, but instead that a) the security proof of CBC mode shows us that knowledge of the the IV does not aid the attacker, assuming that the attacker can't choose the message after knowing about the IV, and b) the observation that each ciphertext block in the message effectively acts like the IV for the rest of the message, hence sending the IV in the clear is no riskier than sending the ciphertext itself.
2d
comment Why cbc-mac is a secure mac with psudo-random but not random IV?
"not secure with a random IV"; is that an IV that the generator of the MAC selects, and is hence translated with the MAC? If so, wouldn't the forger (the guy without the key) also be able to select his own IVs? So, how can a forger use that to come up with a valid (Message, IV, MAC) triplet?
Oct
27
comment Decrypting monoalphabetic substitution key which does not appear in ciphertext
What Paulo was asking was "why does it matter what the ciphertext of h, j, z is?" If you can rederive the plaintext, well, that's generally the goal of cryptoanalysis; the key is generally of interest only because it can be used to allow us to get the plaintext.
Oct
25
comment Is there any probabilistic version of RSA?
@ddddavidee: actually, the PKCS #1.5 signature method is determanistic, and is in common use. However, I would agree that any encryption padding will be nondetermanistic (to prevent someone from verifying a guess on the plaintext, if nothing else).
Oct
22
comment What is the use of Mersenne Primes in cryptography
Two issues with the cited random number paper: a) it doesn't actually talk about Mersenne primes, but numbers of a related form, and b) I don't believe that the random number generators cited are actually cryptographical (that is, indistinguishable from a truly random bit generator).
Oct
21
comment To prove $r_2$ is a uniformly at random value in $Z_n$, where $r_2=r_1 . m$
By computation while they are at the server, you mean the RSA public or private operations? If so, well, what you're doing is known as RSA Blinding; it's a well known technique to allow someone to do the RSA operations without leaking what they're operating on. It's known to be safe.
Oct
21
comment To prove $r_2$ is a uniformly at random value in $Z_n$, where $r_2=r_1 . m$
The answer to that would depend on what you mean by 'security'. You may find it helpful to write down exactly what you mean by that; what security properties are you expecting?
Oct
21
comment To prove $r_2$ is a uniformly at random value in $Z_n$, where $r_2=r_1 . m$
@user153465: the answer to question 1 is "no"; if $m$ is an arbitrary value, consider the case that $m=0$; obviously in that case, $r_2$ is not a uniform random value. For the answer to question 1 to be "yes", we need to put restrictions on the distribution $m$ is chosen from (specifically, that probabilities that $m$ is a multiple of $p$, $q$ and $pq$ must be $1/p$, $1/q$ and $1/pq$)
Oct
19
comment Does the transposition cipher have a network application?
@jake.toString: I suspect it'd be unlikely that anyone would write "we never use transposition ciphers in network applications" unless someone specifically asked.
Oct
19
comment problem with “one time pad”
possible duplicate of How does one attack a two-time pad (i.e. one time pad with key reuse)?
Oct
19
comment Does the transposition cipher have a network application?
I'm not sure how to answer this other than saying that I've never seen a transposition cipher within a network security protocol, and I have a good deal of experience in that area. It's not clear to me why anyone would choose to use a transposition cipher as opposed to one we are confident is semantically secure.
Oct
10
comment Is it possible to demonstrate that md5(x) != x for any x?
One minor nit: the computation about a fixed point assumes a random function. MD5 isn't a random function, and might not behave as one (we don't know any specific way that it doesn't, at least, any way that appears relevant to the question), however that may be due more to our ignorance rather than the internals of MD5. However, it is an appropriate plausibility argument to say "there might be one, and there might not"
Oct
9
comment Do Export Restrictions Still Apply To The Key Length of RC4?
@CodesInChaos: unless you're exporting to North Korea, Syria, or an handful of other countries. If you want to export to those countries, it's a lot more than "fill out some paperwork"
Oct
9
comment How do I generate a number for a lottery and later proves its existence
@KentMuntheCaspersen: I missed where you said that
Oct
8
comment How costly is to find millions of large prime numbers for RSA?
@fgrieu: you're right; I was worried that if the step you take between sieved elements was too small, the discovered primes would be too close. However, if you're looking for primes for this purpose, you'd select a step of circa $2^{900}$ (say) for 1024 bit primes; the resulting primes would have plenty of spread. (To user153465): Doing this would mean that if someone found two primes, factoring the rest would be a lot easier; that's why fgrieu asked if that would be a problem
Oct
8
comment How costly is to find millions of large prime numbers for RSA?
@fgrieu: one requirement is that the product of two of those primes be hard to factor. The problem with those known algorithms is that they generate primes which are numerically close to each other, which means that their product may be easy to factor (say, via Fermat factorization)