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Jul
27
answered Can a 1 byte difference in AES 128 bit keys make huge difference in output?
Jul
23
comment Are specially designed fonts sometimes used in cryptography?
Visual cryptography is not really an encryption method, but more like a secret sharing. Of course you can always consider this as a cipher if you consider one share to be the key and one share to be the ciphertext.
Jul
21
comment Are specially designed fonts sometimes used in cryptography?
Assuming the attacker knows nothing about the encoding algorithm is bad. That's like thinking "if I can't break the system, then no one else ever could". Both are assumptions that have proven to be wrong countless times.
Jul
21
answered Are specially designed fonts sometimes used in cryptography?
Jul
20
comment RSA private key d knowing e,n
Just posting the solution to homework does not really help in understanding how to solve the problem, usually.
Jul
20
comment curious DHE implementation for key exchange
How do you know, that it is not a power of $4$? There is no other way for this to work, unless the server knows the logarithm of $g$ to base $4$. Essentially, this is just DH with the server's side always being static (similar to the public key in ElGamal). The generator used is actually $4$, and what is usually denoted $g^a$ in DH is called $g$ here.
Jul
16
comment What good is a hash accompanying a program?
Considering websites: It can also make a difference if the HTML is transmitted via HTTPS, and the file download is unencrypted.
Jul
16
comment What good is a hash accompanying a program?
The first two sentences indicate, you believe SHA-2 to be vulnerable to finding preimages to a certain hash value (e.g. with the length extension...). This is wrong. As others have already pointed out: SHA-2 is considered one of the stronger hashes (and pretty much state-of-the-art in practical applications). Even MD5 is still considered secure against preimage attacks (but not recommended any more). And that is still enough if you get the hash from a trusted source.
Jul
16
comment Is it a good practice to use plain text for derivation of Keys?
My guess is, that you are trying to re-invent key encapsulation like it is done in hybrid encryption, without defining if you are using symmetric or asymmetric crypto. Right now there is no reason to "hash" the plaintext to derive a key. Comparing to choosing a random one, this only can introduce possible weaknesses without granting any benefit. Especially, since your notation does not actually indicate the use of a hash function, or a PBKDF - which would be necessary, if the plaintext is somehow predictable or easy to guess.
Jul
14
revised Computational indistinguishability: are function parameters known?
added 257 characters in body
Jul
14
comment Computational indistinguishability: are function parameters known?
I did not claim the Mersenne Twister is a good CSPRNG. It is a useful PRNG for simulations etc. where you need randomness with good statistical properties. It is kinda crucial to think of the "CS" part seperately: But I will point that out more explicitly
Jul
13
comment Computational indistinguishability: are function parameters known?
They are not cryptographically secure pseudorandom number generators. They aren't PRFs in the cryptographical sense either.
Jul
13
answered Computational indistinguishability: are function parameters known?
Jul
13
comment Why should the primes used in RSA be distinct?
Newton's method with integer division also works quite good. Complexity should be similar to binary search, but I think it approximates slightly faster.
Jul
13
comment Brute forcing the secret key in Elgamal encryption
"i just did a quick run in python with my code..." Python has arbitrary-length integers by default, Java doesn't, which seems to be the problem.
Jul
13
comment Brute forcing the secret key in Elgamal encryption
If you know the basic elements all fit within the length of int, then you can use long instead, so that you can do the multiplication steps. Arbitrary-length integers are preferable of course, but if this is a one-time assignment, long would be sufficient and easier to handle.
Jul
13
comment How is an epsilon of 1/1000 non-negligible?
There is an error in your definition: To be secure, $\epsilon$ has to be smaller than a negligible function. If the advantage is non-negligible, then it is insecure. Anyway about the example: If it includes a specific $n$, then you can calculate the advantage or an upper bound of it. If the advantage is independent of $n$, it can not be negligible.
Jul
13
comment What is the difference between the standard representants of $\mathbb Z/q\mathbb Z$?
In lattices the elements are vectors and not just integers (more precisely equivalence classes of vectors. This is along the lines of the modulus for integers, but for vectors).
Jul
8
comment Is one-time-pad still secure if the number of 1's in the key is revealed to the attacker?
You definitely loose the information theoretic security. How bad it is depends on your $n$: In a bitstring of length $2x$ and hamming weight $x$, there are ${{2x}\choose{x}} \approx \frac{4^x}{\sqrt{\pi n}}$possibilities, which differs from the full $2^{2x}$ only by the denominator. However, the lower or higher values can give a lot of information about the key.
Jul
7
comment RSA with $\lambda(n)$ or $\varphi(n)$
@fgrieu You're wrong about the "always": The calculation of the modular inverse is done with a smaller exponent by a factor of at least 2. But that does not necessarily imply, that the product $ed$ is smaller (it usually is). Small number example: $n=7\cdot 11 \Rightarrow \varphi=60; \lambda=30$. For $e=11$, we get that $d=11$ in both cases ($11 \cdot 11 = 1$ for both moduli).