poncho
Reputation
48,256
94/100 score
 5h comment What is the branch number of this matrix? Also, the answer depends on the ring that the matrix is based on. This matrix is singular if the matrix elements are $GF(3)$ 8h comment What is the branch number of this matrix? Hint: what's the definition of MDS? 8h comment What is the branch number of this matrix? Can any matrix with a 0 entry be MDS? 10h comment A question about fully homomorphic encryption (FHE) @Vitor: actually, user153465 asked about Fully Homomorphic Encryption; those schemes have infinite multiplicative depth; that is, you are allowed to multiply as many times as you want. There might be a bootstrapping procedure in there somewhere; however that's an implementation issue that we can (at this level) ignore. 10h answered A question about fully homomorphic encryption (FHE) 11h comment Prove that the following MAC is insecure @user3794796: yes, that shows that this is not a secure MAC. Now, it might not be the answer the professor was hoping for; he may have wanted something like "given the MAC of $m_1, m_1$, I can compute the MAC of $F_k(m_1), m_1$", but if so, it's his fault if he picked a question with multiple correct answers. 11h comment Prove that the following MAC is insecure @user3794796: yes; for any secure MAC, with that information, you would not be able to guess (except with trivial probability) the MAC of any message other than the two messages $m_1||m_2$ and $m_3||m_4$ 12h comment Is 3 rounds more secure than 2 in this simplified DES? Actually, I suspect that the professor is expecting you to notice a particularly simple relation between input and output after 3 rounds - what is that relationship? 12h answered Prove that the following MAC is insecure May 1 comment Can I get successful signature verification with just altering message and signature? @dave_thompson_085: that what happens when you change your mind about how to express something, and forget to update the first part of the sentence... May 1 revised Can I get successful signature verification with just altering message and signature? added 2 characters in body Apr 30 answered Can I get successful signature verification with just altering message and signature? Apr 30 answered Is this simple PRNG secure? Apr 29 answered How do attacks on WEP work? Apr 29 comment Elgamal with secret key equal to zero possible? @SEJPM: actually, DJB sets the MSB of the secret key to be 1 so that implementations wouldn't be tempted to "optimize" things by skipping higher order 0 bits in an attempt to make things go faster (and add a timing side channel); he wasn't seriously concerned about someone using the all-0 key... Apr 25 comment Why only one secret value with Shamir's secret sharing? @mikeazo: I came to the same conclusion after I read it a bit more (although my answer there bears some insight into this question) Apr 25 comment Why only one secret value with Shamir's secret sharing? Note that, depending on the $d-1$ shares being known, this 'linear relationship' can be "$S_2$ is this specific value". This sort of thing doesn't happen with Shamir's original method. Apr 24 answered What are the time considerations with regard to security against birthday attack? Apr 24 comment Trying to udnerstand textbook sample solution @Node.JS: how is that $D'$ related to the "real" one? How can Alice reconstruct a working $D$ from $D'$? Hint: $x^{-1} \bmod N$ can be computed, even if you don't know the factorization of $N$. Apr 23 comment is accessing elements of an array in secret order vulnerable to timing attack? One possible concern (which doesn't have anything to do with your question) is that a random permutation iterated twice is not a random permutation; for one, any point has double the expected probability of being a fixed point. It's easiest to see when considering the cycle structure; any odd-length cycle remains an odd-length cycle; any even-length cycle divides into 2 (so a length-8 cycle in the original becomes 2 length-4 cycles). Whether this is a concern depends on what you're using the permutation for...