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Jul
12
comment How is an epsilon of 1/1000 non-negligible?
Could you elaborate on this? As it is, it doesn't answer the question at all.
Jul
9
comment What are the key differences between the draft SHA-3 standard and the Keccak submission?
@mykhal yes, that was already linked (the "it was discussed here" link).
Jun
13
comment Encryption of exe file
Encryption works on anything, not just "text".
Jun
6
comment Are there security issues with adding plaintext length to TLS packets?
Note that your changed protocol will not be compatible to TLS anymore.
Jun
6
comment Share two secrets between two parties
Welcome to Cryptography Stack Exchange. We actually prefer answers which contain the information itself, not just referring to information elsewhere. Could you add a summary of the information to the answer? Otherwise we will convert your answer to a comment.
May
28
comment Is it practical to use a stream cipher in a block cipher mode?
Actually, my answer didn't say "you can't convert a stream cipher into a block cipher", but "the way of converting which was proposed in the question doesn't work". I'll try to edit it.
May
27
comment Effective security of block cipher – equal the key size, or half the key size?
@Dillinur If I remember right, there is a generic quantum algorithm which finds preimages to functions in $O(\sqrt{N})$ time, where $N$ is the number of possible candidates (i.e. $O(\sqrt{2^n}) = O(2^{n/2})$ for $N = 2^n$). Of course, for this to actually work, you need a quite large quantum computer (about $n$ qbits, I think), and also fast enough. Then it depends on the constant factor to see if it is faster than regular brute-force.
May
27
comment Effective security of block cipher – equal the key size, or half the key size?
Also, this only seems to work with a fixed initialization vector (or if you somehow manage to query the block cipher itself), right? (I guess a "chosen-IV-fixed-known-plaintext" attack on CBC allows this.)
May
27
comment Effective security of block cipher – equal the key size, or half the key size?
Maybe the algorithm would be clearer if the first loop used $X$ instead of $T$ as a variable name? (Also, the hash table access is likely not O(1), but $O(\log(2^{n/2})) = O(n)$. Still almost negligible compared to $O(2^{n/2})$.)
May
26
comment Effective security of block cipher – equal the key size, or half the key size?
@J.D. Please add this as an answer (with some details of how this attack works and a reference to a paper or similar).
May
25
comment Is it practical to use a stream cipher in a block cipher mode?
@J.D. consider adding an answer to the question linked in my previous comment, if that is not already covered by one of the answers there.
May
25
comment Can you make a hash out of a stream cipher?
If you have a PRF, why can't we use that directly as the compression function? It looks like unnecessary overhead to first build a PRP from a PRF, and then use Davies-Meyer to remove the P aspect from it?
May
25
comment Is it practical to use a stream cipher in a block cipher mode?
In Converting a stream cipher into a block cipher, this is discussed in a more general way – it looks like there is no way to build a block cipher from just a stream cipher, though if you additionally can use a keyed hash function, it might work. (You'll get a block cipher for much larger blocks, though.)
May
25
comment Is it possible to have identical public keys for different ciphers?
@Nayef it looks like you misunderstood the question ... it is about public keys from different schemes colliding, not about collisions within one scheme (RSA and DSA in your example).
May
19
comment How do we know one-way functions can be iterated?
@fkraiem in a general mathematical point of view, both "input" and "output" of a function can elements of some arbitrary sets. Nothing stringy here. The set of (finite) bit-strings and its subsets are important examples, but not really exclusive ones.
May
1
comment PGP digital signature vs SHA256 HMAC Comparison
keylength.com has some comparisons of key sizes for algorithms.
Apr
25
comment SHACAL-2 vs. AES as underlying block cipher for Secure Hash (aka SHA-256)
Also, you would need to do the key-schedule for each block again, which also adds to the execution time.
Apr
19
comment Why does applying 56-bit DES twice only give 57 bits of security?
@fgrieu I guess we could build the table with only one block of plaintext-ciphertext, and for each of the $2^{48}$ found matches check the second block. This would have work of $2^{57} + 2^{49}$, which I would still consider as 57-bit security.
Apr
15
comment Why is Diffie-Hellman considered in the context of public key cryptography?
@i486 Diffie-Hellman does not directly allow encryption, it is just a key-exchange algorithm (and usually the resulting key should be hashed before being used for anything). You could use that key as a "stream cipher" key stream (i.e. XOR it with a plaintext) and send the result as ciphertext, but then you are actually doing more than just DH.
Apr
12
comment Encryption in which ciphertext is bigger than plaintext?
In cipher designs one usually tries to avoid a (more than constant) increase of the ciphertext.