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17h
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.
1d
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.)
1d
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 $O(2^{n/2})$.)
1d
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).
2d
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.
2d
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?
2d
revised Can you make a hash out of a stream cipher?
Fiestel → Feistel
2d
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.)
2d
answered Is it practical to use a stream cipher in a block cipher mode?
May
25
comment Algorithms for Extracting Linear Equations in Algebraic Cryptanalysis
Welcome to Cryptography Stack Exchange. As mentioned in the automatic "on hold" message, your question is not really clear. At least, I don't understand what you want. Maybe you try to explain it a bit more detailed?
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
19
revised How do we know one-way functions can be iterated?
recursion would be g(n) = g(h(n)) or similar. Using the output of a function as its input is named iterating it.
May
1
comment PGP digital signature vs SHA256 HMAC Comparison
keylength.com has some comparisons of key sizes for algorithms.
May
1
revised PGP digital signature vs SHA256 HMAC Comparison
adding some paragraph breaks
Apr
25
revised SHACAL-2 vs. AES as underlying block cipher for Secure Hash (aka SHA-256)
edited tags
Apr
25
revised SHACAL in SHA-256
edited tags
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
21
awarded  Nice Answer
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.