# Tag Info

## New answers tagged block-cipher

1

AFAIK, no one has proven that AES on a single 128-bit block with a true-random 128-bit key does not provide information theoretic security (such a proof would probably be the end of AES as it would demonstrate a weakness). OTOH, no one has proven that it does. I suppose it is possible that it does, but such a proof is likely to be extremely difficult. Just ...

5

Quite apart from the correction that Reid made (it takes $2^{127}$ attempts to achieve a probability of 50% of finding the right key; with $2^{64}$ attempts, the probability of success is $2^{-64}$), with AES, there is no known way to take advantage of known (or even chosen) plaintext to speed up any brute force search; even with $2^{64}$ chosen ...

2

Your Formulas are alright, but there is some additional information from the exercise/setup: The exercise states, that $F$ should be considered as a blackbox (otherwise you could use the internal stages of $F$, as poncho already suggested). However, as I understand it you can stil evaluate $F$ on any input of your choice. At this point, you can do a couple ...

1

Maybe you have a more specific class of applications in mind, but in my experience there is fairly limited use of cryptography in web applications. The cases I can think of: SSL, in which case the encryption is done by the web server and not by the application (though maybe you don't consider that distinction relevant). The type of encryption is then ...

-1

I don't think stream ciphers are implemented in Web applications ( Actually they are not approved because of weak security) However the protocols like IPsec, SSL and PGP etc do make use of various block ciphers( e.g AES ). Web application security is totally dependent upon the security at individual OSI layers.

4

By a generic attack we also understand an attack that with minimal corrections would apply to every block cipher. For example, suppose you have a (plaintext,ciphertext) pair and test keys by exhaustive search: you apply the keyed cipher $E_K$ to plaintext $P$ for every $K$ and check if you get ciphertext $C$ in response. Quite often, the ciphertext bits ...

3

As explained in a comment: A generic attack is one that works against all block-ciphers (with a given block and key size), without consideration about the structure of the block-cipher. One generic attack for a block cipher of a given block size $b$ bits builds a dictionary of input/output pairs (e.g. from past plaintext/ciphertext), for a fixed key. When ...

3

You could, however the one part that doesn't translate in an obvious manner is the Galois field representation; you would need to pick a field representation for $GF(2^{256})$ and $GF(2^{512})$, because those have not be predefined for those sizes. Here's the issue; GCM does field multiplications internally; that is, it takes two $N$-bit vectors (where $N$ ...

0

As far as I can see you would have (at least) to define a wider polynomial for the multiplication over the GF(256) or GF(512) respectively (See point 2.5 Multiplication). It is unclear if you can do that without touching the overall security of the algorithm. I would not rely on the security of these modified algorithms without extensive research of the ...

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