# Tag Info

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 ...

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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 $N$ ...

3

Yes, you are correct. The modulo operation (in general, not only over polynomials) is defined such that this is always true: $A \equiv A + B\ \ (\bmod\ B)$ Thus, if $A = X^3$ and $B = X^3+X^2+1$, that implies that $X^3$ is equivalent to the sum of $X^3$ and $X^3+x^2+1$, which is $X^2+1$, which is represented by $101$.

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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 ...

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Tweak per message offers good security like IV per message . Basically a tweakable block cipher with same tweak for all messages is same as non-tweakable block cipher. It is deterministic in nature and not considered secure enough in itself. You need to apply a mode of encryption that has been designed for non-tweakable block ciphers to make it secure.

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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 ...

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