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8

A Feistel network is a way of constructing an invertible permutation from a (possibly non-invertible) function. If the function used is pseudorandom and has a large domain, then 3-4 rounds yields a pseudorandom permutation (3 rounds suffice if the adversary can only ask "compute" queries, and 4 rounds are needed if the adversary can ask "invert" queries). ...


5

Yes you could use a hash function as round function, but if you are using the "same key" over all rounds, you are vulnerable to slide attacks. Using a hash function is not a very good idea. Your round function should not introduce biases, should not lead to special differences (attack: differential cryptanalysis), and it should also not be writable as a ...


4

In the substitution step of a typical Substitution-Permutation Network (e.g. in AES SubBytes), the whole state is broken in parts and each part substituted. That's not the case in (the core of) a Feistel cipher, where at each step/round some sizable part of the state is bound to remain unchanged (in order that each step be reversible).


3

You run the algorithm with two different plaintexts (whose difference is usually small – just a few bits, everything else being equal). Wherever these plaintexts lead to different inputs to an S-box (in any layer/round of the algorithm), we call this S-Box “active” (since the other S-boxes produce the same result for both plaintexts, they are called ...


3

I know that all the subkeys $k_i$ are derived from the main key $K$, but how? However the cipher designer feel like. The Feistel design gives guidance as to how the block is processed (and in a way to make inverting the cipher easy), however it gives no guidance as to actually generate the subkeys. The designers can do anything they like, and still ...


2

why it should strictly be bijective instead of injective or surjective? Actually, it is injective and it is surjective; the term bijective just means that it satisfies both the properties of injectivity and surjectivity. Injective does not mean that there is a 'skip' (that's "not surjective"); instead, it states that no two different inputs give the ...


1

In a Feistel network you can use ANY function and it will be invertible. Of course, in order to get security you need the function to fulfill some property. One of the main reasons to use a Feistel network is to get a pseudorandom permutation. For this to work, you need 3 or 4 rounds of Feistel with a pseudorandom function (with tweaks or independent keys at ...


1

First of all, you don't include a key; I'll assume that the sbox is the key. However, even with that assumption, it still doesn't meet the general expected requirements for a block cipher. In the decrypt direction, any one byte of the decrypted result depends only on 16 (!) bytes of the ciphertext block. This can be seen by considering the inverse of the ...


1

We really only need one plaintext-ciphertext pair, but the second can be used as a way to check candidate keys. Make a guess to the final subkey (ie guess all of them). Decrypt the final round of your ciphertext using your key. Store this result and the subkey you used. Repeat for all 2^16 candidates for the final subkey. Make a guess for the first three ...



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