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I am trying to understand how differential Cryptanalysis works. Here I read how to use differential cryptoanalysis for cracking FEAL4 cipher.

The big gotcha with this process is that you cannot just reuse the above differential path to crack rounds 2 and 3. You must build new differential characteristics that only cover the uncracked rounds.

As I understood you cannot use the same differential for cracking all keys. For each round key, you need to use new differential. Could you explain why?

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What are the differentials at the start of round 2 compared to the differentials you started off with? Note the remark about the first subkey being (temporarily) out of reach due to the key whitening step. –  Thomas Mar 10 at 10:28
    
@Thomas Thanks for answer, but I did not get, what do you mean. Do you mean that differential 0x8080000080800000, which used for cracking subkey of last round, leads to the differential 0x02000000 with 100% probability? And so we cannot reveal subkey of round 2? But why we cannot use this differential to reveal subkey of round 3? –  neverwalkaloner Mar 10 at 12:08

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