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5

Did you try Wikipedia? DES consists of 16 rounds of the form: $$L_{i+1} = R_{i}, \quad R_{i+1} = L_i \oplus F(R_i, K_i),$$ which are identical except for the round subkeys $K_i$. (The last round is slightly different, in that the half-blocks $L$ and $R$ are not swapped as they are after all other rounds, but that makes no cryptanalytic difference.) The ...

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Each half of the key is 28 bits long, so there will be $2^{28}$ possible choices for each of them. In the first part of your attack, you start with the known block of plaintext and encrypt it for the first 8 rounds using each possible left half of the key. This gives you $2^{28}$ "half-encrypted" 64-bit blocks. This is less than the birthday bound, so ...

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CD CD is collectively referring to the C and D Registers. Their use and operation while simultaneous is independent. C and D are concatenated together to specify PC2. Permuted Choice 2 PC2 is a selection permutation. You might notice that the first 24 bits of the selected Key are from the C Register (CD(1 to 28)) and the second 24 bits are from the D ...

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