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3DES encryption with independent keys $(K_1,K_2,K_3)$ transforms a 64-bit plaintext block $P$ into ciphertext $C=E_{K_3}(E_{K_2}(E_{K_1}(P)))$, where $E$ and $D$ are DES encryption and decryption. The basic Meet-in-the-Middle attack against this form of 3DES assumes 3 different known plaintext/ciphertext pairs $(P_i,C_i)$, and (theoretically) works as ...


The meet-in-the-middle attack on ciphers like $C=E^2_K(P)=E_{K1}(E_{K2}(P))$ works as follows. Let's assume you're given a known-plaintext pair $(P_1,C_1)$ and a known-plaintext pair $(P_2,C_2)$ You now build a list (by brute-force) containing the pairs $(I,K1)$ for every possible value of $K1$ ($2^{56}$ for DES) with $I=E_{K1}(P_1)$. Constructing this list ...

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