Word Count : 642

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We use the Trappe & Washington simplified-DES (toy) specification: 12-bit block split into L (6 bits) | R (6 bits); a 9-bit key produces four 8-bit round keys by left-rotating the 9-bit key and taking the leftmost 8 bits for k1..k4; the round F expands a 6-bit input r = r1 r2 r3 r4 r5 r6 to 8 bits by E(r)= r1 r2 r1 r3 r4 r3 r5 r6, XORs with the 8-bit round key, splits to two 4-bit halves which feed S1 and S2 (each 4→3 bits: first bit = row, next 3 bits = column), then concatenates the two 3-bit S outputs to a 6-bit f(R,k). Decryption runs the Feistel rounds with the round keys in reverse order; each decryption round maps (L,R) → (R XOR f(L,k), L). (S-box tables, expander and key schedule per the toy cipher spec.) Given: * Ciphertext C = 101000111001 → L₄ = 101000 , R₄ = 111001. * Key K = 101010010 ___ ____ ______ _________ ____.
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