Hello there, I need some help on this cryptanalysis related problem.
A simple 3-round substitution permutation network cipher is shown in Figure 1.
The cipher operates on 8-bit blocks. The 8-bit plaintext block P is XOR-ed bitwise with the 8-bit key K1 before the resulting 8-bit block enters the two first round S- boxes. The remaining key mixing operations are handled similarly.
A substitution box (S-box) is shown in Figure 2. This S-box is used throughout the cipher shown in Figure 1, all 3 S-boxes are identical.
The permutation part of the first two rounds is as shown in Figure 1. The final (third) round does not implement any permutation; the outputs from the final round S-boxes are simply XOR-ed bitwise with the key K4 to produce ciphertext C .
256 plaintext-ciphertext (PC) pairs have been generated using the 3-round cipher and four secret keys (K1, K2, K3, K4). The 256 PC pairs are given in the file PtoC.txt attached below. Plaintexts and ciphertexts are given as integers with the natural binary interpretation. For example the integer 5 represents the 8-bit block 00000101.
The Question: Use Differential Cryptanalysis to recover the final round key K4.
i. develop one or more suitable 2-round differential approximations involving bits of the plaintexts P and bits of intermediate ciphertexts U3 (as shown in Figure 1).
ii. indicate any active S-boxes in your approximations and their biases. Indicate any tables (or other sources) you have used to calculate the biases
iii. give the absolute value of the bias of any 2-round approximation derived above and show how all such biases were calculated.
iv. Explain your specific choice of approximation(s).
Order an Essay Now & Get These Features For Free: