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CM30173: Example sheet 3
6th February
Questions 1 relates to lecture 4. Question 2 relates to lecture 5.
1. Take the SPN from lectures and replace the S-boxes with:
z 0 1 2 3 4 5 6 7 8 9 A B C D E F
πS(z) 7 D E 3 0 6 9 A 1 2 8 5 B C 4 F
For the resulting SPN:
(a) Find the row of the difference distribution table corresponding to input difference
1110
(b) Calculate the full difference distribution table (not by hand!)
(c) Describe a differential attack on this SPN, you should aim to find a differential trail
which has propagation ratio 81
4096
(I believe that there are a variety of these).
(d) Implement your attack (you will need to use your SPN implementation to encrypt
your chosen plaintext-ciphertext pairs).
2. Give the definition of a Feistel cipher. Explain why we do not require f to be injective
and describe the decryption algorithm.