Consider a Feistel cipher with four rounds. Then the plaintext is denoted as P = (Lo, -Ro) and the corresponding ciphertext is C = (L4, R4). What is the ciphertext C, in terms of Lo, RQ, and the subkey, for each of the following round functions?

a. F(R_{Ì-I},K_{Ì}) = 0

b. F(R_{i-1},K_{i}) = R_{i-1}

c. F(R_{i-1},K_{i}) = K_{i}

d. F(R_{i-1},K_{i}) = R_{i-1}®K_{i}

1 This problem deals with a Feistel Cipher.

a. Give the definition of a Feistel Cipher.

b. Is DES a Feistel Cipher?

c. Is AES a Feistel Cipher?

d. Why is the Tiny Encryption Algorithm, TEA, “almost” a Feistel Cipher?

2 Within a single round, DES employs both confusion and diffusion.

a. Give one source of confusion within a DES round.

b. Give one source of diffusion within a DES round.

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