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Weighting This piece of summative coursework is worth 25% of the module grade.

The Feistel cipher is a symmetric block cipher encryption framework which is the basis of many modern day encryption algorithms.   In this coursework you will implement a Feistel cipher system as a hardware component and as a software implementation.  In a Feistel cipher the plaintext, P , to be encrypted is split into two equal size parts L0  and R0  such that P = L0 R0 .  A function F is applied to one half of the plaintext, combined with a key, and the result is XOR’d with the other half of the plaintext.  Feistel ciphers often employ multiple rounds of this scheme. In general the scheme works as follows, for all i = 0, . . . , n,

Li+1  = Ri

Ri+1  = Li - F (Ri , Ki )

To decrypt an encrypted message using this cipher we can apply the same procedure in reverse. For i = n, n i 1, . . . , 0,

Ri  = Li+1

Li  = Ri+1 - F (Li+1, Ki )

For this coursework we are interested in the 16-bit Feistel cipher which uses 4 rounds. The function F (A, B) = A - B . The keys are derived from a single 8-bit key K0  such that,

K0  = b7 b6 b5 b4 b3 b2 b1 b0

K1  = b6 b5 b4 b3 b2 b1 b0 b7

K2  = b5 b4 b3 b2 b1 b0 b7 b6

K3  = b4 b3 b2 b1 b0 b7 b6 b5