You have to implement encryption and decryption with Modi ed Simpli ed DES, as discussed in the class and the

textbook. The permutations IP, P10, P8, and SW, and the functions fk; F, and the S-box S1 are all as described in

the textbook, and can be hardwired into your program. For parts (a) and (b), you use the original S0 box as described

in Stallings 3rd edition. However, for part (c), you have to use a modi ed S-box S0′. In the modi ed S0′,

the rows 0 and 2 are the same as described for the original S0, but the rows 1 and 3 have been switched. So row 1 is

3; 1; 3; 2, and row 3 is 3; 2; 1; 0. Your program should:

take as input a 8-bit block of plaintext and a 10-bit key.

Show the following output (please only print what is being asked for, and nothing else):

(a) the intermediate result after the SW operation while encypting.

(b) the ciphertext.

(c) the intermediate result after the SW operation while decrypting.

(d) the result of the decryption process.

You have to run your programs on the following inputs:

(a) with the original S0: the example from the textbook i.e. the plaintext is 10111101 and the key is 1010000010.

In this case we know the ciphertext should come out to be 01110101, so this is a good way to check that your

program is performing correctly on this input.

(b) with the original S0: the plaintext is 11001110 and the key is 1001100101.

(c) with the modi ed S0′: the plaintext is 00100101 and the key is 1001011001.

Please note that you do not have to actually implement these operations as bit operations. For example, you can store the plaintext as an array of integers.

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