[SOLVED] CS COMP3334 Computer Systems Security 2021/22 Semester 2

COMP3334 Computer Systems Security 2021/22 Semester 2
Tutorial 2 Data Encryption Standard & Mode of Operation
Question 1 Parity bits
Write down the value of all parity bits of below 64-bit DES key.

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0101 011_ 1100 111_ 0000 000_ 0111 110_ 1010 111_ 0001 000_ 1011 101_ 0010 100_
Question 2 S-Boxes of DES
Given the following 48-bit string. 101000001000011110111110001111001010101111011010
Write down the output bit string after passing through the 8 S-boxes. You may use the DES Demo program or SBox.jpg in Blackboard to help.

COMP3334 Computer Systems Security 2021/22 Semester 2
Question 3 DES Encryption and Decryption
In DES, both the encryption and decryption use the same round operation. Suppose a simplified version of DES requires one round of operation only. It can be depicted as below:
Prove that, by applying the same operation on R1L1 with the sub-key K0 during decryption, it will return L0R0.

COMP3334 Computer Systems Security 2021/22 Semester 2
Question 4 Mode of Operation
The encryption scheme used in the following example is monoalphabetic, in which we restrict the possible letter to A P. The key k = BCDEFGHIJKLMNOPA. In other words, for example, A will be encrypted to B. Suppose A P is represented as 0000 1111, respectively. Take all of the blocks as size of 4 bits, i.e., letters are encrypted one by one. Given plaintext IAMBOB, using the following mode of operations, what should be the ciphertext? Assume the initialization vector is C and the initial counter is 1.
a) Cipher Block Chaining (CBC)
b) Cipher Feedback (CFB)
c) Output Feedback (OFB)
d) Counter

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