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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