Question

A particular cipher is implemented by combining the ASCII representation of plaintext characters with pseudorandom bytes (eight-bit binary strings of 1s and 0s) using the XOR function. In the process of encrypting a message, a character in the plaintext, a capital X, is XORed with the pseudorandom byte 10110100.

a. What is the ciphertext (in binary form) generated by the encryption of the character ‘M’? (Please show your work.)

b. How is the plaintext for this encrypted J recovered? (Please show your work.)

Answer 1

ANSWER:

XOR Table

Input: 0,0 Output: 0

Input: 0,1 Output: 1

Input: 1,0 Output: 1

Input: 1,1 Output: 0

Pseudorandom byte: 10110100

ASCII of ‘X’: 01011000

Now performing XOR between X and pseudorandom, we get-

Cipher Text: 11101100

a) ASCII of ‘M’: 01001101

Pseudorandom byte: 10110100

Now performing XOR between M and pseudorandom, we get-

Cipher Text: 11111001

b) ASCII of ‘J’: 01001010

Pseudorandom byte: 10110100

Note that ASCII of J is encrypted text as given in question, so to get plain text we perform XOR-

Plain Text: 11111110

SUMMARY:

For every question we need to get the ASCII of the specified character. The pseudorandom byte is already given in the question. So to find cipher text we just need to XOR the ASCII of character and the pseudorandom byte. Similarly XOR to calculate the plain text.

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