Question

Suppose in order to get a message across enemy lines, we need to choose an initial secret key say, 100110. What will be the decrypted cipher when the original message is 010010011.

(a) 110100

(b) 1001011

(c) 0011010

(d) 011011

I got this question in examination.

I want to ask this question from Cryptography in portion Number Theory and Cryptography of Discrete Mathematics

Answer 1

Correct choice is (a) 110100

For explanation: In order to decrypt the given cipher text, we need to XOR each bit with the corresponding bit in the secret key. As, the cipher text is longer than the secret key, wrap around the remaining cipher text and begin again at the starting. When there will be a chance to decrypt the 7^th bit of the cipher text, XOR it with the first bit of the secret key. Hence, after decrypting every bit in the cipher text, the original message will be 110100.