Question

Assume we encrypt a one “block” message (16 bytes) with a 128 bit OTP key. How long will it take to decipher the message if we are doing 12 x 10^16 XORs per second?

Answer #1

Size of message = 16 bytes

But 1 byte = 8 bits

Size of message in bits = 16 x 8 = 128 bits

Size of key = 128 bits

In OTP, we XOR every message bit with its corresponding key bit

It is given that we are doing 12 x 10^16 XOR operations per second.

Time taken by 1 XOR operation is :

Time taken to try 1 key is :

Now in order to decipher the message we need to try every possible key

Number of possible keys = 2^128

Time taken is :

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