(a) Using Fermat's theorem, find 3²⁰¹ mod 11.

(b) Explain how the Diffie-Hellman key agreement protocol works and what its purpose and main properties are.

Consider a Diffie-Hellman scheme with a common prime q = 11 and a primitive root α = 2.

(i) If user A has public key YA = 9, what is A's private key XA?

(ii) If user B has public key YB = 3, what is the shared secret key K, shared with A?

(c) Briefly explain the Output Feedback (OFB) mode of operation stating its advantages, disadvantages and uses.