Stream Cipher is a kind of symmetric key encryption technique. The question concerns with the stream...

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Stream Cipher is a kind of symmetric key encryption technique. The question concerns with the stream cipher. (a) (b) Explain what is meant by Vernam's one-time Pad. In your answer state when the encryption is perfectly secret. (6 marks) Consider the following cryptosystem with Key Space K={A, B} with pr(A) = 2/3 and pr(B) = 1/3 Plain text space P = {0, 1} with pr(0) =3/5 and pr(1) = 2/5 Cipher Space C = {a, b} with EÃ(0) = a, EÃ(1) = b, E³(0) = b and E³ (1) = a where E (0) denotes the encryption function and pr(.) is the probability function. (i) Compute pr(a) and the conditional probability pr(0|a). (ii) (7 marks) Explain whether the system is a perfect cryptosystem. If not modify the probability distribution of keys so that the system is a perfect cryptosystem. State the reasons for your answer. (7 marks) Stream Cipher is a kind of symmetric key encryption technique. The question concerns with the stream cipher. (a) (b) Explain what is meant by Vernam's one-time Pad. In your answer state when the encryption is perfectly secret. (6 marks) Consider the following cryptosystem with Key Space K={A, B} with pr(A) = 2/3 and pr(B) = 1/3 Plain text space P = {0, 1} with pr(0) =3/5 and pr(1) = 2/5 Cipher Space C = {a, b} with EÃ(0) = a, EÃ(1) = b, E³(0) = b and E³ (1) = a where E (0) denotes the encryption function and pr(.) is the probability function. (i) Compute pr(a) and the conditional probability pr(0|a). (ii) (7 marks) Explain whether the system is a perfect cryptosystem. If not modify the probability distribution of keys so that the system is a perfect cryptosystem. State the reasons for your answer. (7 marks)