3. The Simple Substitution Cipher is a major improvement on the Caesar Cipher.

(a) in what different ways is it an improvement?

If a Simple Substitution Cipher is used (with an unknown key) and we intercept the cipher text OXAO then which of the following four-letter words could be the plaintext: JOHN, SKID, SPAS, LOOT, PLOP, OSLO?

If a Simple Substitution Cipher is used (with an unknown key) and we intercept the cipher text BRKKLK then what do, we know about the plaintext?

Suppose that in the case of intercepting BRKKLK we also know that the plaintext is the name of a country.

What is the plaintext? What important lesson does the BRKKLK example illustrate?

6 .The ciphertext MBR OJFGA SWNTE CNK QJDIL NURW MBR XHMR was obtained by encrypting the plaintext THE QUICK BROWN FOX JUMPS OVER THE GATE using a Simple Substitution Cipher.

(a) How much of the key is determined if this plaintext/ciphertext pair are known?

(b) How many different possible keys could have been used to encrypt the plaintext?

(c) Decrypt the ciphertext MBR TRHLRP WHE HTHV CWND PNEYNE ZNN, which is known to have been encrypted with the same key.

(d) What lesson can be learnt from conducting this exercise?

9. Alice wishes to regularly send Bob a plaintext message P1 or P2. On each occasion she chooses to send either P1 or P2, but on average she chooses the plaintext P1 twice as often as she chooses the plaintext P2 . Each time, Alice uses a (very simple) symmetric cryptosystem, with the same fixed key K, to encrypt the plaintext. When she chooses P1, the ciphertext is C1 = EK(P1); when she choosesP2,the ciphertext isC2 =EK(P2).Suppose that an attacker knows that the only possible plaintext messages are P1 and P2.

(a) Suppose that the attacker does not know that Alice chooses P1 twice as often as P2. What observation will the attacker, who can only see the ciphertexts sent from Alice to Bob, make?

(b) Suppose that the attacker learns that Alice chooses P1 twice as often as she chooses P2. What does the attacker now learn?

(c) Explain how homophonic encoding can be used in this case to make it more difficult for the attacker to learn anything useful from observing ciphertexts.