3. (a) Suppose an attacker has intercepted a ciphertext message that consists of 5 unique letters of the alphabet with 3

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3 A Suppose An Attacker Has Intercepted A Ciphertext Message That Consists Of 5 Unique Letters Of The Alphabet With 3 1 (53.44 KiB) Viewed 37 times

3. (a) Suppose an attacker has intercepted a ciphertext message that consists of 5 unique letters of the alphabet with 33 letters. If an attacker knows that the alphabet has 33 letters, and the cipher used was a substitution cipher, how many different plaintext could there be that would produce this ciphertext message they have intercepted? (3 marks] (b) Consider a Diffic-Hellman key exchange with p = 11,9 = 7. x1 = 3 and Xx = 4. Without using a calculator, compute X, Xaand the secret keys for Alice and Bob, showing details of all calculations 18 marks (e) A financial company allows its clients to transfer money between accounts in the company using messages that consist of three parts, M = M.M.Ms where M specifies the number of the account. from which the money is being transferred, M is the number of the account, into which the money is being transferred, and M shows the actual amount of money being transferred. The messages are encrypted with an AES algorithm operating in the Counter mode, i.e. K-Ex(IV). C = MOK Each message contains three blocks. M. Mo, Ms. which all contain sixteen charac ters Turn over! G1032 MATHEMATICS. CRYPTOGRAPHY () Suppose, an attacker also has an account in this financial company and is able to intercept and modify messages. Imagine now they know the number of the account, into which the money is going. Mg, for a particular message M = CCC that they have intercepted. Explain how they can modify this message so that the amount of money specified in M, is transferred to their own account. [6 marks] (ii) Explain how the use of a MAC would prevent this attack. [4 marks (iii) Above, Ex(M) denotes using a block cipher E with the key K on the message M. It is possible to define a cipher using similar ideas, but using a hash function instead. Describe how to do it, and discuss how the decryption is done. (4 marks (d) A plaintext message M is encrypted using a simple transposition cipher. The period is 6, and the encryption key is the permutation P = (16)(243). The resulting cipher- text is C=ERO SANY BA.EOT.HYENA MR Here each underscore denotes a space between characters. What is the permutation Pl? Give your answer in cycle notation. [2 marks] (ii) Find the plaintext M by decrypting Cusing P! 13 marks