Question 4 (10 marks). Consider security of TLS. (a) (3 marks) Explain the three methods defined for key exchange in TLS and determine their corresponding pre-master secret for each case. b) (2 marks) If ServerKeyExchange is not sent, what is their shared pre-master secret? (c) (3 marks) Two TLS cipher suites are given below TLS DHE DSSWITHAES256CBCSHA256 TLS ECDHE RSAWITHAES256 CBCSHA1 Provide the definitions for those cipher suites (i.e., for key exchange, encryption and MAC) and determine modulo's sizes in the public-key schemes in order to match AES-256 security strength. (Hint. Use the data in Table 1 in Formulae Sheet). What is the overall security strength for each cipher suite? Comments your results. (d) (2 marks) For protecting application data encryption is AES-256 in CBC mode, and MAC is AES-256 CBC-MAC (see CBC-MAC in Formulae Sheet). Assume that CBC-MAC is modified so that a random IV is used each time a tag is computed, and tag - (to, te) where data P if the key is not changed, knowing one pair (P, tag), an attacker can forge 2256 valid tags. 1,... ,pe) where p,'s are 256 bits. If encryption in TLS is removed, show that Question 4 (10 marks). Consider security of TLS. (a) (3 marks) Explain the three methods defined for key exchange in TLS and determine their corresponding pre-master secret for each case. b) (2 marks) If ServerKeyExchange is not sent, what is their shared pre-master secret? (c) (3 marks) Two TLS cipher suites are given below TLS DHE DSSWITHAES256CBCSHA256 TLS ECDHE RSAWITHAES256 CBCSHA1 Provide the definitions for those cipher suites (i.e., for key exchange, encryption and MAC) and determine modulo's sizes in the public-key schemes in order to match AES-256 security strength. (Hint. Use the data in Table 1 in Formulae Sheet). What is the overall security strength for each cipher suite? Comments your results. (d) (2 marks) For protecting application data encryption is AES-256 in CBC mode, and MAC is AES-256 CBC-MAC (see CBC-MAC in Formulae Sheet). Assume that CBC-MAC is modified so that a random IV is used each time a tag is computed, and tag - (to, te) where data P if the key is not changed, knowing one pair (P, tag), an attacker can forge 2256 valid tags. 1,... ,pe) where p,'s are 256 bits. If encryption in TLS is removed, show that