Question 1
Security and Networks Example Questions
Assume the account number is contained in the first block of a message. Assume CBC- mode is used for encryption. Is it possible for an active attacker to change the account
number? Either describe an attack, or explain why no attack exists.
Question 2
Consider the following protocol:
A¡úB:A B ¡ú A:NA
A ¡ú B : {NA}Kab , {Pay Elvis ¡ê5}Kab
[5 marks]
where NA is a nonce and Kab is a symmetric key known only to Alice and Bob. Is this protocol secure? If yes, explain why. If not, give an attack in Alice-Bob notation.
[5 marks]
2