Question

Consider the following protocol that uses a secure public-key encryption scheme E_pk(m), a digital signature scheme Sig_sk(m), and a secure symmetric key encryption scheme E_k(m). Suppose that A has a private key skA and a public key pkA. Suppose that B has a private key skB and a public key pkB. Assume both A and B knows each other's true public key. They both keep their own private keys to themselves. A wants to send B an arbitrary message M. A selects a random secret key K and sends B the following two messages (over an insecure channel):

E_pkB(K || Sig SKA(K))
E_K(M)
For the following statements, state whether each of them is true or false. Briefly explain why (on your answer sheet).
Only B can decipher the contents of the message M, except A (who already knows M).
a. True
b. False

Answer 1

The statement is true because only B can decipher the contents of the message M.

Step-by-step explanation:

The statement is true.

Only B can decipher the contents of the message M, except A (who already knows M). The message M is encrypted using the symmetric key K, which is generated by A and encrypted using B's public key. Since B knows the corresponding private key for his public key, he can decrypt the encrypted symmetric key and then use it to decrypt the message M. A cannot decipher the contents of the message M because he doesn't have access to B's private key.