Alice and Bob would like to use an asymmetric cryptosystem to communicate with each other. They are located in different parts of the country but have exchanged encryption keys by using digital certificates signed by a mutually trusted certificate authority. Which one of the following keys would Bob not possess in this scenario?

Question $4$ options:

$1)$

Alice's public key

$2)$

Alice's private key

$3)$

Bob's public key

$4)$

Bob's private key