To authenticate the receiver $($B$),$ the sender $($A$)$ e$-$mails a challenge message to B$.$ B will use her $\_\_\_\_\_\_$

key to encrypt the challenge message and send it to A$.$ If A is able to use B$\text{'}$s $\_\_\_\_\_\_\_$

key to decrypt and get the plaintext of the challenge message, A has authenticated B successfully.