I am chatting with my friend via Skype chat and I want to know whether it is really my friend Bob on the other side. I have Bob's public key and I am 100% sure that this is real Bob's public key. How can I verify that this is really Bob who is chatting with me?
a] I will ask the guy to send me a file signed by him. Then I will verify the signature.
b] I will send the guy a file signed by real Bob's public key and ask him to verify the file.
c] I will send the guy a file and ask him to sign it. Then I will verify his signature.
I think option A] makes sense. Am I correct? This is just a hypothetical situation.