So he describes a problem hes actually working through when he was "interrupted" to come interview me. He was trying to terminate an SSL connection, modify the stream (for video manipulation) and send it along to the client. I explained public key and how what he wanted to do was called a MitM attack and how he couldn't continue on the stream without the private key of the server. We had a real conversation with me explaining (to a very smart guy) so he could completely understand everything he "needed" to know to solve the problem he was working on.
I got the job. It was the only interview I've had where I really felt they were asking me something relevant to the work and the kind of work I would be required to do. I ended up having to write Symbian OS code, which I had no prior experience in.
TBH most of the comments so far are the interview questions I hate as they have no relevance to anything I care about or will ever be asked to do.