Yogi wants to know about file encryption. He encrypted a file and then wanted to take the key off and put it on a USB key, but he can't find it. Leo says that the certificate is the key. If he can find the certificate, he can copy it. If he were to copy the file without the certificate, no one would be able to get to that file. The idea is that he's encrypting the file so that it can't be opened by anyone who isn't himself, and the way he can prove his identity in this case is by logging into his system. Someone would have to have both his login and the password to access that file.