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The Diffie Hellman algorithm is one that allows two parties to perform a key exchange over an insecure connection. The use a technique where it would be hard for a person watching the communications to reverse engineer the key they share. The end communication is a symmetric one, where the two parties share the same key.
Next let me briefly cover PGP. It stands for pretty good privacy. This is an encryption program that allows users to communicate via secure email. This is a system that uses asymmetric keys.
Finally there is also a newer topic in cryptography called elliptical curve cryptography (ECC). It also uses asymmetric keys. This technique is good for encrypting large amounts of data. I will not go into the mathematics behind this technique right now. Perhaps that is a research topic for another day. Next time I will go over some other mail encryption methods, and also discuss tunnelling.