• A cryptographic system that uses two keys -- a public key known to everyone and a private or secret key known only to the recipient of the message.
• When John wants to send a secure message to Jane, he uses Jane's public key to encrypt the message. Jane then uses her private key to decrypt it.
• Known as PGP - Pretty Good Privacy system.
• Public key cryptography was invented in 1976 by Whitfield Diffie and Martin Hellman.

Important:

• only the public key can be used to encrypt messages and only the corresponding private key can be used to decrypt them.
• it is virtually impossible to deduce the private key if you know the public key.

Problem:

• Must know the recipient's public key to send them something encrypted.
• Hence need some kind of public registry where people can register their public keys...otherwise they have to send it to you.