What are Public and Private Keys?
Both PGP and S/MIME use the RSA cipher system to exchange session keys. RSA used two keys, a public key your friends have and which encrypts email to you, and a private key only you keep and which decrypts email to you. These keys are numbers, around 128 bytes long. You public key is often appended to emails you send, or sent out to other people in PKCS7 certificates. You private key is, for both PGP and S/MIME, protected by a passphrase, or password, which you need to type in once a session in order to decrypt email.
