PGP

PGP provides a mechanism for two people to communication over an insecure channel.

Assumptions:

Here are the steps.

The sender writes the message.
The sender uses MD5 to calculate a 128 bit digital signature for the message.
This is encrypted using the sender's private key and appended to the message.
The sender generates a one time session key and uses this to encrypt the message and the encrypted digital signature using IDEA, a symmetrical cipher.
The sender encrypts the one time session key using the Receiver's public key and appends this to the message.
The sender sends the message, the Receiver gets it
The Receiver decrypts the one time session key using his private key.
The Receiver decrypts the message with IDEA using the session key
The Receiver decrypts the digital signature using the Sender's public key.
The Receiver uses MD5 to calaculate the digital signature, and compares the result to the decrypted signature sent with the message. If they agree, the Receiver has confirmed that the message did in fact come from the sender and that it was not altered in route.