Method
There must exist a shared secret between the sender and receiver, known
as the key.
The sender encrypts a plain text using the secret key, and sends the
cypertext.
The receiver decrypts the cyphertext using the secret key back into
the plaintext.
Problem
How to distribute the keys without interception? Especially for
people with no previous interaction.
Standard
The DES is the standard, used by the government and the banking industry.
Has VERY questionable past.