Symmetric Key Ciphers

Source: RSA and Information Security and Privacy in Network Environments OTA-TCT-606.

Traditional cryptography is based on the sender and receiver of a message knowing and using the same secret key: the sender uses the secret key to encrypt the message, and the receiver uses the same secret key to decrypt the message.

What is the problem with this? How do I set up a communications link with someone in California?

Joseph M. Reagle