Encryption is the process of disguising information to make it unreadable by those who lack the tools to decipher it. This process has been used for centuries to send secret messages. It has been traced back to the Romans and has been actively used by the military ever since. Encryption has become increasingly important as a way to keep information sent over the Internet private.
One common encryption system substitutes numbers for letters. This substitution procedure is called a key. Normally, the sender and the receiver agree on a key, and this key remains secret. Only those who know the key can encode and decode messages. This is called symmetric encryption, since messages use a single key for both encryption and decryption.
Another encryption method, called asymmetric encryption, has become popular for online transactions - you use it every time you check your bank account online or do online shopping. Asymmetric encryption uses a separate pair of keys for each user: a private key known only to the receiver and a public key known by everybody. A sender encrypts messages using the receiver's public key, but only the receiver can decipher it with her private key. Although it sounds more complicated, the principle is the same: a secret key is required to be able to understand the message.
- Encryption (Wikipedia)