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There are many different types of encryption algorithms. They vary in complexity and how secure they are.

The Caesar cipher

A simple method of encryption requires the use of a technique known as the Caesar cipher. The technique was invented by Julius Caesar who lived from 100BC to 44BC. The cipher works by giving a number value to a key. Each plaintext letter is replaced by a new letter - the one found at the original letter's position in the alphabet plus or minus the value of the key.

For example, a key value of +three would change the plaintext message “see you tonight” to the ciphertext message “vhh brx wrqljkw”.

To decrypt the message, the process is reversed.

Caesar ciphers are often shown using a Caesar cipher wheel diagram, to illustrate that when the end of the alphabet is reached it goes around again.

Today, much more complicated algorithms are used to encrypt messages. The Caesar cipher is very easy to decrypt without the key as it is so simple.

Asymmetric encryption - public and private keys

Encryption is of little use if unauthorised users know the key. They can use it to decrypt any message that is encrypted with that key.

One way around this issue is to use an algorithm that generates two keys - a public key and a private key.

A public key can be given to anyone. Anyone can then use this key to encrypt a message. However, the public key cannot decrypt a message - only the second, private key can do that. So long as the private key is never given out, messages will stay safely encrypted.

An encryption method that uses public and private keys in this way is known as asymmetric encryption. There are a number of different asymmetric encryption algorithms that are in use.

Encryption methods that use the same key for encryption and decryption are known as symmetric encryption methods.