Encryption is the process of disguising a message so that it cannot be understood by anyone but its intended recipient. Encryption requires the use of a key. The key is secret as to how the message has been disguised.

Five facts about encryption

Encryption does not prevent someone from intercepting a message - instead it prevents them from being able to understand it.

Unencrypted messages are referred to as plaintext messages. Encrypted messages are known as ciphertext.

A simple method of encryption requires the use of a technique known as the Caesar cipher. 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 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”.

A table containing plaintext and an example ciphertext

To decrypt the message, the process is reversed.

Although this is a good example of encryption, in real life much more complicated algorithms are used to encrypt messages.

Asymmetric encryption - public and private keys

Like all methods of disguise, encryption is of little use if unauthorised users know the key. They can use it to unlock 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. This method is known as asymetric encryption. 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 key (the private key) can do that. So long as the private key is never given out, messages will stay safely encrypted.

Online banking makes use of public and private keys. When a user logs on to their bank, they do so using a public key supplied by the bank. All communication with the bank is encrypted, and only the bank can decrypt the messages using the private key.

Encryption in use

Today, most communications sent via the internet are encrypted in some way:

  • purchases made online are encrypted to try to prevent theft of credit card details
  • tools enable a user to encrypt a document, such as a spreadsheet, before sending it to a colleague via the internet
  • satellite TV transmissions are encrypted to prevent users who are not subscribed from watching TV shows