Substitution ciphers attempt to make messages unreadable by swapping individual letters within the message for other letters. This is a form of encryption that allows any message to be coded and sent using traditional communication methods such as handwriting, e-mail or text message. The Enigma machine, used by the German forces during World War II, used a system based on substitution.
Shift ciphers (also known as Caesar shift ciphers) work by swapping a letter for another one by moving along the alphabet by a fixed number of places. For example, if you shift by 1, then A would become B, B would become C, C would become D, etc. If you shift by 2, then A would become C, B would become D, etc.
Other types of monoalphabetic substitution rely on swapping pairs of letters, but without a pattern, so that A might become J, while B might become X, for example. Messages encrypted in this way are harder to decipher because there are more combinations to try. You can shortcut cracking the code by using frequency analysis - we know, for example, that the most common letter in the English language is e, so the most common letter in the ciphered message was probably substituted for an e.
When working with shift ciphers, type the message into the top box and choose the number of letters by which you'd like to shift. If you are deciphering a message, tick the Decipher checkbox.
When using frequency analysis, the page will show the frequency of each letter in the message in the box on the right. When you click Analyse, it will suggest a set of substitutions based on the frequency of the letters in the message, but you can change the substitutions by clicking on a letter in the right column and selecting an alternative from the list.
You can watch a video on how this page can be used to decrypt (i.e. crack) a message on YouTube. If this topic interests you, why not try the following quiz: Do you have the brains for cybersecurity? (Answers)