1. Introduction to Caesar Cipher

Encryption has been popular for thousands of years. But without a computer or calculator, the encryption method has to be simple enough for a person to do by hand.

The Caesar Cipher in an encryption method that has been around since the days of the Roman Empire.

To encrypt a message, each symbol (letter) is swapped for another letter by a defined offset either upwards or downwards within the alphabet.

The encryption key, then, is the number of shifts to be used.

For example, the letter a becomes d if the key is +3, or it becomes x if the shift is -3. Shifts wrap around if it goes above z or below a.

To decrypt, the cipher is shifted in reverse to recover the original plaintext.

For example the plaintext 'Hello World' with a key of +3 becomes 'Khoor Zruog'

For the Edexcel syllabus, you need to be able to set this out in pseudocode or a flowchart. You may also need to code it in your chosen programming language.