*Ciphers are a great way to play with numbers and arithmetic. They are also a way to explore data representation, and an important part of computational thinking.*

The earliest ciphers were simple **substitution ciphers**, where letters were swapped for different letters or symbols. If we **represent** letters as numbers we can use maths to create ciphers. They are much more flexible. We can easily turn letters into numbers using the following table, for example. This is just a simple version of what character encodings like ASCII and UNICODE do.

We can encrypt whole messages using a maths-based cipher based on this. Here is a simple maths-based cipher. To encrypt a letter:

**STEP 1. **Replace it by its equivalent number.

**STEP 2.** Add 10 to the number.

**STEP 3.** If the result is greater than or equal to 26, then subtract 26.

The last step uses **clock arithmetic**. It works like the hands on a big clock that, here, has 26 hours instead of just 12. When we get to 26, we go back to 0 just like a clock. It disguises patterns.

Let’s encrypt an example word:

CIPHER

It is encrypted by each step as follows:

**STEP 1:** 2-8-15-7-4-17

**STEP 2:** 12-18-25-17-14-27

**STEP 3:** 12-18-25-17-14-1

Representing letters as numbers allows us to make complex ciphers that are hard to crack.

**Things to try**

- Encrypt the following message using this cipher.

I AM A PRISONER

- Write your own message and then encrypt it using the cipher
- Work out the steps to decrypt a message using the cipher. Check it works by decrypting the messages encrypted so far.
- Invent your own number-based cipher. Use clock arithmetic to always end up with a number between 0 and 25. You need a way to decrypt messages too.

**Activity Sheets**

**Puzzle sheet:**Number Based Ciphers Puzzle Sheet [PDF]