Copyright © University of Cambridge. All rights reserved.

'Code Breaker' printed from

Show menu

Many modern codes are based on two very large prime numbers multiplied together.

This problem is based on a code using two different prime numbers less than 10. These two primes have been multiplied together and the resulting number has been used to shift the alphabet forward to new letters, assuming that A is at position 1, B at position 2 etc. For example, if the two prime numbers were 2 and 3, then to make the code, the alphabet would be shifted forward by 6 places. A would become G, B shifts to H and so on.

Which way will you need to shift the letters to decode?

When you have deciphered the code, there will be one word which will remain coded. You can decipher this word by adding the two prime numbers together and shifting the letters again.

Can you find the doubly coded word in this sentence?


You may want to use this interactive code breaker to help you. Shift the red letters by clicking on the red F (forwards) and B (backwards) and then you'll need to use the blue letters in the same way to decipher the doubly coded word.

If you can see this message Flash may not be working in your browser
Please see to enable it.