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Substitution Cipher

Age 11 to 14
Challenge Level

Why do this problem?

This problem offers a statistical activity that has immediate practical application. We have offered a spreadsheet toolkit so that students can concentrate on the analysis of the data without needing to waste time on computation.


Possible approach

"Which letters do you think appear most often in the English language?"
"How could you find out?"

Allow some time for students to think about and share their answers. If they have books with them, perhaps suggest that they take a look to see which letters seem most common at first glance.

If a computer room is available, introduce students to the toolkit and give them time to perform a frequency analysis on some English text (Wikipedia articles are a great source for this). Share results from the frequency analysis. Does everybody find the same letters come out top, and bottom? There is opportunity here for some discussion about the benefits of using longer sample texts.

Then present students with the ciphertext in the problem (available as a text file here).

If a computer room is not available, the ciphertext is available as a worksheet here. Here is a second version of the worksheet with the ciphertext faint so that students can write over it as they go deciphering the message.


Key questions

Can you spot the vowels?

Are there any short words? What might they be?


Possible support

Students could be encouraged to work collaboratively on this problem. There are lots of suggestions to help them get started in the hint.


Possible extension

Students could investigate the frequency of digraphs (pairs of letters such as th or sh) in the English language and consider whether this speeds up the deciphering process.

The Secondary Cipher Challenge and Substitution Transposed offer challenging extensions for students who have worked on this problem and the problem Transposition Cipher.