Time for a little mathemagic! Choose any five cards from a pack and show four of them to your partner. How can they work out the fifth?
Find the frequency distribution for ordinary English, and use it to help you crack the code.
How can Agent X transmit data on a faulty line and be sure that her message will get through?
This problem makes use of students' knowledge about factors and multiples, presented in an intriguing context using the 'hook' of codebreaking.
This problem could follow on from Substitution Cipher where students are introduced to frequency analysis.
Start by performing a frequency analysis on this text:
It is available here as a text file.
Invite the class to comment on what they notice. As they realise that the distribution of letters matches that of ordinary English, invite suggestions as to how the text might have been encoded if not by substitution.
Show the "Peter Rabbit" example from the problem, showing how to encode text using a transposition cipher. This set of PowerPoint slides describes each step of the encryption and decryption process.
Then hand out this worksheet with the text above, and invite students to decode it.
Students could then create their own transposition ciphers and challenge the rest of the class to decipher them.
If computers are available, students might want to use our Cipher Challenge Toolkit which contains a transposition solver.
Why might you choose a 48 character message rather than a 44 character one?
The Secondary Cipher Challenge and Substitution Transposed offer challenging extensions for students who have worked on this problem and the problem Substitution Cipher.
Encourage students to work collaboratively. Working on squared paper is really helpful.