Transposition Cipher

Why do this problem?

This problem makes use of students' knowledge about factors and multiples, presented in an intriguing context using the 'hook' of codebreaking.

Possible approach

This problem could follow on from Substitution Cipher where students are introduced to frequency analysis.

Start by performing a frequency analysis on this text:
 

ttanopnshonstpdeendoaiherltsmnemaihuogrebkedmhsdbendeeetiadenrlottin
tsfbhupltefeonpyolaalnettflveedhhblewlsaieirefutnfnynodakogdtrdlarde
sseibeoetoncoswprmleuhnwaeyhteweiwdasfhlgaodtoalhywnoutx

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.

Key questions

Why might you choose a 48 character message rather than a 44 character one?
 

Possible extension

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

Possible support

Encourage students to work collaboratively. Working on squared paper is really helpful.