Perse Studio

Independent-learning super-curriculum projects: reading, research and ideas shared by Perse students

Cracking the Playfair Cipher : Pratap S, Year 10

Last year, I participated* in the 11th National Cipher Challenge (http://www.maths.soton.ac.uk/cipher2013/), an annual competition organized by the University of Southampton for school students in the UK. Every week over the course of the Michaelmas term, competitors are given pieces of text encrypted with an unknown cipher, to decrypt as fast and as accurately as possible. Each year the hardest challenge is the very last, challenge 8B; this year’s challenge 8B was encrypted using a Playfair cipher. There had been several clues along the way to tell us that it would likely be the Playfair, which allowed us to prepare before the challenge was released. However, that still left the daunting task of working out a way to decrypt Playfair-encrypted text without knowing the key.

The Playfair cipher was devised in 1854 by Charles Wheatstone. It puts the alphabet into a 5×5 grid (‘J’ and ‘I’ occupy the same square). The arrangement of the letters in this grid is the key of the cipher – the sender and receiver of the message both have to know this key.

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Pratap S, Year 10

* Note from teacher: Pratap modestly writes ‘participated’ but in fact he was the joint winner of the Trinity College Prize (awarded to the runner-up in the individual competition) – a hugely impressive feat. This excellent article explains how he approach cracking the most difficult challenge.

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