How did you get on with the Vigenere cipher this week? It is a particularly tricky cipher when you first meet it since standard frequency analysis doesn't help much. Since the cipher alphabet changes from letter to letter you need to analyse frequencies of longer strings - digraphs and so on - and the best way to tackle it is to figure out the repeat length by calculating the index of coincidence. I'll leave you to look that up. The first real attack on that cipher was by Babbage, inventor of the difference engine and father of computing. You can see examples of his work at the science Museum in Kensington. A Polish cryptographer is also credited with the same method of attack (Babbage-Kasiski deciphering), though Babbage was definitely first. On the other hand the Poles were the first to make a serious break on the Enigma cipher and without their input Bletchley would have had a much harder job in the war.
Each of the following 8 teams will win a cheque for £25 for their entry to Challenge 5A. We will be in touch by email to the address you gave us when registering, so do please check it and get back to us promptly so we can process your award.
To everyone else, there is always next week. Good luck,
Harry
Winners of the Part 5A Prizes:
Nixon S Joe L Peter S Grzegorz P from Cardinal Newman College
Amy Beswick Emma Williams from Fairfield High School For Girls
Roddis' Angels from Queen's Elizabeth's High School
Graz H Langer from Runshaw College
Norm Yeung from Magdalen College school
Nina Simister from Dorothy Stringer
Thomas Bodley, Humayun Khan, Praveenan Mathew and Danial Sheikh from St. Joseph's College
Miranda and Jessie from City of London School for Girls
