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Alan Turing’s Cryptographic Legacy

By Keith M. Martin


I’ve always been intrigued by the appeal of cryptography. In its most intuitive form, cryptography is the study of techniques for making a message unreadable to anyone other than the intended recipient. Why is that so intrinsically interesting to so many people?

The answer has at least something to do with our natural human curiosity. We have a fascination for puzzles and mysteries. We love secrets. Cryptography uses secrets to transform messages into puzzles which can then only be solved by anyone else sharing the original secret. To everyone else the puzzle remains a mystery. How wonderful is that?

Cryptography is, however, a deadly serious game. For centuries cryptography has been a tool deployed in times of conflict to protect military communications from being understood by “the enemy”. This is the context in which Alan Turing cut his name as a cryptographer during the Second World War. Turing worked for the Government Code and Cypher School at Bletchley Park and is most famous for his contributions to the demystification of the Enigma encryption machines that the Axis powers used to protect their communications. Turing’s contributions to cryptanalysis, the art of defeating cryptographic schemes, were insightful. In particular, he is credited as being one of the main contributors to the design of the bombe, an electromechanical machine used to search for vital Enigma settings.

The efforts of the men and women of Bletchley Park are widely regarded as having played an important role in drawing the war to a close. Bletchley Park is open to the public and a highly recommended day out. You can see a replica bombe and a striking sculpture of Alan Turing, carved from slate by artist Stephen Kettle.

Alan Turing. Sculpture by Stephen Kettle. Bletchley Park. Photo by Jon Callas. Creative Commons License.

There is much more, however, to both cryptography and Alan Turing’s cryptographic legacy.

I don’t think it’s too shallow to claim that Turing was a genius. To me the strongest evidence is the fact that his work has had significant impact across several different fields of science, cryptanalysis being just one. His work on the theory of computation, along with his Bletchley experience, inevitably drew Turing into the post-war development of early computing machines. Turing was a key player in the initial convergence of theory and practice which enabled the modern computer to emerge in the subsequent decades. Turing was there at the very start. Who knows where our computing journey will end?

What we do know is that modern life would be barely imaginable without the networks of computing devices on which we now rely. We talk, we write, we trade, we bank, we play — all on computers. Our world, which once relied on physical presence and boundaries for its security, is now an open digital one. Without the right precautions we can never be sure, for example, who is taking our money online, what amount they really are taking, and who might be listening in. It’s scary, if you think about it for too long.

The good news is that this digital world can be made secure through the use of, guess what? Cryptography! Significantly, the cryptography used today provides much more than the creation of puzzles from secrets that was first alluded to. The requirement to secure computers has necessitated the development of many different types of cryptography that go far beyond the basic encryption of secret messages that Turing so admirably wrestled with in the 1940s. Modern cryptography also provides services which help to detect unauthorised modification of data. Cryptographic mechanisms can be deployed to assure the source of a digital communication. Cryptography can even be used to create digital analogues of handwritten signatures.

Rather than being a technology only encountered by brilliant mathematicians in the most desperate of times, cryptography is now something that, without even realising, we use every day. We rely on cryptography when we chat on our mobile phones, when we withdraw cash, when we make purchases over the Internet, even when we open our car door. During the Second World War, the Allied Powers nearly didn’t prevail because of the use of cryptography. Now none of us can survive without it.

Even at mass entertainment level, there is cryptography. I smile at the popularisation of the unbreakable encryption technique known as the one-time pad. I see people every day wrinkling their brows during attempts to construct complete specifications of one-time pads from partial information in a newspaper. Perhaps you know these better as Sudoku Squares? You see, we really do love puzzles, mysteries, and secrets. The eternal appeal of cryptography is guaranteed.

I would argue that cryptography is important, useful, clever, and fun, which I think is a charmingly rare combination. I am sure that Alan Turing would agree.

Prof. Keith Martin is Director of the Information Security Group at Royal Holloway, University of London and author of Everyday Cryptography. An active member of the cryptographic research community, he also has considerable experience in teaching cryptography to non-mathematical students, including industrial courses and young audiences. Since 2004 he has led the introductory cryptography module on Royal Holloway’s pioneering MSc Information Security.

OUPblog is celebrating Alan Turing’s 100th birthday with blog posts from our authors all this week. Read our previous posts on Alan Turing: “Maurice Wilkes on Alan Turing” by Peter J. Bentley and “Turing : the irruption of Materialism into thought” by Paul Cockshott. Look for “Turing’s Grand Unification” by Cristopher Moore and Stephan Mertens and “Computers as authors and the Turing Test” by Kees van Deemter later this week.

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