Immediately after humans discovered writing they must also have discovered that concealing information is almost as important as expressing it. They also learned that there is nothing more fascinating than other people’s secrets. The ancient art of cryptography (code-making) has throughout history been matched against the ingenuity of cryptanalysts (code-breakers), sometimes in very dramatic circumstances. The battles of wits, intellects, cunning, mathematical prowess, and, more recently, technology has made the history of cryptology so colourful that it is bound to appeal to everyone’s imagination. It is a great story to tell, and Simon Singh, in The Code Book , tells it very well indeed.
Turn thefirst page and you are taken back to 1586 and introduced to the world of Elizabethan intrigue. Had it not been for code breaking, we learn, Mary Queen of Scots might well have kept her head. Her plot to assassinate Queen Elizabeth and inherit the throne was uncovered by Francis Walsingham, the founder of the British secret service, through the cryptanalysis of his cipher secretary, Thomas Phelippes. Mary was convicted of treason and executed. What follows is a tour de force, presenting the field that starts in about 400 BC in Sparta with the scytale, a device used for communication between military commanders, and ends with quantum cryptography.
The scytale was a tapered baton around which was wrapped a spiral strip of parchment or leather containing the message. Words were then written lengthwise along the baton, one letter on each loop of the strip. When unwrapped, the letters of the message appeared scrambled and the parchment was sent on its way. The receiver wrapped the parchment around another baton of the same shape and the original message reappeared. The next step in the history of ciphers is due to Julius Caesar, who allegedly used, in his correspondence, a simple letter substitution method. The emperor replaced each letter in the message with the letter that followed it alphabetically by three places. The letter A was replaced by D, the letter B by E, and so on. For example, the English word COLD after the Caesar substitution would appear as FROG…. This method is still called the Caesar cipher (regardless the size of the shift used for the substitution).
Singh alternates between cloak and dagger stories and explanations of how ciphers are designed and broken. He takes us from simple Ceasar substitutions, so vulnerable to the analysis of frequency of characters, to more complex, polyalphabetic ciphers. Conceived during the Renaissance and subsequently developed into a fully formed system of encryption in the sixteenth century, the polyalphabetic ciphers were considered unbreakable. Code-makers had a clear advantage over code-breakers for over two centuries. Finally, in the nineteenth century, the polyalphabetic ciphers were broken, among others by Charles Babbage, better known for his Analytical Engine, the first blueprint for what we would now call a computer. Here, Singh, in one of his delightful digressions, tells us also about other ideas of Babbage, such as the cowcatcher - a device that could be fixed to the front of steam locomotive and used to clear cattle from railway tracks.
No book on the history of cryptology is complete without the Zimmermann telegram and Enigma. Thus we are told how America might not have become involved in the First World War if the famous Zimmermann telegram, offering Mexico territorial gains in return for entering the war on the German side, had not been broken by the British intelligence services. The Enigma story is, in my opinion, the best part of the book. The working of the machine is lucidly explained. After reading this part of the book one can only be left in astonished admiration for the pioneering work of a gifted Polish cryptanalyst, Marian Rejewski - the first man to crack Enigma. It is clear that without his efforts and without the cryptanalytic know-how passed by the Poles to the British in 1939, the Bletchley team would not have known where to start. Subsequently, steadily refined versions of Enigma called for novel and ever more powerful cryptanalytic solutions. The formidable mathematical task of breaking increasingly complicated codes led Alan Turing and others to develop the Colossus - one of the fist computers.
With the advent of computers, both codemaking and codebreaking become even more complicated. Every electronic message is a sequence of numbers (e.g. ASCII code) and when confidentiality is required those sequences of numbers must somehow be encrypted in such a way that only the intended recipient can decrypt the message. They are usually combined with another sequence of random numbers called a cryptographic key to produce a cryptogram. Both sender and receiver must have exact copies of the key beforehand; the sender needs the key to encrypt the message, the receiver needs the exact copy of the key to recover the message from the cryptogram. Although such ciphers are very secure they suffer from what is known as the key distribution problem. These random numbers have to be distributed securely and quickly, and until the late sixties that was thought to be impossible without literally carrying the keys around in locked suitcases – a necessity that severely limited the size and bandwidth of secure communications networks. The seventies brought an ingenious mathematical solution: the so-called public-key cryptosystems. Singh provides a simplified but adequate explanation of the underlying mathematical techniques and the history of public-key cryptography. Funnily enough, had he written his book a couple of years earlier he would have attributed the discovery of this new encryption system to the three Americans, Whitfield Diffie, Martin Hellman and Ralph Merkle. However, in December 1997 the British Government officially confirmed that public-key cryptography was originally invented at the Government Communications Headquarters (GCHQ) in Cheltenham. By 1975, James Ellis, Clifford Cocks and Malcolm Williamson from GCHQ had discovered what was later re-discovered in academia and became known as Diffie-Hellman key exchange and the RSA cryptosystem, based on the difficulty of factorising very large numbers. The three British cryptologists, constrained by secrecy, could never cash in on the invention that on the other side of the Atlantic was being turned into a highly profitable business. Singh points out several times that the subject of secrecy is so important that it might be simply impossible to discover what the experts really know.
So if it takes only a couple of prime numbers to hide the most sinister plans of terrorists organisations what sort of line should the law, and law enforcement agencies, take here? What about our civil liberties, do we not have rights to privacy? Singh touches upon this point but it reads more like an excuse to tell the story of Pretty Good Privacy and Phil Zimmermann’s battle with the US government.
Finally Singh makes his leap into the quantum future. The story of quantum cryptography is designed, I guess, to be the book’s grand finale, but unfortunately at this important point the book falls sadly short of the high expectations we have come to have of Singh. This part strikes me as neither well researched nor well explained. Some parts left me flabbergasted. It is shocking to read such nonsense as “…the development of a fully operational quantum computer would imperil our personal privacy, destroy electronic commerce and demolish the concept of national security. A quantum computer would jeopardise the stability of the world…”. After all, quantum computers admit their own one-way functions, and in all probability public key cryptosystems will be implemented at the quantum level long before quantum code-breaking devices become feasible. Moreover, quantum cryptography offers security that is not only superior to anything currently available, but invulnerable even to the power of quantum computing. So there is no need to panic! Another notable omission from Singh’s story of quantum cryptography is the British contribution. it was at the Defence Research Agency in Malvern, not far from GCHQ, that quantum cryptography took its mature experimental shape; without this work it might not have attained feasibility to this day.
Despite my disappointment with the last chapter I think this is a delightful book. Simon Singh is a good story teller. Many anecdotes, such as the one about Charlie Bennett boiling a (dead) turtle in alkali, make it a smooth and a very enjoyable Probably the best book on the history of ciphers since David Kahn’s Codebreakers. Read it!