Secret and Urgent - The Story of Codes and Ciphers. Anon

for a reply to these charges (which he had never received) he would be presently judged guilty in absence and condemned.

      Within the night a fast galley bore Lysander of Sparta south, homeward through the Aegean; the first recorded use of cipher had saved a general and an empire, and set in motion the chain of circumstances that led unbroken to the triumph of West over East under Alexander the Great.

      II

      Suetonius, the Walter Winchell of ancient Rome, says that Julius Caesar kept his fingers on the political pulse of the home city by writing to his friends from Gaul in a cipher that was prepared by shifting each letter of the original clear four places down the alphabet. Habes opinionis meae testimonium, which he wrote to Cicero, would thus come out as MDEHV RSNQNRQNV PHDH XHVXNPRQNZP, allowing for the fact that the Roman alphabet lacked J, K, W and Y.

      Given that the science of solving ciphers had not yet been invented this simple system was enough to protect his correspondence from unauthorized eyes. However, we have the best of reasons for believing that Julius Caesar’s ciphers were neither used very long nor for the conveyance of very important information. Cicero was one of the people in the secret; and Cicero changed political sides, which meant that the great conqueror’s secret was a secret no longer. Moreover, a good many of Caesar’s letters in clear have been preserved. We know from them and from other sources that his usual method of secrecy was to say nothing in writing, but to appoint someone he trusted to carry a message orally.

      The only thing he really contributed to the history of secret writing was the use of his name. A cipher composed by displacing the letters of the alphabet two, three or more steps down the line is still known as a “Julius Caesar cipher” in spite of the fact that there is good evidence this type of communication was in use before he was born.

      III

      The ciphers of Sparta and Rome left no recognizable direct descendents. Roger Bacon’s, whatever one may think of the Newbold decipherment, was an isolated effort; and the interpretation of the records of the rocks took place at a time when cryptography was already highly developed. A certain Abbot Trithemius who wrote an early book on ciphers in Holland, says that Charlemagne used ciphers to communicate with his agents, but the tale stands on about the same basis as the confident medieval assertions that Virgil was a necromancer who could fly through the night on broomsticks. We know as a matter of historical fact that Charlemagne himself never learned to read or write any language till he reached maturity, and that most of the great officers of his court remained illiterate to the end. Any kind of writing would have been cipher enough in such an age.

      And if Charlemagne did use ciphers, it was another case of an isolated effort. The genuine sources of modern cryptography can be traced, vaguely and with some difficulty, to two widely separated medieval springs—clerkly authorship and thieves’ slang. The first may reasonably be looked upon as the beginning of ciphers and the latter of codes, but the two blend and divide like a slow stream passing many islands, and it is not until relatively modern times that they become sharply distinguished.

      It is a little difficult to realize today that during the Middle Ages literature was an extra-hazardous occupation. The Church exercised both a monopoly and a close censorship of it and used that censorship in a manner that seemed to contemporaries highly capricious. When a writer set down his thoughts, he knew for a certainty that sooner or later they would come to the attention of the Church authorities, but not what the result would be. Thus Roger Bacon’s writings were discovered to be highly heretical; he was put into prison. But the works of his pupil, Thomas Aquinas, were found not only good doctrine but extremely valuable, and he was canonized.

      Under the circumstances most medieval writers tried to conceal authorship by some device that would enable them to throw off the mask if the Church approved their work, but keep it up if she did not. One of these devices was that of anagramming the author’s name; Rabelais thus appeared as Messer Alcofribas, just as Monsieur Arouet junior (1.j.) was later to become famous under his anagram of Voltaire. The Newbold decipherment has at least the justification that anagramming was common practice at the time the Bacon manuscript was written.

      A more common device and one more important in the history of ciphers was that of suppressing the vowels in signatures and doubtful passages. There were two methods of doing this. One was to replace the vowels with dots on a regular system, I being represented by a single dot, A by two, E by three and 0 by four. (U, then written as V, was let alone.) Thus “Richard, Roi d’Angleterre” would become R.ch..rd, r..... d’..ngl...t...rr... The other system was to replace the five vowels, in their order, with B, F, K, P and X, making of “Archepiscopus Arnulfus” BRCHFPKSCPPXS BRNXLFXS. The second scheme violated one of the cardinal principles of cryptography by making it difficult to tell a letter of the clear from an enciphered letter; and both were so simple and so well known that they could hardly have deceived anyone, even in the Middle Ages. But as the objective seems to have been not so much complete deception as something to furnish a lawyer’s talking point in case the matter came before an ecclesiastical court, these simple devices seem to have served their purpose.

      At this point there occurs in the history of cryptography one of those gaps which can be crossed only on the slack wire of hypothesis. The earliest known writing concerned with ciphers is a book by one Sicco Simonetta, who was connected with the chancellery of the Sforza Dukes of Milan between 1375 and 1383. It is called the Liber zifrorum; its nature is that of a manual for the diplomatic agents of the duchy. In it Simonetta recommends the use of simple substitution ciphers with suppression of frequencies and the insertion of an occasional code sign. This argues that the Italian courts had already learned the method of solving straight simple substitution ciphers, and requires a brief digression to describe that method.

      IV

      Simple substitution is the ABC and arithmetic of cryptography, and a complete understanding of it and of the method of breaking it is essential to any knowledge of the art. Explanations of the method, surrounded by very good stories, are given both in Poe’s Gold Bug and Conan Doyle’s Adventure of the Dancing Men, but as ninety per cent of all cipher messages are in some form of simple substitution it is worth giving another here.

      Perhaps the simplest form of all is that which assigns a number to each letter of the alphabet in order of occurrence—A = 1, B = 2, C = 3, and so on; or reverses the process, making Z = 1, Y = 2, X = 3. Along with it goes the alphabet of Julius Caesar, which replaces each letter by the one that follows it, two, three or more places down the alphabet. Both are still occasionally met with—usually among school children or pairs of secretive and romantic lovers. Most simple substitution ciphers go beyond this to introduce some slight complication, such as writing the message in conventional signs instead of letters, or using a keyword, something in which no letter is repeated, the key-word being written down, with the rest of the alphabet following and the clear alphabet below:

      In enciphering, the letters of the top line are substituted for those of the lower line. Example: All cats are grey at night becomes NFF WNQP NMO KMOX NQ HBKAQ, or, making the message up into five-letter groups and adding nulls at the end to fill out the last group, as is frequently done:

      NFFWNQPNMOKMOXNQHBKAQVCDZ

      Suppose now that the cryptographer is faced with a message of unknown content. (The groups are numbered for convenience in referring to them.)

      The first step in decipherment is to count the frequency with which each letter appears and to draw up a table of the result. In the present case it is:

      The first observation to be made from this table is that the message cannot be in a transposition cipher. There are not enough vowels and