Читать книгу Secret and Urgent - The Story of Codes and Ciphers - Анон - Страница 5

ARCHAEOLOGISTS believe written languages began as series of pictures representing actions. In the case of all but the Aztec hieroglyphics, and to a lesser degree, the Egyptian, these pictures speedily lost their direct significance, and today all written language is cipher. Its symbols have no intrinsic meaning; they convey an idea only when interpreted according to a system whose secret is shared by the writer and reader. We are apt to lose sight of this today because most people learn to read early in life; but it is only necessary to remember the Middle Ages, when a man who could read was about as rare as a telegrapher is now. When the average citizen received a letter, he had to take it around to someone and have it interpreted, and he would now if he were given a missive written in the dots and dashes which are the special cipher of telegraphy.

One could conceivably learn to speak a language—say Japanese or Arabic—without learning how the symbols in which it is normally written could be translated into sound, though if the same words were expressed in the familiar Latin letters the difficulty would disappear. The art of ciphering or cryptography may be defined by saying it is the process of expressing words that convey an idea to everyone in symbols that convey an idea only to the few persons who share the secret.

It is an extremely old art, which seems to come into being almost spontaneously when—and wherever—a large proportion of the population learns to read. In Egypt, Babylon and medieval Europe, where reading and writing were largely monopolies of a priestly class, there is little or no record of ciphers. The ordinary written language was secret enough. The first certain appearance of the art is among the comparatively well-educated Greeks, who seem to have invented one of the two great classes of ciphers—the transposition cipher, in which the letters of the original message are thrown into some meaningless order, and can be returned to their original arrangement by anyone in possession of the key.

The relatively well-educated ruling class of the late Roman Republic seem to have invented the other great system, the substitution cipher, in which each letter of the original message is replaced by some other letter, symbol or figure. During the dark ages ciphers vanish; when they reappear it is among the Italian city states, where learning is beginning to bloom under the dawn-sun of the coming Renaissance, and as that movement spreads across Europe, it takes the device of ciphered messages with it.

The rise of the general literacy curve in Europe and subsequently in America has been paralleled by a similar rise in the number of persons who know something of ciphers. We are a highly literate nation today; and there are regular cipher departments in four magazines and a good many newspapers, while the many members of an association of intelligentsia spend much of the new leisure trying to puzzle one another with ciphered messages which the recipient is obligated to work out without the aid of the key.

In other words, the thing has become a game, following the course of development normal to such other necessities of primitive life as hunting, camping and fighting with swords. But the progress of invention has made the sword a sport only by turning the serious business of warfare over to the machine; and this is true also in cryptography. The interests of military and diplomatic secrecy can now be served only by elaborate mechanical devices, such as codes, radio that emits “jumbled facsimile” and ciphering machines. The cipherer who uses nothing but his brain, a writing instrument and a clean piece of paper stands about as much chance against them as a good fencer does against a machine gun.

The inclusion of codes in the list of mechanical devices will be noted. Essentially a code is just that, and it always makes cryptographers indignant to hear the terms “code” and “cipher” used synonymously. Anyone who knows the key of a cipher can read it without any apparatus but pencil and paper; but even when the key of a code is known, mechanical equipment is required to interpret the message—a code-book, or code dictionary. The ABC and Bentley’s commercial telegraphic codes are good examples, their dictionaries being volumes that compare favorably as to poundage with Webster’s.

There is also an important difference of central structure between a code and a cipher. In a cipher, every letter of the original message (the clear, it is called by cryptographers) is represented by a letter, figure or symbol of the enciphered message. In a code, a code-word (four, five or seven letters long) stands for a phrase, a sentence or even the whole message of the clear.

Ciphers are thus systematic and can be used to express any thought the sender is capable of putting into words; codes are wholly arbitrary, and can express nothing but the limited number of phrases that can be put into a code dictionary. Every navy in the world uses codes to the exclusion of ciphers. Their arbitrary character presents enormous difficulties to the solver, and the partial solution of the code does not entail the fall of the rest, while the number of things ships can be ordered to do is somewhat limited, and does not demand a great vocabulary.

Most armies, on the other hand, use ciphers. Except for units with large staffs and semi-permanent locations, such as division headquarters, they simply cannot carry code dictionaries around, and if they did would always run the obvious danger of losing them by capture. It is worth noting that naval code-books are bound in lead and, when a warship is in danger of being sunk or captured, it is the officers’ first duty to throw these books overboard.

Diplomacy hesitates between codes and ciphers, generally favoring the latter because of their greater flexibility and the fact they can be used to express delicate shades of meaning. Codes are popular in secret-service work and espionage because they can express more in less space. Long messages are always dangerous to spies, and they really prefer something, like invisible ink, which will give the impression that no message at all has been sent. Criminals (it is an amusing comment on the first two institutions that diplomacy, war and crime are the great sources of secret writing) nearly always use ciphers.

It will be the business of this book to trace, as far as possible, the story of the development of secret writing. Unfortunately that story will be episodic and mainly the tale of successful decipherments. At points where we would most like to have a clear narrative of development we shall find nothing but a vague general outline illuminated by flashes of incident. This is inherent in the nature of such a subject; for we shall not much outrage the probabilities by supposing that the most interesting successes of secret writing have remained secret.

There are certain gaps in the main line of the tale which strengthen this supposition and make it impossible to generalize about cryptography on any but a hypothetical basis. Logically, one would expect the Byzantine Empire to have accomplished something special in ciphers. The later Greeks were a particularly subtle and ingenious race with a strong taste for intrigue, and they had Roman experience to go on. (The early Greek ciphers seem to have disappeared without leaving any traces but a note or two in the works of writers attracted by the curious.) Byzantium was a state in which enough people knew how to read so that it must have been decidedly dangerous to send, in clear text, written messages that anyone wished to keep secret. Yet no literature of the period mentions ciphers, nor have any been preserved.

What makes this all the stranger is that while a general line of development can be traced through the Middle Ages with the suggestion that the Roman method of ciphering was first lost, then rediscovered, bit by bit, the line has such enormous gaps that it may not really exist at all. In one of the earliest manuals on cryptography in existence (di Lavinde’s from the fifteenth century) the use of an enciphered code is recommended: an extremely modern and complex development, which has not been surpassed today. It argues decades and perhaps centuries of effort to defeat decipherers who have become extremely acute at their business. We have not the slightest clue as to how and where that skill was acquired. It may have been at Rome; it may have been at Constantinople; or it may have been at Venice and Genoa. If it was a Venetian-Genoese discovery there is a possibility, rather faint, that ciphers were originally developed in that Near East from which the two great commercial cities drew so much else.

In short, when we begin to investigate the history of ciphers, we are digging in a graveyard whose limits we do not know and where there are headstones only for the failures. Decipherments that have changed the course of history (and they are not a few) are often recorded. The triumphs of encipherment, of messages that got through without being read by interceptors are never mentioned—if for no other reason, because people who have used a cipher successfully wish to keep it secret and use it again.

It is probably for this reason as much as because cryptographers are naturally proud of their own performance that the statement is often made that there is no such thing as an insoluble cipher. Strictly speaking it is not true. Roger Bacon in the early Middle Ages wrote a whole manuscript in a cipher that has thus far defied analysis. It is extremely probable that an insoluble cipher could be produced by mathematical means today.

This is true, however, only if the production of an insoluble cipher and the recording of some relatively short message in it were the only end in view. All ciphers in actual use break down on repetitions, not merely repetitions which can be avoided by careful phrasing in a single message but the necessity of repeating the same words or sentences in several messages. The redundancies of action defeat the best efforts of those who would send secret communications.

Nor is the phrasing always careful, even in a single message. Wherever ciphers are most frequently used, they must be written in a hurry, usually by men without much special training, and always without special apparatus. The effort to break them down, to read the messages they contain, will always be made by experts with ample training, a wealth of time at their disposal and whatever special apparatus they need. No systematic method of defeating this analysis has ever been found or is likely to be.

Moreover, another factor enters here. The only method of delaying expert analysis is by complicating the enciphering process; and complication is fatal. An officer of the British Black Chamber estimates that one-third of all the cipher messages which passed through that department during the World War were garbled; that is, mistakes had been made in the enciphering process. The more complex and safer the cipher, the greater the likelihood of these errors; and in some of the better ciphers they are progressive, so that a single error renders all the rest of the message gibberish, even to the man with the key.

The utmost any modern cipher can hope to accomplish is to force the decipherer to employ his last resources, particularly his resources of time; to delay decipherment until the information obtained by the process is no longer of value. That the information obtained by decipherments will always be of some value is the reason why navies use codes.

But this is already trespassing on matters that should be reserved for the text of this volume.