As to just what constitutes the “perfect” cipher, perhaps it might be said that this description fits any cipher whatever which provides the degree of security wanted for an individual purpose, and which is suited in other respects to that individual purpose. Even a basically weak cipher, in the hands of an expert, can be made to serve its purpose; and the strongest can be made useless when improperly used.
In the present text, we are likely to be found looking at ciphers largely from a military angle, which, apparently, has a more general interest than any other. In time of war, the cryptographic service, that is, the encipherment and transmitting service, is suddenly expanded to include a large number of new men, many of whom know nothing whatever of cryptanalysis, or the science of decryptment. Many of these are criminally careless through ignorance, so that, entirely aside from numerous other factors (including espionage), it is conceded by the various War Departments that no matter what system or apparatus is selected for cipher purposes, the enemy, soon after the beginning of operations, will be in full possession of details concerning this system, and will have secured a duplicate of any apparatus or machine. For that reason, the secrecy of messages must depend upon a changeable key added to a sound basic cipher.
Speed in encipherment and decipherment is desirable, and often urgent; and the conditions under which these operations must often take place are conducive to a maximum of error. The ideal cipher, under these conditions, would be one which is simple in operation, preferably requiring no written memoranda or apparatus which cannot be quickly destroyed and reconstructed from memory, and having a key which is readily changed, easily communicated, and easily remembered. Yet the present tendency, in all armies, seems to be toward the use of small changeable codes, which are written (printed) documents; and, for certain purposes, small mechanical devices.
An enormous number of military cryptograms will be transmitted by radio and taken down by enemy listeners, and even the ordinary wire will be tapped. It is expected that the enemy will intercept dozens, and even hundreds, of cryptograms in a single day, some of which will inevitably be enciphered with the same key. With so much material, knowing the general subject matter, and often exactly what words to expect, or the personal expressions invariably used by individuals, it is conceded that he will read the messages. All that is desired of a cryptogram is that it will resist his efforts for a sufficient length of time to render its contents valueless when he finally discovers them. By that time, of course, the key will have been changed, probably several times, and even the cipher.
With these general facts understood, we may first dispose hastily of the concealment cipher, after which we will examine at greater length the two legitimate types, the transpositions and the substitutions.
CHAPTER II
Concealment Devices
Concealment writing may take a host of forms. Perhaps its oldest known application is found in the ancient device of writing a secret message on the shaved head of a slave and dispatching the slave with his communication after his growing hair had covered the writing. Or, if this appears a little incredible, the ancients have left us records of another device considerably more practical: that of writing the secret message on a wooden tablet, covering this with a wax coating, and writing a second message on top of the first.
In the middle ages we meet a development called puncture cipher; any piece of printed matter, such as a public proclamation, serves as the vehicle, and the cipher consists simply in punching holes with a pin under certain letters, so that these letters, read in regular order, will convey the desired information. It is said that this kind of concealment writing was resorted to in England at a comparatively recent period, to avoid the payment of postage. Postage on letters was very high, while newspapers were permitted to travel free, and the correspondents sent their messages very handily by punching holes under the letters printed in newspapers. Where the sender of a message may also control the preparation of the printed vehicle, any desired letters can be pointed out by the use of special type forms, misspelled words, accidental gaps, and so on.