Represent every letter by two figures, by the intersection of a vertical with a horizontal row. Thus we find that 11 represents a; 34, o; 52, w; 14, d; and so on.
During the Middle Ages secret systems were employed in the operation of telegraphic, military, and naval signals. Torches placed in particular positions at night, flags held in position by day, guns fired at particular intervals, drums beaten in a prearranged way, musical sounds to represent letters, lamps covered by different-colored glass, square holes diversely closed by shutters, levers projecting at different angles from a vertical post—all these were adopted as signals; but secret writing was in most cases a transposition of alphabetical letters.
Schemes of cryptography are endless in their variety. Bacon lays down the following as the “virtues” to be looked for in them: “that they be not laborious to write and read; that they be impossible to decipher; and, in some cases, that they be without suspicion.” Bacon remarks that though ciphers were commonly in letters and alphabets, yet they might be in words. Upon this basis codes have been constructed, classified words taken from dictionaries being made to represent complete ideas. In recent years such codes have been adopted by governments, merchants, and others to communicate by telegraph, and have served the purpose not only of keeping business affairs private, but also of reducing the excessive cost of telegraphic messages to distant points. Obviously this class of ciphers presents greater difficulties to the skill of the decipherer. Figures and other characters have been also used as letters, and with them ranges of numerals have been combined as the representatives of syllables, parts of words, words themselves, and complete phrases. Shorthand marks and other arbitrary characters have also been largely imported into cryptographic systems to represent both letters and words. Complications have been introduced into ciphers by the employment of “dummy” letters or words. Other devices have been introduced to perplex the decipherer, such as spelling words backward, making false divisions between words, etc. The greatest security against the decipherers has been found in the use of what might be called a double code. One of the double-code methods is that after the message has been put into, say, a figure code, to recode it in one in which only words or consonants appear.
Variety is also of great importance. All the world might know the principle upon which a cipher is constructed, and yet the changes may be so numerous as, like those of the Yale lock, to be almost infinite. No cipher can ever be perfect where the same letter, figure, or character is always represented in the same manner; some mode must be adopted by which an endless variety may be secured.
During the time of the Great Commoner, Sir John Trevanion, a distinguished cavalier, was made prisoner, and locked up in Colchester Castle. Sir Charles Lucas and Sir George Lisle had just been made examples of as a warning to “malignants,” and Trevanion had every reason for expecting a similar bloody end. As he awaited his doom, indulging in a hearty curse in round cavalier terms at the canting, crop-eared scoundrels who held him in durance vile, and muttering a wish that he had fallen sword in hand facing the foe, he was startled by the entrance of the jailer, who handed him a letter:
“May’t do thee good,” growled the fellow; “it has been well looked to before it was permitted to come to thee.”
Sir John took the letter and the jailer left him his lamp by which to read it:
Worthie Sir John:—Hope, that is ye beste comfort of ye afflicted, cannot much, I fear me, help you now. That I would saye to you, is this only: if ever I may be able to requite that I do owe you, stand not upon asking me. ’Tis not much I can do: but what I can do, bee you verie sure I wille. I knowe that, if dethe comes, if ordinary men fear it, it frights not you, accounting it for a high honour, to have such a rewarde of your loyalty. Pray yet that you may be spared this soe bitter, cup, I fear not that you will grudge any sufferings; only if bie submission you can turn them away, ’tis the part of a wise man. Tell me, an if you can, to do for you any thinge that you wolde have done. The general goes back on Wednesday. Restinge your servant to command. R. T.
Now this letter was written according to a preconcerted cipher. Every third letter after a punctuation mark, was to tell. In this way, Sir John made out: “Panel at east end of chapel slides.”
On the following evening the prisoner begged to be allowed to pass an hour of private devotion in the chapel. By means of a bribe, this was accomplished. Before the hour had expired, the chapel was empty—the bird had flown.