The words of sentences can, of course, be treated in the same way, and where the alignment from the left gives no results, letters or words can be aligned from the right, or from the center. If columns give no results, diagonals can be inspected, or a zig-zagging line between one column and another.
Experience counts for most, and extensive reading is a vast help. Having seen methods in use, or read the descriptions of methods, we know of some definite thing to look for. Then, too, some of the concealment ciphers have transposition characteristics. This would be the case with the Legrand cipher, which is of the type called “open letter.”
This cipher used a numerical key, which, in turn, was based on a keyword in what seems today a rather odd manner: A keyword CAT, made up of the 3d, 1st, and 20th letters of the alphabet, gives the key 3 1 2 0. Before concealment takes place, a series of word-positions is marked off, and these vacant places are numbered (0 to 9, or 9 to 0), continuing to repeat the ten digits until there are enough of the digits 3, 1, 2, and 0 to accommodate the words of the secret message. This message is then written, word by word, below its digits, beginning with the first digit 3, then going on to find a digit 1, then a digit 2, then a digit 0, then another digit 3, and so on. After the secret message is written into its place, all of the blank positions are filled with connective matter, as in the case of Cardinal Richelieu’s grille-writing. Our later study of transpositions will show approximately how we should go about reading this, once we suspect its use.
So far, we have been considering pure concealment. Many of the classic ciphers, fundamentally of the concealment type, are also substitution ciphers, and their decryptment would follow substitution methods. Of these, perhaps the best known is Bacon’s biliteral cipher, summed up in Fig. 3.
| Figure 3 BACON'S BI-LITERAL ALPHABET A aaaaa IJ abaaa R baaaa B aaaab K abaab S baaab C aaaba L ababa T baaba D aaabb M ababb UV baabb E aabaa N abbaa W babaa F aabab O abbab X babab G aabba P abbba Y babba H aabbb Q abbbb Z babbb S T R I K E baaab baaba baaaa abaaa abaab aabaa N O W abbaa abbab babaa Hold OFf uNtIl you hEar frOm mE agAin. wE May cOMpROmIse. |
Lord Bacon’s cipher presupposes that the encipherer may so control the preparation of his published work that he may prescribe the type to be used for each printed letter, and it is claimed that he actually used his cipher for the preservation of historical secrets, including that of his own parentage. Two fonts of type are required, the letters of one font differing (very slightly) from those of the other font. These we may speak of as the A-font and the B-font, and each letter of the alphabet is given a substitute composed of A’s and B’s, as shown in full in the figure. Before a message, as STRIKE NOW, can be concealed, it must be expressed in A’s and B’s, five of these for each of its letters, as shown, so that a message of 9 letters attains a length of 45. For its concealment, we may use any text whatever whose length is 45 letters, for instance, one whose obvious meaning is the contrary of the secret one: “Hold off until you hear from me again. We may compromise.” The first five letters, HOLDO, are to represent S, the next five, FFUNT, are to represent T, and so on; and the sole purpose of the A’s and B’s is to point out the kind of type which must be used in printing the corresponding letters. In the encipherment of the figure, letters taken from the A-font are indicated by lower-case and those of the B-font by capitals, though it is understood that no such emphatic difference is contemplated in the cipher.
While the average modern person would have no opportunity for employing Lord Bacon’s cipher as described, he has access to an unlimited number of vehicles other than type-difference. Anything, in fact, may serve the purpose, so long as the material is available in two distinguishable forms and in sufficient quantity. Our message of 29 A’s and 16 B’s could be expressed with a deck of playing cards if aces and face-cards are considered to represent B’s. It could assume the form of a fence with 45 palings, in which the B-palings are crooked, damaged, or missing. Ohaver once made use of a cartridge belt in which the A-loops contained cartridges and the B-loops were empty. There is an excellent opportunity here, too, for the compiling of “fake” cryptograms, with A-letters and B-letters distinguished as vowels and consonants, or by the part of the normal alphabet from which they have been taken.
With a biliteral or binumeral alphabet which requires 26 groups, we cannot have fewer than five characters to the group without making groups of different lengths. But another well-known cipher alphabet, devised by the Abbé Trithème for use in much the same way, is triformed, and thus permits that the group-length be reduced to three. The Trithème (Trithemius; Trittemius) alphabet, expressed in digits 1-2-3, was approximately that shown in Fig. 4.