But concealment cipher is not necessarily confined to written and printed matter. Ohaver, in his “Solving Cipher Secrets,” demonstrated the conveyance of messages in the shapes and sizes of stones in a garden wall, or in the arrangement of colored candies in a box; and we read, in fiction, of many similar devices, such as a series of knots tied in a string, or beads strung in imitation of the rosary. Again, we hear of cases in which the arrangement of stamps on envelopes is made to represent the terms of a miniature code. All such devices are, of course, combination-cipher rather than pure concealment, since the stones, candies, and so on, must first be made the substitutes for letters or code terms.
A method of pure concealment, said to have been used by Cardinal Richelieu, involved the use of a grille. Grilles are made of cardboard, sheet-metal, or other flat material, and are perforated with any desired number, size, and arrangement of openings. The Richelieu grille, of approximately the same size and shape as the paper used for correspondence, could be laid over a sheet of paper so as to reveal only certain portions, and the secret message was written on these. The grille was then removed and the rest of the sheet was filled in with extraneous matter in such a way as to present a seemingly continuous text. The legitimate recipient of this message, having a duplicate grille, simply laid this grille over the sheet of paper, and read his message through the apertures.
Concealment cipher goes by various names, as null cipher, open-letter cipher, conventional writing, dissimulated writing, and so on, not always with a difference in meaning, though “conventional writing” does convey somewhat the idea of a tiny code. (In this, casual words have special meanings.)
The name “null cipher” derives from the fact that in any given cryptogram the greater portion of the letters are null, a certain few being significant, and perhaps a few others being significant only in that they act as indicators for finding truly significant letters. To illustrate what is usually meant: Say that your very good friend, Smith, first complains about a radio which he has bought from your neighbor, Johnson, then asks you to take Johnson the following note: “Having trouble about loudspeaker. Believe antenna connected improperly, but do whatever you can.” By reading the final letter of each word, you will find out what Smith actually had to say to Johnson: GET READY TO RUN.
That is the null cipher reduced to its elements, though naturally it can be more skillfully applied. Significant letters may be concealed in an infinite variety of ways. The key, as here, may be their positions in words, or in the text as a whole. It may be their distance from one another, expressed in letters or in inches, or their distance to the left or right of certain other letters (indicators) or of punctuation marks (indicators); and this distance, or position, need not be constant, or regular. Sometimes it is governed by an irregular series of numbers.
Similar devices are applied to whole words. We agree, say, that in whatever communications we send to our accomplice, only the third word of each sentence is to be significant. Desiring to send him the order, STRIKE NOW, we write him as follows: “The building strike is worrying our friends quite a lot. It has now extended to this part of the city.”
A purely concealment cipher may be enveloped in apparent ciphers of other types. The true message is concealed, as usual, in a dummy message, and the whole is enciphered in one of the legitimate systems. It is then hoped that the decryptor, satisfied with having solved the dummy, will look no further. Even more effective would be the device of concealing the message in what appears to be a cryptogram, but is not. It is easy to string letters together in such a way as to make them resemble most convincingly a transposition cryptogram, and in this case it would be hoped that the investigator’s full attention would be given to the hopeless task of decrypting the dummy.
Concerning the decryptment of concealment cipher, we regret to say that cryptanalysis has little help to offer. Fortunately, most of these ciphers depend absolutely on the belief that they will not be recognized as cipher, and once they are so recognized, they present no resistance. In those few cases where the secret message is not at once obvious, it is sometimes useful to arrange the words (or sentences) in columns, or in rows, for a closer inspection.
| Figure 2 I N S P E C T D E T A I L S F O R T R I G L E T H A C K N O W L E D G E T H E B O N D S F R O M F E W E L L |
We have, for instance, an apparent memorandum in which the awkwardness of the wording, or some other factor, has drawn our attention to the possibility of cipher: “Inspect details for Trigleth — acknowledge the bonds from Fewell.” We arrange these words in column form, aligned by their initials, as in Fig. 2, and the third column promptly gives up the secret message STRIKE NOW.