HE number of cryptographic systems capable of formation is practically without limit. So much so, indeed, that if every person in the world were to devise a cipher of his own, no two of them would need to be alike!
Any one not familiar with cryptographic methods might easily be confused by this multiplicity of types. But, fortunately, this seeming infinite variety springs from combinations and adaptions of a relatively small number of principles, which are both easy to understand and apply.
Such of these principles as enable ciphers to be grouped into a number of fundamentally different classes will be recounted in this article. And other principles, governing divisions and subdivisions of these main classes, will be taken up at intervals in subsequent issues of the department.
Without doubt the most important of these basic principles is that of substitution.
In a substitution system the message order remains unchanged in cipher, but the terms of the message are replaced by cipher symbols according to a prearranged alphabet or schedule.
The substitution principle can be applied to letters or other characters, singly or in groups, as well as to words, phrases, and so forth. And the cipher symbols representing these message terms may consist of letters, figures, arbitrary signs, or other characters, in any arrangement.
For example, cipher No. 13, explained below, is a substitution type where each letter of the message is represented in cipher by a symbol of two letters. Again, Charles P. Winsor's cipher in FLYNN'S WEEKLY for September 4, 1926, replaced each pair of message letters by a symbol of three figures.
Finally, to mention still another example, in the code messages in FLYNN'S WEEKLY for April 23 the symbol for each word—or other vocabulary term—is a group of five figures, or a real or artificial code word.
Ranking next in importance to the principle of substitution is that of transposition.
In a transposition system the terms of the message retain their normal value, being merely changed in their order. Ciphers Nos. 14 and 15 in this issue are examples of monoliteral, or single letter, transposition.
No. 10 (J. R. Midford), explained in the recent issue of April 16, illustrates transposition in groups of three letters. The transposition principle can also be applied to words, there being important historical examples of word transposition ciphers.
Still a third fundamental cipher principle is that of nonsignification.
In a system of this class, the message terms retain their natural order and value in cipher, but are interspersed with characters, termed nulls or nonsignificants.
Many types of grille ciphers, where the significant letters or words are written through openings in cards, and the spaces between filled out with nonsignificant letters or words, belong to this class. Our own readers have also been productive in this field. For example, witness cipher No. 5 (M. Walker) described in the January 22 issue, where every third letter is significant.
Finally there are a few ciphers which might be considered as based on the principle of omission, or abbreviation.
In secret writing of this kind, some of the message terms are altogether omitted from the cipher. But those which are retained, hold their original order and original value.
Lodge ritual ciphers, where only the initials or skeletonized outlines of words are used as reminders of their originals, might be placed in this class. "B O, i t cdt ry f insn i t mys o t dg?" in such a cipher might mean: "Brother Officer, is the candidate ready for instruction in the mysteries of this degree?"
The several basic principles just described may be used in any combination to form ciphers of greater complexity. Thus, No. 11, also explained in the issue of April 16, combines the principles of substitution and transposition. And the famous Bacon biliteral cipher—see FLYNN'S WEEKLY for April 25, 1925—may be cited as a classical example of the double substitution variety.
So much, then, for the fundamental principles of cipher structure. Other principles, with further examples, will receive attention in subsequent issues.
This week's ciphers are both of the transposition class, the letters being first transcribed into rows forming columns of equal length, and then taken out of the rectangles so formed in some other order. For example, "MEET ME AT ONCE," forms the following rectangle.
M E E T M E A T O N C E
Taken out by "verticals," the cryptogram would thus become: MMOEE NEACT TE. By " alternate verticals" it would be: MMONE EEACE TT. Or by diagonals: MEMEE OTANT CE. A moment's study will show this.
Can you find the dimensions of the rectangles, and the orders of transposition, in the following two cryptograms?
CIPHER No. 14 (Rudolph L. Lcuckart, Cleveland, Ohio).
TFHWH HOAOO IRRRW SYKKG MOSAO AUBNO YCUDD BITSY EPGHO EHEOU AETWA SRTMR YSOEE
CIPHER No. 15 (John P. Crotty, Jr., Charleston, Massachusetts).
TTLIA ONHEO SRTEH RYIFM DETSE KWFAO STOLO YOHNC ROORB LIGET EETDW ETTOH HRTT
How are you getting along with No. 12 (Fletcher Pratt), the "free subscription" cipher published week before last? We feel that we should get some answers to this one. How about it?
To decipher No. 13 (M. Walker), April 16, first regroup the cryptogram (a) in pairs (b). The differences between the numerical values (A=1, B=2... Z=26 ) of the letters in each pair will then represent by the same values the letters of the message (c).
Thus the difference between B (2) and V (22) is 20, which equals T. Similarly, FN represents H; IN equals E; and so on. The message in full: "There is a tide in the affairs of men, which, taken, at its flood, leads on to fortune. Have your boat ready!"
(a) BVFNI NCUGL ... (b) BV FN IN CU GL ... (c) T H E R E ...
As to next week's department, there is an extra special reason why you should see it.
For, besides some new ciphers, and the answers to those in this issue, it will contain the solutions to the National Puzzlers' League " contest crypts," published at length in the March 19 issue of FLYNN'S WEEKLY.
These are unusually fine.
So do not fail to see next week's issue for something worth while.