Rosario Candela’s recent book, The Military Cipher of Commandant Bazeries, shows the unraveling of one particular cryptogram which, for many years, had baffled the best efforts of all amateurs, and, it is rather suspected, of some few who were not amateurs. The book contains a chapter on general cryptanalysis, and also some cryptograms for solution.

Secret and Urgent, by Fletcher Pratt, is primarily a history of secret writing (a most interesting one, by the way), but contains also a number of examples of cryptanalysis; it also shows a table which the writer has never before seen in published form: a list of English trigrams (three-letter sequences) and the frequency with which they are used in the language. Other examples of decryptment may be found in the Macbeth translation of Langie’s genial little book, De la cryptographie; the appendix to this translation contains the coveted Playfair demonstration, prepared by Lieutenant Commander W. W. Smith of the United States Navy.

Just why so absorbing a subject has been so neglected in a world full of puzzle lovers is hard to understand, especially since the analytic writer, in addition to entertainment, has something to offer of a more serious nature. It is true that trained cryptanalysts are not greatly in demand in peacetime, and that our present corps of cryptographers has a personnel more than ample for providing necessary codes and ciphers, scientifically selected to fit their individual purposes, and safeguarded with suitable protective devices. Yet of what value is the most excellent of ciphers if, at the time of direst need, this cipher, with all of its safeguards, must be placed in the hands of even one man who cannot appreciate its intrinsic value or imagine a need for extra precautions? At any rate, we make our feeble attempt to reach this “one man.” May he learn, at least, that there are reasons for his instructions!

In the planning of the present treatise, all purely historical aspects of secret writing were neglected, and many well-known ciphers whose interest is chiefly historical or literary have either been omitted or given but cursory treatment. Certain other ciphers, representative of types, have been treated at whatever length seemed advisable for bringing out principles; and, with each type discussed, a generous number of cryptograms has been provided, on which the student will be able to test his skill as he learns. The student who masters these fundamentals will be acquainted with the principal forms of cipher, and will be able to solve cryptograms prepared by means of these ciphers provided the cryptograms are of adequate length and based on a language which he understands, or of which he is able to secure understandable specimens. Within limits, he should also be able to analyze and solve such cryptograms without being told in advance what the cipher is. This, we believe, is the kind of text-book desired by the many who desire information about “ciphers.”

Its material, compiled by members of the American Cryptogram Association, has had to be gathered from a great many sources, both within the organization and elsewhere, making it impossible, at times, to give credit where credit is due. Our chief indebtedness, however, is to M. E. Ohaver for a series of articles published during the years 1924 to 1928 in the former Flynn’s Magazine and most unfortunately no longer obtainable from the publishers. Further acknowledgment should be made to Colonel Parker Hitt, whose Manual for the Solution of Military Ciphers, though not available for general distribution, can usually be consulted in large public libraries. We have also borrowed liberally from foreign sources, and members of the association have most generously contributed the results of their original research. For this collaboration and co-operation, the writer is particularly grateful.

CONTENTS

Preface[v]
I.General Information[1]
II.Concealment Devices[4]
III.Transposition Types[9]
IV.Geometrical Types — The Nihilist Transposition[17]
V.Geometrical Types — The Turning Grille[26]
VI.Irregular Types — Columnar Transposition[37]
VII.General Methods — Multiple Anagramming, Etc.[53]
VIII.Substitution Types[68]
IX.Simple Substitution — Fundamentals[69]
X.The Consonant-Line Short Cut
A Method for Attacking Difficult Cases
(by George C. Lamb)		[88]
XI.Simple Substitution with Complexities[99]
XII.Multiple-Alphabet Ciphers — The Vigenère[108]
XIII.The Gronsfeld, Porta, and Beaufort Ciphers[117]
XIV.The Kasiski Method for Periodic Ciphers[127]
XV.Miscellaneous Phases of Vigenère Decryptment[138]
XVI.Auto-Encipherment[146]
XVII.Some Periodic Number-Ciphers[159]
XVIII.Periodic Ciphers with Mixed Alphabets[169]
XIX.Polyalphabetical Encipherment Applied by Groups[185]
XX.Vigenère with Key-Progression[192]
XXI.Polygram Substitution — The Playfair Cipher[198]
XXII.Highlights of Fractional Substitution[209]
XXIII.Investigating the Unknown Cipher[213]
Appendix
English Frequency and Sequence Data[218]
Comparative Table of Single-Letter Frequencies[219]
Chart Showing Normal Contact Percentages (by F. R. Carter)[220]
Chart Showing Frequencies of English Digrams (by O. Phelps Meaker)[221]
Some Foreign Language Data[222]
Bibliography (by W. D. Witt)[224]
The Commonest English Words (by Frank R. Fraprie)[226]
English Trigrams (by Frank R. Fraprie)[226]
English Digrams (by Frank R. Fraprie)[227]
Index[229]