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Overview
This clear and thorough text also provides the reader with 166 specimen codes to solve. This edition for the first time also includes solutions. A special appendix includes final letters, sequence charts, digram, trigram tables, most common words in English and frequencies in English, German, French, Italian, Spanish, and Portuguese.
Product Details
| ISBN-13: | 9780486800592 |
|---|---|
| Publisher: | Dover Publications |
| Publication date: | 10/21/2014 |
| Series: | Dover Brain Games |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 256 |
| File size: | 19 MB |
| Note: | This product may take a few minutes to download. |
| Age Range: | 10 - 18 Years |
Read an Excerpt
Cryptanalysis
A Study of Ciphers and Their Solution
By Helen Fouché Gaines
Dover Publications, Inc.
Copyright © 1956 Dover Publications, Inc.All rights reserved.
ISBN: 978-0-486-80059-2
CHAPTER 1
General Information
The subject which we are about to study is the analysis and solution of cipher, though not including code, which is a very special form of cipher demanding something more than elementary knowledge; nor shall we enter at all into the subject of invisible inks, certainly a most important aspect of secret writing, but belonging to the province of chemistry rather than to that of cryptanalysis. Cipher machines, also, are not within our present scope.
The term cipher implies a method, or system, of secret writing which, generally speaking, is unlimited in scope; it should be possible, using any one given cipher, to transform any plain text whatever, regardless of its length and the language in which it is written, into a cryptogram, or single enciphered message. The process of accomplishing this transformation is called encipherment; the opposite process, that of transforming the cryptogram into a plaintext, is called decipherment.
The word decrypt, with its various derivatives, is being used here to signify the process of solving and reading cryptograms without any previous knowledge as to theirkeys, or secret formulas ; thus the word decipher has been left to convey only its one meaning, as mentioned above: the mechanical process of applying a known key. Our word decrypt, however, is an innovation borrowed from the modern French and Italian writers, and is somewhat frowned upon by leading cryptologists.
The word digram is being used to indicate a two-letter sequence; similarly, we have trigrams, tetragrams, pentagrams, etc., to indicate sequences of three, four, five, etc. letters.
Ciphers, in general, fall into three major classifications:
1. Concealment Cipher
2. Transposition Cipher
3. Substitution Cipher
Minor types, such as "abbreviation," are sometimes included, though, to the writer, these have never seemed to be truly of a cryptographic nature.
In concealment cipher, the true letters of the secret message are hidden, or disguised, by any device whatever; and this type of cipher, as a general rule, is intended to pass without being suspected as the conveyor of a secret communication.
In transposition cipher, the true letters of the secret message are taken out of their text-order, and are rearranged according to any pattern, or key, agreed upon by the correspondents.
In substitution cipher, these original text-letters are replaced with substitutes, or cipher-symbols, and these symbols are arranged in the same order as their originals. There may, of course, be combinations of types, or combinations of several forms belonging to a single type.
The aristocrat of the cipher family is code. This is a form of the substitution cipher which requires the preparation, in advance, of a code book. A series of terms likely to be used in future correspondence (that is, words, phrases, and even sentences) is first gathered into a vocabulary, or "dictionary"; and beside each of these terms is placed a substitute known as a code group, or code word. These substitutes may be groups of letters, or groups of digits, or actual words selected from ordinary language. Very common words or expressions are usually provided with more than one substitute; and nearly always there are substitutes provided for syllables and single letters, so as to take care of all words not originally included in the vocabulary.
No code presents any real, security unless the code symbols have been assigned in a thoroughly haphazard manner. This means that any really good code would have to be printed in two separate sections. In one of these, the vocabulary terms would be arranged in alphabetical order, so that they could be readily found when enciphering (encoding) messages; but the code groups would be in mixed order and hard to find. In the other section, the code groups would be rearranged in straight alphabetical (or numerical) order, so as to be readily found when deciphering (decoding), and the vocabulary terms would be in mixed order. Just what is meant can be seen in Fig. 1, showing fragments from an imaginary code book.
A code of this kind, with symbols assigned absolutely at random, provided it is carefully used (never without re-encipherment) and a close guard kept over the code books, represents perhaps the maximum of security to be attained in cryptographic correspondence; and security, of course, is of prime importance in the selection of a cipher for any practical purpose.
But in considering the relative merits of the various ciphers, it is always necessary to take into account many factors other than security, each cipher being evaluated in connection with the purpose for which it is wanted: Under what conditions must the encipherment and decipherment take place? How must the cryptograms be transmitted? How much of the enciphered correspondence is likely to be intercepted? What degree of security, after all, is absolutely imperative?
A commercial, or other, firm, having a permanent base of operations, and in little danger of being blown to bits by an enemy shell, would not consider the first of these questions from the same angle as the War Department; and the War Department, though considering all of them from several different angles of its own, would still not consider them from the same viewpoint as the State Department.
If messages are to be sent by mail, or by hand, or by telephone, or pasted on a billboard, it is conceivable that a cipher which doubles or trebles their length could still be a practical cipher. For transmission by telephone, the presumption is that the cryptogram must be pronounceable, or, certainly, audible. For written communication, individual purposes have been served by means of pictures.
But when the cryptograms are to be sent by wire or radio, it must be possible to convert them into Morse symbols, either letters or figures, but not intermingled letters and figures. Here, length must be considered, involving questions of time, expense, and the current telegraphic regulations. Moreover, it is conceded that a meaningless text will not be transmitted with absolute accuracy, and a cryptogram which is to be sent by this means must not be of such a nature that ordinary errors of transmission will render it unintelligible at the receiving office.
A factor of particularly grave importance in the selection of a cipher to fit a given purpose is the probable amount of enciphered material which is going to fall into the possession of unauthorized persons. A criminal, who has had to send but one brief cryptogram in a lifetime, might reasonably expect that it will remain forever unread, no matter how weak the cipher. A commercial firm, transmitting thousands of words over the air, is more vulnerable; and the diplomatic office, or the newspaper office, which makes the mistake of publishing almost verbatim the translations of cryptograms which have been transmitted by radio, and thus has surely furnished the cipher expert with a cryptogram and its translation, might just as well have presented him with a copy of its code book.
As to just what constitutes the "perfect" cipher, perhaps it might be said that this description fits any cipher whatever which provides the degree of security wanted for an individual purpose, and which is suited in other respects to that individual purpose. Even a basically weak cipher, in the hands of an expert, can be made to serve its purpose; and the strongest can be made useless when improperly used.
In the present text, we are likely to be found looking at ciphers largely from a military angle, which, apparently, has a more general interest than any other. In time of war, the cryptographic service, that is, the encipherment and transmitting service, is suddenly expanded to include a large number of new men, many of whom know nothing whatever of cryptanalysis, or the science of decryptment. Many of these are criminally careless through ignorance, so that, entirely aside from numerous other factors (including espionage), it is conceded by the various War Departments that no matter what system or apparatus is selected for cipher purposes, the enemy, soon after the beginning of operations, will be in full possession of details concerning this system, and will have secured a duplicate of any apparatus or machine. For that reason, the secrecy of messages must depend upon a changeable key added to a sound basic cipher.
Speed in encipherment and decipherment is desirable, and often urgent; and the conditions under which these operations must often take place are conducive to a maximum of error. The ideal cipher, under these conditions, would be one which is simple in operation, preferably requiring no written memoranda or apparatus which cannot be quickly destroyed and reconstructed from memory, and having a key which is readily changed, easily communicated, and easily remembered. Yet the present tendency, in all armies, seems to be toward the use of small changeable codes, which are written (printed) documents; and, for certain purposes, small mechanical devices.
An enormous number of military cryptograms will be transmitted by radio and taken down by enemy listeners, and even the ordinary wire will be tapped. It is expected that the enemy will intercept dozens, and even hundreds, of cryptograms in a single day, some of which will inevitably be enciphered with the same key. With so much material, knowing the general subject matter, and often exactly what words to expect, or the personal expressions invariably used by individuals, it is conceded that he will read the messages. All that is desired of a cryptogram is that it will resist his efforts for a sufficient length of time to render its contents valueless when he finally discovers them. By that time, of course, the key will have been changed, probably several times, and even the cipher.
With these general facts understood, we may first dispose hastily of the concealment cipher, after which we will examine at greater length the two legitimate types, the transpositions and the substitutions.
CHAPTER 2Concealment Devices
Concealment writing may take a host of forms. Perhaps its oldest known application is found in the ancient device of writing a secret message on the shaved head of a slave and dispatching the slave with his communication after his growing hair had covered the writing. Or, if this appears a little incredible, the ancients have left us records of another device considerably more practical: that of writing the secret message on a wooden tablet, covering this with a wax coating, and writing a second message on top of the first.
In the middle ages we meet a development called puncture cipher; any piece of printed matter, such as a public proclamation, serves as the vehicle, and the cipher consists simply in punching holes with a pin under certain letters, so that these letters, read in regular order, will convey the desired information. It is said that this kind of concealment writing was resorted to in England at a comparatively recent period, to avoid the payment of postage. Postage on letters was very high, while newspapers were permitted to travel free, and the correspondents sent their messages very handily by punching holes under the letters printed in newspapers. Where the sender of a message may also control the preparation of the printed vehicle, any desired letters can be pointed out by the use of special type forms, misspelled words, accidental gaps, and so on.
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.
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.
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."
(Continues...)
Excerpted from Cryptanalysis by Helen Fouché Gaines. Copyright © 1956 Dover Publications, Inc.. Excerpted by permission of Dover Publications, Inc..
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents
PrefaceGeneral Information
Concealment Devices
Transposition Types
Geometrical Types--The Nihilist Transposition
Geometrical Types--The Turning Grille
Irregular Types--Columnar Transposition
"General Methods--Multiple Anagramming, Etc."
Substitution Types
Simple Substitution--Fundamentals
The Consonant-Line Short Cut
A Method for Attacking Difficult Cases (by George C. Lamb)
Simple Substitution with Complexities
Multiple-Alphabet Ciphers--The Vigenere
"The Gronsfeld, Porta, and Beaufort Ciphers"
The Kasiski Method for Periodic Ciphers
Miscellaneous Phases of Vigenere Decryptment
Auto-Encipherment
Some Periodic Number-Ciphers
Periodic Ciphers with Mixed Alphabets
Polyalphabetical Encipherment Applied by Groups
Vigenere with Key-Progression
Polygram Substitution--The Playfair Cipher
Highlights of Fractional Substitution
Investigating the Unknown Cipher
Appendix
English Frequency and Sequence Data
Comparative Table of Single-Letter Frequencies
Chart Showing Normal Contact Percentages (by F.R. Carter)
Chart Showing Frequencies of English Digrams (by O. Phelps Meaker)
Some Foreign Language Data
Bibliography (by W.D. Witt)
The Commonest English Words (by Frank R. Fraprie)
English Trigrams (by Frank R. Fraprie)
English Digrams (by Frank R. Fraprie)
Solutions
Index