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41760779079990.Pdf .CONFIS~Ti;~691 BBS'l'RIOHJB· THE DISTBIBUTION OF THIS SPECIAL TEXT WILL BE RESTRICTED TO REGULARLY ENROLLED EXTENSION COURSE STUDENTS, 'l'O MILITARY PERSONNEL, AND TO OTHER PERSONS COMING WITHIN THE MEANING OF THE PHRASE "FOR OFFICIAL USE ONLY/, ·. ARMY EXTENSION COURSES ~,· ·~~cY-l~~ I!LA)f'~'~ ~ \_ -·----------;-- --·· SPECIAL TEXT No. 166 ADVANCED MILITARY CRYPTOGRAPHY Szcoxn (19'3) EDmox PREPARED UNDER THE DIRECTION OF THE CHIEF SIGNAL OFFICER FOR USE WITH 'I1fE ARMY EXTENSION COURSES .',; ,_ • RSS'fRI<ffRB document contains information affecting the nat1 defense of the United States within the meaning of die · nage Act (U.S. C. -Y,. ~:>· 50: 31, 32). The transmission o · ocument · · or the revelation of its contents in any nl:an any unauthorized person is prohibited. + UNITED STATES GOVERNMENT PRINTING OFFICE ~ (o ~ ~ WASHINGTON : 19'3 . ,. 7.~0NFIDENTIAL Peclassified and approved for release by NSA on 01-14-2014 pursuant to E.O. 1352§ REF ID:A64691 ,·J/ 30 April 1959 Th.is document is re-graded "eEm'!BBrff " of DOD Directive 5200.l dated 8 ~ UP and by autharity of the Directar '2t1:.!i Security ~ncy. ' Paul~w~ S. Willard Colonel, AOC Adjutant Ge.nera.J. • ~· WAR DEPARTMENT, • WAsmNGTON, Februa1"JI S, 1948. Tbis revision of Special Text No. 166, Advanced Milita.ry Cryptog­ raphy (1935), for use with the Anny Extension Courses, is published for the information and guidance of all concerned. BY ORDEB OF THE SlilCBlilTAB.Y OF wAB! :,. G. o. M.ARSHAT,T., General, Ohiejoj8f4. OFFICIAL! JAMES A. ULIO, Major Gemral, 7?i.e Adjut,ant Geneml,. (XI) • SPECIAL TEXT NO. 166 ADVAllTCBD KILITABY CBYPTOGBAPHY 1943 EDITION • Parqrapha Paga Bmcmox I. Introductory remarks __ ---------------------------- 1- 2 1 A. TRANSPOSITION SYSTEMS • II. Monopbase transposition systems-------------------- 3- 9 5 Ill. Polyphase transposition systems _____________________ 10- 12 17 IV. True double transposition ___________________________ 13- 14 20 V. Grilles and other types of matrices ___________________ 15- 24 24 VI. Miscellaneous transposition systems _______________ .-_ 25- 28 39 B. SUBSTITUTION SYSTEMS VII. Polygraphic systems ____ --------------------------- 29- 31 41 VIII. Checkerboard digraphic substitution ________________ - _ 32-- 37 51 IX. Complex substitution systems _______________________ 38- 45 61 C. REPETITIVE AND COMBINED SYSTEMS X. Repetitive systems _________________________________ 46- 49 68 XI. Combined substitution-transposition systems _________ 50- 58 70 D. CRYPTOGRAPHS AND CIPHER MACHINES XII. Cryptographs _____________________________________ 59-- 62 81 XIII. The obsolete U. 8. Army cipher device, type M-94 ____ 63- 69 85 .··~ XIV. Cipher machines ___________________________________ 70- 75 95 lil. CODE SYSTEMS XV. Code systems in general ____________________________ 76- 77 100 • XVI. Enciphered code----------------------------------- 78- 86 102 (DJ) • ··lf·. ., REF ID:A64691 SECTION l INTRODUCTORY REMARKS Paragraph ™sumcS of preceding information _____ ---- - ----- - -------- - ------ - -- - --- 1 Sequence of study___________________________________________________ 2 1. B.&JumGofprecedinginformation.-a. In Special Text No. • 165 (1935), Elementary Military Cryptography, the student was given his first introduction to the study of the more simple means and methods of secret writing. Considerable attention was devofied. .. • to certain preliminary data in the nature of definitions of basic terms employed in cryptography and of the general circumstances surround­ ing the use of cryptography in military communications. The factors determining the influence or effect that the analysis of military cryptograms will have on the tactical situation were discussed some­ what in detail, and it was shown that of these factors the most im­ portant is the degree of cryptographic security inherent in the cryp­ tographic system itself. This was then discussed in detail in con­ nection with related factors involved in the length of time required to solve military crypt.Ogre.ms. Attention was also directed tO infor­ mation bee.ring upon the employment of cryptography in our Army, and the functions, duties, and responsibilities of the various aniis and services concerned in it were set forth. Coming then to a discussion of certain preliminary details of a practical nature, it was shown that systems suitable for the military use must conform to certain more or less rigid require~ents before they can even be considered for such use because of the present-day limit.ations of the a.rt of signal com­ munication in general. b. Having assimilat;e4 all the foregoing dat.a of an introductory • nature, the student then took up the study of the two principal classes of cryptograms: transposition and substitution. Various examples of cipher systems of the transposition class were first illustrated, these starting out with the simplest varieties of monoliteral route trans­ • position and then progressing through more complex types of simple columnar and keyword columnar methods. Only a. hint was con­ veyed as to the existence of far more complicated double and triple transposition systems. The principal disadvantages of transposition methods in general were discussed. Then substitution systems were taken up and after a brief discussion of the nature of alphabets ~ general and of the kinds of cipher alphabets in particular, a few examples of simple monoalphabetic substitution ciphers were given, (1) REF ID:A64691 .... 2 Methods of producing mixed alphabets were illustrated, and the use of sliding basic sequences to derive a set of secondary alphabets was described. Cases of monoalphabetic substitution with variants were presented and their disadvantages from the point of view of crypto­ graphic security were discussed. It was stated that despite a multi­ plicity of values for cipher equivalents, such methods do not yield cryptograms of a high degree of security, and for this reason other methods of producing a multiplicity of values, based upon true poly­ alphabetic methods, are more satisfactory. The use of cipher disks and cipher tables of various sorts was discussed in connection with more complicated types of substitution, and their disadvantages pointed out. Mention was made of methods of increasing the degree of cryptographic security by suppressing or eliminating the manifes­ tations of periodicity in polyalphabetic systems based upon the use of a repeating key. These led to a consideration of the development and use of cryptographs and cipher machines, a few of which were merely mentioned. c. The category of substitution methods under the heading of code systems was then discussed and examples of the various types of code words and codebook arrangements given. The discussion included a comparison of the advantages and disadvantages of cipher and code methods from the point of view of simplicity, rapidity, practicability, secrecy, accuracy, and economy. Considerable attention wa8 devoted to the secrecy requirements of a cryptographic system for military use. d. There then followed a brief discussion of the errors which are alm0st inevitable in cryptographic communication, and of methods for their ·suppression and elimination. Finally, a summary of the funda­ mental rules for safeguarding cryptograms was presented. e. With the foregoing as a background, a review of which is recom­ mended, the student is in a position now to take up the study of more adv8tD.ced cryptographic methods. Special emphasis is to be laid only upon such systems as are practicable for military use. It is necessary to add, however, that cryptography is by no means a static art or science and that viewpoints are always undergoing change; what is regarded as wholly impracticable today may, through some 0 unforeseen improvement in technique, become feasible tomorrow, and it is unwise to condemn a system too hastily. For example, before the World War, and indeed for the :first two years of that conflict, the use of codebooks in the theater of operations was regarded as wholly impracticable.1 Colonel Hitt in his Manual for the Solution of Military Ciphers, published in 1916, says: 1 See, in this connection, Friedman, William F., American Army Field Codes in the American &peditionary Forces During the First World War, Signal Security Service Publication, OCSigO, War Department, Washington, 1942. REF ID:A64691 !. ! 3 The necessity for exact expression of ideas practioally excludes the use of codes for military work, although it is pOBSible that a special tactical code might be useful for preparation of tactical orders. Also, in an official British .Army Manual, of Oryptography prepared in 1914 is found the following statement: Codes will first be considered, but as they do not fulfill the conditions required of a means of secret communication in the field, they need not be dealt with here I at length. ' In the 1935 edition of this text the foregoing quotations were im- mediately succeeded by the following comment: It need only be pointed out in this connection that today code methods pre­ dominate in the secret communication systems of the military, naval, and dip­ lomatic services of practically all the large nations of the world. Nevertheless, it is likely that within the next decade or two the pendulum may once more swing over to the other position and cipher methods may again come·to the fore, a especially if mechanical and electrical cipher machines are perfected so that their operation becomes practicable for general use. It is for this reason, if for no other, that the cryptographer
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