~ ! - ~ 

,, ~ REF ID:A414885~ 

SUBJECT Cipher Printin~elegraph System. ~ 

._AMERICAN TELEPHONE AND TELEGRAPH COMPANY 

BELL ·SYSTEM 

DEPARTMENT OF" DEVELOPMENT ANO RESEARCH 

FRANK B ..JEWETT VICE PRESIDENT 

EDWIN H COLPITTS 

ASSISTANT VICE PRESIDENT 

LYMAN F MOREHOUSE 

EQUIPMENT DEVELOPMENT ENGINEER 

FREDERICK L RHODES 

OUTSIDE PLANT DEVELOPMENT ENGIN-EER 

OTTO B BLACKWELL 

TRANSMISSION DEVELOPMENT ENGINEER 

HOWARD S WARREN 

ELECTRICAL INTERFERENCE ENGINEER 

GEORGE A CAMPBELL 

RESEARCH ENGINEER 

HARRY S SHEPPARD EXECUTIVE ASSISTANT 

11/Iajor William F. Friedman, 

195 BROADWAY 

NEW YORK February 13, 1926 

Office of the Chief Signal Officer, 

W~shington, D. C. 

My dear Major Friedman: 

As requested in your letter of February 10, I am 

sending you a number of additional copies of Mr. Vernam's 

paper on "Cipher Printing Telegraph Systems." I shall be 

glad to furnish more copies if you should need them. 

With kindest regards, I am, 

Attached: 

6 Copies of Paper 

on "Cipher Printing Telegraph Systems. 11 ~· 

GSV:CB 

Sincerely yours, 

@'pp roved for Release bv NSA on 11-18-2014 pursuant to E .0. 1352\3 
• ~F ID:A414BB56. e 

Cipher Printing Telegraph Systems 

For Secret Wire and Radio Telegraphic Communications 

BY G. S. VERNAM1 

Associate, A I E E 

Synopsis.-Tlns ]Japer describes a printing telegraph cipher 

system developed during the ll' 01 ld lV ar for the use of the Signal 

Corps, U. S Arnn;. :J'his system is so de1>igned that the messages 

are in sec1et form from the lune they leave the sender unt'il they arc 

deciphe1 ed a utonwtically at the office of the addressee. If copied 

while en route, the messages Gannol be deciphered by an enemy, 

even though he has full knowledge of the 11wthocli> and appmalns 

INTRODUCTION 

THE purpose of this paper is to discuss briefly certain methods for obtaining secrecy in connection 

with messages sent by wire or radio telegraphy, 

and to describe in particular printing telegraph cipher 

systems that were developed for this purpose during 

the World War. 

The desirability of obtaining secrecy in telegraphic 

communications and the possible advantages of a 

system that would be capable of sending messages in 

such form as to be entirely secret, and which at the 

same time, would be more rapid and accurate than 

the codes and ciphers ordinarily used, were brought 

out in conversations with officers of the Signal· Corps, 

U. S. Army. These discussions made it evident to the 

engineers of the Bell System that it would be very helpful if the well-known automatic features of the printing 

telegraph art could be made available for enciphering 

and deciphering telegraph messages, and could at the 

same time be made practical for use under service 

conditions. 

The engineers recognized that printing telegraphs2 

were rapid and accurate, but were not secret except 

to the extent that their signals could not be read from a 

telegraph sounder. With the general requirements 

for secrecy systems in mind, studies were made of 

printing telegraph systems to determine how their 

messages could be made secret. The result of this 

work was the development of a cipher system that is 

capable of rendering messages e.ntirely secret, is rapid 

and accurate, and is practical to use. 

This "Cipher Printing Telegraph System" was called 

to the attention of the Signal Corps. The Signal 

Corps became very much interested, tested the secrecy 

of communications handled by the system and tried 

1. JDngrnee1, Dept Development and Re:,earch, Am Tel & 

Tel Co 

2. See John l-1 Bell, ''Printmg Telegraph S~·stems," TnANS. 

A. I. l<}. K for 1920, Vol XXXTX, Part 1, p 167, 1rnd 

A. H. Reiher, ' Pnntmg 'Telegraph Sy:,tems Applied to :Message 

Traffic Hanclhng," Tl{Al\'S A I E E for 1922, Vol XLT. p.39 

To be 711 ese11ted at the Mulwinte1 Convention of the A I E. E , 

New York, Feb 8-11, 1926 

1 

used. The operation of the equipment is desc1ibed, as well.as the 

method of using it for sending messages by wire, mail or radio. 

The paper also dii>cusses the prnctical wipossibility of preventing the co7Jying of messages, as by wire tap7nng, and the relative 

advantages of vmious codes and ciphers as regmds speed, accuracy 

and the sec1 epy of their messages. 

* * * * * 

it out between New York and Washington. This 

trial proved that the system could be successfully used 

to send messages secretly and at a speed many times 

faster than by methods previously in use.3 

Each message is automatically enciphered at the 

sending station and deciphered in the same manner 

at the receiving station. The method of ciphering will 

be described later in this paper and is such that under 

certain conditions of use, the messages are rendered 

entirely secret, and are impossible to analyze without 

the key, even if it is assumed that the enemy can 

capture a machine, learn its method of operation in all 

details, and intercept a large number of messages. 

FLEXIBILITY OF SYSTEM 

'I 

___ t1ac.h1ne 

f'erforalor 

_ Ke-1 Tape Tnm-;m1t.ler!I 

--L.~-l'ley Taoeo; 

This method of ciphering can be used with machines 

of various types. The electrically-driven machine 

shown in Fig. 1 was developed during the war particularly for the Signal Corps, U. S. Army. In order t6 

save time in production, standard printing telegraph 

parts were used wherever possible with the result that 

this machine has the appearance of a "start-stop" 

printing telegraph set with some, additional units 

mounted on a shelf at the right end of the table. This 

type of cipher set is particularly suitable for handling 

large,amounts of traffic at high speed. 

3 Note See page 140, "'Heport of the Chief Signal Officer 

to the Secr<Jtary of War" for the year ending June 30, 1919 

/T 

L 
REF ID:A414BB. ' 

2 VERNAM· CIPHER PRINTING TEUJGRAPH SYSTEMS 

If something smaller in size and portable is required, 

the machine shown in Fig. 2 may be used. This 

machine is light and strictly portable as no electric· 

current is required for its operation. It is slower than 

the large machine and requires a knowledge of the 

standard "Baudot" printer code (see Fig. 8) on the 

part of the operator, but' its messages are equally 

secret. 

These machines are considered suitable for general 

use by government departments, business concerns, 

FIG. 2-PoHTABLE CrrHEH 1\1Ac11110J 

etc., for handling confidential messages rapidly and 

secretly. The method of using them can be varied to 

suit conditions and so as to make unauthorized decipherment as difficult as may be necessary up to the 

point where it becomes impossible even for an expert 

cryptanalyst. 

If an appreciable demand exists for machines of 

special sizes or having particular operating features for 

special uses, these can be built to employ the same 

secrecy principle. For example, the functions of 

ciphering and transmitting over a telegraph circuit can 

be combined in one machine, if desired, so that, at the 

sending stat.ion, messages can be simultaneously enciphered and transmitted over the telegraph circuit, 

and so that, at the receiving station, messages can be 

received, deciphered automatically and printed directly 

in plain text; thus avoiding the slight delay caused 

by separately enciphering and deciphering each mesRUIYW TGCZG PIETY RJGUA ELKEJ EZIAO 

ISCFE LCXHF CONEC XELVY DXJBT WFEJM 

HLGDL DDPYD TPGVQ EZAYI LXSZX 

sage. This method is particularly suitable for cases 

· where the cipher equipment can be directly connected 

to a telegraph line or to a radio transmitter and 

receiver and can· be operated by the same personnel. 

If the cipher messages are to be turned over to a 

telegraph or cable company to transmit, they should 

be in written or printed form. For this purpose, the 

cipher machine can be arranged to print the cipher 

messages in group~ of five lette_rs each, spaced to form 

"words." Fig. 3 is a copy of such a message shown 

exactly as it was prepared by the cipher set. Such 

messages can be printed by the machine directly on 

the telegram blank with the address and signature in 

plain English, and if desired, a carbon copy can be 

made at the same time for record purposes. 

PREVENTING ACCESS TO l\IT,,ESSAGES 

There appear to be two general methods for securing 

secrecy in connection with communications, namely, 

(1) by preventing or at least attempting to prevent 

access to the messages or to the lines of communication and,' in the case of telegraphic communications by 

rendering the lines incapable of being tapped, and (2) 

by the use of codes and ciphers with key systems known 

only to the proper parties. 

As regards wire tapping, sensitive alarm devices 

arranged to operate on small changes in the electrical 

constants of the line circuit, are unsuccessful as a 

means of preventing unauthorized parties from obtaining access to the circuit. The electrical condition 

of a long telegraph circuit is continually changing as a 

result of variation in temperature and other weather 

conditions. This fact limits the useful sensitivity 

of any such alarm devices, whereas by using vacuum 

tube amplifiers, a record of the signals passing over a 

circuit can be obtained without appreciably disturbing 

the line' circuit and even without actual contact with 

the wire. 

Telegraph systems have been invented, that will operate successfully on very small lme currents, and which 

use c01ls and condensers to suppress the harmonics in 

the signal impulses, or in other words to avoid sudden 

changes in current value. The currents induced in 

neighboring circuits by such a system would be small, 

so that it would be rather difficult, if ordinary methods 

are used, to obtain a record of the signals by their inductive effect. This can be readily done, however, 

if modern vacuum tube amplifying equipment is used. 

It is also obvious that a record can be easily obtained 

if the wire is tapped. 

Many attempts have been made to obtain secrecy 

during the actual transmission of telegraph messages 

by making them unintelligible. In one system of this 

sort, successive signal impulses are sent alternately 

over two line wires by means of a rotary switch which 

puts the sending key in connection first with one wire 

and then with the other at each movement of the key. 

At the receiving end, the impulses are combined 

through one relay. With this system, the messages 

may be readily copied if both wires are tapped, and it 

is quite possible to decipher the messages even if only 

one wire is tapped. 

Proposals have also been made to use complex 

devices or methods, or so to mutilate the normal impulses that they become unintelligible to anyone tapping the line circuit or intercepting the signals if sent 

, 
• ~F ID:A414BB56. e 

VERNAJ\1. CIPH frn, PRIN'l'ING TELJW rtAPH SYSTEMS 3 

by radio. Any secrecy system of this general class 

can be readily "broken" by anyone having a knowledge 

of the methods used and the ability to assemble and 

operate the necessary apparatus. 

TAPPING DUPLEX AND MULTIPLEX CIRCUITS 

It has also been considered that a full duplex circuit or a multiplex printer circuit, in which messages 

are being transmitted simultaneously in opposite 

directions, could not be tapped and that circuits of 

this character insured secrecy to the communications 

thus being handled. This is not true, however, and 

means have been invented by which a message originating at one station of an ordinary duplex circuit can 

be tapped at any part of the circuit, even though a 

second message is also going over the same circuit 

simultaneously in the opposite direction. This means 

that a multiplex printer circuit, in which as many as 

eight messages, four in each direction, are handled 

simultaneously, may be tapped and a person who is 

familiar with the system can r~adily analyze the multiplex impulses to distinguish between adjacent channels 

· and the letters of each message in each channel. 

An arrangement· for tapping a duplex circuit is 

shown in Fig. 4. A single sensitive polar relay may 

be used to receive the signals from either end of the 

circuit, or by using two such relays, the signals in 

both directions may be read simultaneously. Each 

relay nuiy control a sounder or a suitable recording 

de"."ice. One winding of each relay is connected in 

series with the line, the other winding being connected 

in a circuit from line to ground through an "artificial 

line" composed of adjustable resistances and conWEST STATION TAPPING STATION [AST STATION 

<lensers. The line winding of each relay should have 

relatively few turns and should be of low resistance, 

the other winding having a large number of turns. 

Each artificial line should be adjusted to be substantially equivalent to the impedance of the corresponding 

section of line including that of the terminal station 

equipment multiplied by the ratio of turns of the relay 

windings. 

Signals transmitted from the west station will pass 

through the line windings of both relays at the tapping 

station, a small part of the signal currents also going 

through the lower windings and artificial lines to 

ground. The signal currents pass through both windings of the west relay in series, the magnetic effects of 

the two windings aiding each other, so that the armature of this relay will follow the signals. The same 

signals pass through both windings of the east relay 

in parallel, the magnetic effects opposing and balancing 

so that this relay does not respond to signals from the 

west station. 

In a similar manner, signals from the east terminal 

station will energize the east relay but not the west 

relay at the intermediate station, so that by using suitable recording devices associated with each relay, a 

copy of signals in both directions may be obtained. 

TAPPIN,G A MULTIPLEX CIRCUIT 

This method may be used to tap a multiplex printer 

circuit, in which case a tape record of the form shown 

m Fig. 5 will be obtained. If this is taken, from a 

P- h I__!_ FN + ~ 1--a T -+;.ey+ .q z-l'P~E I -cl 

g p-1 _J_ F p-1-L "Fi+ E FL P- F d'P~CEh j 

!!__ 6.aj 0=-J d -"- b j T f o- J ~ i;.fft E Fi I 

Fm. 5-TAPE R~;cono OF MuLl'lPLEX P1uKT1m S1GNALS 

"double-duplex" circuit alternate letters must be read 

as indicated, to get the message from either channel. 

Every third or every fourth letter must be chosen if 

the circuit is operated by the "triple-duplex" or "quadruple-duplex" method. The individual letters are 

in the ordinary five-unit printer code, the polarities 

of alternate signals being reversed. To decipher such 

a tape, it should be divided into units of five dot 

lengths each. The correct starting point can be found 

in not more than five trials and can be recognized by 

the fact that the letters of each message then form 

sensible combinations. 

CODES AND CIPHERS 

Secrecy, in connection with telegraphic communications, is usually obtained at the present time by means 

of codes and ciphers, the term "code" being applied 

in cryptography to the method in which entire words, 

or phrases of a message are replaced by arbitrary 

groups of letters or numbers usually printed in a code 

book, identical copies being kept by those using the 

code, "cipher" referring usually to a system in which 

the individual letters of a message undergo a change 

either in arrangement or nature. 

It is obvious that the combinations of letters in a 

cipher message will not form pronounceable groups or 

genuine words except occasionally by accident, but 

"code" systems can be arranged to use pronounceable 

artificial "words" or actual dictionary words, if desired. 

This is usually done, as such "code words" are handled 

by the telegraph and cable companies at a cheaper rate 

than the unpronounceable so called cipher "words." 

Each of these two general systems has advantages 

and disadvantages which cause them to be used for 

---~ 
,_ 

~--. 

REF ID:A414BB. 

4 VERNAM: CJPHEH PRINTING 'J'J~LJWHAPH SYS'l'EMS 

certain classes of work, depending upon the conditions. 

The code system has the outstanding advantage, especially for commercial work, of enabling messages to 

be shortened so that the tolls are reduced, and it 

is chiefly for this reason that commercial codes are 

used. Code is not very accurate, as a mistake in a 

single code group or even a single letter may change 

the meaning of an entire message, or necessitate its 

repetition. If secrecy is required, it is necessary to use 

carefully guarded private code books, the maintenance 

of secrecy and accuracy during the printing and distribution of which may cause great trouble. Such 

books must be carefully used to maintain secrecy, and 

must be immediately replaced, sometimes at great 

expense and inconvenience if they should become 

compromised. 

Ciphers, in general, are slower tha;n codes unless 

machines are used, but then they may be very much 

faster. They are more accurate, and depending on 

the system used, cipher messages may be more or less 

secret than code messages. 

There are two general classes of ciphers, known respectively as transposition ciphers and substitution 

ciphers. In the first class, as the name suggests, the 

letters of the original message are rearranged, according 

to a definite system, and in the second class, substitutions for the original letters are made according to 

some prearranged key. In one, the relative positions 

of the letters are changed and in the other, the letters 

themselves. 

TRANSPOSITION CIPHERS 

A transposition cipher may be distinguished from a 

substitution cipher by a study of the frequency of 

occurrence of the letters of the message by comparison 

with a frequency table of the language of the original 

message. Studies which we have made of the frequency of the different letters of the English language 

as they occur in telegrams sent over our private wires, 

indicate that they are used about as shown in Fig. 6. 

It is apparent that some letters are used very frequently, the vowels a, e, i, o, u, forming approximately 

40 per cent. of the total, e being the most commonly 

used letter of all. 4This chart is similar to those used 

by cipher experts. 

In a transposition cipher the letters must be rearranged according to a definite system known to the 

receiving correspondent. Those who make a study of 

ciphers tell us that such systems are usually easy to 

discover, particularly if a considerable number of 

messages are intercepted including two or more of 

exactly the same length. Transposition ciphers are 

not suitable for use with machines. 

SUBSTITUTION CIPHERS 

· In substitution ciphers the order of the letters 

r~mains unchanged, but for each letter is substituted 

4. See "J\1anual for the Solution of Military Ciphers" by 

Lt. Col. Parker Hitt. 

its equivalent in one or more cipher alphabets. For 

example, using the table below, for each letter in the 

plain text alphabet we may substitute its equivalent 

in the cipher alphabet. To decipher, this process 

is reversed. · 

Plain Text-A B C D E F G H I J K L M N 0-P Q R S 

TUVWXYZ 

Cipher -F Q RUKA H G Z SE M LY P 0 B C .J 

VDTXWNI 

'" 0 

fR£.QU£.NCY Or OCCURRENCE Qr LETTERS 

Bind on ii count ot\4,661 let~~ tn pnv;ite wJre \rleqrilm' 

I 

I 

'-'- !_L 

I~ I 0 

I 

f- _f-_ 1 -1--

,_ ·- 0 -- -1-f-•-

- - -1-- -

I ,~ ~ 

I I :ld I 

-- - _L J_L_·~- t:1J ABC DE I" G HI J Kl 111 N 0 P QR S 1 U v WX y: 

Fra. G 

If a chart is prepared from a frequency count of the 

letters in such a cipher message, it will have the general 

appearance of Fig. 6 but the crests will correspond to 

different letters. Messages of this type are readily 

deciphered by an expert even when a "mixed" alphabet 

A11CDEFGHIJKLKNOPQRSTUVWXYZ 

A E a r R A c B ~ w J K z I 0 6 y R D u w s x T v p L 

BGNQTOHAFXLPJSBEKCWUDVURIYZ 

C F Q R U K. A H G Z 8 E M L Y P 0 B t J V D T X W N I 

DR TU ;f B K.W X .PD i" ll Y L ZIV.AS BC Q G H Iii 0 

EA.OKEDSllYVRCWXGBQPJFZUILMHT 

FCHAKSJ"BLP'DIZPYNGUEXRWVTOY 

GBAHWRQLCSZYGUEOVFTIRXPDUKJ 

H ~ F G X Y B C 0 J I H P T K H L A V Z Y W R U D E S 

IYXZPVLSJWRTFAQ.UDNYGKOEIBRC 

JJLSDRFZIHAUBQ.TWXMD:CNKYOPVG 

KKPEFCDYBTUJ.XWHQ.BOSRIJZMLGV 

L Z J K N V I G P F B X T C D R H S 0 Q L Y V B K U A 

M I S L Y X Z M T A Q W C G U V R J P B R H 0 K E D F 

NOBYLGPEKQTE!DUWAFIZVJY.SNXCR 

ONEPZBYOHUWQRVJ.GCKLXTIMJSFD 

PYKOIQ.NV DXBHRFC EllWPZGSJAU 

Q H C B V P G F A N U 0 S J I }~ Z Z X L U '? D Y R VI Q 

RDWCAJUTVYESOPZLUXRKGFHBQ.lN 

SULlJSFEIZGC~Q.BVXWLKDYANPOTH 

TWDVBZ.IBM.KllILHJTPUGYOQCAFSE 

USVDCORXWOKJY l:JIZTFAQEBHGLP 

VXUTQIWPRll:YZVO YGDHNCBLFAJK 

WTRXGLVDUTO?.!ET.:llJSYBPAAFZCQW 

:I V I W A Y T U D B P t, K E X S J R Q. 0 II' G A C N Z Y 

YPYYllHOKERVGUDCFAWITSLJQ.ZBX 

ZLZTOT.MJ GVtFRDUqNHEPKWYXB 

F1a. 7 

is used, such as that illustrated above in which the 

letters of the cipher alphabet are not in the usual alphabetic order. 

By using more than one alphabet, the cipher may be 

made more difficult to "break." The method may 
• ~F ID:A414BB56. e 

VERNAM CIPHER PRINTING 'l'ELEOHAPH SYSTErdS 5 

be described by referring to the "cipher square" shown 

in Fig. 7. In this table, the top alphabet represents 

the plain text, while below it are shown 26 cipher 

alphabets, each designated by a "key" letter given in 

the left-hand column. Some form of key, usually 

a word, is used, the letters of this key word designating 

the alphabets and the order in which they are to be 

used. A different cipher alphabet is used in a repeating 

manner, with each successive letter of the message. 

This type of cipher may be distinguished by the fact 

that the frequency chart is rather flat, the frequency of 

occurrence of all letters being roughly the same. 

-Each cipher alphabet is used repeatedly at regular intervals. By first finding this interval and then studying each alphabet separately, messages of this type 

can be deciphered readily by an expert. 

RUNNING KEY CIPHERS 

If the key used with this type of cipher is made 

very long, so that it never repeats and if any portion 

of this key is never used for more than one message, 

the operation of "breaking" the cipher becomes very 

much more difficult. If, now, in.stead of using English 

words or sentences, we employ a key composed of 

letters selected absolutely at random, a cipher system 

is produced which is absolutely unbreakable. · 

This method, if carried out manually, is slow and 

laborious and liable to errors. If errors occur, such as 

the omission of one or more letters, the messages are 

difficult for the recipient to decipher. Certain difficulties would also be involved in preparing, copying and 

guarding the long random keys. The difficulties with 

this system are such as to make it unsuitable for general 

use, unless mechanical methods are used. 

CIPHER PRINTING TELEGRAPH SYSTEM 

By using machine methods, this type of cipher may 

be made practicable for use. Fig. 1 is an illustration 

of the cipher machine previously referred to, and which 

operates on this principle. As previously mentioned, 

this machine was developed during the recent war and 

adopted by the Signal Corps, U. S. Army. 

J' Certain parts of this machine are the same as those 

used for ordinary printing telegraphs, such as those 

described in recent papers before the Institute. For 

this reason, it will not be necessary to describe in detail the parts which are commonly used in such systems, 

such as the keyboard perforator, the transmitters, and 

printer. 

CIPHER MACHINE-METHOD OF OPERATION 

The messages are first punched in a paper tape by 

means of the keyboard perforator. The code used is 

shown in Fig. 8. This is the well-known five-unit 

printing telegraph code. Each letter is represented 

by a small feed hole and one or more larger holes 

which may be punched in five different positions across 

the tape. Smee in each of these five positions a hole 

may or may not be punched, there are (2) 5 or 32 

possible di~erent combinations in this code of which 

26 are used to designate letters, the other 6 representing 

the so-called "stunts,'' which are the "space,'' "carriage 

return,'' "line feed,'' "figure shift," "letter shift,'' 

and the "blank" or "idle" signal. 

The cipher "key" may take the form of another tape 

of similar form having characters punched in it at 

random and with every tenth character numbered, ' 

so that the tape may be set to any designated starting 

position. The key tapes are prepared in advance, the 

original key being perforated by hand, as by working 

B ~ ~ "' a: :l g ~ 3""~g:5 7 ~4£ 1 & ?2 ::~5·• 

E~Bi5T UCM SRH FG BW ~;;::yz 

1 • • • • • • • • •••••• a· .... ~ •... ! ........ ! .! ...... ! . ! .... !. ! .... ! ...... ! .... ! .. '!' .! .... ! .. ! .... !. ! ...... . 

~ ·. ·:. ·:: :: ·.·. : .. ·. •: :.· 5 • • • • • • • • • • • • • ••• 

-8,9 (, $) '0 1/ AINO KV DL JP OX 

FIG. 8-~AP~~ S11owING PRINTING TEI,EGHAl'HIC Co1rn 

·the keyboard at random, additional copies being made 

automatically by the machine. 

The message tape is passed through a unit known as 

a transmitter, where the holes in the tape serve to 

control the positions of five contact levers, each of 

which makes contact with either of two bus-bars. The 

key tape controls the contacts of a second tape transmitter. The contacts of the two transmitters are 

connected to a set of five magnets or rel~y~\\as shown in 

Fig. 9. Each magnet will be energized if the correspondingly numbered contacts of the two transmitters 

are against opposite bus-bars, but not if they are maFIG. 9 

king contact with similar bus-bars.// In the diagram, 

contacts 1 and 2 of the message transmitter, a;re against 

the left or positive bus-bar, this setting representing 

the letter A. Contacts 1, 4 and 5 of the key transmitter are against the positive bus-bar, representing 

the letter B in the printer code. This will energize 

magnets 2, 4 and 5, which combination represents the 

letter G. 

All of the possible combinations resulting from 

various characters in the two tapes might be shown in 

a cipher square similar to that of Fig. 7 except that it 

would have 32 characters on a side instead of 26. 

The characters of the cipher messages, formed in this 
,.---

REF ID:A414BB • • 

6 VERNAM: CIPHER PlUNTrNG TELEGRAPH SYSTEMS 

way, may be recorded as perforations in a third tape. 

For this purpose a "machine perforator" is used. 

This device is similar in many resP,ects to, a keyboard 

perforator and is shown in Fig. 10. The tape, from a 

reel on the top of the machine, passes through the 

punch block at the front left corner of the machine. 

Here it passes under a die plate and over a group 

of six punches, which may be forced up through the 

tape by the action of an electromagnetic hammer. 

Five of these punches are too short to be acted on 

directly by the hammer and are pushed through the 

tape only when an individual "selecting finger" is interposed between the punch and hammer. The five 

selecting fingers are actuated by five magnets which 

may be controlled by the relays shown in Fig. 9. A 

ratchet-operated star-wheel feeds the tape forward 

after each character has been punched. 

The cipher message tape prepared in this way is unintelligible in form and may be sent to the receiving 

station by messenger or by mail, or if desired, it may 

be transmitted by wire or radio and reproduced by 

another machine perforator at the receiving point. 

The cipher tape is there run through the message transmitter, where its characters combine with those of a 

duplicate key tape to reproduce the original message, 

which will be printed out in page form and in "plain 

text." 

LENGTH OF KEY TAPE 

With the system as described above, the key tape 

must be a:t least as long as the sum of all the message 

tapes used with it, as the messages will lose their secrecy 

to some extent if the key tape is used repeatedly. The 

use of a short repeating key may give sufficient secrecy 

for some uses however. 

A roll of tape eight inches in diameter contains about 

900 feet of tape and would serve to encipher about 

18,000 words counting five printed characters and a 

space per word, without repeating the key. If sent at 

an average speed of 45 words per minute this number 

of words would require 400 minutes or nearly 7 hours 

to transmit. 

In order to reduce the amount of key tape required 

for handling large amounts of traffic, the ''double key" 

system was devised. In this system two key tapes 

are used, the ends of each tape being glued together to 

form a loop preferably about seven feet in circumference. The tapes should differ in length by one character or by some number which is not a factor of the 

number of characters in either tape. A separate transmitter is used for each tape, and the characters of the 

two key tapes are combined, by a method similar to 

that shown in Fig. 9, with those of the message tape 

to form the cipher message. 

The result is the same as though the two key tapes 

were ffrst combined to produce a long single nonrepeating key, which was later combined with the 

message tape. This long, single key is not, strictly 

speaking, a purely random key throughout its length 

as it is made up of combinations of the two original 

and comparatively short key tapes. The characters 

in this key do not repeat in the same sequence at 

comparatively short regular intervals, however, as 

would be the case if only one key tape loop were used. 

The number of characters in this equivalent single 

key is equal to the product of the number of characters 

in the two tape loops, and may easily exceed 600,000 

before ·any part of the key begins to repeat. If 

proper care is taken to use the system so as to avoid 

giving information to the enemy regarding the lengths 

of the two key tape loops or their initial settings and to 

avoid the possibility of ever re-using any part of the resultant single key, this system is extremely difficult to 

break even by an expert cryptanalyst having a large 

number of messages and full knowledge of the- construction of the machine and its method of operation. 

Captain W. F. Friedman, Cryptanalyst of the Signal 

Corps, U.S. Army, has recently invented some modifications• of this system intended to eliminate the loss 

in secrecy that results from using the two more convenient comparatively short repeating key tapes instead of the single long non-repeating key tape. These 

modifications consist of changing at intervals the 

order of connection of the five contacts of one or more 

of the tape transmitters or of adding a third key tape 

and transmitter so arranged that the ·extra key tape 

does not step ahead in unison with the other two key 

tapes, but starts and stops at irregular intervals. 

Either of these methods, properly used, makes' unauthorized decipherment" practically impossible and, 

at the same time, does not unduly complicate the 

machine or its method of operation. 

With the double key tape system, the handling of 

large volumes of traffic is greatly simplified. The 

tapes should be numbered so that the deciphering 

operator can set them at the correct starting point for 

each message, and rules should be adopted so that 

both key tapes will never be set twice at the same 

5. See Patents 1,522,775 of Jan. 1:3, l!J2.5 and 1,.516,180 of 

No_v. 18, 1924. 
• rF ID:A414BB56. e 

VERNAM: CIPHER PRINTING TELEGRAPH SYSTEl'vfS 7 

starting point. Information regarding the proper 

settings for the key tapes for deciphering each message 

must be sent to the deciphering operator. These settings may be prearranged or they may be selected 

arbitranly by the sending operator. In the latter 

case the numbers representing the key tape settings 

should be prefixed to the message. These "key indicators" should preferably be enciphered by running 

them through the machine together with a special key 

tape which is used only for this purpose. 

SPEED OF OPERATION 

This type of machine was operated by the Signal 

Corps over its private wire telegraph circuits. In_ 

service tests made by the United States Army, each 

outgoing message was checked by running it again 

through the machine to decipher and print it, and the 

deciphered copy was then compared with the original 

message, so that each message tape was run through 

twice. A certain amount of time was lost, due to 

setting and resetting the key tapes, checking, etc., for 

each message, but an average enciphering speed of 

10-15 words per minute wa's readily maintained. We 

understand this to be many times faster than those 

manual methods for enciphering or coding, which are 

used where a high degree of secrecy is required. Incoming messages were deciphered at the rate of 30-40 

A considerable number of cipher messages were transmitted over this radio circuit during this demonstration, these messages being automatically enciphered 

before and deciphered after transmission, so that they 

they were absolutely secret, even though transmitted 

by radio. No interference from atmospherics or from 

other radio stations was noticed, all messages being 

received without error. 

In conclusion, we wish to express our appreciation 

of the assistance given us by the officers of the Signal 

Corps and the General Staff, of the United States Army, 

in making tests and trials of cipher printing telegraph 

CL/FFWOOD t-.J J 

ST NEW YOPK 

f1! I 

I 

I 

I 

: C;:ible 

I 

I 

~_:rn ~-.__~ _ __, 

;;. 195 BROADWAY 

NEW YORK 

words per minute. Fw 11-Crncul'I' FOH 

OPERA'l'ION BY RADIO 

This cipher system was demonstrated before the 

deleg~tes to the Preliminary International Communications Conference in October, 1920. During this demonstration, cipher messages were sent over a circuit containing a radio link, as illustrated in Fig. 11. The radio 

eqmpment was the same as that employed a year previous in tests on the operation of multiplex and startstop printing telegraphs by radio and is described elsewhere.6 

6. "'Pnntmg 'l'elcgrnph by liadw" by R. A. Heismg, J ow nnl 

uf the Franklin Institute, Janua!'y 1922, PP- 9,7-101 

systems; and we wish particularly to acknowledge 

our indebtedness to Lt.-Col. J. 0. Mauborgne, of the 

Signal Corps, for his advice in connection with this 

development and for his assistance in arranging to 

have tests made to determine its secrecy and demonstrations and service trials to determine its practicability for Army use. We also wish to express our appreciation of the services rendered by the Cipher Department of the Riverbank Laboratories, Geneva, Illinois, 

and by Col. George Fabyan, the head of these laboratories, in making tests of the secrecy of messages enc:phered in various ways with these m:i.chines. 

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