Center for Cryptologic History

The SIGABA / ECM II Cipher Machine : 

“A Beautiful Idea”
This publication presents a historical perspective for informational and educational purposes, is the 

result of independent research, and does not necessarily reflect a position of NSA/CSS or any other U.S. 

government entity. 

This publication is distributed free by the National Security Agency. 

If you would like additional copies, please e-mail cchpubs@nsa.gov or write to:

Center for Cryptologic History

National Security Agency

9800 Savage Road, Suite 6886

Fort George G. Meade, MD 20755

Timothy Mucklow was a senior historian on the staff of the Center for Cryptologic History following a long career in information assurance at NSA and with the Air Force. After a decade in academia, he served as a military historian at the wing, division, and major command levels and produced

a series of monographs and articles on such topics as information assurance, telecommunications and

computers, and national defense issues, and has offered IA seminars at military facilities around the

world. He received his Ph.D. in 1982. He retired from the CCH in 2013.

 Acknowledgments. The Center for Cryptologic History is grateful to Dr. Craig Bauer, professor

of mathematics at York College of Pennsylvania and editor-in-chief of the journal Cryptologia, for his

assistance with this project, and to LeeAnn Tallman for earlier research.
The SIGABA / ECM II Cipher Machine:

“A Beautiful Idea”

Timothy J. Mucklow

National Security Agency

Center for Cryptologic History

2015

Contents

Introduction ....................................................................................... 1

And Hebern Saw the Wheel .................................................................... 2

Deus ex Machine ................................................................................. 5

Rowlett’s Epiphany .............................................................................. 7

Iacta Alea Est. [The Die Is Cast.] ........................................................... 10

Enter the Navy .................................................................................. 12

A Disagreeable but Rewarding Surprise ....................................................14

SIGABA Is Built ................................................................................... 16

An Impenetrable Machine ..................................................................... 22

The Big Machine That Did ..................................................................... 25

Appendix A: Technical Analysis of SIGABA’s Key Space .................................. 29

Appendix B: The Mechanics of SIGABA...................................................... 32

Notes ..............................................................................................38

Selected Bibliography .......................................................................... 42
SIGABA / ECM II Cipher Machine
1 1

During World War I, both the Americans and the 

Europeans had used manual code and cipher systems 

to secure their critical information. With the advent 

of the machine age, however, it was only a matter of 

time before cryptography became mechanized. Two 

decades later, as the world embroiled itself in another 

major conflict, the use of cipher machines became 

commonplace, and the old craft of cryptology at long 

last became mathematically based.With the dawn of 

this new cryptologic age, a worldwide scramble for 

even stronger encryption began.

Well before the First World War, much work had 

been done to formalize the cryptographic discipline. 

One of those who made a major contribution to 

this process was the 19th-century polyglot Auguste 

Kerckhoffs, who, besides teaching languages, dabbled 

in cryptography. In 1883 he penned a two-essay series,

La Cryptographie Militaire, for the Journal des Sciences 

Militaire, which earned him a niche in cryptographic 

history. Kerckhoffs in his writings had articulated six 

basic principles of cryptography, but he is most commonly remembered for his second, which states that 

the “design of the system should not require secrecy, 

and compromise of the system should not inconvenience the correspondents.”

1

 His argument was that 

neither the elements of an encryption algorithm nor 

the workings of an encryption machine should constitute the basis of a cipher’s security. Instead, an easily changeable key should be the critical component.

In the 1930s, the U.S. Army cryptologist William Friedman and his assistant Frank Rowlett drew

on this simple precept to conceive a cipher machine 

that was easy to use, simple to rekey, and ostensibly 

impossible to break. Then, in a fit of collaboration 

with the Navy, as unprecedented as it was peculiar, 

the two services went on to perfect and jointly field 

their device. To the Army it was known as SIGABA,

to the Navy, ECM (Electric Cipher Machine) II. 

(The technical aspects of SIGABA’s operation can 

be found in Appendixes A and B.) Not only was 

SIGABA the most secure cipher machine of World 

War II, but it went on to provide yeoman service for 

decades thereafter. The story of its development is 

improbable. Its impact was incalculable.

 

Introduction
2

 And Hebern Saw the Wheel

posed, but using a set of cipher wheels whose 

wiring was to be unknown to Friedman ...

Friedman was successful in his attack on the 

system.4

With little effort, Friedman was able to decipher 

every Hebern-enciphered message sent his way. To

do this, Friedman, who had already married cryptanalysis and mathematics, used a statistical approach 

along with a “divide-and-conquer” strategy to break 

the Hebern machine. As Friedman saw it, there were

only two unknowns, the cipher wheel (with twentysix characters) and the rest of the machine. Since the 

cipher wheel stepped regularly and predictably, the 

rest of the machine could be considered a constant.

It was then a simple matter of sequentially analyzing 

the individual components.

Friedman realized from experience that the regular advancement of the Hebern cipher rotors, like all

machines of that era, posed an intrinsic cryptologic

vulnerability that was mathematically exploitable. He

conjectured that to make a cryptographically sound

machine, one would need a countermeasure to address

predictable rotor movements. But just how such pseudorandom movement could be achieved consumed

Friedman’s thoughts for years. Friedman’s solution,

when it finally came to him around 1926, was conceptually simple and the technology proven.5

 It occurred to

him that paper tape similar to that used by telegraphers

Mr. Edward Hebern of Madera, California, is 

generally credited with inventing the first American 

electromechanical rotor cipher machine.

2

 According to lore, he came up with the idea around 1912 

while serving a term in the state penitentiary for 

horse thievery. If there were any connection between 

breaking rocks and his cryptographic epiphany, 

Hebern never offered one. By the end of 1915 he 

had built a working model, filed a patent, and was 

knocking on the doors of potential buyers.3

 It was 

not long before the U.S. Navy began showing interest in Hebern’s device, and in the years that followed 

the Navy would buy and test a number of his successive machines. As something of a historical twist,

one of Hebern’s early employees, Agnes (Meyer) 

Driscoll,went on to become a luminary of American 

cryptology in her own right.

Half a century after Hebern set out to peddle 

his original device, the Army cryptologist Frank 

Rowlett acknowledged in his memoirs that the 

Hebern cipher machine was an important invention in pre-World War II American cryptography. 

Rowlett, nevertheless, went on to add, 

Navy cryptographers asked [William] 

Friedman for his assessment of the security 

it afforded ... [T]he Navy [supplied] a set 

of messages prepared exactly in accordance 

with the procedures which they had pro-
3

And Hebern Saw the Wheel

could also be used to dictate the movements of the cipher wheels. Friedman

reasoned that it would be relatively easy to

generate long sequences of random fiveunit keys and that the tape itself was easily replaceable and relatively inexpensive. 

Holes punched in a tape would permit

feeler contacts to turn an electrical current on or off, causing a cipher machine’s

rotors to step. Thus, with each key stroke, 

randomly placed holes in a five-group tape

would produce an apparent random stepping for one or more of the cipher rotors.

The cipher text is printed on a small tape. 

The decryption process simply works in

reverse. Key tape could be manufactured

in secure facilities, distributed to the

appropriate recipients, and inserted into

the machine according to rigidly imposed,

pre-established schedules.6 

William Friedman with AT&T printing telegraph, 1920 (courtesy 

of the George C. Marshall Foundation, Lexington, Virginia)

Edward Hebern and his electromechanical rotor cipher machine 

(courtesy of Ralph Simpson, CipherMachines.com)
4

SIGABA / ECM II Cipher Machine

Five-group punched hole paper telegraphic tape

The crypto rotor wheels William Friedman envisioned for his device 

were flattened cylinders with an alphabet around the circumference. 

One face of the cylinder had twenty-six spring-loaded copper pins 

protruding from it; the other face had twenty-six flush copper contacts. 

Inside each cylinder was a wire maze connecting the electrical contacts 

on one side to the pins on the other. 

Thus, an electrical impulse beginning with, say, the letter A on one 

side might connect to H on the other side and so on around the wheel in 

random fashion. Several cylinders serially juxtaposed on a spindle sideby-side could further scramble impulses. (Image courtesy the online 

Crypto Museum, www.cryptomuseum.com) 
5 5

Deus ex Machine

not—recognize the unique contribution to cryptology presented by randomly stepping rotors. There 

remain today bundles of letters between Friedman 

and the patent office, often in duplicate and triplicate copies, meticulously detailing the merits of his 

respective patents years after the initial patent applications were filed.10 In spite of his repeated petitions,

patent office officials sitting only blocks away from 

Friedman obdurately continued to presume that any 

cipher rotor machine was just an unimaginative derivation of the Hebern machine. 

 In a letter from Friedman to the U.S. Patent 

Office dated December 5, 1934, regarding Patent 

Serial No. 682,096, he stated, “the cipher key here 

serves as the physical embodiment of the ‘keying 

principle’ ... and that its sole purpose is to serve as 

the controlling element in effecting the displacements of the cipher wheels in a variable manner. 

Contrast this situation with that in Hebern.”

11 In 

the Hebern cipher machine, the complexity is the 

machine’s wiring, not the initial setting of the cipher 

wheels or the placement of the rotors. Friedman 

continued, “Referring now only to the mechanism 

for displacing the cipher wheels, in Hebern there is 

embodied no such thing as a cipher key which corresponds to a ‘keying principle which is variable in 

character’ because the mechanism for displacing the 

cipher wheels is absolutely fixed.”

12 In the Hebern 

cipher machine, the cipher wheels advanced in the 

On April 23, 1932, Friedman revealed his idea of 

randomly stepping rotors to his three junior cryptanalysts: Frank Rowlett, Solomon Kullback, and Abraham Sinkov.

7 This foursome at the time constituted 

the bulk of the U.S. Army’s cryptanalytic organization, the Signal Intelligence Service (SIS). At the 

meeting Friedman enthusiastically explained that 

“the inherent weakness of all such devices” (where 

the devices were cipher machines that determined 

their own rotor stepping pattern) is that “the keying 

mechanism is a part of the device itself.”

8 He then 

went on to draw for them several rough sketches of a 

cipher machine with a “key tape transmitter” that, in 

conformity with Kerckhoffs’ second principle, would 

separate keying material from the cipher machine. 

His colleagues were duly impressed with his visionary countermeasure; certainly Friedman had good 

reason to be pleased with his own inventiveness.

William Friedman, over the course of his life, 

demonstrated a penchant for collecting cryptographically related patents. Ultimately he was granted 

thirty of them.9 Like inventors before and since, he 

discovered that dealing with the U.S. Patent Office 

was neither a pleasant nor an alacritous proposition.

In fact Friedman’s own experiences with government red tape proved to be an ongoing source of 

exasperation that consumed his time and sapped his 

energies. As many times as he explained it to them,

patent office officials simply could not—or would 
6

SIGABA / ECM II Cipher Machine

these movements are regular or 

periodic in character, and controlled by ratchet mechanisms 

internal to the device itself. In the 

present invention [Friedman’s],

these movements are controlled 

by the cipher key transmitter in an aperiodic manner ...

It will be recognized that the 

Hebern structure has the inherent weakness of all such devices 

where the keying mechanism is 

a part of the device itself. Periodical recurrence of movements 

is a natural characteristic of all 

such mechanisms and the predictable factor thus introduced 

defeats the essential purpose.

14

As the months turned into years,

it probably occurred to Friedman that 

his purposes might have better served 

by addressing his letters to Santa Claus.

From the time Friedman had identified the weakness in contemporary 

rotor cipher machines and had conceptualized an effective countermeasure, 

the subsequent refinements to his pseudorandom stepping rotors were entirely 

evolutionary. As Friedman’s design for 

his new devices became more sophisticated, they manifested themselves 

under a succession of names, including M-134, M-134A, M-134-T1, and 

M-134-T2. When Friedman’s M-134 

Converter went into production, it “consisted of the 

chassis, the machine itself, an assembly of wheels,”

a plugboard, and a punched tape transmitter (reader).15 Drawing on a pool of ten different rotor wheels,

the M-134 used five wheels at a time, but it was 

the paper key tape that provided movement to the 

respective rotors and gave it cryptographic strength.

same regular manner for every message. Friedman’s

complaint was in response to a letter from the Patent 

Office, posted June 6, 1934, which states that Friedman’s claims were “rejected on Heburn [sic].”

13

In still another letter from Friedman to the Patent Office, dated April 3, 1934, he again contended,

In Hebern the movements or displacements 

of the code wheels are purely mechanical;

Friedman’s M-134 converter. Note plugboard 

at upper left.
7 7

Rowlett’s Epiphany

with vulnerabilities. Too, Rowlett really doubted 

the overall practicality of Friedman’s key tape transmitter. He surmised that in stressful environments 

operators might easily tear or misuse key tapes. Just 

as disturbing to him was the prospect of their succumbing to the temptation to reuse the key tapes 

and their starting places. Rowlett was also concerned 

about numerous issues associated with Friedman’s

machine. Not only were Friedman’s key tapes lengthy, 

but they required all parties on a particular network

to maintain the same large inventories of keying 

material. The distribution of and the accountability 

for tape alone would entail untold manpower and 

resources, not to mention acquiring secure facilities 

for its storage. 

Besides his reservations about Friedman’s key 

tape transmitter mechanism, Rowlett just didn’t like 

making the paper key tapes for the M-134 and would 

do almost anything to return to the more exciting work of breaking into Japanese ciphers. Given 

Rowlett’s affinity for machine solutions, he sought 

a means of automating this impossible assignment.

With a burst of inspiration, he decided that it might 

be possible to use one set of rotor wheels to generate the random stepping movement for the M-134 

cipher rotor wheels. If he could find a way to do that,

the entire process would become much easier and 

undoubtedly more secure. He would later admit in 

an interview,

Frank Rowlett, who had received several years of 

cryptanalytic training under Friedman, was given 

the unenviable task of creating the quantities of key 

tape necessary to drive Friedman’s electric cipher 

machines. Rowlett recorded why and how in 1934 

he had been assigned the job of making key tape. He 

said, “Now Friedman’s thought was that we ought 

to get cracking on the construction of these tapes so 

that when the machines came off the assembly line”

they could be sent out to field stations right away. 

Unfortunately, the machine Friedman had designed 

for the process of generating key tapes was beset with 

problems, and Rowlett spent much of his time trying 

to make it work properly. Rowlett continued with a 

touch of irony: “I got stuck with the job of making 

the tapes because I had a little bit more practice in 

mechanical things than the rest of the group and I 

think they were smarter than I because they didn’t

let it be known.”

16 Being the dedicated mathematician that he was, Rowlett set out to make the key 

as random as possible. Since theory and practice are 

frequently at odds with one another, he discovered 

that producing key tape proved to be a more difficult 

undertaking than Friedman had envisioned.

Rowlett struggled with the problem of key generation until he became completely frustrated. It 

was obvious to him that producing loops of paper 

tape with holes randomly punched throughout their 

lengths was too labor intensive and probably rife 
8

SIGABA / ECM II Cipher Machine

instead of the tapes. Well, I thought this 

was a pretty powerful thing and I just was so 

enthusiastic about it because it looked like 

I was getting out of this impossible task, I 

went to tell Friedman about it.18 

Armed with enthusiastic conviction, Rowlett 

approached his boss about scrapping the whole key 

tape transmitter concept and moving on to something more cryptographically sound. After permitting Rowlett only briefly to present his discovery, 

Friedman would hear no more. He categorically

refused to believe it possible to create a machine 

capable of generating its own random stepping.

19

Friedman, who was every bit as hard-headed as his 

nemeses over at the Patent Office, remained wedded to using replaceable paper key tape. After all, his 

salient argument for pursuing a patent for his key 

tape transmitter—which caused the cipher rotors 

to advance irregularly—was that any machine that 

determined its own stepping would be in violation 

of Kerckhoffs’ second principle and, therefore, vulI don’t know that I ever was confronted with 

a more hopeless task than making these 

[Friedman’s] devices work and do what 

was needed and I soon became desperate. 

It didn’t take more than a month for me to 

realize that I was fighting a real losing battle 

here, and as you are apt to do in the case 

of where necessity becomes very evident you 

try to figure out some better way of doing 

things and I was dreaming about how rotors 

could be made to do other [things] and 

decipher [messages].17

As the drudgery of developing key tapes became 

more tedious, the solution came to him.

I thought it would be a helluva good idea if 

we replaced these key tapes with a second set 

of rotors which in effect would generate five

screens of impulses equivalent to the holes 

and no holes in the five levels of the tape and 

use this assembly of five additional rotors 

Key tape for the M-134 converter
9

Rowlett’s Epiphany

nerable to cryptanalysis.To what degree the loss of a 

potential patent clouded the respectable Friedman’s

thinking is not known.

Frank Rowlett, besides being intellectually brilliant, was a tenacious man and not the sort to abandon a cause in which he genuinely believed. Time 

and again, he requested of his employer the opportunity to fully explain his concept. Each attempt 

made Friedman all the more formal—to the point of 

being brusque—and their strained working relationship slowly approached a breaking point. Years later, 

Rowlett recounted,

This went on for, oh, I guess six, eight, ten 

months, I confronted Friedman ... Friedman 

was still reluctant and finally out of a real fit 

of desperation I said, either Mr. Friedman I 

don’t know what I’m talking about or I know 

what I’m talking about and you don’t understand me ... I think we’ve got to clear this up 

because I’m going to have to quit that job. I 

just can’t meet your requirements.20

Then being on something of a roll, Rowlett 

went on to inform the unsmiling Friedman that 

he would have to go above Friedman’s head to the 

Army’s chief signal officer if Friedman did not give 

his proposal a fair hearing. This of course was a 

risky gamble for a family man in the depths of the 

Great Depression. Friedman was now faced with 

an ultimatum and had to weigh the respective merits of retaining a capable and loyal assistant or losing face to his subordinate. Reluctantly, he offered 

Rowlett an opportunity to fully lay out his concept.21 

Frank Rowlett, besides being 

intellectually brilliant, was a 

tenacious man and not the 

sort to abandon a cause in 

which he genuinely believed.

Frank Rowlett
10

Iacta Alea Est.*

concept marked a gigantic step in cryptography, and 

his prudent acquiescence to Rowlett changed the 

course of history. 

Surprisingly, once Friedman fully accepted 

that he had been wrong, his attitude toward his 

junior changed entirely, almost. Rowlett went on to 

recollect,

 . . . I had never before found him so friendly and so agreeable to work with. He still 

retained his “boss-employee” attitude, but I 

could see that as he reached a more comprehensive grasp of the principles I had discovered, he was accepting me as a professional 

cryptanalyst rather than as a student.

Before the day was out, Friedman told Rowlett,

“I want you to start immediately on drafting patent specifications, and I will work directly with you 

in developing these new principles into their most 

advantageous form.” The odious work of key tape 

generation was put on hold for the time being.

24 Patents come. Patents go. Patents come again.

By summer’s end in 1935 Friedman and Rowlett 

had refined the details for a wholly new cipher 

machine, the M-134-C, which synthesized the 

principles of Rowlett’s stepping maze and Friedman’s earlier M-134s. They also managed to com-

* The die is cast. plete a draft patent (shown opposite) for what was to 

Rowlett knew this would be his sole opportunity to 

sell his stepping maze concept and came to the meeting well prepared. One after the next he responded 

to Friedman’s sometimes insightful, sometimes petty 

questions with rock-solid, unambiguous answers.

At the end of the tension-filled session, Friedman 

was forced to pause and reconsider some of his own 

cherished cryptographic perceptions. Not only did 

Rowlett’s invention run counter to Friedman’s previously published opinions against the incorporation 

of keying logic into a cipher machine, but it made 

Friedman’s beloved pet project, the M-134, obsolete. 

Following Rowlett’s presentation, there was a long 

and pregnant silence, after which Rowlett was coolly

dismissed from the room. Ominously, Rowlett heard 

nothing from his boss for the rest of the day. As the 

afternoon slowly wore on, Rowlett mentally cleaned 

out his desk and began to consider his prospects for 

future employment.22

Years later Rowlett recalled the incident: “Well,

next morning he [Friedman] came in, eyes shining, 

just all excited and he says, we’re going to do this.”

He called it a “beautiful idea ... and he went up to 

see the Chief Signal Officer with stars in his eyes to 

try to sell this new idea.”

23 A night of due reflection 

had forced Friedman to recognize that Rowlett’s
11

become SIGABA. (Army cryptographic 

equipment of that period was given code 

names beginning with SIG [for signals] 

followed by randomly chosen letters.) 

Unfortunately for the cryptographers, the 

nadir of the Depression had been reached,

and the Army’s budget for cryptographic research and development had been 

entirely depleted by Friedman’s M-134 

device. Thus, any fielding of SIGABA 

was out of the question. Still, the Army 

did have a small sum which could be allocated for retrofitting Friedman’s existing 

M-134s. This they used to construct a 

number of “add-on” devices to be used 

with the existing M-134s.25 The so-called 

SIGGOO (M-229) component replaced 

Friedman’s “key tape transmitter” with a 

less robust version of Rowlett’s stepping 

maze. 

The SIGGOO assembly consisted of 

a three-rotor setup in which five of the 

keyboard inputs were live, as if someone 

had pressed five keys at the same time on 

an ENIGMA. These outputs were “gathered up” into five groups as though all the 

letters from “A” to “E” were wired together. 

In that way the five signals on the input 

side would be randomized through the 

rotors and come out the far side, ensuring 

power in one of five lines. The SIGGOO 

rotors, therefore, could be controlled with 

a day code, or key, eliminating the need 

for paper tape keying material. Because 

of cost constraints, though, not all of the 

model M-134s could be retrofitted with 

SIGGOOs, and large numbers of them continued to 

rely on Friedman’s original paper tape transmitters.26

The M-134 Converter modification with its five

rotor cipher wheels, the three rotor wheel SIGGOO 

add-on, and a 26-wire plugboard was hardly elegant 

in appearance and resembled a comic-strip Rube 

Goldberg device. It was, nonetheless, the strongest 

device in the shadowy world of cryptology and easily surpassed anything the British or the Axis Powers 

had at their command. Since one cannot know what 

he doesn’t know, neither Friedman nor Rowlett at 

the time truly appreciated just how advanced their 

new cipher machine really was.

Iacta Alea Est.

William Friedman and Frank Rowlett’s draft patent of 

August 1935 for what was to become SIGABA
12

“had been unsatisfactory [as well and] they now had 

a lot of development money but didn’t have any ideas 

to invest the money in and for goodness sakes did 

the Army have something ... any good ideas at all.”

29 

Friedman, who was initially taken aback by this 

admission, sat up in his chair and brightly suggested that he might indeed have something to share. 

But first he would have to gain permission from his 

senior leadership. 

Later that month on the 21st, Lieutenant 

Wenger and a colleague, John W. McClaran, met 

with Friedman and Rowlett at the Munitions Building. There, the Army proudly shared its plans for 

SIGABA with the Navy. Lieutenant Wenger, who 

among his colleagues was hardly known for animated 

expression, remained true to form and demonstrated little indication that he was impressed. Ten days

later, on Halloween, another OP-20-G complement 

composed of Wenger, McClaran, and John Harper 

ambled across the bridge for another meeting with 

Friedman. Again, Friedman explained Rowlett’s

plan for the stepping maze and how it could control cipher rotor movements.30 The following day 

a still larger contingent of naval officers assembled 

in Friedman’s office for a briefing on the SIGABA 

concept. Throughout this last presentation, Friedman’s guests exhibited an uneasy institutional politeness and left with Wenger’s same general lack of 

enthusiasm. 

 Enter the Navy

As the torrid summer in 1935 lost its grip on 

pre-air conditioned Washington, DC, it was apparent to no one that the sister services were on a cryptographic collision course. While Friedman and 

Rowlett occupied themselves with fine tuning a 

patent request for SIGABA, the Navy’s OP-20-G 

organization was considering the acquisition of new 

cipher machines. The Navy cryptographers for several years had relied on Hebern machines to meet 

their communication security needs, but by the mid1930s they began to doubt the machines’ integrity 

and balked at a contract to procure more of them.

Quietly, Lieutenant Joseph N. Wenger, acting chief 

of the Navy’s cryptologic organization, OP-20-G,

and his colleagues began looking farther afield for 

something more secure.

27

Early in October Wenger made the fateful decision to seek out Friedman’s advice and walked over 

the little-used pedestrian bridge that linked the Navy 

and Munitions buildings on the Mall in Washington. Clearly, the Navy was in dire straits if it admitted to needing any advice from the Army. As Frank 

Rowlett remembered it, “in one of the rare periods 

of consultation between [the] navy organization and 

the army organization I think it was Joe [Wenger]...

[who] told Friedman that the navy would been [sic] 

real disappointed with the [existing] contract.”

28 

Lieutenant Wenger went on to tell Friedman that 

the new machine Hebern was trying to sell the Navy 
13

Enter the Navy

tated. He knew it was a good idea, a very good idea,

and became all the more convinced not only that it 

would work but that it offered secure communications beyond anything thought possible at the time. 

With SIGABA’s future on indefinite hold, Friedman and his colleagues returned to solving Japanese 

ciphers and producing paper key tapes for those 

M-134s not fitted with SIGGOOs (since the standard M-134s and those with SIGGOOs were not 

compatable, they were used on separate networks).

Lieutenant Joseph N. Wenger, acting chief of 

the Navy’s cryptologic organization OP-20-G 

(pictured later in his career)

Rowlett, after several anxious months, had 

received no feedback from OP-20-G and began to 

pester Friedman for word about the Navy’s plans for 

SIGABA. Friedman finally tired of these queries 

and confessed that Wenger had reported that “there 

were certain ... operational difficulties and that 

they just weren’t sure the idea would work.” To this,

Rowlett mused, “well, I thought the doggone thing 

would work and maybe they weren’t as smart as they 

thought they were.”

31 Rowlett had reason to be irriWenger told Friedman that 

“they now had a lot of 

development money but 

didn’t have any ideas 

to invest the money in 

and for goodness sakes 

did the Army have 

something ... any 

good ideas at all.”
14

 A Disagreeable but 

 Rewarding Surprise

machine was. Wright responded that he believed

it was Wenger.

33

Frank Rowlett was no little upset by this news.

Not only had he been excluded from SIGABA’s

development, but he saw the credit for his invention going to another man. At best Ham Wright

had been misinformed, and at worst Wenger was

hoping to pass off the rotor maze as his own idea. In

either case, Rowlett decided to get to the bottom of

the matter as quickly as possible. As soon as Wright

left, Rowlett hurried down the hall to tell Major

William Reeder what had just occurred.34 Reeder, 

who, as Friedman’s superior, was the head of the

Signal Intelligence Service, needed to be informed

of this turn of events. That Reeder already seemed

to be aware of the Navy’s work on SIGABA came

as the second disagreeable surprise of the day. Just

how long Reeder had been keeping this to himself is unclear. While he had not gone to Rowlett

with the news, Reeder was perfectly straightforward about it when asked. In spite of this omission, Rowlett still considered Reeder to be a good

supervisor and a friend. Several months later, Major

Reeder, in trying to make amends, went to Rowlett

with something that he wanted to hear. The Navy

would soon be taking delivery of a working model

of their new cipher machine and Rowlett was invited to help test it.35

Four years later, in 1939, with one war spreading 

across Europe, another was looming on the Pacific 

horizon. In anticipation of the latter conflict, Rowlett 

and Navy Lieutenant Commander Wesley (Ham) 

A. Wright were working on the Japanese PURPLE 

analog cipher machine (used to decrypt the Japanese 

diplomatic cipher designated as PURPLE) when a 

shocking revelation came to light. Rowlett recounted 

this incident in his memoirs:

... we were speculating about what type 

of cryptographic mechanism the Japanese might have used to produce such an 

unconventional substitution system. “Maybe they’re using something like what we’re 

planning to use in our new Navy cipher 

machine”, Wright remarked. “Let me 

explain it to you, and you can give me your 

opinion of it”. He then started to sketch out 

for me the cryptographic circuitry of for [sic] 

the new Navy cipher machine.

32

Rowlett with an increasingly sickening realization recognized the new Navy cipher machine

as his own invention. He was particularly stunned

since back in 1935 he had learned through Friedman that the Navy had rejected the idea as being

impractical. Without revealing that he had conceived of the cryptographic principles involved,

Rowlett asked Wright whose idea the cipher
15

A Disagreeable but Rewarding Surprise

the stepping magnets of both mazes.”

38 He was 

also fascinated by the additional set of rotor wheels 

that the Navy had included. When he and Friedman shared the stepping maze concept with the 

Navy, the design incorporated a plugboard similar 

to that on the German ENIGMA machine. At first 

the Army cryptographers did not perceive “... the 

advantage of the extra set of rotors that the Navy 

had introduced ... We [Friedman and Rowlett] 

preferred the plugboard. The Navy for some reason didn’t like plugboards but this was not a point 

... to quibble about.”

39 With either the plugboard 

or the index rotors, SIGABA represented the pinnacle of cryptography, and both the Army and Navy 

were confident in its ability to resist all assaults.40

Those who attended the demonstration at 

the Navy Building had every reason to bask in the 

warmth of self-congratulation. It had been a long 

and unlikely path from Friedman’s and Rowlett’s

collaboration to the Navy’s manifestation of their 

concept, and Rowlett couldn’t help but reflect with 

a little nostalgia on the old ways of doing business:

“... before, you had to have a different set of codebooks and these became onerous, but with the new 

secure cipher machines, all the cipher clerk needed 

was a box of rotors and a little pamphlet that told 

him how to use the rotors, which made the concept 

... very practical.”

41 As elated as all of them were, 

none present could then conceive the extent to which 

their accomplishment would alter the future. 

On 3 February 1940 the Army’s primary SIS 

cryptologists—Friedman, Rowlett, Sinkov, and 

Kullback—trooped over to the Navy Building to see 

the prototype of the Electric Cipher Machine Mark 

II (SIGABA). Following an informal demonstration 

of the device, during which Friedman and Rowlett 

could hardly contain their excitement, they were

finally given the opportunity to put it through its 

paces. It was a case of love at first sight. Rowlett later 

said, “... it was the most beautiful thing to look at 

from where I stood and I couldn’t keep my hands off 

it and of course the Navy was delighted to find somebody as enthusiastic about it as I appeared to be.”

36

The Navy had exceeded any expectations Rowlett 

had entertained, and he wanted to explore every 

facet of the Navy’s engineering triumph. He and 

Friedman were all the more heartened by the Navy’s

willing acknowledgment of the Army’s visionary role 

behind SIGABA. (The Army designated the combination of machines as the M-134-C, which was also 

applied to the later, more mature SIGABAs. For the 

purpose of this paper, the M-134-C machine will be 

referred to by its more popularly used Army short 

title, SIGABA, whenever possible.) 37 

Rowlett, besides being taken with the purely 

mechanical aspects of the machine, was also quite 

interested in SIGABA’s wiring scheme. He later 

said, “I was very curious about the circuitry that 

they’d decided on in terms of the association of the 

contacts on the in-plates of the control maze with 
16

SIGABA Is Built

the Army mandated their inclusion for SIGABAs

deployed outside of the continental United States.42

The quest to further enhance ECM/SIGABA

would likely have continued for some time had not the

Battle for France, then raging in Europe, convinced

Navy officials that the time to field their new cipher

machine had arrived.43 As the Navy prepared to let

contracts for full-scale production of SIGABA, the

cryptologists at OP-20-G and their counterparts in

the War Department were fully confident that SIGABA/ECM II could deliver the degree of security they

sought. This optimism was founded in the combination of six factors identified by Navy cryptographer

Lieutenant Laurance Safford and paraphrased below:

1. SIGABA/ECM II included a sufficient 

number of cipher wheels in the maze to generate an astronomical number of “alphabets”

and starting points. The five-wheel cipher 

maze of the ECM and Combined Cipher 

Machine (CCM) provided 11,881,376 

alphabets for each arrangement of code 

wheels. (The ENIGMA, by contrast, offered 

only 17,576.) 44 [CCM was a specially modified, less sophisticated SIGABA that was 

interoperable with the British mainstay 

TYPEX machine.]

2. The SIGABA/ECM II included ten 

reversible cipher wheels in a set. This proFriedman and Rowlett’s idea behind SIGABA 

had been elegantly simple, but engineering it into 

practical reality proved to be a formidable undertaking for the Navy. From late 1936 until January 1941,

Navy cryptographers and their prime contractor, the 

Teletype Corporation, developed a series of prototype machines, each building on the lessons gleaned 

from its predecessor. Early prototypes were prone to 

failure due to environmental conditions such as heat,

humidity, and vibration, but as the SIGABA design 

matured, these problems and others were resolved.

Successive models were smaller, lighter, faster, and 

more reliable. 

By the time the ECM Mark II (SIGABA)

neared its birthing stage, the resulting device proved

to be both electromechanically robust and cryptographically strong. Variant models could operate

on 115-volt alternating electricity or 24-volt direct

current/battery power to suit the respective needs

of the sister services. Production-run SIGABAs

achieved an impressive 60 words per minute capability. To allay any Army reservations about SIGABA’s reliability under austere conditions, Navy

engineers offered emergency, manually cranked

attachments. These saw almost no actual use in

field operations. Another option was a forty-pound

thermite bomb for SIGABAs destined to go into

harm’s way. While the Navy steadfastly prohibited

these emergency destruction devices aboard ships,
17

SIGABA Is Built

A sketch of the wheel control unit, the “control rotor bank,” separate from the cipher unit, 

called the “cipher rotor bank.” Note at bottom reads, “Wheel control unit and cipher wheel 

unit are identical in design and interchangeable. Wiring to be arranged as indicated.” 

It is similar to the diagrams Friedman showed the Navy during 1935. 

(Friedman Collection, NSA/CSS accession #47270, box 10, folder 5)
18

SIGABA / ECM II Cipher Machine

4. The stepping of the alphabet maze was

controlled by an independent source—in this

case by both the five index rotor set and the

five control rotor set.47

5.The use of a multiplicity of stepping actions

(5,855), dependent solely on the key, instead of

only one in other cipher machines.48

6. The replication of code wheel sets—both

“effective” and “reserve”—with prompt

change of code wheels in case of known compromise, and a periodic change as an added

security measure. The inclusion of a back-up

set of fifteen wheels (five index rotors, five

control rotors, and five cipher rotors) was

taken to dispel any lingering doubts as to the

absolute security of SIGABA.49

vided 9,667,680 possible wheel orders, making an adversary’s “trial and error” solutions 

impractical, if not impossible.

45 (The CCM 

also offered ten reversible cipher wheels per 

set.) [There is a common misconception 

that SIGABA rotors could rotate either 

forward or backward.While the direction of 

travel for SIGABA rotors was forward only, 

all fifteen rotors could be flipped in either 

direction to maximize their cryptographic 

potential.]

3. The use of aperiodic stepping of cipher

wheels, instead of regular or modifiedregular stepping motion, precluded all known

analytical solutions and prevented “short-cut”

solutions with captured cipher wheels.46

SIGABA rotor maze. Note the five small ten-pin wheels that replaced the 

Army’s original plugboard.
19

SIGABA Is Built

ships of the 14th Fleet, with another 100 in storage awaiting distribution to capital ships and other 

units. Twenty-five more were ashore undergoing 

depot-level maintenance. Fortunately, the machines 

destined for the doomed vessels were still in the 

warehouse on 7 December 1941 and thus escaped 

destruction. A hundred of these Pearl Harborsurplus machines were hastily transferred to the 

Army and later used in North Africa. To meet the 

demand of producing more than fifty devices per 

month, contracts were let with additional manufacturers. By 1943 5,730 ECM Mark II/SIGABAs 

were in service and more than 300 per month were

being delivered. The only feature distinguishing the 

Army and the Navy machines was the service-unique 

designation on the name plates. For the first time 

in the nation’s history, the Army and Navy enjoyed 

cryptographic interoperability.

51

With America’s involvement in the war a near 

certainty, the Army identified the necessary funds to 

assist the Navy in procuring SIGABAs in numbers.

The Army’s first lot of 459 machines was fielded in 

June 1941.With a due sense of urgency, it distributed these SIGABAs to upper echelon headquarters in 

the CONUS and to selected organizations in American possessions in the Far East. Naval units in the 

Atlantic theater were given priority because forces in 

the Pacific had already been equipped with the older 

ECM Is.50 Of course some sensitive sites like Corregidor, Guam, and Pearl Harbor necessitated the 

highest degree of security for the signals intelligence 

information they processed and thus received the 

new cipher machines as well. The Navy was in the 

process of outfitting its capital ships of the Pacific 

fleet with ECM Mark IIs (SIGABAs) when Pearl 

Harbor was bombed. Ninety-six had been issued to 

Women assembling SIGABA crypto rotor wheels
20

SIGABA / ECM II Cipher Machine

was between six months and two years. In 1943 the 

average WAVE managed to solder the connections 

for fourteen wheels per day. One actually assembled 

a record twenty-two wheels on her shift.The average 

for male shipyard electricians had been seven wheels 

per day before the decision was made to assign them 

to other duties. Midway through the war, the Navy 

women alone had wired more than 150,000 rotor 

wheels. Remarkably, there was not a single configuration error and only one instance where a wheel had 

been mislabeled! 52 Before the rotors were shipped 

to the field, each one underwent more than 2,800 

operations before they were certified for use. When 

SIGABA production ceased after World War II, 

some 10,060 machines were in the inventory along 

with over 450,000 crypto wheels to support them.

According to the U.S. Army’s account of the 

SIGABA’s development, History of Converter 

M-134-C, “each of these experimental and adopted 

models is in the direct line of cryptographic development which culminated in Converter M-134C.”

53

In keeping with Army procedures, the major constituent subcomponents of the SIGABA encryption system were given their own alphanumeric 

designations.54 Below are examples of SIGABA 

crypto-nomenclature.

SIGABA Converter M-134-C chassis, less the 

rotors

SIGKKK Maintenance instructions for 

converter M-134-C

SIGQZF Crypto-operating instructions for 

converter M-134-C

SIGBRE General instructions for converter 

M-134-C

SIGIVI Cipher basket unit for converter 

M-134-C

Set of 10 rotors as designated in the 

current key list

Keying data in the current key list

While trusted civilian contractors such as the 

Teletype Corporation manufactured and assembled 

the SIGABA chassis, the all-important crypto rotor 

wheels remained a military in-house activity. Uniformed Navy personnel and civilian workers fabricated rotors from stock materials at the Washington 

(DC) Navy Yard (later at the Nebraska Avenue Naval 

Station).The most security-sensitive work of actually wiring the rotors was performed by more than 200 

WAVES (Women Accepted for Volunteer Emergency Service). WACs (members of the Women Army 

Corps) at Arlington Hall performed the same task 

for the Army. Since each cipher machine required 

a minimum of two complete sets of fifteen cryptographic rotors, these military women were pressed to 

meet growing production quotas. On occasion when 

the Army’s requirements overwhelmed its capacity to produce them, the Navy lent a willing hand 

in providing additional rotors. Because SIGABA/

ECM IIs saw heavy use, the life expectancy of a rotor 

The most security-sensitive 

work of wiring the rotors was 

performed by more than 

200 WAVES (Women 

Accepted for Volunteer 

Emergency Service).WACs 

(members of the Women 

Army Corps) ... performed the 

same task for the Army. 
21

SIGABA Is Built

 IBM SIGABA key generator used at headquarters to produce daily key settings 55

As though SIGABA were not already sufficiently sound cryptographically, the Army and 

Navy took additional measures to ensure the system’s robustness. Unlike the cipher systems used by

the British and the Axis Powers, SIGABA’s daily 

settings were generated by yet another rotor-based 

cipher machine rather than humans. Army and Navy 

key generators produced each month’s daily settings 

printed on a single sheet of paper. Thus, the key was 

easy to transport, easy to use, easy to destroy, and, if 

necessary, easy to replace. 
22

 An Impenetrable Machine

second was the CCM, a SIGABA variant made at 

the ECM Repair Shop exclusively for joint Allied 

communications. Because of cost, only 631 of these 

models were made. The third, most common and 

most cost-effective, was the “X” Adapter manufactured by the Teletype Corporation in Chicago. 

Forty-five hundred of these were sent to depot-level 

maintenance facilities for installation. All three of 

these options used Typex-configured cipher rotors.

By the war’s end, nearly all U.S. military communications facilities could process joint-Allied secure 

communications traffic.57

 In spite of SIGABA’s enormous cryptographic

strength, the sister services harbored a slight but

healthy anxiety about the enemy’s cryptanalytic

capabilities and continued to upgrade SIGABA.

What if the Japanese or Germans were enjoying

success against SIGABA similar to the Allied success against ENIGMA? The notion could not be

blithely dismissed. Thus, during the latter part of

the war, the Allies sought means to validate the

integrity of their cryptographic efforts. Intercepted

enemy messages were closely scrutinized, and prisoners of war were examined for any hint that U.S. 

cryptography had been compromised. Even before

Berlin fell, it had become clear to the Allies that the

Nazis had exploited some lower-level U.S. cipher

systems when there had been lapses in COMSEC

discipline. On the other hand, no evidence emerged

Throughout World War II, SIGABA offered

America an inestimable advantage. While decrypted

Axis communications channeled a continual flood

of actionable intelligence to U.S. decision makers,

SIGABA denied the enemy a similar resource. The

collaborative nature of SIGABA’s design enabled

Army and Navy forces to coordinate their complex,

joint operations in complete secrecy. That the services

were operating identical machines turned out to be

particularly important in the early hours of the war

when the distribution of machines and rotors was

not yet complete. During critical engagements in the

Philippines, Java, Australia, and North Africa, Army

and Navy crypto-maintenance personnel shared components back and forth as necessity demanded.There

were also emergency situations when the Navy and

Army used each other’s equipment to ensure the

unimpeded flow of secure wartime communications.56

The need for Anglo-American interoperability similarly hastened Allied cooperation in fielding cryptographic solutions. Later modifications 

permitted specialized, less sophisticated ECM 

Mark IIs [SIGABAs] to interoperate with British Typex cipher machines in support of joint U.S./

British/Canadian operations. This functionality was 

achieved through three different means. The first of 

these, the ECM Adapter (CSP 1600), was produced 

at the Washington Naval Yard ECM Repair Shop 

where 3,500 were made available for retrofit. The 
23

An Impenetrable Machine

that was an interesting item in itself. They 

did have some success with the Hagelin.

Rowlett continued,

...we had truckload after truckload of German cryptographic equipment. ... We had 

some of the technical reports right up to the 

solution of code books and ciphers of other 

countries, photographic copies of second 

story jobs that they had performed on safes 

and embassy code rooms ...This was gone 

over, carefully evaluated and assessed and a 

series of reports produced which you might 

find under the term TICOM [Target Intelligence Committee] Reports.63

With the return of peace in late 1945, the victorious Allies began compiling exhaustive studies on 

Axis technologies and capabilities. One of these, the 

Seabourne Report, was a series of technical treatises 

drafted by German subject matter experts. Volume 

XIII of this report detailed the Nazis’ successes 

against Allied cryptographic systems. According 

to interviews with senior officials of the Luftwaffe 

Signal Intelligence Service contained in the report,

the Germans revealed they had made no headway 

against the British Typex cipher machine, which 

was greatly inferior to SIGABA. The Luftwaffe 

cryptologists interestingly did not address SIGABA 

specifically, and their American counterparts were

reluctant to press for answers lest they raise unwanted questions from their former enemies. Considering the immense disparity between SIGABA and 

Typex, it is a certainty that the Nazis made no 

inroads into the “American Big Machine.”

64

The official War Diary of the German Signal 

Intelligence Group again seems to validate the findings of the other inquiries. Entries made between 

February and November 1944 again strongly suggest that the Axis made no inroads into SIGABA.

While their cryptologists were reading “un-Steckered” Croatian ENIGMA machines (early commercial model ENIGMA machines without a plugthat the Germans had made any headway against

SIGABA.58

Deciphered Japanese traffic also indicated that 

they had not broken into Allied ciphers. An intercepted JN-A-20 message, dated 24 January 1942,

from the naval attaché in Berlin to the Vice Chief 

of Naval General Staff Tokyo afforded a comforting 

revelation. In it the naval attaché said he considered 

“joint Jap[anese]-German cryptanalytical efforts” to 

be “highly satisfactory,” since the “German[s] have 

exhibited commendable ingenuity and recently 

experienced some success on English Navy systems,”

but are “encountering difficulty in establishing successful techniques of attack on ‘enemy’ code setup.”

59 In another decrypted JN-A-20 message, the 

naval attaché wrote home that he had “...discovered 

that Heine [German] CI [Cryptographic] organization totals 800 persons and is ... receiving unsatisfactory results on American Communications.”

He went on to report, “Since last year when Italy 

capitulated, English and American countermeasures 

have become more vigilant due to interpreting the 

CI situation.”

60 The Japanese in their own internal 

communications confessed that they had made no 

real progress against American cipher systems and 

that the Americans were becoming even savvier 

about the security of their cryptographic operations.

Following V-E Day, Friedman and his associates were anxious to discover just what the Axis 

cryptanalysts had known. According to Rowlett, 61

We also were very much interested in what 

results ... have been achieved by the Germans on the ECM or the SIGABA. And 

we got the answer to that. ... they talked to 

the fellow who was in charge of what they 

called the American Big Machine.

62 See, 

they’d identified the Big Machine as the one 

jointly used by the Army and Navy, and they 

couldn’t tell we were using different rotors 

or other things because their cryptanalytic 

understanding was just not at that level; and 
24

SIGABA / ECM II Cipher Machine

board or Steckerbrett), British transposition ciphers,

Yugoslavian ciphers, and a variety of others, they 

undertook efforts against an American five-letter 

system. A translated notation in March 1944 reads,

“A study was begun of a group of 5-letter messages from U.S.A. links presumably were enciphered 

with a machine of unknown type.”

65 These doubtlessly were SIGABA messages. With each successive month the War Diary reflected that no progress was made on the five-letter American cipher 

system. Then in September 1944, with the Allied 

forces steadily advancing against the Germans, the 

War Diary includes,“U.S. 5-letter traffic: Work discontinued as unprofitable at this time.” The Third 

Reich’s cryptologists ostensibly decided to focus on 

ciphers they could exploit.66

 During his detention by the Allies, the German cryptographic mathematician and POW Dr. 

Erich Huettenhain was asked what work was done 

on British and American ciphers. He answered that 

most of the successes were diplomatic and “Most 

of the American strip cipher was read.”

67 When 

he was asked what type of cipher machines were

broken, he responded, “The main machine broken was the American Hagelin which was broken 

only when [an] error occurred.” He continued,

“[c]ommon and regular solutions [were] impossible”

against the Hagelin machine. Only when a soldier 

or sailor grew lazy and neglected to change the daily 

key were the Germans able to leverage their way 

into a Hagelin-based cipher.

68 If the Germans could 

exploit only a lower-level Hagelin machine on occasion, there was no possibility that they could even 

begin to unravel SIGABA. Dr. Huettenhain, when 

pressed about other Allied cipher machines, said, “I

know of no other type of American machine, but 

the British Typex is known. It was not broken, and 

so far as we know cannot be solved unless the wheel 

positions are known.”

69 Dr. Huettenhain continued,

We have the ENIGMA [sic] which is similar to the Typex, and as we believe that the 

ENIGMA cannot be solved, no great effort 

was made to solve Typex. Typex has seven 

wheels and we therefore believe it to be 

more secure than our ENIGMA. ENIGMA when used according to instruction is 

unbreakable. It might be broken if a vast 

Hollerith complex is used but this is only

slightly possible.

70

After months of interviews, none of the cryptologic POWs could offer any information about

SIGABA. They spoke candidly, even proudly, about

their successes against British and American ciphers,

and why not? Had they broken SIGABA, surely they

would have been all the more delighted to regale

their captors with their cryptanalytic prowess. Little

did they suspect that their own cherished ENIGMA

machines had been entirely compromised. Extensive evidence gleaned from the Japanese after the

war indicated that they had made even less progress

against SIGABA than the Germans had.71
25 25

 The Big Machine That Did

tion of intelligence activities, which enabled economies of scale and a synergy of efforts. Thus, by late 

1942 the sister services were able to streamline the 

production and dissemination of processed intelligence information. Whether Safford and Wenger 

had envisioned the new, centralized operational 

model when they championed the development of 

SIGABA back in 1935 is not known, but it is consistent with Wenger’s overall grand designs for the 

Navy’s radio intelligence function. The Army similarly benefitted from SIGABA’s implementation at 

its central cryptologic facility at Arlington Hall Station near Washington, DC. 

SIGABA also supported Allied intelligence

and military operations. The most striking example

of this was its role in the Battle of the Atlantic.

From collection platforms in Britain and at sea,

radio signals collected from German U-boats were

enciphered by American-operated SIGABAs and

routed chiefly by undersea cable to Washington.

There, the Navy processed the four-rotor ENIGMA traffic on the cryptoanalytic “bombes” at the

Nebraska Avenue station. Hours later, critical,

actionable wartime intelligence relating to U-boat

operations was again encrypted on SIGABAs and

sent back across the Atlantic to Allied forces.While

the United States and the United Kingdom enthusiastically shared most of their cryptologic secrets

with one another, this did not apply to SIGABA.73

SIGABA, besides merely securing critical information, made possible a major shift in the way 

America’s intelligence organizations conducted their 

business. Before the summer of 1941, the flow of 

information from far-flung radio collections sites 

back to Washington took days to several weeks.

The sensitive nature of intercepted data, analysis,

and reports necessitated that stringent methods be 

employed to ensure their confidentiality during transit. Since the United States in the 1920s and 1930s 

did not have absolute faith in its own cryptographic 

devices, the services had to rely on physical security 

to coordinate their classified communications. This 

meant that messages had to be laboriously typed 

onto onion-skin paper and then forwarded via costly 

registered air mail to their destinations.72 Given the 

paucity of air service in those days, however, most 

mail was couriered to awaiting naval ships which 

carried it back stateside. Army elements stationed in 

the continental United States relied on couriers as 

well as the postal system. For those abroad in such 

places as China, the Philippines, and Panama, the 

Army process mirrored that of the Navy. As war 

became imminent, the demand for a rapid, secure 

means of transmission became paramount, and 

SIGABAs were distributed to higher priority customers as quickly as they could be produced.

One of the concomitant benefits of secure radio 

and telegraph communications was the centraliza-
26

SIGABA / ECM II Cipher Machine

The United States for various reasons regarded

any information about the SIGABA machine as so

sensitive that it did not share any of its principles

or details with the British. SIGABAs deployed to

British military facilities were operated and stored

in secure enclaves to which host-nation personnel

were not permitted. Joint U.S.-UK tactical communications in the Pacific Theater were passed

along circuits using the Enigma-like British Typex

machine; during the latter part of the war the

Allies used the CCM to coordinate joint activities.

Throughout World War II high-level communications to and from Roosevelt and Churchill passed

through SIGABA-based circuits. Messages from

Downing Street were forwarded to the American

embassy in London where they were encrypted and

sent to Washington, DC, where they were rendered

into plaintext and directed to the White House. 

When peace returned in the summer of 1945,

more than 16 million Americans were wearing the 

uniform of their country; two years later those numbers had dwindled to slightly more than 1.5 million, and the nation was awash in surplus military 

materiel. Cryptographic equipment such as M-209 

cipher machines and M-90 devices could be purchased for a nominal sum. Not for sale, however, 

were the 10,060 SIGABAs which had successfully

defied the best efforts of the Axis Powers. Postwar 

technical analysis of German and Japanese cryptologic capabilities put SIGABA’s principles in a perspective that the Americans themselves were only

just then coming to appreciate. Studies suggested 

that SIGABA was so much more technologically

advanced than had been thought that its principles 

needed even more protection after the war than during it.74 Army and Navy cryptologists were not so 

much concerned that an adversary might be able 

to exploit SIGABA if he were privy to its design;

rather, their chief concern was that an enemy could 

use its cryptographic principles to protect his own 

communications.To this end the sister services promulgated policies that mandated that all SIGABAs 

be guarded twenty-four hours a day by armed military personnel.75 This had been the usual practice 

during the war in overseas locations, and the services 

did not want a relaxation of security with the return 

of peace.

Army and Navy cryptographers, still concerned 

that SIGABA’s principles might be compromised,

undertook to remove SIGABAs from geographical areas where they might be compromised and 

replaced them with SIGRODs.76 SIGROD was a 

transportable, electromechanical, keyboard-operated cryptographic machine capable of enciphering and deciphering message traffic at the rate of 

forty to fifty words per minute. It was nearly identical cryptographically to the joint U.S./UK CCM as 

well as the British Typex machine. Smaller, lighter, 

and much cheaper to maintain than SIGABA, the 

five-rotor SIGRODs were capable of processing top 

secret information and, importantly, if compromised 

would not disclose the sensitive cryptographic principles embodied in SIGABA.77 SIGABAs were

phased out incrementally and replaced with the less 

powerful machines. Nevertheless, SIGABAs continued to be used at higher level headquarters for 

processing the nation’s most closely held secrets and 

were stockpiled in heavily secured facilities against 

emergency situations when they might be needed 

again. When the Korean War broke out, SIGABAs 

were used extensively at higher echelons because of 

their dependability and high degree of security. For 

the remainder of the 1950s, the brainchild of Friedman and Rowlett could be found in military higher 

headquarters and critical message centers around 

the world.78 During the course of twenty years,

SIGABA had processed millions of classified messages, contributed to the saving of countless lives,

shortened the agony of war, and helped to advance 

the cause of freedom. That it altered the course of 

history goes unquestioned.

In 1956 a grateful Congress awarded $100,000 

to William Friedman for his contribution to SIGABA and for other cryptologic achievements. Two 
27

The Big Machine That Did

cratic parody of itself, the Patent Office on 16 January 2001 granted a patent for SIGABA—some sixty-six years after its conception and thirty-two years 

after William Friedman’s death.

years later a similar sum was granted to Laurance 

Safford for his World War II cryptographic work; it 

would be another eight years before Frank Rowlett 

received his reward.Then, in something of a bureauThe SIGABA/ECM II 
28

SIGABA / ECM II Cipher Machine
29

letters/numbers on the rotors could be “on top” for 

each of the 10 wheels, giving another 2610 possibilities. Configuring the control and cipher rotors in the 

machine in their correct forward/reverse orientations 

and the correct letter “on top” positions would take 

an exhaust of (210)(10!)(2610) ≈ 5.2 × 1023 for their 

initial settings. SIG ABA code clerks and operators 

used key charts published monthly to determine the 

wheels’ forward/reverse orientation, placement position in the rotor maze, and the “on top” settings of 

each rotor for each message.

The third bank of cryptographic wheels contained in the rotor maze was the five index rotors 

which sat next to the control rotors and was the closest to the keyboard. The index rotors were smaller 

than the other rotors and had only ten contacts on 

each side. Unlike the other ten rotors, the index rotors 

did not “rotate/advance” during operation but were

set daily to initial values dictated by the published 

key chart. While it was physically possible to insert 

these rotors in either the forward or reverse position, the index rotors were placed forward throughout World War II. Like the other rotors, each index 

rotor could be placed in any of the five slots in the 

index rotor bank.This capability afforded an exhaust 

of 5! = 120 to find the correct index rotor permutation. From June 1945 onward, however, the index 

rotor configuration did not change, and the setting for

The SIGABA machine (U.S. Patent 6175625) 

had five cipher rotors, five control rotors, and five

index rotors for a total of fifteen rotors. All fifteen 

rotors, in banks of five, were encased in a removable module or rotor maze on top of the 100-pound 

machine. The cipher and control rotors were interchangeable and greatly added to the overall security 

against a brute force attack. Each rotor had twentysix contacts on one side wired to twenty-six contacts on the other side. These cipher rotors could be 

placed in any order in the five slots. They could also 

be inserted forward facing or reverse. The initial settings for the rotor placement were dictated by a code 

book, and changed daily. 

Adjacent to the bank of five cipher rotors were

the five control rotors. Not only were the cipher 

and control rotors interchangeable, but it took only

seconds to rearrange them in a new configuration.

In Friedman’s own words, “Wheel control unit and 

cipher wheel unit were identical in design and interchangeable.” Even if the enemy had captured the 

rotors and knew the wirings of each rotor, they still 

had to exhaust over 10! = 3,628,800 different combinations of cipher/control rotor locations. Furthermore, the enemy would have to determine if each 

rotor was placed forward or backward.This increased 

the key space by a factor of 210

, since there were two 

choices for each of the ten rotors. Also, any of the 26 

 Appendix A: Technical 

 Analysis of SIGABA’s Key Space
30

SIGABA / ECM II Cipher Machine

each of the 10 and 10! possibilities for each of the 5.

So the total number of configurations for the system 

would be (26!)10(10!)5 ≈ 2992.8

. One need not count 

rotor reversed orientation settings or consider which 

value is “on top,” because such changes are already 

represented by one of the wirings considered. Some 

wirings would produce identical encryptions, but the 

effect on the already massive key space is minimal.

Even with this enormous theoretical key space, 

the Army and Navy took no chances when they suspected (later proved false) a physical compromise of 

SIGABA in the last months of the war in Europe. In 

a surge effort, the services produced an entirely new 

set of fifteen rotors for each of the 10,060 machines

in the inventory. In retrospect, given the state of Axis

cryptologic prowess, this was not warranted. All the

same, General Eisenhower, who personally ordered the

fielding of the new rotors, was not willing to imperil the

success of military operations or the lives of his troops.

Thus, at the war’s end, each SIGABA was equipped

with two sets of rotors.

The SIGABA’s wiring scheme is provided on 

the next page.

each index rotor was provided by the daily key list. An

enemy without knowledge of how it was implemented

would have to consider reversals (another factor of 25

) 

and which value was “on top” (another factor of 105

).

Therefore, finding the correct setting for each of the

fifteen rotors would take (210)(10!)(2610)(5!)(25

)(105) ≈

2.0 × 1032 attempts, which is approximately 2107.3 possible key combinations. Once the index rotors were

no longer permuted daily in 1945, this dropped to

(210)(10!)(2610)(25

)(105) ≈ 1.7 × 1030, which is 2100.4

trial decryptions. This was surprisingly good by the

modern Advanced Encryption Standard (2128, 2192, or

2

256) and Data Encryption Standard (256) key sizes.

Of course, as long as the enemy did not recover the machine, the wirings in each rotor added to 

the security. For each of the ten cipher and control 

rotors there were 26! theoretical ways for them to be 

wired. A large number of these wirings would have 

been avoided since it does not seem random to have 

rotors that mapped A Æ B, B Æ C, C Æ D, etc. But 

these “nonrandom” wirings should be included in the 

theoretical possibilities, since they are valid wiring 

combinations. If an enemy did not know any of the 

rotors’ wiring, there would be 26! possibilities for 
31

Appendix A: Key Space

SIGABA wiring scheme
32

 Appendix B: The 

 Mechanics of SIGABA

The fact that each message used a different message indicator (initial setting) meant that

each message for that day would start in a different place along the scrambling maze. While reusing the scrambling maze does not mean the enemy

could immediately read the messages, it did weaken

the cipher system. Certainly, overuse of the highlevel SIGABA system could have led to German

or Japanese cryptanalytic capabilities against SIGABA, but the Americans avoided this by relying on

the tactical Hagelin machines to conduct much of

their chatter. Furthermore, to conduct an exhaustive attack against the SIGABA machine would

take 248.4 trial decryptions to get the rotors in the

right order. Overuse would weaken the encryption

system, not break it.

The twenty-six outputs from the control rotors 

are bundled together into nine different input signals that are then connected to the index rotor bank.

At most, there could be four live wires going into 

the index rotors. At the minimum, there could be a 

single live wire. In the diagram pictured in Appendix 

A, there are three live wires going from the control 

rotor bank to the index rotor bank.

The index rotors, denoted I1

 through I5, further

scramble the inputs from the control rotors. Once the

signals travel through the five index rotors, they are

bundled by adjacent pairs into five wires that connect

to each one of the five cipher rotors. If the wire comRefer to the wiring diagram in Appendix A for 

more information about SIGABA’s cryptographic 

wiring.

The SIGABA machine works in a relatively 

simple manner. Four input signals are activated with 

each keystroke. These four signals go through the 

five control rotors. These rotors (see SIGABA wiring scheme on previous page) are designated as S1

through S5

 in the diagram. After each keystroke, the 

center, or third, control rotor (S3

) will step forward 

one letter. After every twenty-six keystrokes, both 

the third (S3

) and the fourth (S4) control rotor will 

step. After every 676 keystrokes, the second (S2

),

third (S3

), and fourth (S4) rotors will step. The first 

(S1

) and fifth (S5) rotors are stationary. This ensures 

that the impulse signals from every keystroke are 

scrambled in a different fashion. Rotors S3

, S4, and 

S2

 comprise a 17,576-long counter. That is, after S3

does a full cycle, rotor S4

 steps. Then, after S4 finishes a full cycle (and S3

 has completed twenty-six 

full cycles), rotor S2

 will step. By the time S2 has gone 

through its full cycle, the whole machine has enciphered 17,576 letters, which is only 214.1

, making this 

the “cycle length” for the control rotor scrambling 

maze. After 17,576 enciphered letters, the machine 

will start over, essentially reusing the same scrambling system.This was why the rotor placements had 

to be changed daily. 
33

Appendix B: Mechanics

Each letter of the message travels through only the

five cipher rotors, C1

 through C5 in the diagram. The

control rotors and index rotors exist only to dictate the

stepping of the five cipher rotors. The plaintext letter

from the keyboard travels via wire to the left-hand side

of the cipher rotor bank.The signal wire is then scrambled through the five cipher rotors and emerges on the

other side of the cipher rotor bank as an encrypted letter. The cipher text is then printed on a small tape. The

decryption process simply works in reverse. 

ing from the index rotor bank is live, then the cipher

rotor it is connected to will step forward one letter. 

Otherwise, it will not move. For each keystroke, there

can be between one and four live wires coming from

the index rotor bank and going into the cipher rotor

bank. In the diagram, there are exactly two wires that

are active: wire number two and wire number five. 

This means that both the second (C2

) and fifth (C5) 

cipher rotor will step, but the other three will remain

stationary for this particular keystroke.

Five pages from the SIGABA user’s manual Crypto-Operating Instructions for Converter M-134-C
34

SIGABA / ECM II Cipher Machine
35

Appendix B: Mechanics
36

SIGABA / ECM II Cipher Machine
37

Appendix B: Mechanics
38

1. David Kahn, The Codebreakers: The Story of Secret 

Writing. New York: MacMillan, 1967, 235.

2. Kahn, Codebreakers, 415.

3. Jack Levine, United States Cryptographic Patents,

1861-1989 (Terre Haute, IN: Cryptologia, 1991),

85.

4. Frank B. Rowlett, The Story of Magic: Memoirs of

an American Cryptologic Pioneer (Laguna Hills,

CA: Aegean Park Press, 1998), 69. Also Captain Laurance Safford, United States Navy OP20-S-5, History of Invention and Development of

the Mark II ECM, SRH-360, 19-21 (National 

Archives and Records Administration [NARA]:

RG 457, box 1124).

5. Army Security Agency, History of Converter

M-134-C, Vol. 1, or SRH-359 (Washington, DC: 

Army Security Agency, n.d.), 18.

6. Ibid., 51-60.

7. Friedman Collection, NSA/CSS Archives accession #47270, box 14, folder 1.

8. Ibid.

9. Levine, United States Cryptographic Patents, 115.

10. Friedman Collection, 1920-1960, NSA/CSS 

Archives accession #47270, box 14, folders 1-2.

11. Ibid., folder 1.

12. Ibid.

13. Ibid.

14. Ibid.

15. Frank B. Rowlett, Oral History Interview 1974,

OH-1974-01, Part B, 45c, Ft. Meade, MD: Center for Cryptologic History (CCH).

16. Ibid., 45c-45d.

17. Ibid., 38.

18. Ibid., 39.

19. Ibid., 39j.

20. Ibid., 40.

21. Ibid., 39-40.

22. Ibid., 40; Rowlett, Story of Magic, 97.

23. Rowlett, Oral History Interview 1974, OH-1974-

01, 40, Ft. Meade, MD: Center for Cryptologic

History. 

24. Ibid.; Rowlett, Story of Magic, 97-98.

25. Heather Ellie Kwong, thesis: Cryptanalysis of the 

SIGABA Cipher, San Jose State University, 2008.

26. http://www.sccs.swarthmore.edu/users/08/ajb/tmve/

wiki100k/docs/SIGABA.html.

27. Laurance Safford, History of Invention and Development of the Mark II ECM, SRH-360, NARA

RG 457, Box 1124. The ECM Mark I was the

Navy’s primary cipher machine from May 1936 to

January 1942. It was also the Navy’s first attempt

at an “in-house” cipher machine. In spite of ECM

Mark I’s formidable cryptographic strength, the

Navy remained uneasy about its vulnerability to

exploitation. The Mark I arguably was the most

powerful cryptologic device extant, but at the time

the cryptographers at OP-20-G had little appreciation of this fact. The ECM Mark I embodied

elements of contemporary Hebern machines and

then was heavily modified by Donald Seiler and

Laurance Safford. These novel cryptographic

enhancements placed the Mark I in a league of

its own, well above the Hebern machine. Seiler

also went on to perform much valuable engineering work on the ECM Mark II’s development.

According to Safford, the Mark I was beset with

Notes
39

Notes

numerous mechanical deficiencies that required

almost constant maintenance to keep it operational. This fact alone was probably the largest,

immediate driving factor in the Navy’s quest for

a new machine. When he returned from sea duty

and was briefed by Wenger on his interview with

Friedman and Rowlett, Safford readily embraced

the SIGABA principles and pressed for its

development.

28. Rowlett, Oral History Interview, 1974, OH-1974-

01, 41, Ft. Meade, MD: Center for Cryptologic

History. 

29. Ibid.

30. Safford, History of Invention and Development of

the Mark II ECM, 24-25, 29.

31. Rowlett, Oral History Interview 1974, OH-1974-

01, 45n, Ft. Meade, MD: Center for Cryptologic

History; Army Security Agency, History of Converter M-134-C, vol. 1, SRH-359, Washington,

DC: Army Security Agency, n.d., 139.

32. Rowlett, Story of Magic, 143.

33. Army Security Agency, History of Converter 

M-134-C, 138-139; Rowlett, Story of Magic, 143.

34. Rowlett, Story of Magic, 143-144.

35. Ibid., 144.

36. Rowlett, Oral History Interview 1974, OH-1974-

01, 46, Ft. Meade, MD: Center for Cryptologic

History.

37. On 1 August 1941 the sister services formally

adopted the SIGABA/ECM II as their joint 

cipher machine. Each agreed not to share any 

information whatever about the machine to any 

outside person or organization.

38. Rowlett, Oral History Interview 1974, OH-1974-

01, 46, Ft. Meade, MD: Center for Cryptologic

History.

39. Ibid., 47.The Navy’s Donald Seiler had given due 

consideration to plugboards and rejected them in 

favor of the index rotor wheels even before the 

first ECM Mark II had reached the blueprint 

stage. Safford, History of Invention and Development of the Mark II ECM, 39. Safford, Chief OP20-G, relates that besides being inconvenient to 

the user, plugboards in the past had not prevented the initial solutions of six different cipher 

machines the Navy had tested. He also eschewed 

them because user errors in plugging had directly 

led to compromise of the key, not to mention the 

effect on the reliability of communications. The 

Navy’s greatest argument in favor of rotors over 

plugboards was that rotors offered 49 times more 

stepping combinations than the latter (SRH 

360, 28). German and American cryptographers 

independently of each other conceived of using 

plugboards as enhancements to their respective 

cipher machines.

40.Rowlett, Oral History Interview 1974, OH-1974-

01, 45, Center for Cryptologic History, Ft. Meade, 

MD.

41. Ibid., 46.

42. Safford, History of Invention and Development 

of the Mark II ECM, 40; The Navy deemed the 

thermite emergency destruction devices too dangerous to use aboard ships where fire is the greatest threat. Navy tests of thermite bombs reduced 

the ECM’s critical components to molten metal 

in 97 seconds. Safford, History of Invention and 

Development of the Mark II ECM, 61. This notwithstanding, the Army had few qualms about 

fielding both thermite and TNT charges atop 

SIGABA’s 800 lb. security cabinets aboard tactical communications vehicles. There is no record 

of an unintended mishap associated with these 

devices; even so, the Navy’s concerns were partly 

vindicated by more than one Army incident of a 

near discharge. 

43. Safford, History of Invention and Development of

the Mark II ECM, 42; 37-52 passim.

44. Ibid., 6; the ECM Mark I, despite the Navy’s

reservations about it, was still more powerful 

than any cipher device fielded by either the British or the Axis Powers.

45. Ibid., 6-8.

46. Ibid.

47. Ibid.

48. Ibid.

49. Ibid.

50. Safford, History of Invention and Development of

the Mark II ECM, 54.

51. Ibid., 55-56.

52. R. A. Ratcliff, Delusions of Intelligence: Enigma,

Ultra, and the End of Secure Ciphers (New York: 
40

War II as Revealed by “TICOM” Investigations and 

Other Prisoner of War Interrogations and Captured 

Material, Principally German, vol. 5, The German Air Force Signal Intelligence Service. Army 

Security Agency, 10 October 1946.

65. War Diary of the Signal Intelligence Group, February–November 1944 (NSA/CSS Archives,

accession #5411, box G22-0303-3), 64.

66. Ibid.

67. Erich Huttenhain, interview, 10 July 1945 

(NARA: RG 457, box 1006).

68. Ibid.

69. Ibid.

70. Ibid.

71. Ibid. The Japanese Signal Intelligence Service, 17 

October 1952 (NARA: RG 457, box 1129).

72. Ratcliff, Delusions of Intelligence, 178; Pre-War 

Radio Intelligence Activities in the Philippines,

63, CCH Files; Timothy Mucklow, Federal History Journal, 2011, 59.

73. Army-Navy Joint Policy Concerning Distribution and Disclosure of Cryptographic Design of the 

ECM-M134C, 26 June 1942; Safford, History of

Invention and Development of the Mark II ECM, 

57-58.

74. Safford, History of Invention and Development of

the Mark II ECM, 110-111, 115-116; Ratcliff, 

Delusions of Intelligence, 168.

75. Letter, Col. George A. Bircher, SC, to CG, AAF, 

Washington, DC, Attn: Ch, Sec, Air Communications Office, 9 May 46, sub: Policy on Storage 

of Converter M-134-C:

 . . . 2. This Agency does not consider that 

the cessation of hostilities justifies any relaxation in security regulations pertaining to 

cryptographic material and has noted with 

alarm a tendency towards such relaxation 

during recent months as has been evidenced 

by an increase in the number of physical and 

cryptographic compromises.

3. Since the cryptographic principle and 

design of Converter M-134-C is in the 

sole possession of the United States and it 

is considered to be the best cryptographic 

device of its type, it is not advisable to jeopardize its use or storage under unsatisfacCambridge University Press, 2006), 81. Also Safford, History of Invention and Development of the 

Mark II ECM, 52.

53. Army Security Agency, History of Converter 

M-134-C, 7.

54. Ibid., 147-158, 247-248.

55. The Navy generated its SIGABA key at the 

Nebraska Avenue Naval Station where it operated its ECM Mark II maintenance facility. 

Army cryptographers produced SIGABA keys

at Arlington Hall. Later in the war, the Army 

moved its SIGABA maintenance and training 

functions to Vint Hill Farms near Warrenton,

VA. 

56. Safford, History of Invention and Development of

the Mark II ECM, 31.

57. Ratcliff, Delusions of Intelligence, 105, 164, 168,

177. By the war’s end, more than 8,000 SIGABAs had the capability to interoperate with the 

British Typex machine.

58. J. G. Seabourne, The Signal Intelligence Service 

of the German Luftwaffe, vol. 13. November 24,

1945 (National Archives and Records Administration: RG 457, box 976). The Navy’s anxiety about its own ECM Mark I led Wenger to 

approach Friedman about the Army’s cryptographic research—Rowlett, Oral History Interview 1974, OH-1974-01, 45m—in the first 

place. The sister services’ continued development 

of, and their absolutely rigorous application of, 

COMSEC doctrine demonstrate their concern 

over enemy attempts to break into U.S. crypto 

systems.

59. JN-A-20 messages, 1942 (National Archives 

and Records Administration: RG 457, box 1006,

temporary folder).

60. Ibid.; Ratcliff, Delusions of Intelligence, 201-203.

61. Rowlett, Oral History Interview 1974, OH-1974-

01, 105.

62. Ratcliff, Delusions of Intelligence, 202. Several different German cryptologic organizations named 

SIGABA as the “Big Machine.”

63. Rowlett, Oral History Interview 1974, OH-1974-

01, 158-159.

64. Seabourne, Signal Intelligence Service. Also see 

report: European Axis Signal Intelligence in World 

SIGABA / ECM II Cipher Machine
41

because it simply could not keep pace with modern high-speed telecommunications. Research 

based on newly acquired information suggests 

that SIGABA was taken out of the inventory to keep the advanced technology out of the 

hands of the Soviets. Because every SIGABA 

and rotor had been accounted for and because 

Army and Navy cryptologists had much confidence in the integrity of SIGABA principles,

they were confident that SIGABA’s secrets 

could be retained; History of Converter M-134-C, 

5 May 1950 included in CCH copy of SRH 

360, 7-10 (included in CCH copy of SRH 360).

SIGABA and its temporary successor SIGROD were slowly replaced in the 1950s by the 

TSEC/KL-7 (ADONIS/POLLUX). The new 

cipher machine was an electronic-mechanical 

hybrid that employed a programmable cipher 

rotors/bezel assembly (eight rotors/thirty-six 

pins), cams, and vacuum tube technology along 

with a novel re-flexing principle. It was phased 

out of the U.S. military inventory in the early

1980s.

tory security conditions. It is considered 

that eliminating the necessity for a 24-hour 

armed guard, regardless of geographical 

location, even if the converter were stored 

in a CH-76 in a locked code room, would 

result in such a condition, and that no justification exists for undergoing such a risk. . . . 

Ratcliff, Delusions of Intelligence, 168.

76. History of Converter M-134-C, 5 May 1950,

included in the CCH copy of SRH 360, 1-10.

77. Ibid., 7; Extracts: Annual Report, Security Division, CSAS-80, Fiscal Year 1948 (included in 

CCH copy of SRH 360), 1-5; Excerpt: Tab A10,

“Staff Study Concerning the Issue of SIGROD 

Machines to all Military Attachés,” Annual 

Report, Sec Div, Tech Staff, FY 48 (23 April 

1947) 1948 (included in CCH copy of SRH 

360), 1-4; Extract: Tab A7, “Staff Study on the 

Introduction of the SIGROD,” Annual Report,

Security Division,Technical Staff, FY 48 (3 February 1950), 19 (included in CCH copy of SRH 

360).

78. Until recently, cryptologic historians were under 

the impression that the end for SIGABA came 

not because of any cryptographic weakness but 

Notes
42

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43

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SRMN = U.S. Navy Discrete Records of Historical 

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National Security Agency, 1956.

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Signal Intelligence Service (Later Signal Security

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Forces Security Agency,” 12 October 1950. NSA/

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Related Publications

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