Preface

One of the missions of the National Secu

(NSA) is to protect classified information wheth

processing, or transit. Collectively, informa

security (INFOSEC) is the development and a

hardware, software, and doctrine. The “in tran

called communications security (COMSEC), 

the underlying information is protected fro

exploitation, disruption, or misrepresentat

available only to authorized recipients. This broc

cradle-to-grave story of highly successful cr

equipment for teletypewriter (TTY) commun

TSEC/KW-26 and the people who developed, pr

fielded it. (The italicized words are defined in t

glossary.)

Teletypewriter COMSEC

The changes in communication technology l

the introduction of the KW-26 date back to 19

introduction of the Start/Stop method of sy

printing telegraph equipment by Charles L. Kru

son, Howard Krumm. Until that time synchron

telegraph systems employed constant length c

five-element Baudot. However, these systems r

accurate means for maintaining synchronis

electromechanical transmitting and receiving 

“Start/Stop” overcame this drawback by resync

the start of each character, making it no longer 

accurately control the speed of the instruments.

1
Alphabet #2 (IA#2). Start/stop Teletype be

worldwide commercial applications about 

instrument employed a typewriter-style ke

printed the received signals directly onto paper t

requiring the intermediate use of perforated ta

end of the system. It was capable of working a

forty words per minute.1 For the first time, anyon

type could send messages without needing a ski

or for that matter any attendant at the receivi

system was so successful that it became the dom

of handling record traffic, but not securely.

In 1917 a young AT&T research engineer nam

Vernam, working on new developments in teleg

up with a novel (and relatively simple) scheme fo

TTY.2 He mixed two Baudot-coded punched 

(hole being a “+,” no hole a “-”): one tape co

plaintext message and the other the “key.” Thes

added “modulo 2” (“exclusive or”) in a mec

reader producing output cipher signal. At the re

the incoming cipher is added (modulo 2) with a 

the identical key producing the plain text. I

unpredictable and “flat” (i.e., probability of a “+

of “-” = 1⁄2), then the cipher text is “unbreakable” 

to cryptanalysis). The Vernam logic:

2

Modulo Two Addition (*)

plain key cipher

+ * - 

- * + 

+ * + 

- * - 
Signal School in 1918. They added two (modulo 

prime eight-foot tape loops, making a secondar

one million key characters. The Signal Corps s

interest. It wasn’t until WWII that the Signal Co

it for high-level TTY traffic (SIGTOT). In the 

marketed the Vernam system for commer

applications. Though an engineering succes

commercial failure. Cable companies and 

continued to use old-fashioned codes instea

encryption.3

Krumm and Vernam opened new vist

encryption, a far cry from the technology of the 

paper” codes and ciphers for hand-keyed Morse 

that time. A competing technology for se

messages was invented soon after Vernam: 

mechanical rotor machines. Though they oper

and were more complex devices, their operatio

in key management considerably outw

production/destruction of mounds of one-

Invented by four men in four different countries 

an American, a German, a Dutchman, and a S

rotor machines were the principal means of mes

before and through WWII for governments and 

During WWII the U.S. used commercial o

machines, e.g., Hagelin M209, SIGABA, SI

government-produced key.5

Lessons Learned from WWII 

The stunning successes of Allied cryptologist

the Enigma6 and Purple did not go unhee

Department of Defense after the war. The mili

3
Corps officers and noncoms.

In-house development of improved off-line r

to replace the WWII rotor machines was initiat

sought a high-speed on-line OTT equipment to 

British 5-UCO currently being used in the 

community. The British enjoyed great succe

system because, being fully synchronous, 

electrically regenerated on tandem high freq

radio links. Like predecessor U.S. equipment 

used “mountains” of key tape to operate on a 24

shear magnitude of generating, certifying, and de

tape and the cost of distribution and accountab

their use to the most sensitive traffic. On the pl

systems provided traffic flow security (TFS) a

directly with commercial circuits. Anoth

capability of the 5-UCO was that the operator a

end could maintain crypto-synchronization if th

suddenly changed by “walking up and down” t

(one character at a time or one bit at a time). Th

avoided the operationally cumbersome task of a 

With the merger in 1949 of the service 

agencies into the Armed Forces Security Age

creation of NSA in 1952, jurisdiction over U.S. c

was vested in NSA. This extended to major pla

the Department of Defense. Throughout, a prim

was communication security for record traf

Barlow, chief of the Record and Data group, i

high-priority research on viability of electronic en

TTY, namely:

4
This was a decided break from past: in techn

rather than relays and wired rotors; in cr

keystream generators rather than one-time ta

and in key management punch cards rather tha

wired rotors. To be retained in the replacement o

was the Vernam modulo-2 convention and its 

security (TFS) capability.

The proposed TSEC/KW-26 development wa

the above requirements, i.e., to provide TFS a

security for all classifications of record traffic.

Development Effort7

When the requirements for secure TTY were 

most mature electronic technology was th

vacuum tube. Tubes had limitations, howeve

their reliability and power consumption. A

transistor had been invented (1947), it was n

“prime time.” Therefore, the search was on for a

solution. The R/D engineers sought out l

industrial partners in digital technology to 

cryptographic concepts generated in house.

The KW-26 graphically illustrated this sy

young engineer working in Howard Barlow’s 

intrigued with the research on Bi-Magnetic cores 

college classmate, Lyle Thompson at Burroughs 

Research Center. Lyle described how they 

multiple winding torroidal magnetic cores 

Magnetic Elements (BIMAG).8 With appropria

nections, they performed the binary logic and w

implement shift registers. This spawned ongoi

5
Joe Hannan, a recent electrical engineering gra

Zeigler, and two Army technicians. Joe’s team b

a “proof of concept” model based on the Dick 

Marsh cryptologic and I/O guidance by Fra

Joe’s supervisor, for the plethora of com

interfaces the KW-26 would face. Charles Napie

novel common fill device (CFD), a first of the b

daily crypto-variable. It used an IBM card p

RemRand format (45 vice 80 columns, 

rectangular holes). His design, fabricated in the 

shop, was incorporated into the breadboard. In 

operator inserted the daily key card into the CF

the door securely, locking the card in plac

doctrine did not permit card re-use. Therefore, 

requirement, the card was severed upon reopen

The completed breadboard model require

telephone relay rack for each transmitter a

terminal. In 1954 TCOM, the NSA telecom

organization, successfully tested the system 

looped at an overseas site. It demonstrated the 

electronic TTY encryption. The system employe

devised Koken crypto-algorithm used with a 

TTY input. As the breadboard was being built 

Joe Hannan prepared the specification for 

development contract.

Engineering Development Contract9

The Request for Proposal (RFP) was released 

in 1953 to the Burroughs Corporation. Its objec

demonstrate

6
• availability (continuous operation)

• in/output compatibility with associat

communication systems

The RFP was based on Fibonacci vice Ko

Their proposed cryptologic design was base

BIMAG, from the Fibonacci shift registers to the 

combining elements. The CFD was governmen

After evaluation of the Burroughs bid from co

and technical risk, NSA awarded the developm

in 1953. The initial design indicated that B

superior to vacuum tubes on reliability 

consumption (though the KW-26 clock rate w

slow, less than 5 kHz, the vacuum tube pent

the BIMAG elements were approaching their de

The first model was delivered in 1955. 

demonstrated BIMAG as a viable technology fo

logic. The torroidal cores of exposed magne

mounted on “islands” along the edge of the pl

board and hand-wired while on the circuit boa

Burroughs and the NSA engineers recogniz

hurdle, i.e., the need for more practical packagin

circuits in the final engineering development p

units also were successfully evaluated on lo

circuits.

A follow-on contract for Final Engineering D

Models was awarded to Burroughs in 1955 to 

above deficiencies. During this phase each core w

as a potted module with twelve I/O pins conn

internal windings. Up to eighty of these potted m

mounted on large multilayer printed circuit 

7
Environmental tests showed these mode

intermittent loss of crypto-synchronization

temperature tests (up to 125 degrees F) revea

ceramic capacitors were shorting out at 

temperatures. Erie, the manufacturer, upon 

testing discovered a “bad batch” had been 

Burroughs. As a result, Erie instituted new qu

procedures, and Burroughs instituted incoming 

No further problems occurred. The R/D p

released the final design to the COMSEC 

engineers for procurement of operational equip

interim Joe Hannan and Tony Russo particip

ongoing operational evaluation with TCOM.

Operational Trials 10

Final engineering development units were o

evaluated on the CRITICOMM network (the title 

the JCS to those communications supporting 

SIGINT mission). The KW-26s performed very 

for the problem of dealing with transmission ano

lacked the ability to “walk” up and down the ke

had been possible with the 5-UCO. This limitat

overcome by inserting a twenty-one-bit delay in 

(TSEC/HW-8) between the incoming line and t

KW-26. Initially the operator would set the de

and bring up the link. If the path delay shifted 

receive operator would search for the correct d

maintaining crypto-synchronization (garbled t

of course, have to be re-sent). This was effecti

percent of the time, thus avoiding the time

8
restart coordination with the sending station. N

Navy purchased approximately 450 HW-8s.

Services’ requirements for numerous modes 

significantly increased the cost, schedule, and th

of the original engineering design. Of the mode

combinations possible, it is doubtful more than 

were ever used. One option, the so-called speed 

delayed the production deliveries at a tim

equipment was critically needed. This option was 

operate via the low-speed transoceanic direct cu

which were in their last days of service. Other c

permitted operation through AT&T mechanical 

repeaters, also being replaced in the 1960s.

Production

The NSA COMSEC Production organization

• coordinated DoD and Community bu

• issued RFPs

• handled contract management 

• coordinated acceptance tests

9
• published manuals

• performed key management 

• carried out inventory control

• handled decommissioning effort

The initial production order for 1,500 for C

and related SIGINT requirements was awarded t

in 1957. During this initial procurement there w

pressure from other services’ programs f

Subsequent procurements running from the 

produced over 14,000 units. These were procu

shore facilities and shipboard use, Army and t

tactical point-to-point service, and in 

environments by the Defense Communicatio

State, and CIA.

A KW-26 system (transmitter or receiver) co

800 cores and approximately 50 vacuum-tube dr

occupying slightly more than one half of a sta

rack. Most of the space in the rack and most of th

power were required for the special-purpose v

circuits needed to provide compatibility with 

input and output circuits configurations.

Operational Use

The KW-26 was the workhorse cryptosys

point-to-point circuits of the CRITICOMM netw

also used it to provide traffic flow and cont

to/from SCA SIGINT sites to in-house NSA a

services used KW-26 to secure their tactical to

10
decommissioned by NSA, being replaced b

advanced solid-state data encryptor, TSEC/KG-

Parting Lessons Learned

In the 1950s the requirements process

command decision authority within the Do

intelligence community, subjected the multi-us

11

Fig. 2. TSEC/KW-26C, electronic synchro

teletypewriter security equipment
in preparing specifications, as well as a better m

incorporate engineering changes after operation

the extent possible, the handoff from R/D to 

Engineering should strive to be a “build to pr

Having the ability to slave all the KW-26 trans

communications center to a master clock would 

useful tool to reduce preventive maintenance a

availability. 

Epilogue

The KW-26 truly was a “value added” adjun

point-to-point TTY communication circuits

operational and maintenance viewpoint, it w

rigors of sixteen-inch guns being fired 

experienced very few operational outages. 

operational objectives for a diverse set of users

those in the CRITICOMM network and in the ge

(GENSER) community. For over twenty-five ye

the DoD and the intelligence community, thr

Cold War.

Decommissioning lasted several years. NS

the services and other “holders of reco

“demilitarization process” in returning the se

ponents to the Information Assurance Director

other locations for secure destruction. (These we

on the printed circuit boards and the CFD, wh

information about the combining and crypto

remains (e.g., the aluminum chassis, power s

after having their labels removed, were melte

discarded as scrap.

12
When the last days of the first KW-26 becam

TCOM sent the first units to the NSA archives 

preservation. The parting words of the prin

transmission system engineer:

I can think of no equipment that advan

cause of secure 100 WPM Tele-type

service more than the KW-26. It also 

that electronic on-line encryption w

only feasible but actually enhanced 

performance.11

13

Fig. 3. The KW-26 in operation
3 (U) Ibid., 397, 411, 421

4 (U) Ibid.

5 (U) Earl Flowers, David Boak: unpublished notes

6 (U) Center for Cryptologic History, The Bombe: Pre

Cryptanalysis (Ft. George Meade, MD: National Security 

7 (U) Albert J. Meyerhoff, et al., Digital Application

Devices (New York: John Wiley and Sons, 1960)

8 (U) Ibid.

9 (U) Joseph Hannan: unpublished notes

10 (U) Richard A. Day: unpublished notes

11 (U) Ibid.

Acknowledgments

Contributors Reviewers

Richard A. Day Robert Alde

Don Burkness Howard Barlow

Stanley Disson (Burroughs) David Boak

Earl Flowers Jerry Moskowitz

Joseph Hannan Richard Proto

Tom Prugh

14
encryption algorithm is merely the access

mathematical process, to encrypt and decrypt m

Asymmetric Key Cipher – Also known 

private key cryptography system.

Authentication – Security measure d

establish the validity of a transmission, m

originator, or a means of verifying an 

authorization to receive specific categories of inf

Baudot code – A synchronous code in whic

length bits represent one character. Note 1: The 

which was developed circa 1880, has been rep

start-stop asynchronous International Alphabe

No. 2). Note 2: IA No. 2 is not, and should not 

as, the Baudot code. Note 3: The Baudot code has 

used in teletypewriter systems. 

Cipher – Any cryptographic system in wh

symbols or groups of symbols represent units of 

in which units of plain text are rearranged, or bo

Cipher text – Enciphered information.

Code – System of communication in whi

groups of letters, numbers, or symbols repres

plain text of varying length.

Common Fill Device (CFD) – One of 

devices developed to read-in, transfer, or store k

Communications security (COMSEC) 

and controls taken to deny unauthorized persons 

derived from telecommunications and to 

15
authenticate telecommunications. COMSE

includes, but is not limited to, key, equipm

documents, firmware, or software that e

describes cryptographic logic and other items t

COMSEC functions.

CRITICOMM – The title assigned by the 

communications supporting the national SIGIN

Cryptography – Art or science con

principles, means, and methods for rend

information unintelligible and for restoring 

information to intelligible form.

Cryptosecurity – Component of COMSE

from the provision of technically sound crypto

their proper use.

Cryptanalysis – Operations performed in 

encrypted messages to plain text without initial k

the crypto-algorithm and/or key employed in th

Cryptology – The study of both crypto

cryptanalysis. 

Cryptosystem – Associated INFOSEC item

to provide a single means of encryption or decry

Decipher – Convert enciphered text to p

means of a cryptographic system.

Decrypt – Generic term encompassing 

decipher.

Emissions security – Protection res

measures taken to deny unauthorized persons 

derived from intercept and analysis of co

16
and encoding.

Encryption algorithm – Set of ma

expressed rules for rendering data unintelligible 

a series of conversions controlled by a key.

End-to-end encryption – Encryption of in

its origin and decryption at its intended destina

intermediate decryption.

Fill device – COMSEC item used to transfer 

in electronic form or to insert key into a crypto-e

Hard copy key – Physical keying mater

printed key lists, punched or printed ke

programmable, read-only memories (PROM).

Information Assurance (IA) – 

operations (IO) that protect and defend infor

information systems by ensuring their availabil

authentication, confidentiality, and nonrepud

includes providing for restoration of informatio

incorporating protection, detection, and reaction 

Information System (IS) – the entire in

organization, personnel, and components for th

processing, storage, transmission, display, di

and disposition of information.

Information systems security (INFOS

ISS) – Protection of information syste

unauthorized access to or modification of 

whether in storage, processing or transit, and 

denial of service to authorized users, inclu

17
encrypting or decrypting electronic sign

determining electronic counter-countermeasu

(e.g., frequency hopping or spread spectru

producing other key.

Key card – Paper card, containing a pattern 

holes, that establishes key for a specific cryptone

time.

Key stream – Sequence of symbols (or their 

mechanical equivalents) produced in a mach

manual cryptosystem to combine with plain tex

cipher text, control transmission security p

produce key.

Off-line cryptosystem – Cryptosystem 

encryption and decryption are performed inde

the transmission and reception functions.

On-line cryptosystem – Cryptosystem 

encryption and decryption are performed in asso

the transmitting and receiving functions.

One-time cryptosystem – Cryptosystem 

key used only once.

One-time tape – Punched paper tape use

key streams on a one-time basis in certa

cryptosystems.

Plain text – Unencrypted information.

Security fault analysis – Assessme

performed on Information System (IS) h

determine the security properties of a device wh

fault is encountered.

18
message. 

Teleprinter – A teletypewriter that can only 

and does not have a keyboard for transmission.

Teletypewriter (TTY) – A printing 

instrument that has a signal-actuated mec

automatically printing received messages. Note 1

have a keyboard similar to that of a typewriter 

messages. Radio circuits carrying TTY are c

circuits” or “RATT circuits.” 

Traffic-flow security – Measure used to 

presence of valid messages in an on-line cryp

secure communications system.

TSEC nomenclature – System for identify

and purpose of certain items of COMSEC mater

Work factor – Estimate of the effort or tim

a potential perpetrator, with specified ex

resources, to overcome a protective measure.

Zeroize – To remove or eliminate the key fr

equipment or fill device.

(Sources: Federal Standard 1037c; National 

Systems Security Glossary – NST ISSI4009)

19