From:" Military Communications: A Test for Technology" Center for Military History

Throughout the war, communicators from both sides were engaged in a conflict on an electronic battlefield that ultimately affected the outcome of battles fought by ground combat troops. Monitoring an adversary's plans, tracking his movements, deceiving and manipulating him, and disrupting his communications -- all were elements of a deadly contest of electronic warfare. (1)

The principal contenders on the electronic battlefield were, on the one side, the men of the U. S. Army Security Agency (ASA) and the South Vietnamese Special Security Technical Branch and, on the other, teams of Communist intercept operators, called technical reconnaissance agents. Handpicked for loyalty and intelligence, the men of both sides were deadly proficient, continuously perfecting the art of Electronic Warfare. To counter that effectiveness, communicators of the opposing armies developed elaborate proceduresand sophisticated equipment in a defense against electronic warfare called communications security.

Traditionally, offensive electronic warfare ranged from disrupting an enemy's communications to covertly monitoring them for intelligence. During the conflict in South Vietnam both sides restricted more overt forms of electronic warfare so as not to interfere with or compromise their collection of intelligence. Electronic warfare in South Vietnam thus was waged largely in its most covert form. Motivating, training, and equipping communicators to defend against that hidden threat tasked the ingenuity of cryptographers and the leadership of signal officers on both sides throughout the course of the war.

Although the South Vietnamese had some American radio intercept equipment left behind by the French Army, they had little success with it. Thus, in 1958 President Diem requested electronic warfare assistance from the United States to locate clandestine Communist radio stations in South Vietnam. The U. S. Intelligence Board resisted hazarding the loss of highly classified electronic warfare equipment to the Communists until President Kennedy, responding to an increasingly urgent need to penetrate a burgeoning Viet Cong insurgency, directed the Army Security Agency on 29 April 1961 to send men to assist and train the South Vietnamese in conducting radio direction finding. On 13 May the first contingent of American electronic warfare specialists arrived in South Vietnam. (2)

During General Maxwell D. Taylor's visit to South Vietnam in the fall of 1961, he noted problems with the collection and reporting of signal intelligence. Responding to Taylor's troubling news, Assistant Secretary of Defense (International Security Affairs) John H. Rubel directed the Secretary of the Army and the Director of the National Security Agency to improve American electronic warfare operations in

South Vietnam. Before tackling that job, the Army re- equipped the teams in South Vietnam with more reliable radios and established a separate Operations and Intelligence Net for disseminating intelligence to advisers. The paucity of signal intelligence was less easily remedied. Signal Corps engineers dispatched from the U. S. Army Electronics Command at Fort Monmouth found that equipment assigned to the teams, although the best available, was designed for operations in Europe and was ill suited for the peculiar propagational characteristics of Southeast Asia. (3)

Since it was impossible reliably to track the extremely high angle sky waves produced by Viet Cong high- frequency radios in the tropic atmosphere over South Vietnam, monitoring teams were limited to intercepting transmissions located in the range of a ground wave -- about five miles, or some 5 percent of the available targets. After modifying the antennas of the intercept equipment, the engineers returned to the Electronics Command Laboratories at Fort Monmouth to develop better equipment.

Working with engineers from the Army Security Agency, they experimented with completely new approaches to radio intercept. They discovered that by taking direction- finding equipment aloft in an aircraft, they not only eliminated terrain and vegetation barriers, but they also raised their receiving antennas into the path of the radio waves from enemy transmitters and gained additional mobility. In March 1962 three specially modified L- 20 airplanes flew the first operational missions against Viet Cong transmitters in the III Corps sector, and within a month airborne direction- finding teams had located the transmitters of six major Viet Cong headquarters. (4)

The several coups, attempted coups, and countercoups during the first half of the 1960s interfered with the development of South Vietnamese intelligence organizations and the conduct of clandestine activities, against the Viet Cong. The party in power always suspected that those operations might be turned against it rather than against the Viet Cong. Because of that suspicion, it was not until 1964 that the position of communications intelligence officer (J- 7) was established on the Joint General Staff to supervise clandestine operations carried out by the military. Even then, assignment of electronic warfare teams and equipment to field units where their use could be most effective came slowly. Not until 1968 were the first three South Vietnamese divisions provided with small electronic warfare units, called technical detachments. Another year passed before the rest of the divisions received detachments and each corps was assigned a fixed radio direction- finding station. (5)

The dramatic evolution and success of the U. S. Army's electronic warfare program stunted the development of South Vietnam's program during the 1960s. Within a short time after arriving in South Vietnam, the U. S. Army Security Agency took control of all electronic warfare activities. The Army's efforts were supplemented in January 1962 by the arrival of forty- three marines from the 1st Composite Radio Company (USMC).

Conducting both offensive and defensive electronic warfare programs, and tasked with a training and advisory missions as well, the Army Security Agency's 3d Radio Research Unit was stretched thin. In early 1963 the Army Security Agency assigned the 7th Radio Research Unit the defensive communications security mission for U. S. units in South Vietnam. Thereafter, the 3d Radio Research Unit concentrated on offensive electronic warfare, while providing direction and staff supervision to the 7th Radio Research Unit. Assigned to the U. S. Army Security Agency, Pacific, the two radio research units operated under the staff supervision of the MACV deputy chief of staff for intelligence and received logistical support from the U. S. Army Support Command, Vietnam, and the U. S. Army Security Agency Materiel Support Command in Virginia.

With the American troop buildup of 1965 came a requirement to improve the organization for electronic warfare. As each division and its separate brigade arrived with its own direct- support unit of the Army Security Agency, it became more difficult to control the American electronic warfare effort from the agency's Pacific headquarters in Hawaii. In mid- 1966 the Army Security Agency deactivated the 3rd Radio Research Unit and formed a new headquarters, the 509th Radio Research Group, to manage all Army electronic warfare operation in South Vietnam. Under the operational tasking of the Military Assistance Command, the 509th Group received command direction and technical support from headquarters of the Army Security Agency, Pacific, in Hawaii and logistical support from the U. S. Army, Vietnam. (6)

Designed to give cryptologic support to a field army, the 509thGroup had four major radio research components: the 303rd Battalion, the 313th Battalion, the 8th Field Station, and the 224th Aviation Battalion. It also had responsibility for the 101st Radio Research Company (formerly the 7th Radio Research Unit) which had a countrywide communications security mission and provided direct support to the two major U. S. headquarters in South Vietnam, the Military Assistance Command and the U. S. Army, Vietnam. The 303d Battalion provided similar support to the II Field Force headquarters at Long Binh and controlled the Army Security Agency's companies and detachments supporting the combat divisions and separate brigades assigned to the II Field Force; and the 313th Battalion, located at Nha Trang, performed a corresponding role in support of the I Field Force

Allied Offensive Electronic Warfare

To the envy of many -- especially communicators in the 1st Signal Brigade -- and to the aggravation of a few, the Army Security Agency's units in South Vietnam enjoyed an unrivaled independence. ASA battalion commanders were serving and reporting to so many different headquarters that they never had to submit entirely to one authority. Avoiding domination by field combat commanders and retaining strong ties with their parent organization, the Army Security Agency in Washington, the agency's field commanders could speak out more freely concerning their own areas of expertise and could experiment with new techniques without fear of interference.

Through continued trial and error, field technicians improved airborne directionfinding techniques. By the spring of 1966 the Joint Chiefs of Staff had approved a fleet of fifty- seven Army and forty- seven Air Force aircraft to support the program begun by the 3d Radio Research Unit. The old L- 20's, redesignated U- 6's, were supplemented by Army U- 8's and Air Force C- 47's, all carrying direction- finding equipment. To coordinate the tasking and scheduling of missions between Army and Air Force airborne radio direction- finding units located throughout South Vietnam, in June 1966 the MACV intelligence staff established a coordination center, jointly manned by representatives of the 509th Radio research Group and the 6994th Security Squadron, the Air Force unit responsible for electronic warfare in South Vietnam. (7)

Although South Vietnamese Army pilots had begun flying direction- finding missions with American instructors in 1963, the South Vietnamese program was ineffective until the early 1970s, when the Vietnamization program afforded them more sophisticated aircraft and more opportunity. The rapid growth of the South Vietnamese program, matched by the equally rapid decline of the American role, enabled the South Vietnamese eventually to provide 95 percent of the intelligence gained from airborne electronic warfare. After the 1973 cease- fire, with aircraft losses increasing from improved enemy air defenses and deteriorating logistical support within the South Vietnamese armed forces, South Vietnam's airborne program took a precipitous decline. (8)

Although American airborne direction- finding efforts overshadowed the ground program, the Army Security Agency's direct- support units were essential to American electronic warfare. Operating against a variety of enemy units, they identified targets for further exploitation by airborne teams. Since they worked against the same Communist targets every day, they acquired the best understanding, of local Communist communications. Building an informal enemy communications order of battle, the commander of every detachment knew which enemy units provided the most important information and which had the worst security discipline.

Analysts familiar with local communications patterns could garner important information. Realignment of radio nets and relocation of terminals often indicated changes in the enemy order of battle or forecast an impending operation. Disappearance of an enemy station after an allied attack obviously suggested success. American intelligence officers attempted to confirm i the observations of astute electronic warfare analysts with information from collateral intelligencc - captured documents or prisoners of war. (9)

The commander of the direct- support electronic warfare units on the ground had the responsibility of maintaining the vital link between the entire electronic warfare community and the tactical commander and his staff. He had to bridge the chasm between the esoteric science of cryptology and the practical concerns of the battlefield command post. Detachment chiefs of direct- support units advised tactical commanders of the enemy order of battle and of the electronic warfare resources, both ground and airborne, that were available to support particular operations.

Even after equipment and techniques improved, American electronic warfare efforts remained limited against short- range enemy communications, both telephones and low- powered radios. Teams were unable to move close enough to wiretap enemy lines or monitor short- range radio without jeopardizing the security of men and equipment. (10)

The Americans increasingly turned to unattended equipment to fill that role. In 1966 the United States began to build an electronic barrier of acoustic, seismic, and radio sensors across the northern border of South Vietnam, the panhandle of Laos, and the eastern regions of Thailand to detect North Vietnamese infiltration. A group of American scientists, who had assembled secretly in the summer of 1966 to consider means for harnessing American technology for the war, had conceived of the ambitious approach. To implement it, Secretary of Defense McNamara formed the Defense Commnunications Planning Group on 15 September 1966 under Lt. Gen. Alfred D. Starbird, the director of the Defense Communications Agency. (11)

Using the technical resources of the Electronics Command and several other governmental laboratories and commercial manufacturers, the Defense Communications Planning Group spent $670 million to develop and produce large stocks of sensors camouflaged as pieces of vegetation to use in the barrier, which had come to be called the McNamara Line. Seeded from the air throughout the Laotian panhandle in the fall of 1967, the sensors transmitted the sounds of enemy activity to aircraft orbiting the region. At an Infiltration Surveillance Center at Nakhon Phanom in northeastern Thailand, the Seventh Air Force plotted the recorded detections and launched air strikes against the most promising targets (12)

In South Vietnam the sensors were being installed in conjunction with fortifications and barriers along the Demilitarized Zone when Khe Sanh came under siege in early 1968. General Westmoreland diverted the sensors to ring the Marine base. They were so successful in warning of enemy movements and identifying targets for artillery and air support that General Westmoreland obtained permission to postpone the completion of the McNamara Line to use the sensors in tactical operations. By 1969 the Military Assistance Command had installed sensors on perimeters of military installations, along main convoy routes, and across principal enemy avenues of approach in the border areas of South Vietnam. Tactical units and Special Forces teams monitored the sensors. To aid in the communication of readings from remote areas, a radio relay aircraft orbited the tri- border area west of Pleiku and a ground relay operated from the top of Nui Ba Den.

Although the McNamara Line had only limited success in halting infiltration or in strategic interdiction, the sensors were used effectively by tactical units. While it was impossible to know whether the source of a sensing in a remote area was enemy activity, the sensors pinpointed the area of a sensing. Local units could then investigate the cause. (13)

Although offensive American electronic warfare activities in South Vietnam were devoted primarily to finding the enemy and collecting intelligence, occasionally other approaches were tried. A few attempts were made at manipulative communications deception, a technique in which false transmissions were deliberately made on one's own communications nets to mislead an eavesdropping enemy. For example, the Americans would transmit false strength reports to indicate the weakening of a base's defenses, thereby enticing the enemy into a trap. On several occasions following manipulative communications deception, the enemy did attack and suffered heavy casualties. Imitative deception, involving a more difficult procedure of entering the enemy's communications network to transmit false or misleading information, was never attempted by Americans in South Vietnam.

Pointing out that the enemy used his radios seldom and that combat operations were brief, General Westmoreland responded that except for the Ia Drang battle there had been little opportunity for jamming enemy communications. The Army attempted ground- based jamming only twice, once at Pleiku and Nha Trang to override the broadcast of Viet Cong propaganda intended to ncite a Montagnard revolt. The only formal jamming program was that employed by the Navy against enemy communications and radar signals in North Vietnam to suppress antiaircraft defenses. (14)

It was not easy to overcome the enemy's communications defenses. American electronic warfare operators were always looking for intelligence on enemy communications procedures and equipment. A special detachment from the 509th Radio Research Group reviewed every captured document and report of interrogation of prisoners and defectors for clues to the operation of enemy communications. If an interrogation report indicated that a prisoner or defector might be a source of additional communications information, specially trained interrogators from the Radio Research Unit continued questioning him. Often those special interrogation reports elicited requests for additional questioning on particular points. Another special unit, the 18th Signal Detachment, which was assigned to an operating branch of the MACV intelligence staff called the Combined Materiel Exploitation Center, studied captured communications equipment and provided intercept operators with a valuable guide to the operational range and characteristics of Viet Cong communications. (15)

The Communist Defenses

The enemy's security did not crack easily. A wary people by nature, the North Vietnamese had cloaked the insurgency in South Vietnam in a pervasive secrecy. Captured communications security directives indicated that as early as 1962 the Communists were taking a serious and sophisticated approach to protecting their communications. For example, they permitted unencrypted transmissions only on news broadcasts. (16)

Besides the Communists' natural prediliction for secrecy, an unswerving adherence to directives and plans within the ranks enhanced the security of Communist communications. Communist radio operators were more likely to follow rigid security directives than their more highly educated American contemporaries, who tended to interpret rather than follow directions and to look for shortcuts which sometimes compromised communications security. Because the Communists maintained tight discipline over their troops, they also could plan and rehearse operations in precise detail with the confidence that each step would be carried out exactly as directed, a fact that lessened the need for communications for command and control during those operations. Less communications, in turn, meant fewer opportunities for Americans or South Vietnamese to use electronic warfare against Communist communications.

In addition to strictly regulating communicators, the Communists educated other staff officers to maintain communications security. They enjoined them to avoid using electrical transmission whenever courier service was available. Any information transmitted by radio had to be protected even after it was no longer classified, lest an enemy discover it in an unencrypted form and break signal codes by comparing the text of encrypted and unencrypted communications. Perhaps the greatest impetus to enemy communications security was the universal belief throughout the ranks that the Army Security Agency was practicing electronic warfare against Communist communications. (17)

The Central Office for South Vietnam distributed codes, call signs, and frequency assignments to the signal staffs at each military region headquarters, where signal representatives from units and province. picked up extracts of the information pertaining to their own operations. From those extracts each radio operator copied into a notebook only that information that applied to him. Although much less efficient than the American practice of mimeographing an entire package of communications information, called Signal Operating Instructions, and giving it wide distribution, the enemy methods were far safer. By strictly controlling distribution, signal officers not only limited the amount of signal information vulnerable to capture but also made it impossible for radio operators to enter any nets in which they did not belong, a practice that could cause breakdowns in net discipline and security.

The Viet Cong assigned professional cryptographers to every regiment and every province. Since radio operators were kept physically separated from the cryptographers and were even forbidden to associate with them during off- duty time, radio operators had little opportunity to handle or have knowledge of any unencoded classified information. Besides restricting access to important cryptographic material, the compartmentalization of communicators and cryptographers precluded the inadvertent transmission of classified information in the clear by a careless radio operator. Because of the sensitive nature of cryptographers' work, they were usually handpicked Communist Party members who had received intensive training at a special school in Hanoi. (18)

Even in low- level units without cryptographers, radio operators followed precise operational procedures that made communications more secure. The Communist practice of communicating with each station on a net only at a scheduled time and on a prescribed frequency -- and of varying those times and frequencies periodically -- made it difficult for an enemy to intercept a particular station by continually monitoring a single frequency. The enemy made mandatory the use of international procedural words, called prosigns, to communicate signal information.

Using these shortcuts meant that transmitters were on the air a shorter time and were thus less exposed to enemy intercept. How diligently the various procedures were followed depended to a great extent on the quality of the supervision rendered by local signal staff officer. Most were quite harsh in enforcing communications discipline. In their reprimands of violators they even equated laxity with disloyalty to the Communist cause. When voice radio came into wide use on enemy nets during the late 1960s, it became more difficult to control those speaking directly, usually extemporaneously, on communications channels. Signal officers issued voice codes, but operators frequently neglected to use them or developed their own unsecure brevity codes. Because voice radio operators usually received less training than Morse code radiomen, quality of communications and net discipline were also lower on voice nets.

As the Americans improved their airborne direction- finding techniques, net discipline and operational procedures became as important as cryptography to the Communists. They knew that even if the Americans could not read the text of a station's traffic, they could still home in on the signal emitter of the transmitter. Suspecting that American radio direction finders guided B- 52 bombers that were dropping their large bombs with deadly accuracy on the transmitters serving major Communist headquarters, in 1965 commanders began ordering their signal officers to set up transmitters as far away from supporting headquarters as possible. In addition enemy radiomen learned to recognize by sight and sound the types of aircraft used for airborne radio direction finding and to shut off their transmitters when they were in the area .( 19)

U. S. Communications Defense

Americans had greater difficulty enforcing communications security. In South Vietnam some Americans were lulled into a false sense of security by believing that encoding was unnecessary because the enemy was unable to understand English. While the Viet Cong planned operations in great detail to minimize communications during actual execution, Americans, valuing flexibility and spontaneity, relied heavily on radio and telephone communications to make lastminute adjustments to plans and to control fast- moving airmobile operations. Perhaps from a sense of isolation in a strange land, American radio operators chattered incessantly on their nets. Most felt it their job to make frequent communications checks and to notify higher headquarters concerning even the most unimportant happenings in the field. Coupled with American disdain for secrecy, the heavy use of communications made Americans lucrative targets for electronic warfare unless cryptographers found means to protect their communications. (20)

During the 1950s American advisers made virtually no attempt to protect their communications. Only cumbersome manual codeing procedures were available to them. Nor was there any check to determine whether South Vietnamese or American communications were being compromised. Not until late 1960, after a disturbing report by an Army Security Agency inspection team from Hawaii illuminated communications security deficiencies in South Vietnam, were the first steps taken to assign responsibility for improving communications security in Southeast Asia.

The Military Assistance Advisor Group assigned to the chief signal adviser staff supervision for communication security matters, and the Joint Chiefs of Staff directed the Army Security Agency to provide cryptologic support to the advisory group. In the spring of 1961 the Army Security Agency sent a team to monitor

telephone circuits on the combined South Vietnamese- American switchboard in Saigon. In November of the same year, a mobile team set up monitoring operations in support of the advisor detachment at the I Corps headquarters in Da Nang. By March 1963, when the 7th Radio Research Unit became responsible for communications security in South Vietnam, ten teams were monitoring wire and radio circuits throughout the country. (21)

The monitoring program revealed compromises of classified information. Alerted to the vulnerability of communications, General Paul D. Harkins directed the MACV Assistant Chief of Staff for Communications- Electronics, Lt. Col. Philip S. Pomeroy, to establish a position on his staff for an assistant for communications security. Colonel Pomeroy made the newly arrived 39th Signal Battalion the cryptographic distribution authority for all American cryptographic equipment, a formidable mission involving distributing, repairing, and accounting for all cryptographic machines and documents in South Vietnam.

During the next two years the battalion handled three generations of communications security equipment. Off- line cipher machines, installed with the Operations and Intelligence Net in 1962, were replaced the following year by online machines. (22) They, in turn, were replaced in 1964 by a more rugged and reliable on- line machine. Although many American units in South Vietnam continued to use older sets on low- level nets for several years, the on- line system eventually became standard for U. S. tactical teletype communications. As the offline system was replaced, it was turned over to the South Vietnamnese. (23)

From a concern to help South Vietnamese communicators as well as from a desire to protect American comunications over South Vietnamese channels, signal advisers of the Military Assistance Advisory Group pressed for American cryptographic support for the South Vietnamese. In addition to the compromise of classified information, nonsecure South Vietnamese comunications offered the enemy an excellent start in breaking codes that protected secure American systems transmitting identical information.

With the assistance of the 39th Signal Battalion, the signal staff of the Military Assistance Command conducted classes for the South Vietnamese in the use of cryptographic equipment. Officers from the 3d Radio Research Unit inspected the facilities in which the devices were to be installed and assisted the South Vietnamesc in meeting stringent physical security rcquircments. In Septemher 1962 the first secure South Vietnamese nets came on the air. By the spring of 1964 all high- level communications of the Joint General Staff and the corps headquarters were secured by an on- line teletype system, and every division and regiment had received off- line devices for use on tactical Morse code nets. (24)

As voice radio and telephone replaced teletype and Morse code as the primary modes of American communications, officers of the Army Security Agency could take little consolation in improvements to the cryptographic equipment for teletype or Morse code nets. Although devices existed for on- line protection of telephone conversations, they were not widely available in South Vietnam because they were still being tested and were extremely expensive. Only a constant awareness of the vulnerability of voice communications and careful use of operations codes could defend against breaches ot telephone security. In the absence of proof that the Viet Cong were intercepting American transmissions, advisers were unconvinced of a real threat and rarely used operations codes. While the security experts of the 7th Radio Research Unit could provide communications security training for operators of teletype and Morse code nets, who were profsessional communicators, they were unable to reach everyone with access to a telephone or voice radio.

The problem was destined to get worse. As American combat units began deploying to South Vietnam during 1965, every combat leader from squad to division would have the battlefield replacement for the ubiquitous American telephone: an FM voice radio. Simple to operate, the radios were conveniently at hand to maintain constant command and control, to request air end artillery fire support, and to seek logistical support. With the. high power setting habitually used by Americans, the FM voice radio had tremendous range and transmitted its signal in all directions. Those qualities -- reliability, convenience, and range -- made the FM voice radio a lucrative target for Viet Cong electronic warfare technicians.

The proliferation of these radios throughout South Vietnam, moreover, made it simple for the Viet Cong to capture or steal them to use in electronic warfare operations. As new U. S. units arrived in South Vietnam during the mid- 1960s, the 7th Radio Research Unit warned them of the dangers of relaxing communications security. They played recordings of security breaches detected in the monitoring program and taught classes to radio operators on security precautions to be taken to protect American communications. But continued monitoring of American radios and telephones demonstrated the futility of those efforts.

Since the 7th Radio Research Unit could monitor only about 6 percent of all American communications in South Vietnam, the Army Security Agency studied the security of tactical communications in one unit, the 1st Cavalry Division, to assess the seriousness of the situation. For the last three months of 1965, soon after the division arrived in South Vietnam, the 7th Radio Research Unit provided a contingent of monitoring teams to augment the division's own 371st Radio Research Company. Selection of the 1st Cavalry Division for the study was fateful, for the teams were soon to find themselves monitoring the communications of units engaged in the most savage fighting yet experienced in the war -- the first American engagement with North Vietnamese troops in the Ia Drang valley.

Monitoring 10,902 voice, teletype, and Morse code transmissions during the three weeks before the battle, the team noted little concern for fundamental communications security precautions. The air cavalrymen rarely used available authentication systems to protect against imitative deception. (25) Even though many net frequencies and call signs had been compromised by transmission in the clear, signal officers seldom changed Signal Operating Instructions, which assigned frequencies and call signs to divisional units.

Although the commander of the 371st Radio Research Company reported the findings and made remedial suggestions, the division's signal officers had no time to tighten communications security before the division became hotly engaged in battle. Then the monitoring team found that American communications security in areas near the enemy not only failed to improve, but sometimes worsened. Restricting the program to voice communications, the teams monitored 28,023 voice transmissions during the month- long fight. They found that once the battle was joined, communications security was completely ignored. Sensitive information was broadcast in the clear and critical messages accepted without any authentication challenge. The only attempt to protect information was the occasional use of an unauthorized, homemade code and easily compromised point- of- origin systems. (26)

Neither approved operations codes nor off- line security devices were ever used during the battle. Officials in the Army Security Agency and the National Security Agency were most alarmed that communicators in frontline combat units had failed to use the security measures available to them: off- line coding machines andauthorized authentication and operations codes. Communicators and commandersthroughout South Vietnam were less surprised by the findings; they saw in them a reflection of practices prevalent throughout tactical units. Off- line encryption methodstook time and on the battlefield American commanders felt that time was more crucial than security.

The Ia Drang findings provided new impetus to a prior request by the commander in chief, Pacific, for the National Security Agency to develop speech security equipment for tactical units in South Vietnam. Initially reluctant to produce a cryptographic device for the combat infantryman that would undoubtedly be captured, the National Security Agency in January 1966 nevertheless agreed to develop a security device for portable radios.

The National Security Agency and the U. S. Army Electronics Command designed the PRC- 77, a modified PRC- 25 that could be connected to the speech security gear. The findings from the Ia Drang also influenced a decision by the Defense Communications Agency to expand the Automatic Secure Voice Communications System (AUTOSEVOCOM) to include narrowband terminals for tactical units. (27)

While awaiting the development of new equipment, officers of the Army Security Agency in Southeast Asia turned their attention to ways of supplementing conventional monitoring for security violations with more preventive approaches. They advised units about techniques to protect communications. Through selective monitoring ,they next tested and reported to the commanders, how well specific recommended security practices worked. The results of monitoring were then applied in future plans.

When the United States began installing the infiltration detection system, the McNamara Line, the U. S. Army, Pacific, sent a five- man team to the Infiltration Surveillance Center in Thailand to determine how to keep the highly classified project secret. After two months of monitoring telephone circuits and voice radio nets, the team obtained names, locations, organizations, and security and communications plans for the entire operation. To avoid further compromises, they recommended that communications concerning the project be restricted to secure teletype as much as possible.

They even advised discontinuing the use of the code names for the project. Improvements were gradual and subtle. Although the new preventive approach did not offer any dramatic solution to communications security problems, it did give electronic warfare specialists the opportunity to influence operational planning.

Even though the 1st Cavalry Division's experience in the Ia Drang made commanders aware of the deplorable state of comnunications security, they still felt that most communications security measures were unnecessary and restrictive. They were not convinced that the enemy was monitoring their communications. They felt that practicing communications security meant sacrificing the tactical flexibility and control provided by extensive open communications.

While more sympathetic to the importance of communication is security, division signal officers agreed with their commanders that the conflict between operational efficiency and fundamental signal security measures appeared irreconcilable. Cross- attachment of units and daily interaction with support aircraft based hundreds of miles away made it virtually impossible to issue compatible codes to all forces participating in an action or to change frequencies and call signs often.

After changing the call signs on all the radio nets in the 1st Cavalry Division during the Ia Drang battle, the division signal officer, Lt. Col. Tom M. Nicholson, discovered that he had caused so much confusion that he had to return to former call signs to reestablish basic command and control. The revision of a division's Signal Operating Instructions and their distribution to very unit -- a task usually handled by the division radio officer -- was so prodigious that it usually demanded the full attention of the division signal officer's staff for an entire week. Those Signal Operating Instructions were sometimes compromised by loss or capture even before they could be fully distributed.

Rather than limiting access to communications to improve security, signal officers were under pressure to keep open as many channels as possible to ensure that warnings and emergency requests were quickly received and disseminated. Since the Americans passed information about planned air and artillery bombardments to South Vietnamese troops and civilian authorities to preclude accidents and civilian casualties, any attempt to encode those warnings by using unfamiliar call signs might cause confusion that could lead to casualties. Similarly, the American medical evacuation system relied on rapid clear communications. Even such vital battle information as unit location and numbers of casualties was transmitted in the clear on medical evacuation nets. No signal officer dared change the standard frequency or call sign -- DUSTOFF -- used to summon medical evacuation helicopters throughout South Vietnam. (28)

Powerless to change many of the routine procedures and practices that contributed to a lack of communications security, the men of the 7th Radio Research Unit concentrated on warning commanders of violations which, if monitored by the enemy, would jeopardize lives or give advance warning of an operation. Although some warnings were heeded and compromised plans changed, many were ignored. Even when enemy ambushes followed unheeded warnings, few commanders would admit that the Communists were reacting to intercepted American communications.

Enemy Offensive Electronic Warfare

Aside from isolated intelligence reports, usually based on the claims of enemy prisoners, there was little conclusive evidence that the Viet Cong and North Vietnamese were exploiting American communications. Credit for finally producing confirmation of the effectiveness of the enemy's communications intelligence effort rested with a platoon of infantrymen from the 1st Brigade, 1st Infantry Division and an investigation team from the 509th Radio Research Group.

On 20 December 1969 the infantrymen overran the camp of a technical reconnaissance unit, known as A3, assigned to the Viet Cong's Subregion 1 on the outskirts of Saigon and captured twelve members of the team with their equipment and logs. Four days later the target exploitation team briefed the MACV commander, General Creighton W. Abrams, Jr., on the startling results of the interrogations of the enemy radio interceptors. (29)

Captured during the last days of 1969, the A3 Technical Reconnaissance Unit was living proof of the spectacular evolution of the Viet Cong's technical reconnaissance during the preceding decade. Although relatively low in the organizational hierarchy, the little unit was manned by a well- equipped team of experienced professionals guided in their work by doctrinal publications and procedural manuals evincing a keen insight into the weaknesses of American and South Vietnamese communicators and into the means of exploiting those weaknesses. Working with the attention to detail and ingenuity that had become trademarks of the Viet Cong's technical reconnaissance operation, the A3 team was found with over 1,400 handwritten copies of voice transmissions.

The team had been monitoring voice and Morse code traffic of American and South Vietnamese units operating in Subregion 1 for several years. Their equipment was simple and well maintained: two captured PRC- 25's and one captured PRC- 77 for monitoring FM voice traffic, and one Chinese Communist R139 receiver and seven small commercial transistor radios for monitoring AM Morse traffic. With precisely engineered antennas the intercept operators were employing the equipment at far beyond its normal range limitations.

Study of the logs kept by the A3 team chief indicated that members of the team knew more about the communications of local American and South Vietnamese units than did most allied communicators. They even knew the voice characteristics and communications habits of many of the radio operators working in the area.

After following the American nets for several years - a continuity no U. S. signal officer on a one- year tour enjoyed - the Viet Cong intercept operators had discerned various exploitable patterns. Having heard the confusion on American and South Vietnamese nets when frequencies and call signs were changed, they learned to adjust to new Signal Operating Instructions more quickly than the communicators in the nets. Knowing that each unit had a limited block of frequencies for switching among its nets, when Signal Operating Instructions were changed the intercept operators would simply monitor each frequency in a division's assignment block for recognizable voices and then begin reconstructing the nets. Sometimes their American adversaries, in attempting to reorganize the nets quickly, would make the intercept team's adjustments even easier by giving the frequencies and call signs in the clear to confused radio operators.

Since radios were used extensively to coordinate the planning and conduct of joint ground and air operations, the A3 unit focused on air nets both as lucrative sources of operational intelligence and as keys to reconstructing other nets. Working against the 1st Cavalry Division, for example, the Viet Cong listened each evening to transmissions of the 11th Aviation Group, the division's helicopter support unit, to learn which units would be airlifted into battle the following day and what their destinations would be. During those nightly warning orders to the pilots, even the command frequencies of the supported ground units were passed by a simple frequency designation code which the Viet Cong broke in the first week of its use. Undoubtedly many air assault landings were ambushed using intelligence gathered from the nightly reports.

When the distribution of secure- voice equipment to the 11th Aviation Group put an end to that source of intelligence in the summer of 1969, the Viet Cong merely turned their attention to other divisional nets. The Air Liaison Net, on which medical and special aviation support was arranged, still operated in the clear to give stations without security equipment an opportunity to request assistance. The most fertile source of all was the Air Warning Net; it broadcast information concerning air strikes, artillery barrages, and impending enemy attacks to every fire base and to all aircraft flying through the area.

Besides receiving prior warning, of maneuver and fire support plans, the Viet Cong also learned from the Air Warning Net whether their own attack plans had been compromised and whether American and South Vietnamese units were being alerted. Even more important was advance warning of pending strikes by B- 52's. After monitoring a B- 52 warning, the Viet Cong knew that they had between ten and twenty minutes in which to dispatch a courier to a nearby radio station and send warnings to other Viet Cong units in the area before huge, 750- pound bombs rained from the sky.

Although the phenomenal success of the A3 team came as a great surprise to most American tactical commanders, the very preoccupation of the Viet Cong with the defense of their own communications had long before convinced the men of the 509th Radio Research Group of the value that the Viet Cong placed on electronic warfare and led them to conclude that they needed to develop a strong offensive capability. Apparently recognizing the increased effectiveness of electronic warfare when applied covertly, the Viet Cong went to great lengths to conceal the fact that they had any capability at all. Carefully shielded by security forces, technical reconnaissance soldiers were segregated from other Viet Cong troops and forbidden to acknowledge the nature of their work. Technical reconnaissance information was transmitted only by courier or else encrypted in a highlevel code for radio transmissions (30)

The capture of the A3 unit finally gave substance to a threat that officials of the Army Security Agency had previously been able to define only vaguely. More than simply exposing the activities of one unit, the revelations illuminated the whole obscure history of Viet Cong electronic warfare and corroborated previously unsubstantiated evidence. Logs containing entire texts of American messages copied by the A3 unit made credible the capabilities and successes that commanders had once dismissed as unfounded pessimism by advocates of communications security. The cool professionalism of the intercept operators on the A3 team, if universal throughout the technical reconnaissance forces, would explain both the success and secrecy of Viet Cong electronic warfare. The documents captured with the team contained doctrinal principles and instructions reflecting a refinement in the art of electronic warfare that could have developed only over many years.

The French had suspected as early as 1952 that a Viet Minh radio intercept unit was listening to their communications from a site just east of Hanoi. By the early 1960s the Viet Cong had organized small strategic intelligence cells to conduct radio interception against high- level South Vietnamese communications. In 1963 those cells were organized into the 47th Technical Reconnaissance Battalion, which operated under the direction of the Intelligence Section of the Military Staff of the Central Office for South Vietnam. Meanwhile, in local Viet Cong units communicators using captured radios were experimenting with various forms of electronic warfare on their own. Monitoring of South Vietnamese logistical communications nets became an important source of information for Viet Cong units planning to ambush convoys.

During attacks they monitored South Vietnamese communicators to determine where the defenses were weakening and if reinforcements were being summoned. Some Viet Cong provincial units kept captured GRC- 9's on the command frequency of local South Vietnamese units to learn of proposed attacks and bombing missions. After learning the operating procedures of the South Vietnamese nets, emboldened Viet Cong communicators even practiced jamming and imitative deception against the South Vietnamese. News of electronic warfare successes spread throughout the Viet Cong's ranks, and by 1964 some province committees were even conducting informal training programs in electronic warfare for their communicators. (31)

Encouraged by that success, but also worried lest the activities in the field units alert the South Vietnamese and disrupt the flow of high- level communications intelligence being collected by the technical reconnaissance battalion, the Central Office for South Vietnam decided in 1965 that the entire electronic warfare effort needed direction and organization and convened a special intelligence conference to discuss the topic. The conferees called for a combined tactical and strategic technical reconnaissance organization reaching down to provincial and regimental levels. In addition to giving direct tactical support to local units, technical reconnaissance squads with the regiments and provinces were to send intelligence to a technical reconnaissance platoon stationed at the military region headquarters. There, it would be analyzed for strategic intelligence to be sent to the Central Office for South Vietnam. The technical reconnaissance battalion was to manage the entire operation and to provide communications for the clandestine network. (32)

By mid- 1966 most of the technical reconnaissance organizations planned during the conference were in operation, and the original plan was being broadened to include cells at district level. Viet Cong communications officials scoured the ranks for the most intelligent and reliable soldiers to be sent to technical reconnaissance schools established at the Central Office for South Vietnam and the regional headquarters. Numerous informal training programs took place at the unit level, and many Viet Cong radio operators became part- time intercept operators. In response to a dramatic increase in targets resulting from the arrival of American units, the entire Viet Cong electronic warfare program continued to expand. (33)

Technical reconnaissance grew so quickly, from 179 men in 1964 to over 1,500 men in early 1967- that Viet Cong leaders worried that the high performance and secrecy of the program might be decreasing. The battalion at the Central Office for South Vietnam, which had been the nucleus of the whole effort, had been drained of its most experienced men to serve as cadres in the field and instructors at the school. Since the quality of the battalion's management of the entire technical reconnaissance operation seemed to be suffering, the chief of the Military Intelligence Bureau at the Central Office for South Vietnam in February 1967 ordered the unit deactivated and its mission and personnel transferred to a newly formed staff section of the Central Office for South Vietnam, the Technical Reconnaissance and Intelligence Department. Thereafter technical reconnaissance developed an organizational structure involving over 4,000 elite troops enjoying the highest priority of support and protection. (34)

With the additional authority implicit in that arrangement, the new technical reconnaissance organization was able to get more English linguists to use against the Americans. Rather than simply adding to the total body of communications intelligence, the intercept of American communications dramatically improved the quality of the effort against the South Vietnamese. Information monitored on American nets supplemented and confirmed information obtained from South Vietnamese nets. To pit American and South Vietnamese communicators against each other, Viet Cong cryptologists also caused suspicions between the two allied forces.

Exploiting parallel nets of U. S. advisers and their South Vietnamese counterparts, where one net might be encrypted and the other insecure, technical reconnaissance agents reconstructed entire coding systems by comparing encrypted and plain text traffic. Although high- level operations codes were never broken, the two- front attack on communications aided the Viet Cong in breaking the low- level codes used by the South Vietnamese prior to 1969 and in more easily understanding the simplistic, unauthorized codes often used by ingenious, but naive, Americans who thought that they could fool their adversary by cloaking sensitive information in uniquely American references. (35)

Although the Communists had decided to forsake more overt electronic warfare techniques, such as jamming, so as not to interfere with the monitoring program or to put targeted communicators on guard, Viet Cong communicators sometimes employed jamming and communications deception. The jamming usually consisted of whistling, humming, or playing music while an American station was attempting to communicate. More common than jamming were attempts at communications deception, a variation of which was commonly practiced during unit movements to cover the displacement of headquarters, a period of extreme vulnerability to attack, and to make it more difficult to find units in their new positions. While a headquarters moved, its radio station would remain behind and continue passing traffic as if nothing ad changed.

When the relocation was completed, a new station would come on the air at the new location, but to confuse any intercept, the old station would continue transmitting for several weeks. There were no documented cases of the Viet Cong's attempting more direct manipulative deception by deliberately passing information on their own nets with the intention that it be intercepted. There were, however, numerous examples of imitative deception, where the Viet Cong entered an American or South Vietnamese radio net posing as an authentic station. Success or failure In the deception usually depended on whether the receiver of the message used uthentication codes to challenge the validity of the person transmitting the message. (36)

Viet Cong deceptions were most successful when rapid response by the receiving station was required. Entering fire control or air support nets, they would request a halt to fire that was hitting their positions or even call in fire on American or South Vietnamese positions. Helicopter pilots, who rarely employed authentication codes, sometimes found themselves drawn into traps by false radio messages or smoke grenades, the latter a means by which ground troops marked their location for helicopters. The Viet Cong used smoke grenades both to ambush helicopters and to divert American and South Vietnamese fire. For the Viet Cong, visual signals had the additional virtue that using them required no fluency in English. (37)

The Viet Cong also tapped telephone lines stretched between guard bunkers on the perimeters of American and South Vietnamese camps and fire support bases to garner intelligence and conduct deception operations. In several cases they successfully learned the nature of perimeter defenses by calling outposts for strength reports. In the most successful imitative deception operation, a Viet Cong killed an American perimeter guard at the Da Nang Air Base and used his telephone to direct the base defense unit to the north side of the base just as a large Viet Cong force was about to attack from the south. Meeting little resistance, the force caused $15 million in damage to the base and its planes. (38)

Convinced of the success of Viet Cong electronic warfare by the revelations made in the wake of the capture of the technical reconnaissance team, American commanders began to take communications security more seriously. Rejecting former misconceptions concerning the adverse effects of communications security measures on operations, Maj. Gen. Elvy B. Roberts, the commander of the 1st Cavalry Division, admitted, ''The fear has always been that airmobile operations would get so tied up it would not be worth the sacrifice one would have to make in effectiveness of airmobile operations.... I find the fear of it to be like many other fears - more imagined than real". (39)

With the support of commanders, signal officers were able to enforce previously disregarded communications directives. Once signal officers were given the authority to instruct net control stations to prohibit stations from entering nets unless they used authorized operations codes and authentication tables, the use of point- of- origin systems and informal codes ended. A pocket- size wheel device made of laminated plastic containing operations and authentication codes developed in 1969 by the National Security Agency specifically for use in South Vietnam also gained wide acceptance once radio operators and staff officers were forced to use it. (40)

Speech security equipment, developed as a cooperative venture of the National Security Agency and the Army and introduced in mid- 1968, was less readily accepted. Fragile connecting cords and insufficient spare parts caused some initial logistical problems. The Electronics Command sent teams to South Vietnam to train radiomen to operate the new sets, but commanders found that operators experienced problems in communicating between secure and nonsecure stations. Those using nonsecure radios, unable to monitor communications between secure radios sometimes unknowingly interfered with secure communications.

When signal officers established separate secure and nonsecure nets, operators, incorrectly believing that the secure devices significantly reduced range, would use only the nonsecure net. Until the introduction of secure retransmission devices in 1970, secure nets were unable to take advantage of the added range provided by airborne retransmission. Perhaps the greatest disadvantage of the secure devices was the twenty- four pounds they added to the burden of the radio telephone operator. Only the heightened awareness of the enemy's threat to communications instilled by the capture of the technical reconnaissance team and pressures by signal officers overcame those objections.

Within six months of the team's capture, the percentage of radios using portable security devices increased from 53 to 83 percent, and those using vehicular security devices increased from 52 to 90 percent. Use of voice and teletype security equipment and the new operations codes effectively ended the Viet Cong threat to American communications. Even though the Communists captured some devices, they were unable to use them due to the daily rekeying of devices on American nets. (41)

In retrospect it is difficult to determine which side was most successful in waging the electronic war. During the early 1960s the enemy clearly monitored South Vietnamese communications with impunity, and from the arrival of U. S. combat units until the introduction of voice security equipment, Viet Cong technical reconnaissance agents enjoyed similar success against American voice radio nets. Because the Communists' principal electronic warfare weapons were small commercial transistor receivers and stolen or captured radios rather than special electronic intercept equipment, their success appeared in some measure more noteworthy than that achieved by the Americans with their sophisticated eavesdropping devices.

American largesse was one source of American weakness. An abundance of easily operated voice radios in the hands of operators indifferent to proper security precautions gave the Viet Cong's technical reconnaissance agents more and easier targets. Undoubtedly many American operations were compromised and lives lost due to Communist exploitation of loose radio procedures. American operators, on the other hand, not only had the handicap of fewer targets but also faced an enemy whose habit of making detailed plans and executing them without deviation reduced his reliance on electronic communications. When the Communists did communicate, they did so with an appreciation for their adversary's electronic warfare capability and a corresponding adherence to stringent communications procedures and safeguards.

In the final balance, technological superiority gave the Americans the edge in both the offensive and defensive aspects of the electronic warfare struggle. Airborne direction finding, for example, provided American operators an ability to locate and track enemy units and afforded a means of determining enemy order of battle and locating targets to be attacked. Rather than automatically homing on radio emissions to pinpoint a unit's radio, as the Americans could do with their directionfinding equipment, the Communists had to look for clues to an American unit's location in the text of the message traffic From the defensive standpoint, once the Americans finally improved the security of their voice communications, Viet Cong technical reconnaissance agents lost their targets. By then, however, Viet Cong technical reconnaissance teams had done their damage during the years of the most critical fighting. And American plans to withdraw from South Vietnam denied the opportunity for the United States to exploit fully its technological superiority in the electronic war.

The End

Notes:

1) Although U. S. intelligence agencys restrict the definition of electronic warfare to activities involving electronic countermeasures, electronic countercountermeasures, and electronic support measures, in this volume the term is used more broadlv to embrace operations involved in maintaining communications security and conducting electronic surveillance.

2) See Chapter 1 for a discussion of early efforts to obtain electronir warfare assistance for South Vietnam. Msg, CINCUSARPAC, DAIN 137177, to DA, ACSI, 23 Jul 5S, CMH/ USARPAC, 82d USASA Special Operations Unit in SVN, 26 Nov 63, p. 3, 73A3330/ 40; Memo, Asst Secy of Defense (SO) for Asst Secv of Defense (ISA), 12 May 61. sub: Intelligence Assistance for South Vietnam, file 1- 1855- 61, ISA 091.3 VN, 64A2382/ 54. Both in SVNRC.

3) Memos, JCS for Asst Sctry of Defense (ISA), 5 Dec 61 and 10 Dec 61, sub: Beef- up Status Report, file I- 17639/ 61. 413.44 VN, 64A2382/ 43, WNRC; USARPAC, 82nd USASA Special Operations Unit in SVN, 26 Nov 63, pp. 7- 8, Trip Rpt, Army Chief of Staff, 10- 1, Jun 62, p. 7, file l54189, 66A3138/ 82, WNRC.

4) USARPAC, 82d USASA Special Operations Unit in SVN, 26 Nov 63, pp. 7- 8; MACY, Briefing for Maj Gen Joseph A. McChristian, Airborne Radio Direction Finding, J2/ D004167/ 67, 319- 75- 054/ 20, WNRC; MS, Lt Col Robert M. Burch, Tactical Electronic Warfare Operations in SEA, 1962- 1968, Project CHECO, 10 Feb 69, K717.0413- 93, OAFH

5) Lung, Intelligence, pp. 61- 62, 125- 26.

6) 3rd Radio Research Unit, Annual Historical Rpt, Jan- Mar 64, 73A3330/ 11, WNRC. For its accomplishments during the period 13 May 1961 to 31 December 1962, the 3rd Radio Research Unit won the first Meritorious Unit Commendation awarded to an Army unit since the Korean War.

7) Memo, Asst Secy of Air Force (R& D) for Vice Chief of Staff, USAF, 10 Jul 62, sub: Direction Finding in Counterinsurgencv Operations, 66A3138/ 86; Memo, DA. ACSI- CI, for Col Thomas W. Riley, Jr., USA, JCS Project Officer, 19 Sep o2. sub: Improvement of High Frequency Radio erection Finding Capabilities, South Vietnam. 66A3L38/ 89; Ltr, ASA to ACSI. DA. 10 Nov 64, sub: Formal Programming tor Airborne Platform, tile 373.1, 66A320/ 64. All inWNRC.

8) Lung, 1ntelligence, pp. 126- 27, 141- 42.

9) Lewy, America in Vietnam, p. 35; Memo, Gen W. C. Westmoreland for Chief of Staff, U. S. Army, 30 Mar 65, sub: Evaluation of the Memorandum Prepared by DA Concerning the Situation in the Northern Provinces of RVN, CS091VK (30 Mar 65). 68A3305/ l, WNRC.

10) MFR, Special Activities Section. Thailand, Operation Branch, 8 May 62, sub: Thailand Opera- tions, ACSI file 6- 2412, 66A3201/ 155, WNRC; MACV Directive 381- 45, 5 Apr 69, sub: Exploitation of Enemy Landline Communications Systems, CMH.

11) For an account of the work of the Defense Communications Planning Croup and the use of sen- sors in Southeast Asia, see Paul Dickson, The Electronic Battlfield (Bloomington, lnd.: Indiana University Press, 1976).

12) MACV History 1968, pp. 911- 34, CMH.

13) Ibid., 1969, ch. 7, pp. 1- 15, CMH.

14) Rpts, JCS 222/ 994, 6 Sep 66, through JCS 222/ 994- 6, 22 Nov 66, sub: Jamming VCPAVN Field Communications, JACO (1966), DA; 6, NARS; Alfred Price, Instruments of Darkness: The History of Electronic Warfare (New York: Charles Scribner's Sons, 1978), pp. 265- 68.

15) C- E- Technical Intelligence, Command Communications, September 1967, pp. 72- 76, C. M. H.

16) CICV Rpt, ST 67- 006 pp 6- 8 Cap Doc, 24 Aug 67, TIR RVN 114- 67, file 17) Cap Doc, 28 Jun 67, TIR RVN 98- 67, file 516- 02( 77) TIRRVN 67, A69- 21/ 2, ASA.

18) Intel Rpt, 25 Dec 67, TTIR 31- 67, file 516- 02( 77) TTIRVN 67, A69- 21/ 2, ASA; IFFV, PERINTREP 43- 67, 22- 28 Oct 67, an. C, 70A499/ 8, WMRC.

19) Cap Doc, 28 Sep 68, TIR RVN 245- 68, file 516- 02( 77) TIR VN 5 (68), A70- 7/ 3; Cap Doc, 22 Aug 67. TIR RVN 112- 67, file 516 -- 02( 77) TIR VN 2, A69- 21/ 2. Both in ASA.

20) Memo, DA, Asst Secy of Armv (REWED) for Chiet of staff, 14 Apr 61, sub: Communications Effec- tiveness in Future Combat, Ltr, 3d Bde to 4th Div, 18 AD r 69, sub: Combat Operations AAR- Operation

21) USARPAC, 82d USASA Special Operations Unit in SVN, l6 Nov 63, pp. 12- 13.

22) Off- line encryption and decryption take place independent of the communications process; on- line encryption and decryption occur while a message is being transmitted and received. The off- line system uses a typewriter that produces an encrypted tape for transmission over a teletype or Morse code circuit, while an on- line system automatically encodes and decodes regular teletype pulses as they are being processed by transmitting and receiving equipment.

23) USARPAC, 82d USASA Special Operations Unit in SVN, 26 Nov 63, pp. 5- 14, MACV J- 6 history, FY 1962- 63, pp. 2- 3, file GF- 3, 338- 75- 1009/ 63, WNRC.

24) MS, HQ, USARPAC, History of the U. S. Army Buildup and Operations in the Republic of Viet nam (RVN), 1 Jan 61- 31 Jan 63, pp. 166- 67, and ibid., Feb 63- 31 Dec 63, pp 231- 35, both in CMH MACV J- 6 History, FY 1962- 63. p 5, MSG, MAAG, Saigon, DAIN l36493, to CINCVAC, 2 Aug 61 sub: Communications Security Assistance to Vietnam CMH.

25) Authentication codes, the radio operator's password system, were used to challenge the legitimacy of stations passing traffic in a net.

26) Point- of- origin systems were based on the use of encoded reference points from which locations could be identified. For example "CP located 600 meters northeast of point Alpha"

27) Chief of Staff of the Army Memorandum 94- 66, Chief of Staff of the Army to JCS, 33 Feb 66, sub: Tactical Voice Security, Incl to JCS 222/ 934; Memo, Dir, NSA, for Dep Secy of Defense, 19 May 66, sub: Secure Tactical Voice Communications. Incl to JCS 222/ 953- 1; Memo, DA for Dep Secy of Defense, 29 Jun 66, sub Secure Tactical Voice Communications, JCS 2221953- 2. All in JACO (1966), DA/ 4, NARS.

28) Capt. Dennis E. Whitmer, "Aviation Communications Security," Aviation Digesl, May 1968, pp. 10- 13.

29) Ltr, 509th Radio Research Gp to ASA, 26 Mar 70, sub: "Historical Monograph on Project Touchdown, w/ 1 Incl and 16 Tabs", CMH, contains the complete study of Project Touchdown on which this account is based. The tabs contain transcripts of the interrogations and translations of the documents. A short, but thorough, discussion of Project Touchdown is contained in MACV, Lessons Learned No. 79: Enemy Exploitation of Allied Tactical Communications, 8 Mar 70, AD 508351, DDC.

30) Ltr, USARV to 1st Sig Bde et al., 20 Nov 69, sub: Operations Security, w/ lncl, 72A6443/ 26, WNRC.

31) MACV 1- 2, Log entry 8- 99- 65, 30 Aug 65 Cap Doc dtd 7 Jul 64 p. 8, A76- 332/ 15, NSA CICV Rpt, ST 67- 006 pp. 8- 9

32) Ltr, USARV to 1st Sig Bde et aL, 20 Nov 69, sub: Operations Security, pp. 1- 5; CICV Rpt, VC/ NVA Electronic Warfare Capability (ST 67- 061), 1 Jul 67 (hereafter cited as CICV Rpt, ST 67- 061), pp. 10- 11,71A4237/ 12, WNRC.

33) Ltr, USARV to 1st Sig Bde et al., 20 Nov 69, sub: Operations Security, p. 2; CICV Rpt, ST 67- 61, pp. 12- 14.

34) Ltr, USARV to 1st Sig Bde et al., 20 Nov 69, sub: Operations Security, p. 2; MACV, Counterinsurgency Lessons Learned No. 64: Imitative Communications Deception (hereafter cited as Lessons Learned No. 64), 15 Sep 67, p. 2, AD 505525, DD

35) The number thirty- nine, for example, was routinely expressed as Jack Benny's age and became a much- used numerical reference point. NSA, Deadly Transmissions, Dec 70, pp. 12- 13, K370.04 6, OAFH.

36) CCIV Rpt, ST 67-() 61, pp. 2- 6; MACV, Lessons Learned No. 64.

37) CCIV Rpt, ST 67- 061 p. 6, MACV Lessons Learned No. 64, p. 5.

38) MACV Enemy Offensive Electronic Warfare Lessons Learned No. 64, pp. 3- 4; NSA, Deadly Transmissions, p. 10.

39) Quote from MS, Lt Col Norman E. Archibald, Tactical Communications, 1st Cavalry Division

40) Maj Gen Thomas M. Rienzi, Debriefing Rpt, 4 Jun 70, pp. 46 - 48, HRC 314.82, CMH; I FFV Daily Staff Journal, 24 Oct 67, item 5, 70A478/ 33,

41) Memo, JCS for Asst Secy of Defense (I& L). 25 Nov 67, sub: Secure Tactical Voice Communications, JCS 222/ 953 4, JACO (1965), DA/ 2, NARS; ORLL, 1st Sig Bde, 31 Jul 70, p. 9, 72A7128/ 5 WNRC, Rienzi Debriefing Rpt, p. 45.