The VENONA Intercepts

During the Second World War, the US intercepted thousands of Soviet communications under a top secret NSA program.

A decrypted VENONA intercept referring to the Rosenbergs

In June 1941, Nazi Germany invaded the USSR, and the Soviet dictator Joseph Stalin became an ally of the UK’s Winston Churchill and shortly later the USA’s Franklin Roosevelt. It was an uneasy alliance. Churchill in particular never trusted Stalin, and Stalin trusted neither the UK nor the US. But the blunt reality was that Soviet forces were a vital part of the war against the Nazis, and so the Allies supported Russia heavily with aid and equipment through the war. Churchill famously summed it up: “If Hitler invaded Hell, I would make at least a favourable reference of the Devil in the House of Commons.” Both sides, however, knew that it was a marriage of convenience, and both sides expected the alliance to collapse once the war was over.

As a result, the Soviets were kept at arm’s length, and Stalin was not included in a number of discussions between Roosevelt and Churchill. Most crucial of these was the atomic bomb. The effort to build a nuclear weapon had its start when Albert Einstein wrote a letter to Roosevelt informing him that the recently-discovered process of fission in a uranium atom could potentially produce an explosive of unimagined power. Roosevelt in turn appointed a “Uranium Committee” to look into the matter, which later resulted in the Manhattan Engineering District. The Manhattan Project clandestinely recruited most of the best physicists in the world to produce a workable atomic bomb.

Stalin was told none of this. But the Soviet KGB already had a pretty good spy network inside the US, and word soon began filtering back to Moscow about the super-secret project. Stalin ordered it be given top priority, and in the end, his network of spies and informants provided enough information that when the USSR detonated its first atomic bomb in 1949 (codenamed Joe-1), it was an almost exact duplicate of the American “Fat Man” that had been dropped on Nagasaki. The Cold War was on in earnest.

The KGB was, by 1941, a sophisticated and experienced intelligence network, and while the Americans had very few active agents inside the heavily-restricted Soviet Union, the KGB was able to take advantage of the open American and British democracies to recruit a wide network of agents and informants. During the Great Depression of the 1930s, capitalism was being widely viewed as a failure, and alternative ideologies like socialism and communism were seen as viable alternatives and were widely embraced (at least until the true nature of Stalin’s regime became apparent). So the Russians had a deep pool of left-liberals to fish from. Some of their informants were just naive idealists. Others thought that the US should not withhold information from their Soviet allies in wartime. And some thought that in the expected postwar world neither superpower should be able to have such a military advantage over the other. So the KBG had no trouble finding scientists, military personnel and others who were willing to pass on information.

In what was the standard practice followed by spy agencies all over the world, these informants were given codenames to hide their identity, and passed all their information to intermediaries through various clandestine methods. These go-betweens (who all had codenames of their own) then passed it on to higher-ups in the network, and finally it would be sent to Moscow by courier or by ciphered radio messages.

These, naturally, were coded with a high-security system. Communications between the Russian Embassy in the US and KGB headquarters in Moscow went by wireless radio or by cable telegram, and the United States as a matter of routine kept intercepted copies of every message that was sent. The Soviets were however using a type of cipher system known as a “one-time pad”, which used a book of random number or letter sequences and which, if used properly, could not be broken by any type of cryptanalysis, even in theory. And so, while the US compiled thousands of Soviet messages during the war, it could not read any of them.

Then, in 1942, the Russians made a mistake: pressured by the rapid Nazi advances into the USSR, one of the clerks in Moscow erroneously re-issued sections from a 35,000-page cipher pad that had already been used for previous messages—making all of the messages that were subsequently enciphered with those repeated sections, between 1942 and 1945, vulnerable. It was the break that the US needed. The Army’s Signal Intelligence Service, forerunner to the NSA, began a special project called VENONA to attempt to use the new information to break into the previously-unreadable Soviet messages.

The work took years. Many of the messages could only be partially deciphered, and many could not yet be read at all. Nobody yet knew who all the Soviet codenames (some 350 of them) were referring to. It was not until 1946 that a few of the messages dating from 1943 became readable enough to at least extract the information that the Soviets had a large network inside the US, and they were interested in political developments (Stalin was always afraid that the US and UK would cut a peace deal with Hitler that would allow the Nazis to continue their war on the USSR) as well as military work on jet engines, radar, missiles, proximity fuses, etc. But most of all, Stalin was interested in the American work he called ENORMOUS—the Russian codename for the Manhattan atomic bomb project.

Deciphering the messages was extremely hard work, however, and it wasn’t until 1946, when the war was already over and Stalin already possessed America’s atomic secrets, that the full picture began to emerge. The VENONA intercepts made it clear that the Soviets had several agents and informers inside the super-secret Manhattan Project itself, and both the American FBI and the British MI-6 turned their efforts to identifying them.

The British determined in 1949 that one of their scientists who had worked with the Americans at Los Alamos, named Klaus Fuchs, had been passing information to the Russians. (VENONA referred to Fuchs by the codename CHARLES and then later as REST). Klaus was arrested and sentenced to 14 years.

The Americans, meanwhile, determined that physicist Ted Hall (VENONA codename MLAD), who had helped design the bomb, and machinist David Greenglass (codename KALIBRE), who had helped make the special explosive implosion lenses, had been passing their knowledge of the project to the Soviets through an agent which VENONA referred to as ANTENNA and later as LIBERAL (and one intercept also identified LIBERAL’S wife as ”Ethel”). The FBI investigation concluded that LIBERAL was Julius Rosenberg—Greenglass’s brother-in-law.

The Soviets, meanwhile, learned about the program in 1949 when the US shared some of its VENONA information with MI-6, where it fell into the hands of double-agent Kim Philby. But by now it was too late for the KGB to do anything except warn some of its operatives (including Philby’s partners Donald Maclean and Guy Burgess) that their cover may have been blown.

Although by 1955 much of the Soviet spy ring was eventually identified using information from VENONA, the NSA concluded that the fact that they could read some of the Soviet ciphered messages was far too important a secret to be revealed publicly, and so it was decided not to make any of the VENONA information available to any investigator or prosecutor or to reveal it in court. Indeed, the project was so highly classified that not even President Truman was aware of it: he was briefed on some summaries of VENONA but was never told the source of this information.

Some members of the spy ring, such as Greenglass and the Rosenbergs, were convicted on the basis of other evidence that was obtained by the FBI outside of VENONA sources, and no VENONA information was used at the Rosenberg’s trial. Other suspects, such as Hall, had been identified through VENONA but did not have enough evidence outside of the intercepts for a conviction. And some of the codenames mentioned in the VENONA intercepts have still not been identified.

The NSA’s work on the VENONA intercepts was ended in 1980, when the contents of nearly all of the messages which could be broken had been determined and it was decided that nothing useful could be further extracted. By this time, reports had already begun to leak out that the Americans were able to read at least some of the wartime Soviet messages.

Beginning in 1995, then, the NSA finally acknowledged the program, and the first 49 deciphered VENONA texts were publicly released, which covered some of the atomic spy rings including the Rosenbergs. Over the next several years, in a series of six stages, the entire collection of 3,000 decrypted texts was declassified and made publicly available.

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