Amid the confusion of war, Moscow had sent out duplicate sets of additives to various Soviet installations around the world. When the cryptanalysts discovered that the same series of additives had been used more than once, they had all the leverage they needed to break the Soviet cipher system. Having used guesswork to deduce the additives for a Soviet message intercepted in one part of the world, they could test those same additives against the massive backlog of messages intercepted in other parts of the world. Sooner or later the same additives would appear and another message could be deciphered. It was an excruciatingly tedious task with less than perfect results. Since only a portion of the code book had been salvaged, many of the 999 five-digit groups used by the Soviets were missing. Knowing the additive might yield the proper five-digit group, but if that group could not be found in the code book, the word remained indecipherable. Whole passages were blanks, and the meaning of other phrases could be only vaguely grasped.

Because of the laborious nature of the task, years would elapse between the actual transmission of a Soviet message and its decoding by the Armed Forces Security Agency. The first big break did not come until 1949, when the cryptanalysts found a duplicate additive in the New York-to-Moscow channel and were able to decipher enough of a Soviet message to identify it as the text of a 1945 telegram from Churchill to Truman. Checking the message against a complete copy of the telegram provided by the British Embassy, the cryptanalysts confirmed beyond doubt that a Soviet spy had somehow been able to obtain the verbatim text - cable number and all - of a private communication between two heads of state.

Shortly after the end of the war a brilliant American cryptanalyst named Meredith Gardner, from the U.S. Armed Forces Security Agency (the forerunner of the NSA), began work on the charred remains of a Russian codebook found on a battlefield in Finland. Although it was incomplete, the codebook did have the groups for some of the most common instructions in radio messages - those for "Spell" and "Endspell." These are common because any codebook has only a finite vocabulary, and where an addresser lacks the relevant group in the codebook-always the case, for instance, with names-he has to spell the word out letter by letter, prefixing with the word "Spell," and ending with the word "Endspell" to alert his addressee.

Using these common groups Gardner checked back on previous Russian radio traffic, and realized that there were duplications across some channels, indicating that the same one-time pads had been used. Slowly he "matched" the traffic which had been enciphered using the same pads, and began to try to break it. At first no one would believe him when he claimed to have broken into the Russian ciphers, and he was taken seriously only when he got a major breakthrough in the Washington-to-Moscow Ambassadorial channel. He decrypted the English phrase "Defense does not win wars!" which was a "SpelllEndspell" sequence. Gardner recognized it as a book on defense strategy published in the USA just before the date the message was sent. At this point, the Armed Forces Security Agency shared the secret with the British, who at that time were the world leaders in cryptanalysis, and together they began a joint effort to break the traffic, which lasted forty years.

Operation BRIDE (as it was first known) but later DRUG and VENONA, as it was known in Britain, made painfully slow progress. Finding matches among the mass of traffic available took time enough. But even then there was no certainty the messages on each side of the match could be broken. The codebook was incomplete, so the codebreakers used "collateral" intelligence. If, for instance, they found a match between the Washington-to-Moscow KGB channel and the New York-to-Moscow trade channel, it was possible to attack the trade channel by using "collateral," information gathered from shipping manifests, cargo records, departure and arrival times, tide tables, and so forth, for the date of the message. This information enabled the codebreakers to make estimates of what might be in the trade traffic. Once breaks were made in one side of a match, it provided more groups for the codebook, and helped make inroads on the other side.

The British and Americans developed a key device for expanding the VENONA breaks. It was called a "window index." Every time a word or phrase was broken out, it was indexed to everywhere else it appeared in the matched traffic. The British began to index these decrypts in a more advanced way. They placed two unsolved groups on each side of the decrypted word or phrase and after a period of time these window indexes led to repetitions, where different words which had been broken out were followed by the same unsolved group. The repetition often gave enough collateral to begin a successful attack on the group, thus widening the window indexes. Another technique was "dragging." Where a"SpelUEndspell" sequence or name came up, and the cryptanalysts did not know what the missing letters of the spelled sequence were, the groups were dragged, using a computer, across the rest of the channels, and out would come a list of all the repeats. Then the cryptanalysts would set to work on the reverse side of the repeat matches, and hope to attack the "Spell/Endspell" sequence that way.

It was an imperfect art, often moving forward only a word or two a month, and then suddenly spilling forward, like the time the Americans found the complete text of a recorded speech in the Washington Ambassadorial channel. Often terrible new difficulties were encountered: one-time pads were used in unorthodox ways, up and down, or folded, which made the process of finding matches infinitely more problematic. There were difficulties, too, with the codebooks. Sometimes they changed, and whereas the Ambassadorial, GRU, and trade channels used a straightforward alphabetically listed codebook, rather like a dictionary, so that the codebreakers could guess from the group where in the codebook it appeared, the KGB used a special multivolume random codebook which made decrypting matched KGB channels a mindbending task. The effort involved in VENONA was enormous. For years both GCHQ and NSA and MIS employed teams of researchers scouring the world searching for "collateral"; but despite the effort less than 1 percent of the 200,000 messages we held were ever broken into, and many of those were broken only to the extent of a few words....

Years later, I arranged for Meredith Gardner to visit Britain to help us on the British VENONA. He was a quiet, scholarly man, entirely unaware of the awe in which he was held by other cryptanalysts. He used to tell me how he worked on the matches in his office, and of how a young pipe-smoking Englishman named Philby used to regularly visit him and peer over his shoulder and admire the progress he was making. Gardner was rather a sad figure by the late 1960s. He felt very keenly that the cryptanalytical break he had made possible was a thing of mathematical beauty, and he was depressed at the use to which it had been put.

"I never wanted it to get anyone into trouble," he used to say. He was appalled at the fact that his discovery had led, almost inevitably, to the electric chair, and felt (as I did) that the Rosenbergs, while guilty, ought to have been given clemency. In Gardner's mind, VENONA was almost an art form, and he did not want it sullied by crude McCarthyism. But the codebreak had a fundamental effect on Cold War attitudes among those few indoctrinated officers inside the British and American intelligence services. It became the wellspring for the new emphasis on counterespionage investigation which increasingly permeated Western intelligence in the decades after the first break was made. More directly, it showed the worldwide scale of the Soviet espionage attack, at a time when the Western political leadership was apparently pursuing a policy of alliance and extending the hand of friendship. In the British traffic, for instance, most of the KGB channel during that September week was taken up with messages from Moscow detailing arrangements for the return of Allied prisoners to the Soviet authorities, groups like the Cossacks and others who had fought against the Soviet Union. Many of the messages were just long lists of names and instructions that they should be apprehended as soon as possible. By the time I read the messages they were all long since dead, but at the time many intelligence officers must have been struck by the sense that peace had not come in 1945; a German concentration camp had merely been exchanged for a Soviet Gulag.

Frank Rowlett then described to me the cryptanalyst who was working to decipher the KGB code, Meredith Gardner. Gardner, said Rowlett, was unusual and brilliant, not only as a cryptanalyst but also as a linguist. He spoke six or seven languages and was one of the few Western scholars who read Sanskrit. Until the outbreak of World War II he had taught languages at a university in the Southwest. On joining the ASA, Gardner taught himself Japanese in three months, to the amazement of his colleagues. Throughout the war he had worked on breaking the Japanese codes, and after the war was over Rowlett had put him on the KGB codes. Rowlett told me I'd find Meredith Gardner to be a shy, introverted loner, and that I'd have a hard time getting to know him.

The ASA offices were at Arlington Hall, across the Potomac from the District of Columbia, in Virginia, at what used to be a girls' school. Gardner met me in one of the brick-and-wood-frame buildings, and as we sat down to talk I soon realized that Rowlett's description of him was accurate. Gardner was tall, gangling, reserved, obviously intelligent, and extremely reluctant to discuss much about his work or whether it would progress any distance beyond the first fragments that the FBI had already received. I asked him how I could be of assistance to him; he seemed not to know. I told him I was intensely interested in what he was doing and would be willing to mount any sort of research effort to provide him with more information; he simply nodded. I offered to write up a memo about one of the message fragments because I thought the FBI might have a glimmer of understanding of the subject matter being discussed by the KGB; he was noncommittal.

From that day on, every two or three weeks I would make the pilgrimage out to Arlington Hall. Meredith Gardner was indeed not easy to know, and was extremely modest about his work, but eventually we did become friends. Neither the friendship nor the solution to the messages was achieved overnight, but steady progress was made. Little by little he chipped away at the messages, and I helped him with memoranda that described what the KGB might be referring to in some of them. The ASA's work was further aided by one of the early, rudimentary computers.

Even now, long after his extraordinary achievements were publicly acknowledged, few Americans have heard of Meredith Knox Gardner, who has died aged 89. Yet his superb talent as a cryptanalyst led to one of his country's most important penetrations of Soviet diplomatic codes. Among other benefits, it unmasked the wartime spy rings which passed the atomic bomb secrets to Moscow. Years later, when a US government commission released details of the Venona project, it had no hesitation in dubbing Gardner "a genius".

Even before entering the second world war, the United States had cracked one of Japan's top-secret codes, and so discovered that Tokyo had, in its turn, broken some Soviet codes. Washington promptly established a unit of the army signal corps to attack the secret communications of what was then one of its allies.

The job was made slightly easier because Russia had to pass to America's wartime censors duplicates of all its coded cable traffic from the US. The authorities then retained every message sent between 1942 and 1946. In the standard diplomatic custom, Moscow used one-time pads to make its codes unbreakable. But analysis of hundreds of preambles to the main text showed that seven messages had been encrypted with the same key.

It emerged that, when the Nazis were at the gates of Moscow in December 1941, the KGB cryptographic unit had doubled its output of one-time pads by printing an extra copy of each. These duplicates were sent to the KGB, Soviet embassies and consulates, army and navy intelligence, and the Soviet trade organisation - it was the inclusion of the trade organisation that made the first breakthrough possible.

American cryptanalysts were able to retrieve the plain-language cargo manifests of Soviet ships which had sailed from American ports with wartime supplies. They then compared them with the trade organisation's coded messages of the same period. Piece by piece, they reconstructed both the original Soviet cipher books and the duplicate one-time pads with which they were super-encrypted.

It was at this point that Gardner came into the picture. A shy and reclusive figure, he was born in Okolona, Mississippi, and had graduated from the universities of Texas and Wisconsin, before teaching languages at both institutions. With an astounding armoury of language skills - from German and Spanish to Lithuanian, Sanskrit, old and middle high German, and old church Slavonic - he had been recruited just after Pearl Harbor. Initially, he worked on German ciphers, but then amazed his colleagues by mastering Japanese in three months, and subsequently dealing with that traffic.

At the end of the war, he was moved to the Russian section, where he began studying out-of-date Soviet codebooks (probably stolen by FBI agents) to work out the structure of Moscow's current codes. His first real success came in December 1946, when he decrypted a signal referring to the Manhattan Project, which the Russians called Enormoz. Six months later, he decoded a long message containing dozens of cover names (many of which have still not been broken).

Painstaking crosschecking by FBI agent Robert Lamphere revealed that the person code named "Liberal" in KGB messages was Julius Rosenberg. It also emerged that "Charles" was Klaus Fuchs, "Hicks" was Guy Burgess, "Homer" was Donald Maclean and "Stanley" was Kim Philby. The decrypts starkly disclosed the unrealised extent of Soviet wartime espionage in the US.

Gardner's work remained unknown to anyone outside the confines of the US national security agency, and stayed under wraps for years after the Venona project was wound up in 1980. The first revelation that it had ever existed came in 1987, when the former MI5 agent Peter Wright referred to it in his autobiography, Spycatcher. Wright claimed that Gardner had started with a charred copy of a Soviet code book retrieved from a Finnish battlefield, which enabled him to discover some of the commonest five-figure instruction groups used in radio communications.

Meredith Knox Gardner, a linguist and puzzle solver whose skill at deciphering codes played a pivotal role in the Rosenberg spy case, died on Aug. 9 at a hospice in Chevy Chase, Md. He was 89.

Fluent in French, German, Greek, Italian, Japanese, Latin, Lithuanian, Spanish and Russian, Mr. Gardner arrived here early in World War II to work for the Army Signal Intelligence Service, a predecessor of the National Security Agency.

He spent the war poring over messages between Germany and Japan. After their defeat, his focus turned to the Soviet Union. He was assigned to help decode a backlog of communications between Moscow and its foreign missions. The project, named Venona, was based in northern Virginia. In recent years, the National Security Agency has made public some of the exploits of the code breakers.

Starting in 1939, the Signal Intelligence Service intercepted thousands of Soviet communications, but they were not studied while America and Russia were allied. By 1946, Mr. Gardner was among the hundreds of people trying to decode the accumulated messages.

On Dec. 20, 1946, Mr. Gardner discerned that a message from 1944 had contained a list of the leading scientists on the Manhattan Project, which developed the atomic bomb. More months of toil by Mr. Gardner and his colleagues turned up a reference to an agent code-named Liberal who had a wife named Ethel.

''Liberal'' and his wife were Julius and Ethel Rosenberg, who were convicted of conspiracy to commit espionage. They were executed in 1953....

Because the Venona project was not disclosed in detail until the mid-1990's, its work was never widely recognized. In a speech in 1997, Senator Daniel Patrick Moynihan of New York said the deeds of Mr. Gardner and Mr. Lamphere, in particular, were contributions ''that Americans have a right to know about and to celebrate.'' Mr. Lamphere died in February.

Despite his pride in having helped to smash an espionage ring, Mr. Gardner was sorry that the Rosenbergs were put to death. Mrs. Gardner said her husband's reasoning was that ''those people at least believed in what they were doing.''

Meredith Gardner, who has died aged 89, was the American codebreaker responsible for breaking the ciphers that led to the arrests of the atom spies Julius and Ethel Rosenberg and the break-up of the Cambridge spy ring.

During the late 1940s, Gardner was the main cryptanalyst working on the Venona material, messages sent between the KGB's Moscow Centre and its agent handlers abroad using the theoretically unbreakable one-time pad system.

Fluent in French, German, Greek, Italian, Japanese, Latin, Lithuanian, Russian and Spanish, Gardner joined the United States Army's codebreaking organisation, the Signals Security Agency, early in the Second World War.

He worked initially on German ciphers and then on Japanese super-enciphered codes, in which messages were first encoded in five-figure groups taken from a code book and then enciphered by adding a series of randomly produced figures, known as an additive, which was taken from a second book.

The techniques needed to break these messages were to be extremely useful to Gardner when, in late 1945, he was assigned to work on what was then known simply as "the Russian problem".

KGB messages were produced in exactly the same way as Japanese super-enciphered codes. But where the Japanese gave the codebreakers a way in by repeatedly using the same sequences of additive, the Russian system did not.

As its name suggests, the additive appeared on separate sheets of a pad. Once a stream of additive had been used, that sheet was torn off and destroyed, making the message impossible to break.

But because of wartime shortages, the Russians made a critical mistake. Several pads had been duplicated and sent to different KGB stations abroad. Although it was still not easy to break, the American codebreakers had the help of a partially burned codebook abandoned in Finland.

The intelligence which they produced represented the crown jewels for the FBI and MI5, vital information on the many Soviet agents operating against the West. The breakthrough came on December 20 1946, when Gardner read part of a message that contained a list of leading scientists working on the Manhattan Project, the development of the atomic bomb.

Sent two years earlier, the message was the first hint that there might be Soviet spies working at the Los Alamos atomic weapons plant. By August 1947, when he produced his first report on the Venona messages, Gardner had found a number of agents all referred to by codenames.

One, codenamed Liberal, had appeared in six separate messages. The only clue to his identity lay in the name of his 29-year-old wife. Gardner, a lifelong fan of crossword puzzles, was particularly pleased with the way in which he had come up with the wife's name.

The KGB cipher clerk had spelt the name of Liberal's wife out in single letters but there were only three groups, the first representing E and the third L. "I had never come across a three letter meaning in the spell code," he later recalled. "Then I said: ah, but they anticipate sending a lot of English text, and the most common word in the English language is 'the'."

The name of Liberal's wife was Ethel, one of the key clues that led to the uncovering of Julius and Ethel Rosenberg who, in 1953, were sent to the electric chair.

Another wartime Venona message, which Gardner was finally able to break in 1949, revealed that a scientist at Los Alamos codenamed Rest, was passing atomic secrets to the Russians. The message contained a Los Alamos report written by the British scientist, Klaus Fuchs.

By now Fuchs was working at the British atomic research establishment at Harwell, Oxfordshire. He was questioned by MI5 officers and five months later admitted that he had passed atomic weapons information to Moscow. He pleaded guilty in court and, on March 1 1950, was jailed for 14 years.

Probably the most important atom spy that Gardner uncovered was never prosecuted. Theodore Hall, codenamed Mlad (Young), was the star pupil at Harvard of Professor John Van Vleck, an authority on quantum theory who had been secretly recruited by Robert Oppenheimer, the scientific head of Los Alamos, to help to design the atomic bomb.

Recommended by Van Vleck, Hall was soon put in charge of the team designing the implosion trigger for one of the experimental bombs. With access that even Fuchs and certainly not Rosenberg enjoyed, Hall was the best source the KGB had inside Los Alamos.

Although the FBI knew that Hall was Mlad, he was never put on trial, largely because, unlike Fuchs and Rosenberg, he never confessed and the American authorities were wary of making public the evidence from Venona until 1995. Hall left America for Britain where he became a leading Cambridge scientist.

But probably Gardner's greatest triumph was to break the message that led to the demise of the Cambridge spy ring. It was in January 1949 that Gardner broke a number of messages from the KGB station at the Soviet consulate-general in New York to Moscow Centre referring to an agent with the covername Homer.

This spy had been in Washington in mid-1945 and gained access to the secret messages between Truman and Churchill on the fate of the leaders of the Polish Home Army.

It was Donald Maclean, a senior British diplomat, who had been posted to Washington the previous year. Crucially, in mid-1945, his American wife Melinda was pregnant and living with her mother in New York.

Kim Philby, who was posted to Washington as an intelligence liaison officer shortly after the messages on Homer were deciphered, described how the FBI concluded that it could be one of 6,000 people.

"It had so far occurred neither to them nor to the British that a diplomat was involved, let alone a fairly senior diplomat," he said. "Instead, the investigation had concentrated on non-diplomatic employees of the embassy."

But slowly, MI5 narrowed those names to a handful of people who would have had access to the top-secret exchanges between London and Washington. Then in April 1951, the Venona decoders found the vital clue in one of the messages. For part of 1944, Homer had had regular contacts with his Soviet control in New York, using his pregnant wife as an excuse. The names had been narrowed down to just one - Donald Maclean.

Tipped off by Philby, who had access to the Venona material, he fled to Moscow with Guy Burgess, another of the Cambridge spies. The hunt for the so-called Third Man, who had tipped them both off, began and very soon Philby fell under suspicion. It was the beginning of the end for the Cambridge five, made up by Anthony Blunt and John Cairncross.

Meredith Knox Gardner, an Army Signal Intelligence Service code breaker whose work on encrypted KGB messages to and from Moscow during and after World War II led to the exposure of Soviet agents who spied on the U.S. atomic bomb project, has died. He was 89.

Gardner died of complications from Alzheimer's disease Aug. 9 at a care facility in Chevy Chase, Md.

Gardner's work included the discovery of lists of code names in telegrams sent by the Soviet consulate in New York to Moscow from 1943 to 1945. It led directly to the unmaskings of Klaus Fuchs, the German-born scientist convicted of spying for the Soviets; Julius and Ethel Rosenberg, who in 1953 were executed for espionage; and the British intelligence officer Kim Philby, who after defecting to Moscow in 1963 said he had been a Soviet spy for two decades.

Within the intelligence community, Gardner was said to have been a living legend, and his work in penetrating Soviet codes is widely considered to have been one of the great U.S. counterintelligence coups of the last half-century. But he remained unknown to the public for more than 50 years until 1996, when he emerged from anonymity to tell his story at a conference on the decrypting operation, code-named "Venona." At that conference, then-Sen. Daniel Patrick Moynihan (D-N.Y.) introduced Gardner as an unsung hero of the Cold War.

Describing his discovery of code names in the Soviet cables sent from New York to Moscow during and after the war, Gardner told the Venona conference: "That smelled of espionage. Otherwise, why would you go to the trouble of using something other than someone's real name?"

In December 1946, his suspicions were all but confirmed when he decrypted a New York-to-Moscow cable sent two years earlier containing the code names of several leading scientists who had been working on the Manhattan Project, which was the U.S. effort to build an atomic bomb. This had been the most secret of all U.S. projects during the war, and in the postwar period the atomic bomb was the key to the balance of world power. Not until 1949 would the Soviets detonate an atomic bomb.

Gardner, a gifted linguist who was fluent in German, Old High German, Middle High German, Sanskrit, Latin, Greek, Lithuanian, Spanish, French, Italian, Russian and Japanese, came to Washington early in World War II to work as a civilian for the Army Signal Intelligence Service, a predecessor of the National Security Agency....

Over subsequent months, as more Venona cables were decrypted, it became clear that Moscow had recruited dozens of agents at various levels of government, and the FBI was directed to follow up leads. Robert Lamphere was the FBI agent named liaison officer with Venona. He and Gardner developed a symbiotic relationship in which Gardner gave Lamphere lists of agents named in the Venona cables, while Lamphere gave Gardner information that might be helpful in further decryption.

This led to a massive manhunt for spies in the late 1940s and early 1950s and is said to have contributed to the communist-baiting excesses of Sen. Joseph McCarthy. Among the other Soviet agents mentioned in the Venona documents were David Greenglass, the younger brother of Ethel Rosenberg who received a 15-year prison sentence for passing along information about the atomic bomb; and Theodore Alvin Hall, who was recruited as a 19-year-old Harvard student to work on the bomb and was then said to have passed along the vital secrets of this work to the Soviets. Hall, who was never formally charged, died in Cambridge, England, in 1999.

Information from the Venona operation also led to the exposure of Kim Philby's British comrades in espionage, Guy Burgess, Donald Maclean and Anthony Blunt.

From at least two sources, the Soviets learned that their U.S. espionage net had been discovered. One was Philby, the British intelligence officer and double agent. He was posted in Washington in 1949 and had a habit of dropping in on Gardner's Venona operation.

The other was Bill Weisband, a Russian emigre who was hired as a linguistic adviser for Venona. Gardner occasionally consulted him on points of Russian grammar. At the Venona conference, the National Security Agency declassified tapes of the confession of a Los Angeles aircraft worker who identified Weisband as his KGB handler. Weisband was fired from Venona and later served a one-year prison sentence for contempt of court for refusing to testify about Communist connections. He died in 1967. U.S. counterintelligence officials said they are convinced he was a Soviet spy.

In 1972, Gardner retired from NSA. The Venona operation was shut down in 1980.

In retirement, Gardner lived quietly in a modest condominium on Connecticut Avenue in Washington, where he traced his Scottish genealogy and did the daily crossword puzzle in the Times of London, which is reputed to be the most difficult in the world.