BOOK REVIEW : A Techno-Thriller Zeroes in on Nazis : SEIZING THE ENIGMA: The Race to Break the German U-Boat Codes, 1939-1943 <i> by David Kahn</i> Houghton Mifflin $22.95, 282 pages

One of the untold secrets of World War II is the clandestine operation against Nazi submarine wolf packs in the North Atlantic, a campaign conducted with an arsenal of unlikely weapons: “bombes” and “kisses,” “cribs” and “bans,” “gardening” and “romsing,” all of the improvised tools and techniques of Allied cryptographers who cracked the coded orders that directed the U-boats to their targets.

The secret is revealed by David Kahn in “Seizing the Enigma,” a work of military history that is also a techno-thriller of the first order, a tale of combat and adventure on the high seas, and an unlikely war story in which the real (if unsung) heroes include the chess masters and math prodigies and scholars of dead languages who labored in an English country manor called Bletchley Park to break the codes of the Third Reich.

“Seizing the Enigma” focuses on the naval applications of ULTRA, the Allied code-breaking operation that defeated Enigma, Nazi Germany’s secret communications technology.

According to Kahn, an historian of cryptography and author of “The Codebreakers,” the achievements of the men and women who served in ULTRA were among the most heroic--and the most decisive--of the war.

“ULTRA was the greatest secret of World War II after the atom bomb,” Kahn writes. “It was one of the great intellectual achievements of the century, no less remarkable because it was achieved against a secret produced by men rather than of nature.”

Earlier books about ULTRA have focused on its use against the Luftwaffe in the Battle of Britain. Here, for the first time, Kahn tells the story of the naval version of Enigma, which withstood the efforts of Allied cryptographers during the early and crucial years of the war when Nazi submarines were harrying the life-sustaining convoys from America. The urgent struggle of the Allies to defang the Nazi wolf packs is what gives “Seizing the Enigma” its breathless pace and its frequent moments of high drama.

“The submarines were sinking more ships than Britain and the United States were building,” Kahn explains. “The codebreakers understood that unless they could solve the naval Enigma and ascertain the U-boats’ movements in advance, the British were in grave danger of losing the Battle of the Atlantic, with possibly fatal consequences for the nation.”

According to Kahn, it took revolutionary technological innovation, mathematical and linguistic genius, and sheer physical courage to win the intelligence war. At Bletchley Park, an elite if bookish army of code-breakers labored around the clock to decipher the intercepted radio traffic of the German navy, relying on cutting-edge technology, brute intellectual force, occasional flashes of insight and sometimes just good luck to decode an intercepted radio message and thereby spare a convoy from submarine attack.

“‘There’s nothing like real blood,’ the cryptanalysts said to one another,” Kahn writes, “referring to solving the life-or-death messages that the Germans were trying hard to keep from them.”

Yet the code-breakers and their primitive computers were impotent against Enigma without one crucial piece of information--the ever-changing “keys” that German wireless operators used to set their Enigma machines. So the Royal Navy went to extraordinary lengths to secure the keys, and their exploits provide some of the most vivid scenes in “Seizing the Enigma”--or, indeed, any account of World War II.

A young naval officer named Ian Fleming, for example, proposed a caper worthy of James Bond himself--a team of Royal Navy commandoes would crash-land a captured German bomber into the sea near a German warship, then overpower their rescuers and steal the Enigma keys.

Fleming’s scheme was never put into action--but plenty of other missions, only slightly less exotic, were mounted, including the stalking and ambush of German weather ships and the capture of U-boats on the high seas, all of which yielded the crucial keys to Enigma.

Kahn is equally adept at explaining the mathematical underpinnings of cryptography, discussing the strategic implications of counter-submarine warfare, and spinning a blood-and-guts yarn of hand-to-hand combat on the high seas.

In “Seizing the Enigma,” a book with both punch and subtlety, he manages to accomplish all three at once. It’s a must-read for espionage buffs and students of World War II, but it has something to offer any reader who enjoys a ripping good story.

Next: Richard Eder reviews “This Is the Life” by Joseph O’Neill (Farrar Straus Giroux).