Seizing the Enigma (27 page)

By the summer of 1940, with the Battle of Britain in full force, the Germans were using ships for weather observation. On July 23, the
Adolf
Vinnen
was positioned north of Iceland. Three weeks later, the
Hinrich Freese
sailed to relieve her. On September 14, the
Sachsen
sailed as a replacement for two stations on Greenland that the British had closed. She was commanded by Captain Otto Kraul, an experienced whaler.

The demand for weather information continued to grow. The Naval War Staff had declared that an “augmented weather reporting service is necessary” for Operation
SEALION
—the invasion of Britain—and that it was to be provided by weather buoys, U-boats, and fishing vessels. U-boat Command objected, saying that its submarines should not be endangered by such secondary missions and should be used exclusively in their primary function: to sink enemy ships. But the war staff overruled it, and U-boats were required to transmit regular weather reports.

These, however, could not be fully satisfactory. In the first place, they were transmitted from each U-boat’s operating area, not from the weather factories in the northern and western Atlantic. In the second place, they lacked the regularity of reports from a ship in a single location. Even the subsequent postponement of
SEALION
did not end the need for weather information; it would be needed for the raids on British shipping that German surface forces were to undertake.

In the middle of November 1940, however, the navy found itself facing a crisis in weather ships. Three vessels that it had obtained for that purpose proved inadequate. And two of the four northern weather-reporting ships had been sunk. At 1:10
A.M.
on October 24, the signal station at the Stadlandet headland on the coast of occupied Norway reported cannon fire and searchlight beams out at sea; the
Adolf Vinnen
had been sunk by a British submarine, survivors said. On November 16, the
Hinrich Freese
, visiting the German meteorological station on the Arctic island of Jan Mayen, radioed frantically: “Am being chased by enemy vessels.” Nothing further was heard, even when she was ordered to report, and the navy concluded that she had been lost. Then an accident took the
Sachsen
out of service for a while, leaving only one weather ship operating in the north.

Now greatly pressured, the navy obtained, on November 23, four fishing vessels for use as weather ships. They had the capacity to be rebuilt for cruises longer than the three weeks or so that sufficed for fishing. One of them was the fourteen-year-old, steam-powered, 139-foot-long
München
, named for the lively capital of Bavaria. She had to be fitted out with additional water tanks, reserve fuel bunkers, more space for provisions, room for three or four more men, and radios. The
Sachsen
was mentioned as a norm: she carried a 150-watt transmitter, a 40-watt portable transmitter, two portable receivers, one panoramic receiver, and one broadcast receiver. The
München
was worked on at the Seebeck dockyard in Bremerhaven, which estimated six weeks for the job. Her sand ballast was replaced by poured concrete. Marine engineers thought at first that the vessel should be strengthened for the ice. But Kraul advised that this was not necessary if the
München
did not plan to seek shelter in estuaries where she might be frozen in. This advice reduced the time needed for rebuilding.

The high command of the navy declared specifically that weather ships were not warships but auxiliary vessels belonging to the train of the
Kriegsmarine.
As a consequence, the original civilian crews were retained. But some new personnel, such as the weather observers and some of the radiomen, were members of the navy. They wore mufti but were given armbands saying “
Deutsche Wehrmacht
” that they were to wear in case of enemy contact. Despite the noncombatant nature of the weather ships, the navy acquiesced in the proposal made by the Naval Group Command North after it lost the
Adolf Vinnen
and the
Hinrich Freese
: that the weather ships be armed for self-protection. But because of the haste to get the ships to sea, the
München
had time only to have a machine gun installed. The vessels were equipped with cipher systems for communicating the results of their observations to Germany.

One was the
Wetterkurzschlüssel
, or Short Weather Cipher. This booklet, issued in October 1940, was printed on absorbent pink paper in water-soluble ink. Its 21 pages consisted mostly of tables. Each
table condensed weather observations and locations into a single letter. These substitutions reduced the time it took to transmit a message from minutes to seconds, decreasing congestion on German radio circuits as well as the chance of enemy interception and direction-finding. The observer aboard ship converted his measurements into these letters in the order prescribed (see the
Appendix
for an example). He then added his ship’s two-letter signature. The time of the observation was presumed to be within an hour of the radioing of the message.

The Germans knew that the meaning of these messages would quickly be ascertained by British meteorological cryptanalysts, donating valuable information to Britain. To prevent this, and to maintain security in general, the Germans enciphered their abbreviated weather messages in the Enigma. Each weather ship thus carried an Enigma and its associated manuals and key lists. The latter specified the Enigma settings that were to be used each day for a month throughout the German navy; sometimes two or even three months’ worth of keys had to be carried, depending on the length and dates of a cruise. The keys specified the three rotors to be placed in the machine in left-to-right order, the position of the alphabet rings on each rotor, the setting of the three rotors, and the plugboard connections. Also included were the indicators book, which listed the three-letter message-grade indicators that, when tapped out on a set-up Enigma, yielded the message key—the position to which the rotors were turned at the start of the encipherment of a nonweather message—and the bigram tables.

Weather messages abbreviated this procedure. The Short Weather Cipher listed twenty-six three-letter groups that served as ready-made message keys. Each was represented by a letter: EDM, for example, by A. The encipherer chose one of these positions, say, that represented by Q. Then, with the rotor order and alphabet ring position specified in the current Enigma keying document, he turned the rotors until the three letters that Q represented appeared in the windows in the machine’s lid. Then he enciphered his Short Weather Cipher text; MZNFPED plus, say, MR as signature, might become
TVUOIGMVX. The encipherer prefixed the key indicator Q to this and, in front of that, a WW, meaning
Wetterkurzspruch
or “short weather message.” The radioman, who was also the encipherer, would tap this out on a specified frequency in 15 to 20 seconds. He sent it “blind”—not addressed to any station. But any station that received the message was to acknowledge receipt. If the ship received no confirmation, she repeated her transmission fifteen minutes later on a different frequency. The main weather stations became Norddeich, for weather ships operating south of Iceland, and Kootwijk, in the Netherlands, for those north of Iceland; more often than not, however, other stations replied. The receiving station deciphered the message using Enigma, added a time group, and teletyped the Short Weather Cipher letters to the navy’s weather central, which reconverted them into the original data.

One of the major determinants of Europe’s weather is the polar front: the boundary between the warm moist air of the south and the cold dry air of the Arctic. Another is the North Atlantic Drift, the remnants of the Gulf Stream. To determine their locations and movements, the Germans favored two observation areas. One was some 300 miles east and slightly north of Iceland, a watery square 54 miles on a side designated as AE 39 on the
Kriegsmarine
grid. The other was the square AB 72, in the Norwegian Sea some 550 miles northeast of Iceland. Its southernmost boundary was at 71° north latitude, the parallel that cuts the North Cape, the northernmost point of Europe. The weather ships worked these areas for three to five weeks, excluding the time required to go to and return from their stations. The
Coburg
, for example, sailed on February 10, 1941, and returned on March 28; the
Ostmark
left port on April 2 and returned on May 6.

While the
Ostmark
was still at sea, the
München
received orders to relieve her. This was to be the
München
’s second cruise and the second time she had replaced the
Ostmark
in AE 39. And conditions would be better than on the first cruise, which had put the vessel in
the freezing North Atlantic from the middle of February to the middle of March. She sailed in the evening twilight of Thursday, May 1, from Trondheim, the fortified seaport in whose ancient cathedral kings of Norway are crowned, steaming slowly in the gathering darkness down the wide fjord. She had 40 miles to go to reach its mouth, and another 30 until, after passing the chains of islands that paralleled the coast, she rocked to the swell of the open sea.

Her crew of about twenty consisted of some seamen who had worked on her as a fishing trawler and who ran the ship and some navy sailors who had been assigned to her for weather, radio, or weapons duties. One of them was the young weather observer Fritz Rebelein, who had been plucked from his barracks in Wilhelmshaven apparently at random to be trained in meteorological observation. Another was draftee Heinrich Wiggeshof, not yet 20. A ham radio operator in peacetime, he had been sent to the navy’s communications school at Flensburg and then assigned to the
München
for his
first duty afloat. Both had completed the first cruise, but this time Rebelein had a premonition that he would not come back.

Once the
München
had reached her assigned square east of Iceland, Rebelein and the other observers took measurements twice a day. They encoded the results in the Short Weather Cipher and gave them to one of the radiomen. He enciphered this in the Enigma, for which he, like all
Kriegsmarine
vessels in the Atlantic, had the Home Waters cipher net keys for May. And because the
München
’s replacement might be delayed, or the ship for some other reason might have to stay at sea longer than the scheduled twenty-five days, she also carried the keys for June.

The Germans were not the only ones paying attention to the weather ships. The solutions of naval Enigma traffic made possible by the documents from the
Krebs
included messages from the weather ships and acknowledgments of their reception. The messages often told of the weather ships’ movements to and from their stations.

In his little office in Hut 4, Harry Hinsley, who analyzed the cryptanalysts’ output especially for trends and for information on specialized (nontactical) matters, read these intercepts along with all the others. One day in the spring of 1941, a “passing thought” brought to his consciousness something that had remained beneath the surface of his mind: the weather messages were enciphered in Enigma. He asked himself whether the Germans would be so foolish as to put the cipher machine used for the whole German navy and its accompanying key documents on an isolated, vulnerable weather ship. But then, in a “mad moment,” he concluded, “Good God, this must be right!” It further dawned on him that if the weather ships were out at sea for a long time, they had to carry Enigma keys for several months. Perhaps with Ian Fleming’s unfulfilled idea of seizing cipher documents from a lured-out air-sea rescue boat and the success of the
Krebs
episode in the back of his mind, Hinsley thought of capturing Enigma papers from a weather ship.

The young intelligence officer discussed his idea with Clive Loehnis, Bletchley’s liaison with the Admiralty, particularly its Operational Intelligence Centre. The O.I.C. was impressed by the idea, but it recognized the chief risk. If a weather ship radioed that it was being boarded, the Germans would probably change the keys, perhaps including the rotor wirings, and possibly even abandon the Enigma for another system. On the other hand, the O.I.C. and B.P. observed that, despite the destruction of two weather ships through British action, the Germans had not seemed to fear that their cryptographic documents had been compromised and had left their systems in place. Perhaps their complacency would continue. So the O.I.C. and B.P. concluded that the prize was worth the risk. Even if no cryptographic papers were captured, the Germans would be deprived of a source of weather reports. In addition, Hinsley knew, there were two weather ships on northern stations at a time, doubling the chances for capturing cipher documents and wiping out German weather ship information.

This clinched it for the intelligence types. With these arguments in hand, they approached Captain Haines, assistant director of the O.I.C. for radio intelligence. Operational questions like these came first to him. No doubt after satisfying himself of the necessity of such a “pinch” and the possibility of its succeeding, he discussed it with operational officers of the Admiralty. They concluded that such a seizure, while tricky, was not impossible.