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Almost certainly the best of the nonbelligerent cryptanalysts, and perhaps one of the best in the war, was that of the precarious neutral, Sweden. ...

Early in 1940, just before the German occupation of Norway, Nazi agents there, who were concentrated in the German-Norwegian shipping lines and in the large fishing and fish-processing firms, were ordered to pass back information on ship movements and weather. They disguised the data as sales prices, offers, and tonnage reports on fishing ...

But the Norwegian authorities had intercepted the telephone calls ... They sent recordings to Sweden, where Segerdahl discovered that the five-digit "prices" actually represented the transposed and monalphabetically enciphered numbers of ships in Lloyd's Register. The solutions enabled Norway to break up at least one of the rings ...

The Swedes also read messages in other German systems — a double transposition for the military attache and two substitution systems for the troops. The latter gave them an unexpected peek into the sex habits of German soldiers. The Wehrmacht provided women from the Baltic states and concentration camps as prostitutes for the occupation forces in Norway, and the vessels were naturally awaited with great eagerness.

Their arrivals and departures formed the subject of excited communication between units, and not infrequently a radioman in a port from which a ship had just sailed would recommend one of the girls to a fellow signalman in the port to which the ship was headed. The reasons were sometimes quite specific, and the Swedes came to think that they knew the girls almost as well by cryptologic means as the soldiers did by carnal.

David Kahn
"Duel in the Ether: Neutrals and Allies"
The Codebreakers:
The Story of Secret Writing