[WalterBright]

Hand them a newspaper. Plenty of text there to use as a pad. The code breakers would need to know both the newspaper used, and the algorithm used to select letters from the paper in order to crack it. Or use a magazine to provide the algorithm.

Every U-Boot mission gets another newspaper. Every U-Boot has a different newspaper. There weren't that many U-Boots, so this would be manageable.

Even decoding one U-Boot's transmissions would not compromise the others.

[sidewndr46]

The U-Boats have plenty of other operational failures. Whenever U-Boats would be potentially damaged by the Allies, they'd put a notice in at one of the British listening stations. When the U-boat came into port, it'd radio in advance that it was damaged and might need special provisions (couldn't steer well, etc.) or even a tug to make it into the harbor. This all happened in German on the HF radio, which propagates really well. At least to Britain and possibly all the way to the mainland US.

The Allies basically got free reports of exact damage they inflicted on subs this way. On the other hand if a report didn't show up in a few days, they probably sunk it.