Signal Intelligence: The Kriegsmarine begins distributing special decryption algorithms (Sonderschlussel) for each individual U-Boat. They will not actually be applied in the field until mid-November 1944. If applied correctly, they are very difficult to crack, especially if cribs are scarce. It will take 5,300 bomb hours to drop one, the Sonderschlussel, in April 1945. Only three Sonderschlussels will be cracked, and all three by the US Navy’s OP-20-G.
