Signal Intelligence: After 5,300 hours of bombing, the first Nazi Sonderschlussel code is broken. Only three Sonderschlussels will ever be cracked, and all three by the US Navy’s OP-20-G. The Kriegsmarine had begun distributing special decryption algorithms (Sonderschlussels) for each individual U-Boat in June 1944. However, they were not actually applied in the field until mid-November 1944. If applied correctly, they are very difficult to crack, especially if cribs are scarce.
