(name omitted for security purposes)
Decrypter - Kriegsmarine transmissions
Hut 8, Bletchley Park
December 1942
Since Admiral Karl Doenitz changed the Enigma ciphering procedures for the U-boats in February 1942, we hadn't been able to crack the new code. I can still remember my colleagues' expressions when I walked in after the change in ciphers. I asked them what happened, but they just shook their heads. We could intercept all the information we wanted, but we couldn't decrypt it. We knew this day was coming ever since we captured material about the development of the new Enigma machine, but we didn't know when it was going to hit us.
The new German ciphering machine (codename M4) used a four-rotor system, making the cipher much more complex than the previous three-rotor version. We gave the new cipher the codename "Shark", instead of "Dolphin", since it was so much stronger.
From then on, we've worked feverishly to crack this problem - the Kriegsmarine surface units and Baltic submarines still use the old three-rotor procedures, but the information we can glean from them has been very limited. The submarine trackers aren't able to get much information from us, and they have had to rely on agent reports, direction finding, and aerial reconnaissance. It's been a step backwards. Our automated keyfinding hasn't worked for ten months.
That was until three members of the HMS Petard succeeded in capturing one of the codebooks for weather reports in October. After that, we could work backwards to penetrate the new code using the plain-text from the codebooks.
Now we have broken the Enigma code again! We can decrypt German radio signals and send them straight to the Operational Intelligence Centre in London by teletype. The increase in German U-boats and the wolf pack tactics that they have adopted has also allowed us to intercept more radio signals, so we can decrypt their orders and reports.
Instead of locating individual submarines with direction finding, we can again take the fight back to the Germans by figuring out their plans and tactics.
We've been able to teleprint the positions of 10 U-boats to the Operational Intelligence Centre today, and there will be more to come tomorrow.
We are back on track. The Germans will have to look out.
