The main Enigma ciphers used by the U-boats are listed below. The Bletchley codename for a cipher is given first, with the Kriegsmarine name in brackets.
Dolphin (Heimische Gewässer, later Hydra)
Dolphin came into service at the beginning of the war. It was used by all U-boats (and ships) in "home waters" (an area which included the Atlantic) until 5 October 1941 (see Shark).
Bletchley broke Dolphin from 1 August 1941 until the end of the war.
Used by the Atlantic and Mediterranean U-boats from 5 October 1941. Introduced because of fears about security, especially the threat from the British Secret Service, which was highly (but overly) respected by the Kriegsmarine. Shark used three-rotor Enigma (M3) until 1 February 1942, when it switched to the four-rotor version (M4).
Bletchley broke Shark in M3 form. The M4 version was only broken on three days before 13 December 1942. From then until the end of August 1943, it was generally broken, but quite often late. From September 1943 onwards, Shark was normally broken within about 24 hours.
Used by the Mediterranean U-boats from June 1943 to October 1944. Turtle was broken from June 1943 onwards.
Used by the Northern U-boats (based in Norway) from 25 June 1944 to the end of the war. Narwhal was broken from September 1944.
[No British name] (Thetis)
The cipher for U-boats on tactical training exercises in the Baltic. Thetis was never broken.
Used by U-boats in the Black Sea from October 1943 to August 1944. Grampus was broken from October 1943.
Used by supply ships and U-boats in the Far East from September 1941. Sunfish was intermittently broken from August 1943.
Other naval Enigma ciphersThese included:
The cipher for fleet operations. Barracuda was used from May 1941. It was never broken.
Bonito was first broken in May 1944, and generally solved from July 1944.
- Der Schlüssel M Allgemeine Bestimmungen (M. Dv. Nr 32/3)
- British Intelligence in the Second World War: Its
Influence on Strategy and Operations,
F. H. Hinsley, with E. E. Thomas, C. F. G. Ransom and R. C. Knight,
volume 2 (London, 1981), Appendix 4; volume 3(1) (London, 1984), Appendix 3.