Break of Naval Enigma
In 1939 only two people thought
Naval Enigma could be broken: Frank Birch, Head of German Naval Section,
and Alan Turing.Birch because it had to
be broken.Turing because it would be so interesting to break it!.
First main breaks of naval Enigma
Hut 8 could not read Dolphin traffic without delay until June and July 1941, when it used keys captured from the
weather ships München and Lauenburg. Decrypts were translated by Hut 4, which then sent their full text by
teleprinter to the Admiralty's Operational Intelligence Centre (OIC) in London. Intelligence from the decrypts
enabled the OIC to re-route many convoys past the few U-boats (about 20) then in the North Atlantic.
Re-routeing convoys on the basis of 'Ultra' saved many lives and hundreds of thousands of tons of vital shipping.
Hut 8 broke Dolphin cryptanalytically from August 1941 onwards. It was
helped because the order in which the
rotors were inserted in Enigma changed only every two days. If a crib was available, a bombe run on the second
day could therefore find the day's settings in under 20 minutes, saving much precious bombe time.
Manual weather ciphers broken by Bletchley's weather section in Hut 10
provided many cribs. The Atlantic
U-boats made numerous weather reports, encoded on the Wetterkurzschlüssel before encipherment. From
February 1941 on, Hut 10 broke general weather signals, enciphered with the naval manual meteorological
cipher, which incorporated the U-boats' weather reports. In early May 1941, Bletchley received a copy of the
1940 edition of the Wetterkurzschlüssel from München and U-110. This enabled Hut 8 to reconstruct the
U-boats' weather signals, and so obtain a second source of cribs. In addition, identical signals on subjects such
as mine-clearing were sometimes sent using naval Enigma and another manual cipher, the Werftschlüssel
('dockyard cipher'). When Hut 4 broke the signals using Werftschlüssel, Hut 8 had more cribs.
Hut 8 suffered a massive reverse on 1 February 1942 when a new Enigma machine
(M4) came into service on
Triton (codenamed Shark by Hut 8), a special cipher for the Atlantic and Mediterranean U-boats. The
combination of M4, Shark and a second edition of the Wetterkurzschlüssel proved devastating. Bletchley Park
became blind against Shark for over 10 months. Fortunately, M4's fourth rotor (beta) was not interchangeable
with rotors I to VIII. Beta increased M4's power by a factor of 26, but rotors could still only be mixed in 336
(8x7x6) different ways - not 3,024 (9x8x7x6).
At one setting of beta, M4 emulated M3, which was M4's undoing. Three members
of the British destroyer HMS
Petard seized the second edition of the Wetterkurzschlüssel from U-559 on 30 October 1942, before it sank
near Port Said. Hut 8 once again had cribs, which it could run on three-rotor bombes, the only type available.
The U-boats were using M4 in M3 mode when enciphering the short weather reports. A three-rotor bombe run on
60 rotor combinations therefore took only about 17 hours instead of the 442 hours (18 days) required if M4 had
used its full potential.
On 13 December 1942, Bletchley teleprinted the OIC the positions of over
12 Atlantic U-boats, on dates from 5
to 7 December, as established from Shark weather signals. Hut 8 had penetrated M4 Shark with the help of the
weather broadcasts broken by Hut 10. Intelligence from Shark, although sometimes badly delayed, played a
critical part in the Battle of the Atlantic, perhaps saving from 500,000 to 750,000 tons of shipping in December
1942 and January 1943 alone.
Hut 8's use of the Wetterkurzschlüssel against Shark was short-lived.
A third edition of the weather short signal
book took effect on 10 March 1943, depriving Hut 8 of cribs. Bletchley had feared that the change would blind it
for several months, but by using short signal sighting reports (made by U-boats in contact with convoys and
encoded from the Kurzsignalheft) as cribs, Hut 8 re-entered Shark again on 19 March and broke it for 90 out of
112 days before 30 June. Kurzsignalheft short sighting reports also used M4 in M3 mode - and the
Kurzsignalheft had also been recovered from U-559.
British and US Navy four-rotor bombes entered service in June and August
1943, respectively, but some July
and August Shark keys took up to 26 days to solve. However, from September on, Shark was generally broken
within 24 hours. At the end of 1943, work on Shark was transferred to the US Navy's Op-20-G codebreaking unit
in Nebraska Avenue, Washington, DC, because the US Navy had over 50 bombes by mid-November.
The main role for naval Ultra was probably in re-routeing convoys, but
it was used in many different ways. The
US Navy employed Ultra offensively in 1943 and 1944 to sink many of the important supply U-boats (Types XB
and XIV, such as U-118, U-233 and U-460), which applied a multiplier effect to the U-boats by replenishing them
The Wetterkurzschlüssel and Kurzsignalheft were retrieved from U-559
by Lieutenant Anthony Fasson, Able
Seaman Colin Grazier (both were posthumously awarded the George Cross - Britain's second highest award for
gallantry) and 16 year-old Tommy Brown (who survived to receive the George Medal). Without their bravery,
Shark would not have been broken before four-rotor bombes came into service, if at all. The Allies (Britain,
Canada and the United States) would not then have established naval supremacy in the Atlantic until the
second half of 1943 at the earliest, which would have probably delayed the D-Day Normandy landings until
1945. Few acts of courage by three individuals can ever have had so far-reaching consequences. Without Ultra,
the U-boats would still have been defeated in the long run, but the cost in human life in the global conflict would
have been even more terrible than it was.