Lecture on Naval Enigma

The Breaking of German Naval Enigma
by Tony Sale | Back to Naval Enigma Index |

U Boat Contact Signals

In March 1943 the German Navy changed the Weather Short Code Book and Bletchley Park, which had been relying on Short Weather messages to break Shark, the U-Boat Naval Enigma cipher, was blacked out.

The code breakers realised that it might be possible to use U-Boat contact signals to get back into Shark.

The contact signals from the U-boats were short signals sent by radio to report sightings of possible Allied targets. They were short to avoid the radio signals being direction found from the UK.

The contact signals were composed by the U-boat operator using a Short
Signal code book and then enciphered on his Enigma machine.

The signals were only 22 characters long.

Since Bletchley Park had captured a copy of the Short Signal Code Book,
Bletchley Park could also recreate the codes used to describe the position
and direction and speed of the convoy, this gave what Bletchley Park called
a "crib", the guessed plain text of the intercepted cipher message.

The essence of breaking the day's settings from these short signals was
that in order for the messages to be sent quickly, the Germans used a
simple system for the message setting. This message setting is the
positions to which the Enigma wheels are to be turned to decrypt the
message and the Enigma operator just looked up an indicator/message setting
pair in his K Book (Kenngruppenheft Nr 1), included the indicator in the
message heading and enciphered the short signal starting with the wheels
at the messages setting.

Bletchley Park had captured a K Book so from the indicator in the message
header could just look up the corresponding message setting.

Thus from the intercepted short signals the code-breakers knew the relative
start positions of the messages but not the base configuration of the Enigma
machine. But they did know that all the messages were sent on the same
base configuration. This allows the messages to be written out on squared
paper offset by the message settings.

When sufficient messages had been received and set out in this way it was
possible to construct a menu to be run and on the Turing Bombe. This device
then found the base settings of the Enigma machine for that day.

A re-created set of Short signals

By establishing a possible convoy course across the North Atlantic, a set of messages can be created as if they had come from the U boats at various points along the path of the convoy. (These are the intercepts I created for Mick Jagger's film "Enigma")

These are the intercepted U-Boat contact signals as they would have come in from the Scarborough "Y" station.

Message setting from K Book

SC28/04/43 0940 7369 A348
LQB 0910/28/04/43 QGMI VVEE SERQ YGBW IAHK HW.......BHB

SC28/04/43 1140 7369 A356
TMF 1110/28/04/43 MLWP EOIG VUWY USNT AHFT WW.......LLG

SC28/04/43 1230 7369 A378
DLP 1205/28/04/43 RTVQ HSLL WKDS DAEK ABNO NG.......HPW

SC28/04/43 1335 7534 A406
JKT 1310/28/04/43 BZKN YRIY AUBD KFRJ EMBR DC.......RHO

SC28/04/43 1435 7369 A420
BHK 1405/28/04/43 QUWP YNLE LGEY DEZS BXYW XA.......LNU

SC28/04/43 1530 7534 A457
SAT 1510/28/04/43 RMRO RYOC ZJLB LNWQ XEUL TG.......LJT

SC28/04/43 1635 7369 A465
GLS 1605/28/04/43 LGRH CETW DLKZ WYVT ZBDN KL.......AIC

SC28/04/43 1705 7534 A487
HPX 1635/28/04/43 OBPF ARLK SZTK XGGW BFTT GL.......TRX

SC28/04/43 1725 7369 A505
NFT 1710/28/04/43 YWGH LYOO SZOO MXOW UHCU DE.......RMU

SC28/04/43 1825 7482 A528
BUW 1805/28/04/43 AIYE ULYF EUEE BFHR VMDV ET.......OGO

SC28/04/43 1855 7534 A537
OGV 1835/28/04/43 TSVB PZHM CZVM AGUF CZVE AP.......BPZ

SC28/04/43 1940 7369 A549
LFR 1910/28/04/43 VNPZ REZZ KUBG KHYO DCCG HX.......JRK

SC28/04/43 2020 7482 A556
QGX 1945/28/04/43 QTFH JSHV WTAD GBRB PYYQ ND.......RBN

SC28/04/43 2035 7534 A573
RVN 2005/28/04/43 LLUU GHKV TUYQ ICEP NNAZ DV.......SLR

SC28/04/43 2100 7369 A582
ULF 2035/28/04/43 HTBY GZWV WYKI SFUW RCLY LG.......BRF

SC29/04/43 2135 7278 A610
KPZ 2110/28/04/43 MIGU TMNR OAAP XHWR HYLG CL.......EUP

SC29/04/43 2220 7482 A636
YXW 2145/28/04/43 NGYB VZRP NTTH WCFM PECF LP.......BZR

SC29/04/43 2235 7534 A659
WPM 2205/28/04/43 XBFU LVES BJVN MFNP OLRC IB.......BFQ

SC29/04/43 2305 7369 A672
WLH 2235/28/04/43 ULQT QGZJ QDDK UGPI WQRC ZI.......DWX

SC29/04/43 2335 7278 A693
PNC 2310/28/04/43 YGXN PBAQ CUUL EHEB EGYC VW.......JJT

SC29/04/43 0005 7482 A712
DVZ 2345/28/04/43 FHLR XYUA SHLS NPCP WDXK EO.......EON

SC29/04/43 0035 7534
UNT 0005/29/04/43 ULQZ OEXS ULGL EFEA HSBC IR.......PKE

SC29/04/43 0105 7369 A751
FSZ 0035/29/04/43 LZET JPJR QFSY TAUH GKYN VU.......RYA

SC29/04/43 0140 7305 A783
RCU 0110/29/04/43 UDIF XIUB FZLR EGFS LFKZ QU.......BDG

SC29/04/43 0215 7560 A823
QKG 0145/29/04/43 NYCJ WSVI QDYK MXII RQFR EV.......HTS

	The intercepts give the time of origin of the U-Boat contact signal. The position of the convoys are reported to Bletchley Park. A radio direction fix on the U-Boat signal determines which convoy has been sighted.
	The German grid map of the North Atlantic can be used to get the German co-ordinates reference for the convoy.

Look up in the captured German Short Signal Book the code letters for,
Sighted Convoy, Grid ref 1, Grid ref 2, Convoy Direction, Convoy Speed.
Ignore the last two letters which are the unknown U-Boat reference.
At the first sighting, position is BD1491, direction 70 degrees, speed 10 knots.

This gives as the crib for the first signal:

CKSA KAFV MWZJ QKDX QRZA ??

SC28/04/43 0940 7369 A348
LQB 0910/28/04/43 QGMI VVEE SERQ YGBW IAHK HW.......BHB
..................CKSA KAFV MWZJ QKDX QRZA ??

In a similar way, cribs are worked out for each of the intercepted
contact signals.
Sort the Intercepts by arrival order within first letter of
the message setting looked up in the German K Book.

Setting the intercepted messages in depth.

The code breakers would have to start each group
not knowing which one would come out best.

BHB.....QGMI VVEE SERQ YGBW IAHK HW
BPZ.....TSVB PZHM CZVM AGUF CZVE AP
BRF.....HTBY GZWV WYKI SFUW RCLY LG
BZR.....NGYB VZRP NTTH WCFM PECF LP
BFQ.....XBFU LVES BJVN MFNP OLRC IB
BDG.....UDIF XIUB FZLR EGFS LFKZ QU

In fact the "B"s group is best and here they are set in depth.

Along the top of the squared paper are consecutive alphabets in lower case.
Above these are marked the Enigma wheel turnover positions.

Then the intercepted messages with cribs underneath, are written out
offset by the third letter in the message setting.

Now the codebreaker looks for and rings, vertical pairs of the same letter
pairs preferable in the same vertical area on each message and preferably
between wheel turnover positions.
(Because the Enigma machine is symetrical, A enciphers as R
is the same as R enciphers as A)

The short horizontal lines show the parts of the crib which have the greatest
confidence of being correct.

Constructing a Menu for the Turing Bombe.

There are three messages, BRF, BZR and BFQ which have a vertical overlap
between the turnover points for wheels 1 and 3, so start with this area.

At offset "t" (along the top alphabets), in BRF, D -> U. In BFQ, A -> U.
Also at offset "v" in BFQ, A -> V and in BZR, K -> V.

Thus remembering that K -> V is equal to V -> K, then: D - U - A - V - K is a connected fragment of a menu.

The last message, BDG, gives the link F -> D which makes a strong Menu for the Bombe.

The Menu information is passed to the Bombe room as four letter groups. The first two letters are the letters involved, the next two letters are the offset.

This Menu then reads:

UAFT AVFV VKZU GKET KFSS FDEU DUST FSFS SXEU YSZT.

Run the Turing Bombe.

Note: the main bombe program is over 100Kb so takes a few minutes to load into a browser on a 56K Modem.

To return from the Bombe program, use the "Go Back" top menu button on your browser.

Click here to run Turing's Bombe

When the Bombe stops it shows wheel order 534,
wheel positions DMQ,
and derived Steckers of AD DA FF GB KM SS UU VT XI YR.

This was considered a strong stop with one confirmation (AD DA)
and three self Steckers.

Getting the day's Enigma settings from the Bombe drop.

This is definitely not for the faint hearted and may cause your brain
to overheat!

The Bombe has discovered a set of core wiring positions which satisfy
the Menu given some Steckers as shown in the stop.

But the Bombe has assumed Ring settings of ZZZ. What the codebreaker
has to do is to deduce the actual Ring settings and the remaining Stecker
connections used by the U-Boat operators to encipher the contact signals.

This gives the complete Enigma settings for the day and means all other
3 rotor Naval messages can just be decrypted, including the vital Short
Weather signals.
This simulation of a Naval Enigma shows you how to do this.

Click here to run a Naval Enigma simulation.

Follow the instructions in the right hand panel.

If you manage to get through it you will find that the settings for Naval Enigma for the day were: Wheel Order: 534, Ring settings: XMJ, Steckers: AD GB KM VT XI YR ZH NQ OL CE

So now any other U Boat messages within the 24 hrs can be just deciphered, including the vital Short Weather messages from which the entries in the new Short Weather Code Book can be re-created.

This page was originally created by the late Tony Sale
the original curator of the Bletchley Park Museum