88 JANUARY2021|COMPUTERSHOPPER|ISSUE395
keyforadaywouldconstituteonlyofthewheel
order(whichrotorsto use,andtheirorder),the
ringsettings(wherethealphabeticringoneach
rotorshouldbeplaced)andtheplugsettingsof
theplugboard.
Theoperatorof amachinewould picka
random rotororder (suchas‘XVF’)and useit
to encode asecondrotor order(such as ‘LPR’).
Thesecondrotororder was usedto encryptthe
finalmessage.Totransmit amessage,the first
rotor orderwas transmittedin plain-text (‘XVF’),
thenthe encodedsecondrotororder wassent.
Finally,thecyphertextofthe messagewas sent.
Theresultwasthat every messagewasencoded
withadifferent rotorsetting.
Turing knewfrom the Polishsystem howthe
Enigmawas wired, butasthemessage indicator
systemhad beenreplacedhe knew that he
wouldhaveto find adifferent wayofgetting
intothecode.He decided touse message cribs,
which relied on aflaw inhumanity rather thana
flawinthe encryptionsystem.
From previous messages, Turing andhis team
knew that certainstations wouldalways senda
broadcast thatstarted with thesame phrase,
suchas“Theweatherreportfortoday is...”and
with this inmind, theycouldtakethe coded text
and knowhow thefirst characters would decode.
Sincethey knewthat no letter encryptedcould
bethe same letterdecrypted,thenumberof
combinations possible wasreduced massively.
Of course, finding the initialmessage was hard –
so hardthat Bletchley Park had an entire team
of peoplededicated to finding cribs. Once acrib
had beenfound, there wasstillaneedtouse
brute-forceattack–toattempt every possible
combination of rotorsettingsand plugboard.
To do this, Turing devised the Bombe. This
electro-mechanicaldeviceacted like 36 Enigma
machines, automatically stepping through every
single possiblerotor setting on atraditional
three-rotor Enigma machine. Thekey pointhere
was that the Bombe was rewired based on the
informationretrieved from the crib, using a
‘menu’ provided by the cryptographic team.
As the Bombe worked itsway through every
permutation of rotor settings, an electrical
current would either flowornot flow through
the system, and thiswas checked by the
Bombe’s comparator unit. Using this method, it
was possible to check for alogical contradiction,
ruling out particularrotor settings; if there was
no contradiction, themachinewould stop and
therotorsettingscouldbenoteddown.These
couldthen betestedbyhandonaTypex
machinemodifiedtoworklikeanEnigma
machine.Meanwhile,theBombecouldbe
startedagain,lookingforthenextpossible
solution,untilthe codehadbeenbroken.
Gordon Welchman, anothercryptologist,
made anadjustment tothe Bombe, addinga
diagonal boardatthe rear,whichmade itmore
efficient inits attack onEngima cyphers.Codes
were being broken faster thanever, and the
informationretrievedwas morepertinent.
Admittedly, thisis anover-simplification of
how hardEnigma provedtocrack–ittook alot
of hardwork andagreatdeal of genius.Still, this
in essenceis howthe mysteriesofthe Enigma
machine were unravelled.
DEVELOPINGTHE PARK
As the successes with Enigmacontinuedto pile
up, so too didthe requirementfor morestaff
andmore roomat thePark, aswell asincreased
levels ofsecurity. Spacewasrelatively easyto
come by and huts were quicklyconstructed from
wood(andlater on,brick huts werebuilt)to
house teams ofcryptographers,each designated
with breaking adifferenttypeofEnigma code.
Hut6, which wasrun by GordonWelchman,
was designated withbreaking thearmy and air
forceEnigmamachines.Hut8,runbyAlan
Turing,wasdesignedtobreakthenavalEnigma
encryption.TheGermannavywasworriedthat
Enigmawasbeingbrokenand,asaresult,they
usedamoresecuresystemofhandingout
messageindicatorsandswitchedtousinga
four-rotorEnigmamachine–buttheymadea
crucialmistake.
Thefour-rotormachinehadalockoutwheel
onitthatmeantitcouldsendandreceive
traditionalthree-wheelEnigmamessages.Before
thefour-rotorEnigmamachinewas infull
service,aboatsentamessageusingit.Realising
whatthey’ddone,theGermanboatcrewresent
themessageusingthethree-rotorsetting.The
twomessagesenabledtheBletchleyParkstaffto
workoutthewiringofthefourthwheelwithout
everhavingseenthemachine,andbuildthe
necessaryBombetobreakthecode.
Turinghimselfwasresponsibleforbreaking
thenavalcode.However,gettingcribswas
provingdifficult,untilmilitary actionhelped
him.AcodebookcapturedonU-110gavethe
BletchleyParkteamalistoftheKurzsignalen
(ShortSignalCode),whichwhere 22 characters
longandusedtoreportsightingsofpossible
Alliedtargets.Thecodesreducedtransmission
timesand,therefore,the likelihood of being
located through direction-findingtechniques.
Then, on29th-30thOctober1942, U-5 59
was depth-charged and forcedto surface. Three
British sailors–Able Seaman ColinGrazier,
Lieutenant Francis Anthony Blair Fasson and
NAAFI canteen assistantTommy Brown –swam
to the sinking U-boatandcapturedcode books
with allthecurrentU-boatEnigma keys,plusthe
Wetterkurzschluessel(WeatherShort Code
Book). Withthese code books,Turing andhis
teamhad avital source of cribs tolookout for.
THE FIRSTCOMPUTER
While themain focusof Bletchley Parkwas
breakingEnigma, thecryptologistswerealso
taskedwithbreakingthe Lorenzcypher,whose
traffic was knownasTunny. Lorenzmachines
were farlarger andmore complex than Enigma.
Theywere teleprinters that output messages
not astextbutasaseriesofcharacters,each
composedin five-bit baudot code. Toencrypt
thedata, the Lorenzmachinegeneratedfive
pseudo-random bits, which were‘Exclusive
Shopper’sDeputy EditorJim Martin turns on
thereplicaBombe, helped by areal-life Wren
who served at Bletchley during the war
Alan Turing’s office as he left it,complete
with the tea mugchained to theradiator
