ISSUE395|COMPUTER SHOPPER|JANUARY2021 89
Ian Fleminghad along
volvement with Bletchley
rk,and its influence can be
tinthe James Bond novels
ORed’(XORed)withtheplaintext.TheXOR
operationlooksatpairsofbits,andoutputsa1
ifoneofthecharacters isa1–otherwise,the
outputis0. Therefore, 1XOR 0=1, but 1XOR
1=0. Each characteroutputintheLorenz
machinewould beXORedwiththe pseudo-
randombits,so 10010 XOR11001= 0101 1.
The important thing tobearin mind with XOR
operationsisthat XORingthe outputwiththe
pseudo-randombits reversesthe encryption,so
01011 XOR 11001= 10010.
The Lorenzmachine generated its pseudo-
randombits through aset of10pin wheels,
wherethe encryptionkeywasdeterminedbythe
startingpositionofthe wheels.Therewerefive
pin wheelsthatsteppedregularly(chi) and five
that steppedirregularly(psi). Theirregular
stepping of thepsi wheels wascontrolledbytwo
extrawheels,called motor wheels. The Lorenz
machinedouble-encryptedthe data –first
generatingafive-bitpseudo-random codefor
the chi wheels,then generating asecond five-bit
pseudo-randomcode forthe psi wheels.The
order in whichthese were applied,in encrypting
anddecrypting,wasnot important.
STATISTICALATTACK
WT Tutte,acode-breaker working at Bletchley
Park, discovered how to crack Lorenz using
statisticalmethods to breakboth parts of the
key: thepin settings forthe wheels(known as
wheelbreaking) and the wheel setting (the
starting positions of each wheels). The wheel
breaking was often done by hand,using Tutte’s
rectanglingmethod, which involvedwriting
down the bits of message in arectangle and then
analysing the results statistically for patterns
that gave awaythe pin positioning.
Once this had been determined, the starting
positions of the wheels could be workedout.
Tutte worked outthat thepsi wheels were a
weak pointinthe encryption, as they moved
irregularly; the output from these wheels was the
same for many characters.Itwas important to
start with the regularly stepping chiwheels.
Tutte alsodiscovered that you could work
on pin wheels in pairs. So, while there were 22
million possible wheel start positions, there were
just 1,271 possible starting combinations for the
first chi wheels. Duetothe way that the system
worked,guessingthefirsttwochiwheelswould
removethefirsttwoencryptedbitsthroughout
theentire message. It was then possible,using
thepartiallydecryptedcode, toanalysethe
result statistically lookingfor patterns,wherebya
character was repeatedtwicein arow –suchas
inthe English word‘look’.Thisis statisticallyvery
likelyinGerman words, too.
Assuming thefirst twowheelswereset
correctly, there were only 598combinations
ofstartingpositions for thelast twowheels.
Oncetheywere set,thisleft just 29 positions for
thethird wheel,which meant that only 1,89 8
startingcombinationswere neededto breakthe
chiwheels.Oncethiswas complete,breakingthe
psi wheels wasconsiderably easier.
The only problem thenwasto automate
theproblem, which iswhere code-breaker Max
Newmancamein.Newmanbelieved itwas
possibletobuild amachine toperformsome
ofthe calculations automatically. The initial
machines,named Heath Robinson, proved
unreliablebutwereagoodstartingpoint
forTommyFlowers,whodecidedyoucould
buildanelectronicmachine for reading
cipher text, performing the necessary
calculationsandoutputtingthe results on
aprinter –in otherwords, thefirst computer,
called Colossus.
Initially, its jobwas toopticallyread in
cypher text characters,and countrepeated
charactersonencodedmessagesfor allthe
combinations of thechi wheels, based onits
programmed wheel settings,outputtingthe
results to aprinter.Thelines with thehighest
counts aboveasetstatistical value were the
most likely chi wheel settings. These werepassed
on toto ateamthat wouldbreak the easier psi
valuesbyhand and checkchisettings manually,
decodingmessageson areplica Tunnymachine.
Operating at 5,000characters persecond,
Colossus wascapable ofworkingout thechi
settings inlessthan 30minutes. However,it
was also discoveredthat Colossuscouldbe
Ian Fleming is best knownfor hiscreation
JamesBond,but during thewar he wasin
part of theNaval Intelligence Division
and heavily involved withBletchley Park.
Fleming cameupwith several
plans to retrieve German
code books, including
Operation Ruthless, which
was never implemented.
The plan wastocapturea
German plane and fill it full
of British pilots dressed as
Germans. They’d crash the
plane intothe sea, wait for
aGermanshiptorescue
them, overpower the crew
and then take overthe
rescue boat. They’d shoot
the crew, dump them
overboard andreturnto
England with thecode books in-hand. It
soundslike aplot in aJames Bond novel.
Later in the war, Fleming was involved in
Operation Mincemeat, which put false
documentsinthehands of the Germans,
leading them astray as
tothe real focus of
D-Day.The information at
Bletchley Parkwas keyin
proving that the Germans
fell for thetrap.
Read more abouthis
work inFrom Bletchley
with Loveby Mavis Batey,
vailableatthe Park.
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inv
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FROM BLETCHLEYWITH LOVE
Ian Fleming and the code breakers
TonySale demonstrates his
workingreplicaof Colossus, the
first computer ever built
c
