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Colossus Book Cover
Quote from Financial Times
Colossus: The Secrets of Bletchley Park's Codebreaking Computers
Colossus
Colossus at wartime Bletchley Park.


[030109-31-]
B. Jack Copeland.^1


Table of Contents

  * Introduction
  * The Tunny machine
  * A sample decrypt
  * Central figures in the attack on Tunny
  * Breaking the Tunny machine
  * Turingery
  * Tutte's statistical method
  * Heath Robinson
  * Flowers, neglected pioneer of computing
  * Colossus
  * Misconceptions about Colossus
  * Postwar
  * Colossus and the modern computer

[030109-01-]
Colossus and two operators from the Women's Royal Naval Service,
Dorothy Du Boisson (left) and Elsie Booker.^2

[030109-03]
The Lorenz Schlusselzusatz (cipher attachment) was code-named 'Tunny'
by the British.^3

                             [030109-24]
An Enigma machine with its wheels, lamps and plugboard exposed. Once
 the operator had inserted the correct wheels for the day, he closed
                          the inner lid.^4

                             [030109-21]
Teleprinter code. Holes in the punched paper tape correspond to
crosses in the teleprinter code.^5

                             [030109-22]
Punched paper tape containing the letters COLOSSUS in teleprinter
code.^6

[030109-06]
The Tunny radio network of the German Army,
March 1943 - July 1944.^7
[030109-26]
The Mansion, Bletchley Park - wartime headquarters of the Government
                      Code and Cypher School.^8
[030109-27]
Tunny with its twelve encoding wheels exposed.^9

Introduction

Colossus, the first large-scale electronic computer, was used against
the German system of teleprinter encryption known at Bletchley Park
as 'Tunny'. Technologically more sophisticated than Enigma, Tunny
carried the highest grade of intelligence. From 1941 Hitler and the
German High Command relied increasingly on Tunny to protect their
communications with Army Group commanders across Europe.

Tunny messages sent by radio were first intercepted by the British in
June 1941. After a year-long struggle with the new cipher, Bletchley
Park first read current Tunny traffic in July 1942. Tunny decrypts
contained intelligence that changed the course of the war in Europe,
saving an incalculable number of lives.

The Tunny machine was manufactured by the German Lorenz company.^1
The first model bore the designation SZ40, 'SZ' standing for
'Schlusselzusatz' ('cipher attachment'). A later version, the SZ42A,
was introduced in February 1943, followed by the SZ42B in June 1944.
'40' and '42' appear to refer to years, as in 'Windows 97'.

The Tunny machine

Tunny was one of three types of teleprinter cipher machine used by
the Germans. (The North American term for 'teleprinter' is
'teletypewriter'.) At Bletchley Park (B.P.) these were given the
general cover name 'Fish'. The other members of the Fish family were
Sturgeon, the Siemens and Halske T52 Schlusselfernschreibmaschine 
('Cipher Teleprinter Machine'),^2 and the unbreakable Thrasher.^3
Thrasher was probably the Siemens T43, a one time tape machine. It
was upon Tunny that B.P. chiefly focussed.

The Tunny machine, which measured 19" by 151/2" by 17" high, was a
cipher attachment. Attached to a teleprinter, it automatically
encrypted the outgoing stream of pulses produced by the teleprinter,
or automatically decrypted incoming messages before they were
printed. (Sturgeon, on the other hand, was not an attachment but a
combined teleprinter and cipher machine.) At the sending end of a
Tunny link, the operator typed plain language (the 'plaintext' of the
message) at the teleprinter keyboard, and at the receiving end the
plaintext was printed out automatically by another teleprinter
(usually onto paper strip, resembling a telegram). The transmitted
'ciphertext' (the encrypted form of the message) was not seen by the
German operators. With the machine in 'auto' mode, many long messages
could be sent one after another--the plaintext was fed into the
teleprinter equipment on pre-punched paper tape and was encrypted and
broadcast at high speed. Enigma was clumsy by comparison. A cipher
clerk typed the plaintext at the keyboard of an Enigma machine while
an assistant painstakingly noted down the letters of the ciphertext
as they appeared one by one at the machine's lamp-board. A radio
operator then transmitted the ciphertext in the form of Morse code.
Morse code was not used with Tunny: the output of the Tunny machine,
encrypted teleprinter code, went directly to air.^4

International teleprinter code assigns a pattern of five pulses and
pauses to each character. Using the Bletchley convention of
representing a pulse by a cross and no pulse by a dot, the letter C,
for example, is *xxx*: no-pulse, pulse, pulse, pulse, no-pulse. More
examples: O is ***xx, L is *x**x, U is xxx**, and S is x*x**. (The
complete teleprinter alphabet is shown in Appendix 1: The teleprinter
alphabet.) When a message in teleprinter code is placed on paper
tape, each letter (or other keyboard character) takes the form of a
pattern of holes punched across the width of the tape. A hole
corresponds to a pulse (cross).

The first Tunny radio link, between Berlin and Athens/Salonika, went
into operation on an experimental basis in June 1941.^5 In October
1942 this experimental link closed down, and for a short time it was
thought that the Germans had abandoned the Tunny machine.^6 Later
that same month Tunny reappeared in an altered form, on a link
between Berlin and Salonika and on a new link between Konigsberg and
South Russia.^7 At the time of the allied invasion in 1944, when the
Tunny system had reached its most stable and widespread state,^8
there were 26 different links known to the British.^9 B.P. gave each
link a piscine name: Berlin-Paris was Jellyfish, Berlin-Rome was
Bream, Berlin-Copenhagen Turbot (see right-hand column). The two
central exchanges for Tunny traffic were Strausberg near Berlin for
the Western links, and Konigsberg for the Eastern links into Russia.^
10 In July 1944, the Konigsberg exchange closed and a new hub was
established for the Eastern links at Golssen, about 20 miles from the
Wehrmacht's underground command headquarters south of Berlin. During
the final stages of the war, the Tunny network became increasingly
disorganised.^11 By the time of the German surrender, the central
exchange had been transported from Berlin to Salzburg in Austria.^12

There were also fixed exchanges at some other large centres, such as
Paris.^13 Otherwise, the distant ends of the links were mobile. Each
mobile Tunny unit consisted of two trucks.^14 One carried the radio
equipment, which had to be kept well away from teleprinters for fear
of interference. The other carried the teleprinter equipment and two
Tunny machines, one for sending and one for receiving. This truck
also carried a device for punching tapes for auto transmission.
Sometimes a land line was used in preference to radio.^15 In this
case, the truck carrying the Tunnies was connected up directly to the
telephone system. (Only Tunny traffic sent by radio was intercepted
by the British.)

As with the Enigma, the heart of the Tunny machine was a system of
wheels (see right-hand column). Some or all of the wheels moved each
time the operator typed a character at the teleprinter keyboard (or
in the case of an 'auto' transmission from a pre-punched tape, each
time a new letter was read in from the tape). There were twelve
wheels in all. They stood side by side in a single row, like plates
in a dish rack. As in the case of Enigma, the rim of each wheel was
marked with numbers, visible to the operator through a window, and
somewhat like the numbers on the rotating parts of a combination
lock.

From October 1942 the operating procedure was this. Before starting
to send a message, the operator would use his thumb to turn the
wheels to a combination that he looked up in a codebook containing
one hundred or more combinations (known as the QEP book). At B.P.
this combination was called the setting for that particular message.
The wheels were supposed to be turned to a new setting at the start
of each new message (although because of operator error this did not
always occur). The operator at the receiving end, who had the same
QEP book, set the wheels of his Tunny machine to the same
combination, enabling his machine to decrypt the message
automatically as it was received. Once all the combinations in a QEP
book had been used it was replaced by a new one.

                                          T     +     N     =     M
  M     +     N     =     T
                                          *     +     *     =     *
  *     +     *     =     *
                                          *     +     *     =     *
  *     +     *     =     *
                                          *     +     x     =     x
  x     +     x     =     *
                                          *     +     x     =     x
  x     +     x     =     *
                                          x     +     *     =     x
  x     +     *     =     x

                                           Adding N to T produces M
  Adding the letter N to the
      letter M produces T

The Tunny machine encrypted each letter of the message by adding
another letter to it. (The process of adding letters together is
explained in the next paragraph.) The internal mechanism of the Tunny
machine produced its own stream of letters, known at B.P. as the
'key-stream', or simply key. Each letter of the ciphertext was
produced by adding a letter from the key-stream to the corresponding
letter of the plaintext.

The Tunny machine adds letters by adding the individual dots and
crosses that compose them. The rules that the makers of the machine
selected for dot-and-cross addition are simple. Dot plus dot is dot.
Cross plus cross is dot. Dot plus cross is cross. Cross plus dot is
cross. In short, adding two sames produces dot, and adding a mixed
pair produces cross. (Computer literati will recognise Tunny addition
as boolean XOR.)

For example, if the first letter of the plaintext happens to be M,
and the first letter of the key-stream happens to be N, then the
first letter of the ciphertext is T: adding M (**xxx) and N (**xx*)
produces T (****x).

The German engineers selected these rules for dot-and-cross addition
so that the following is always true (no matter which letters, or
other keyboard characters, are involved): adding one letter (or other
character) to another and then adding it again a second time leaves
you where you started. In symbols, (x + y) + x = y, for every pair of
keyboard characters x and y. For example, adding N to M produces T,
as we have just seen, and then adding N to T leads back to M (see
right-hand column).

[030109-09]
Thomas H. Flowers, creator of Colossus.^10

This explains how the receiver's Tunny decrypted the ciphertext. The
ciphertext was produced by adding a stream of key to the plaintext,
so by means of adding exactly the same letters of key to the
ciphertext, the receiver's machine wiped away the encryption,
exposing the plaintext again.

For example, suppose the plaintext is the single word 'COLOSSUS'. The
stream of key added to the plaintext by the sender's Tunny might be:
WZHI/NR9. These characters are added serially to the letters of
'COLOSSUS':

               C+W  O+Z  L+H  O+I  S+/  S+N  U+R  S+9.

This produces

                              XDIVSDFE

(as can be checked by using the table in Appendix 1). 'XDIVSDFE' is
transmitted over the link. The Tunny at the receiving end adds the
same letters of key to the encrypted message:

[030109-05]
Tunny. Wheels 1-5 are the psi-wheels, wheels 6 and 7 are the
motor-wheels, and wheels 8-12 are the chi-wheels.^11
[030109-23]
A wheel cam in the operative and inoperative positions.^12
ET
    The Essential Turing (Oxford University Press, 2004) gives a
 comprehensive account of the Bletchley Park codebreaking operation,
          including Turing's own description of the Bombe.
[030109-34-]
The Battle of Kursk was a turning point of the war.^13

               X+W  D+Z  I+H  V+I  S+/  D+N  F+R  E+9.

This uncovers the letters

                              COLOSSUS.

The Tunny machine in fact produces the key-stream by adding together
two other letter streams, called at B.P. the psi-stream and the chi
-stream (from the Greek letters psi (ps) and chi (kh)). The psi-stream
and the chi-stream are produced by the wheels of the Tunny machine.
Let us consider the wheels in more detail.

The twelve wheels form three groups: five psi-wheels, five
chi-wheels, and two motor wheels. Each wheel has different numbers of
cams (sometimes called 'pins') arranged evenly around its
circumference (the numbers varying from 23 to 61). The function of
the cam is to push a switch as it passes it, so that as the wheel
rotates a stream of electrical pulses is generated. The operator can
adjust the cams, sliding any that he selects sideways, so that they
become inoperative and no longer push the switch when they pass it
(see right-hand column). The wheel now causes not a uniform stream of
pulses as it turns, but a pattern of pulses and non-pulses--crosses
and dots. The arrangement of the cams around the wheel, operative or
inoperative, is called the wheel pattern.

Prior to the summer of 1944 the Germans changed the cam patterns of
the chi-wheels once every month and the cam patterns of the
psi-wheels at first quarterly, then monthly from October 1942. After
1 August 1944, wheel patterns changed daily. The changes were made
according to books of wheel patterns issued to Tunny units (different
links used different books).

It is the patterns of the cams around the wheels that produces the
chi-stream and the psi-stream. Whenever a key is pressed at the
keyboard (or a letter read in from the tape in 'auto' mode), it
causes the five chi-wheels to turn in unison, just far enough for one
cam on each wheel to pass its switch. Depending on whether or not
that cam is operative, a pulse may or may not be produced. Suppose,
for example, that the cam at the first chi-wheel's switch produces no
pulse and the cam on the second likewise produces no pulse at its
switch, but the cams on the third and fourth both produce a pulse,
and the cam on the fifth produces no pulse. Then the pattern that the
chi-wheels produce at this point in their rotation is **xx*. In other
words, the chi-stream at this point contains the letter N. The five
psi-wheels also contribute a letter (or other keyboard character) and
this is added to N to produce a character of the key-stream.

A complication in the motion of the wheels is that, although the
chi-wheels move forward by one cam every time a key is pressed at the
keyboard (or a letter arrives from the tape in auto mode, or from the
radio receiver), the psi-wheels move irregularly. The psis might all
move forward with the chis, or they might all stand still, missing an
opportunity to move. This irregular motion of the psi-wheels was
described as 'staggering' at B.P. Designed to enhance the security of
the machine, it turned out to be the crucial weakness.

Whether the psi-wheels move or not is determined by the motor wheels
(or in some versions of the machine, by the motor wheels in
conjunction with yet other complicating factors). While the psis
remain stationary, they continue to contribute the same letter to the
key. So the chis might contribute

                           ...KDUGRYMC...

and the psis might contribute

                           ...GGZZZWDD...

Here the chis have moved eight times and the psis only four.

A sample decrypt

To OKH/OP. ABT. and to OKH/Foreign Armies East, from Army Group South
IA/01, No. 411/43, signed von Weichs, General Feldmarschall, dated 25
/4:-

Comprehensive appreciation of the enemy for "Zitadelle"

In the main the appreciation of the enemy remains the same as
reported in Army Group South (Roman) IIA, No. 0477/43 of 29/3 and in
the supplementary appreciation of 15/4. [In Tunny transmissions the
word 'Roman' was used to indicate a Roman numeral; '29/3' and '15/4'
are dates.]

The main concentration, which was already then apparent on the north
flank of the Army Group in the general area
Kursk--Ssudsha--Volchansk--Ostrogoshsk, can now be clearly
recognised: a further intensification of this concentration is to be
expected as a result of the continuous heavy transport movements on
the lines Yelets--Kastornoye--Kursk, and Povorino--Svoboda and
Gryazi--Svoboda, with a probable (B% increase) ['B%' indicated an
uncertain word] in the area Valuiki--Novy Oskol--Kupyansk. At present
however it is not apparent whether the object of this concentration
is offensive or defensive. At present, (B% still) in anticipation of
a German offensive on both the Kursk and Mius Donetz fronts, the
armoured and mobile formations are still evenly distributed in
various groups behind the front as strategic reserves.

There are no signs as yet of a merging of these formations or a
transfer to the forward area (except for (Roman) II GDS [Guards]
Armoured Corps) but this could take place rapidly at any time.

According to information from a sure source the existence of the
following groups of the strategic reserve can be presumed:- A) 2
cavalry corps (III GDS and V GDS in the area north of Novocherkassk).
It can also be presumed that 1 mech [mechanised] corps (V GDS) is
being brought up to strength here. B) 1 mech corps (III GDS) in the
area (B% north) of Rowenki. C) 1 armoured corps, 1 cavalry corps and
probably 2 mech corps ((Roman) I GD Armoured, IV Cavalry, probably
(B% (Roman) I) GDS Mech and V Mech Corps) in the area north of
Voroshilovgrad. D) 2 cavalry corps ((B% IV) GDS and VII GDS) in the
area west of Starobyelsk. E) 1 mech corps, 1 cavalry corps and 2
armoured corps ((Roman) I GDS (B% Mech), (Roman) I GDS Cavalry,
(Roman) II and XXIII Armoured) in the area of Kupyansk--Svatovo. F) 3
armoured corps, 1 mech corps ((Roman) II Armoured, V GDS Armoured,
(B% XXIX) Armoured and V GDS Mech under the command of an army
(perhaps 5 Armoured Army)) in the area of Ostrogoshsk. G) 2 armoured
and 1 cavalry corps ((Roman) II GDS Armoured, III GDS Armoured and VI
GDS Cavalry) under the command of an unidentified H.Q., in the area
north of Novy Oskol.

In the event of "Zitadelle", there are at present approximately 90
enemy formations west of the line Belgorod--Kursk--Maloarkhangelsk.
The attack of the Army Group will encounter stubborn enemy resistance
in a deeply echeloned and well developed main defence zone, (with
numerous dug in tanks, strong artillery and local reserves) the main
effort of the defence being in the key sector Belgorod--Tamarovka.

In addition strong counter attacks by strategic reserves from east
and southeast are to be expected. It is impossible to forecast
whether the enemy will attempt to withdraw from a threatened
encirclement by retiring eastwards, as soon as the key sectors
[literally, 'corner-pillars'] of the bulge in the frontline at Kursk,
Belgorod and Maloarkhangelsk, have been broken through. If the enemy
throws in all strategic reserves on the Army Group front into the
Kursk battle, the following may appear on the battle field:- On day 1
and day 2, 2 armoured divisions and 1 cavalry corps. On day 3, 2 mech
and 4 armoured corps. On day 4, 1 armoured and 1 cavalry corps. On
day 5, 3 mech corps. On day 6, 3 cavalry corps. On day 6 and/or day
7, 2 cavalry corps.

Summarizing, it can be stated that the balance of evidence still
points to a defensive attitude on the part of the enemy: and this is
in fact unmistakable in the frontal sectors of the 6 Army and 1
Panzer Army. If the bringing up of further forces in the area before
the north wing of the Army Group persists and if a transfer forward
and merging of the mobile and armoured formations then takes place,
offensive intentions become more probable. In that case it is
improbable that the enemy can even then forestall our execution of
Zitadelle in the required conditions. Probably on the other hand we
must assume complete enemy preparations for defence, including the
counter attacks of his strong mot [motorised] and armoured forces,
which must be expected.^14

The right-hand column contains is a rare survivor--a word-for-word
translation of an intercepted Tunny message.^15 Dated 25 April 1943
and signed by von Weichs, Commander-in-Chief of German Army Group
South, this message was sent from the Russian front to the German
Army High Command ('OKH'--Oberkommando des Heeres). It gives an idea
of the nature and quality of the intelligence that Tunny yielded. The
enciphered message was intercepted during transmission on the 'Squid'
radio link between the headquarters of Army Group South and
Konigsberg.^17

The message concerns plans for a major German offensive in the Kursk
area codenamed 'Zitadelle'. Operation Zitadelle was Hitler's attempt
to regain the initiative on the Eastern Front following the Russian
victory at Stalingrad in February 1943. Zitadelle would turn out to
be one of the crucial battles of the war. Von Weichs' message gives a
detailed appreciation of Russian strengths and weaknesses in the
Kursk area. His appreciation reveals a considerable amount about the
intentions of the German Army. British analysts deduced from the
decrypt that Zitadelle would consist of a pincer attack on the north
and south flanks ('corner-pillars') of a bulge in the Russian
defensive line at Kursk (a line which stretched from the Gulf of
Finland in the north to the Black Sea in the south).^18 The attacking
German forces would then attempt to encircle the Russian troops
situated within the bulge.

Highly important messages such as this were conveyed directly to
Churchill, usually with a covering note by 'C', Chief of the Secret
Intelligence Service.^19 On 30 April an intelligence report based on
the content of the message, but revealing nothing about its origin,
was sent to Churchill's ally, Stalin.^20 (Ironically, however, Stalin
had a spy inside Bletchley Park: John Cairncross was sending raw
Tunny decrypts directly to Moscow by clandestine means.^21)

The Germans finally launched operation Zitadelle on 4 July 1943.^22
Naturally the German offensive came as no surprise to the
Russians--who, with over two months warning of the pincer attack, had
amassed formidable defences. The Germans threw practically every
panzer division on the Russian front into Zitadelle,^23 but to no
avail, and on 13 July Hitler called off the attack.^24 A few days
later Stalin announced in public that Hitler's plan for a summer
offensive against the Soviet Union had been 'completely frustrated'.^
25 Zitadelle--the Battle of Kursk--was a decisive turning point on the
Eastern front. The counter attack launched by the Russians during
Zitadelle developed into an advance which moved steadily westwards,
ultimately reaching Berlin in April 1945.

Central figures in the attack on Tunny

[030109-30-]
           Flowers' photo from his wartime ration book.^15

Colossus was the brainchild of Thomas H. Flowers (1905-1998). Flowers
joined the Telephone Branch of the Post Office in 1926, after an
apprenticeship at the Royal Arsenal in Woolwich (well-known for its
precision engineering). Flowers entered the Research Branch of the
Post Office at Dollis Hill in North London in 1930, achieving rapid
promotion and establishing his reputation as a brilliant and
innovative engineer. At Dollis Hill Flowers pioneered the use of
large-scale electronics, designing equipment containing more than
3000 electronic valves ('vacuum tubes' in the US). First summoned to
Bletchley Park to assist Turing in the attack on Enigma, Flowers soon
became involved in Tunny. After the war Flowers pursued his dream of
an all-electronic telephone exchange, and was closely involved with
the groundbreaking Highgate Wood exchange in London (the first
all-electronic exchange in Europe).

[030109-16]
Max Newman. Head of the Tunny-breaking section called the 'Newmanry',
    Newman was in charge of the Colossi. He went on to found the
      Computing Machine Laboratory at Manchester University.^16
[030109-11]
Colonel John Tiltman (right), with Alastair Denniston, Head of the
Government Code and Cypher School from 1919 (left), and 'Vinca'
Vincent, an expert on Italian ciphers. Tiltman achieved the first
break into Tunny.^17

[030109-17]
Alan M. Turing. Turing made numerous fundamental contributions to
code-breaking, and he is the originator of the modern
('stored-program') computer.^18

[030109-13]
William T. Tutte.^19 [030109-29]
A British recording of the start of a Tunny message (8 August
1941).The message begins //// //MAR THA99 GUSTA V99LU DWIG9 9OTTO
99BER TA99L UDWIG 99GUS TAV99 OTTO9.^20

Max H. A. Newman (1897-1984) was a leading topologist as well as a
pioneer of electronic digital computing. A Fellow of St John's
College, Cambridge, from 1923, Newman lectured Turing on mathematical
logic in 1935, launching Turing^26 on the research that led to the
'universal Turing machine', the abstract universal stored-program
computer described in Turing's 1936 paper 'On Computable Numbers'. At
the end of August 1942 Newman left Cambridge for Bletchley Park,
joining the Research Section and entering the fight against Tunny. In
1943 Newman became head of a new Tunny-breaking section known simply
as the Newmanry, home first to the experimental 'Heath Robinson'
machine and subsequently to Colossus. By April 1945 there were ten
Colossi working round the clock in the Newmanry. The war over, Newman
took up the Fielden Chair of Mathematics at the University of 
Manchester and--inspired both by Colossus and by Turing's abstract
'universal machine'--lost no time in establishing a facility to build
an electronic stored-program computer. On 21 June 1948, in Newman's
Computing Machine Laboratory, the world's first electronic
stored-program digital computer, the Manchester 'Baby', ran its first
program.

John Tiltman (1894-1982) was seconded to the Government Code and
Cypher School (GC & CS) from the British army in 1920, in order to
assist with Russian diplomatic traffic.^27 An instant success as a
codebreaker, Tiltman never returned to ordinary army duties. From
1933 onwards he made a series of major breakthroughs against Japanese
military ciphers, and in the early years of the war he also broke a
number of German ciphers, including the army's double Playfair
system, and the version of Enigma used by the German railway
authorities. In 1941 Tiltman made the first significant break into
Tunny. Promoted to Brigadier in 1944, he went on to become a leading
member of GCHQ, GC & CS's peacetime successor. Following his
retirement from GCHQ in 1964, Tiltman joined the National Security
Agency, where he worked until 1980.

Alan M. Turing (1912-1954) was elected a Fellow of King's College,
Cambridge in 1935, at the age of only 22. 'On Computable Numbers',
published the following year, was his most important theoretical
work. It is often said that all modern computers are Turing machines
in hardware: in a single article, Turing ushered in both the modern
computer and the mathematical study of the uncomputable. During the
early stages of the war, Turing broke German Naval Enigma and
produced the logical design of the 'Bombe', an electro-mechanical
code-breaking machine. Hundreds of Bombes formed the basis of
Bletchley Park's factory-style attack on Enigma. Turing briefly
joined the attack on Tunny in 1942, contributing a fundamentally
important cryptanalytical method known simply as 'Turingery'. In
1945, inspired by his knowledge of Colossus, Turing designed an
electronic stored-program digital computer, the Automatic Computing
Engine (ACE). At Bletchley Park, and subsequently, Turing pioneered
Artificial Intelligence: while the rest of the post-war world was
just waking up to the idea that electronics was the new way to do
binary arithmetic, Turing was talking very seriously about
programming digital computers to think. He also pioneered the
discipline now known as Artificial Life, using the Ferranti Mark I
computer at Manchester University to model biological growth.^28

William T. Tutte (1917-2002) specialised in chemistry in his
undergraduate work at Trinity College, Cambridge, but was soon
attracted to mathematics. He was recruited to Bletchley Park early in
1941, joining the Research Section. Tutte worked first on the Hagelin
cipher machine and in October 1941 was introduced to Tunny. Tutte's
work on Tunny, which included deducing the structure of the Tunny
machine, can be likened in importance to Turing's earlier work on
Enigma. At the end of the war, Tutte was elected to a Research
Fellowship in mathematics at Trinity; he went on to found the area of
mathematics now called graph theory.

Breaking the Tunny machine

From time to time German operators used the same wheel settings for
two different messages, a circumstance called a depth. It was thanks
to the interception of depths, in the summer of 1941, that the
Research Section at B.P. first found its way into Tunny.

Prior to October 1942, when QEP books were introduced, the sending
operator informed the receiver of the starting positions of the 12
wheels by transmitting an unenciphered group of 12 letters. The first
letter of the 12 gave the starting position of the first psi-wheel,
and so on for the rest of the wheels. For example, if the first
letter was 'M' then the receiver would know from the standing
instructions for the month to set his first psi-wheel to position 31,
say. At B.P. this group of letters was referred to as the message's
indicator. Sometimes the sending operator would expand the 12 letters
of the indicator into 12 unenciphered names: Martha Gustav Ludwig
Otto... instead of MGLO..., for example (see right-hand column). The
occurrence of two messages with the same indicator was the tell-tale
sign of a depth.

1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30
J  S  H  5  N  Z  Y  M  F  S   0   1   1   5   I   V   K   U   1   Y   U   4   N   C   E   J   E   G   P   B
J  S  H  5  N  Z  Y  Z  Y  5   G   L   F   R   G   X   O   5   S   Q   5   D   A   1   J   J   H   D   5   0
0  0  0  0  0  0  0  f  o  u   g   f   1   4   m   a   q   s   g   5   s   e   k   z   r   0   y   w   h   e
31 32 33 34 35 36 37 38 39 40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60
M  N  T  Q  M  A  0  U  4  Y   L   1   Q   I   J   L   Y   V   I   N   U   B   2   3   R   5   W   E   V   G
B  K  S  U  C  B  T  T  O  5   E   4   T   S   L   E   3   F   G   Z   Y   U   H   V   H   3   H   E   E   0
s  a  y  t  l  g  t  q  t  q   w   q   u   a   b   w   c   w   m   x   l   v   t   s   v   b   u   0   1   g
61 62 63 64 65 66 67 68 69 70  71  72  73  74  75  76  77  78  79  80  81  82  83  84  85  86  87  88  89  90
Q  I  2  4  5  G  R  J  M  L   C   Y   5   0   H   K   A   S   1   I   S   5   X   U   N   S   R   Z   Z   B
T  G  2  H  H  1  Q  J  X  V   K   1   B   J   M   K   2   O   M   Z   Y   V   I   N   3   H   M   C   3   D
u  m  0  m  p  s  x  0  e  n   e   r   3   j   4   0   u   x   a   q   t   m   3   j   q   z   p   1   r   t
91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120
D  B  B  1  C  L  S  Q  H  H   U   H   5   X   D   0   F   N   3   J   3   V   O   C   A   D   J   C   D   N
U  Q  3  4  Z  R  2  M  R  M   O   H   5   J   Q   P   W   U   E   Y   C   P   R   G   1   L   D   A   T   I
c  c  5  q  1  o  e  j  v  4   1   0   0   p   v   p   v   j   g   v   y   4   1   h   m   3   5   f   b   r

The first 120 characters of the two transmissions attacked by
Tiltman. The letters shown in green are the result of 'cancelling
out' the key by adding the two transmissions together.^21

So when on 30 August 1941 two messages with the same indicator were
intercepted, B.P. suspected that they had found a depth. As it turned
out, the first transmission had been corrupted by atmospheric noise,
and the message was resent at the request of the receiving operator.
Had the sender repeated the message identically, the use of the same
wheel settings would have left B.P. none the wiser. However, in the
course of the second transmission the sender introduced abbreviations
and other minor deviations (the message was approximately 4000
characters long). So the depth consisted of two not-quite-identical
plaintexts each encrypted by means of exactly the same sequence of
key--a codebreaker's dream.

On the hypothesis that the machine had produced the ciphertext by
adding a stream of key to the plaintext, Tiltman added the two
ciphertexts (see right-hand column). If the hypothesis were correct,
this would have the effect of cancelling out the key (since, as
previously mentioned, ((x + y) + x) = y). The resulting string of
approximately 4000 characters would consist of the two plaintexts
summed together character by character. (This is because 
(K + P) + (K + P) = ((K + P) + K) + P = P + P, where K is the key, P
is the plaintext, and K + P is the ciphertext.)

[030109-07]
Tutte deduced the design of the Tunny machine from the pair of
intercepts shown above.^22

[030109-33]
Alan Turing. 'We might have lost the war without him.'^23
[030109-32]
The stricken U-110 shortly after depth charges blasted it to the
surface. A Royal Navy boarding party captured the U-110's Enigma
machine.^24

Tiltman managed to prise the two individual plaintexts out of this
string (it took him ten days). He had to guess at words of each
message, and Tiltman was a very good guesser. Each time he guessed a
word from one message, he added it to the characters at the right
place in the string, and if the guess was correct an intelligible
fragment of the second message would pop out. For example, adding the
probable word 'geheim' (secret) to characters 83-88 of the string
reveals the plausible fragment 'eratta'.^29 This short break can then
be extended to the left and right. More letters of the second message
are obtained by guessing that 'eratta' is part of 'militaerattache'
(military attache), and if these letters are added to their
counterparts in the string, further letters of the first message are
revealed. And so on. Eventually Tiltman achieved enough of these
local breaks to realise that long stretches of each message were the
same, and so was able to decrypt the whole thing.

Adding the plaintext deduced by Tiltman to its corresponding
ciphertext revealed the sequence of key used to encrypt the messages.
These 4000 characters of key were passed to Tutte and, in January
1942, Tutte single-handedly deduced the fundamental structure of the
Tunny machine. He focussed on just one of the five 'slices' of the
key-stream, the top-most row were the key-stream to be punched on
tape. Each of these five slices was called an 'impulse' at B.P. (In
the 'Colossus' punched tape shown earlier, the first impulse is ****
xxxx, the second is x*x***x*, and so on.)

The top-most impulse of the key-stream, Tutte managed to deduce, was
the result of adding two streams of dots and crosses. The two streams
were produced by a pair of wheels, which he called 'chi' and 'psi'.
The chi-wheel, he determined, always moved forward one place from one
letter of text to the next, and the psi-wheel sometimes moved
forwards and sometimes stayed still. It was a remarkable feat of
cryptanalysis. At this stage the rest of the Research Section joined
in and soon the whole machine was laid bare, without any of them ever
having set eyes on one.

Turingery

Now that Bletchley knew the nature of the machine, the next step was
to devise methods for breaking the daily traffic. A message could be
read if the wheel settings and the wheel patterns were known. The
German operators themselves were revealing each message's setting via
the 12-letter indicator. Thanks to Tutte's feat of
reverse-engineering, the wheel patterns were known for August 1941.
The codebreaker's problem was to keep on top of the German's regular
changes of wheel-pattern.

In July 1942 Turing invented a method for finding wheel-patterns from
depths--'Turingery'. Turing was at that time on loan to the Research
Section from Hut 8 and the struggle against Naval Enigma.^30
Turingery was the third of the three strokes of genius that Turing
contributed to the attack on the German codes, along with his design
for the Bombe and his unravelling of the form of Enigma used by the
Atlantic U-boats.^31 As fellow codebreaker Jack Good observed, 'I
won't say that what Turing did made us win the war, but I daresay we
might have lost it without him'.^32

Turingery was a hand method, involving paper, pencil and eraser.
Beginning with a stretch of key obtained from a depth, Turingery
enabled the breaker to prize out from the key the contribution that
the chi-wheels had made. The cam-patterns of the individual
chi-wheels could be inferred from this. Further deductions led to the
cam-patterns of the psi- and motor-wheels. Once gained via Turingery,
this information remained current over the course of many messages.
Eventually the patterns were changed too frequently for any hand
method to be able to cope (there were daily changes of all patterns
from August 1944), but by that time Colossus, not Turingery, was
being used for breaking the wheel patterns.

 M   N   T        M + N              N + T

 *   *   *          *                  *

 *   *   *          *                  *

 x   x   *          *                  x

 x   x   *          *                  x

 x   *   x          x                  x

                            DMNT                 
Forming the delta of 'MNT' by adding adjacent letters.^25

Basic to Turingery was the idea of forming the delta of a stream of
characters. (Delta-ing a character-stream was also called
'differencing' the stream.) The delta of a character-stream is the
stream that results from adding together each pair of adjacent
letters in the original stream. For example, the delta of the short
stream MNT (sometimes written DMNT) is produced by adding M to N and
N to T (using the rules of dot-and-cross addition explained
previously). The delta of MNT is in fact TM, as the table in the
right-hand column shows (the shaded columns contain the delta).

[030109-35]
'Delta-ing' and 'Turingery' were Turing's fundamental contributions
to the attack on Tunny.^26
[030109-02]
For the full story of Colossus see Colossus: The Secrets of Bletchley
Park's Codebreaking Computers (Oxford University Press, 2006 & 2010).
 Contains 2 chapters by Flowers and first-hand accounts by 17 of the
                    Bletchley Park codebreakers.
ACE_smallfile
   For more information about the ACE, see Alan Turing's Automatic
  Computing Engine: The Master Codebreaker's Struggle to Build the
Modern Computer (Oxford University Press, 2005). Contains material by
Turing and his contemporaries. Available online at Oxford Scholarship
                               Online.

The idea of the delta is that it tracks changes in the original
stream. If a dot follows a dot or a cross follows a cross at a
particular point in the original stream, then the corresponding point
in the delta has a dot (see the table). A dot in the delta means 'no
change'. When, on the other hand, there is a cross followed by a dot
or a dot followed by a cross in the original stream, then the
corresponding point in the delta has a cross. A cross in the delta
means 'change'. Turing introduced the concept of delta in July 1942,
observing that by delta-ing a stretch of key he was able to make
deductions which could not be made from the key in its un-deltaed
form.^33

Turingery worked on deltaed key to produce the deltaed contribution
of the chi-wheels. Turing's discovery that delta-ing would reveal
information otherwise hidden was essential to the developments that
followed. The algorithms implemented in Colossus (and in its
precursor Heath Robinson) depended on this simple but brilliant
observation. In that sense, the entire machine-based attack on Tunny
flowed from this fundamental insight of Turing's.

How did Turingery work? The method exploited the fact that each
impulse of the chi-stream (and also its delta-ed form) consists of a
pattern that repeats after a fixed number of steps. Since the number
of cams on the 1st chi-wheel is 41, the pattern in the first impulse
of the chi-stream repeats every 41 steps. In the 2nd impulse the
pattern repeats every 31 steps--the number of cams on the 2nd
chi-wheel--and for the 3rd, 4th and 5th impulses, the wheels have 29,
26, and 23 cams respectively. Therefore a hypothesis about the
identity, dot or cross, of a particular bit in, say, the first
impulse of the chi will, if correct, also produce the correct bit 41
steps further on, and another 41 steps beyond that, and so on. Given
500 letters of key, a hypothesis about the identity of a single
letter of the chi (or delta-ed chi) will yield approximately 500/41
bits of the first impulse, 500/31 bits of the second impulse, 500/29
bits of the third, and so on--a total of about 85 bits.

In outline, Turing's method is this. The first step is to make a
guess: the breaker guesses a point in the delta-ed key at which the
psi-wheels stayed still in the course of their 'staggering' motion.
Whatever guess is made, it has a 50% chance of being right. Positions
where the psis did not move are of great interest to the breaker,
since at these positions the deltaed key and the deltaed chi are
identical. (The reason for this is that the deltaed contribution of
the psis at such positions is *****, and adding ***** to a letter
does not alter the letter.) Because the key is known, the letter of
the deltaed chi at the guessed position is also known--assuming, of
course, that the guess about the psis not having moved is correct.
Given this single letter of the deltaed chi, a number of bits can
then be filled in throughout the five impulses, by propagating to the
left and right at the appropriate periods.

Now that various bits of the delta chi are filled in, guesses can be
made as to the identity of others letters. For example, if one letter
of the delta chi is *???* and the corresponding letter of the delta
key is *xxx* (C), the breaker may guess that this is another point at
which the psis stood still, and replace *???* in the delta chi by *
xxx*. This gives three new bits to propagate left and right. And so
the process continues, with more and more bits of the delta chi being
written in.

Naturally the breaker's guesses are not always correct, and as the
process of filling in bits goes on, any incorrect guesses will tend
to produce clashes--places where both a cross and a dot are assigned
to the same position in the impulse. Guesses that are swamped by
clashes have to be revised. With patience, luck, a lot of rubbing
out, and a lot of cycling back and forth between putative fragments
of delta chi and delta psi, a correct and complete stretch of delta
chi eventually emerges.

Tutte's statistical method

[030109-38]
Ralph Tester - head of the Tunny-breaking section called the
'Testery'.^27



[030109-10-]

Newman.^28



[030109-19]
A British 'Tunny machine', used in the deciphering process. The racks
of electrical equipment imitated the actions of the German Tunny.^29



[030109-44]
Machines in the Newmanry at Bletchley Park for processing punched
tape.^30



[030109-39]
The intercept station at Flowerdown.^31



[030109-40]
Jerry Roberts, leading codebreaker in the Testery.^32



Tunny could now be tackled operationally, and a Tunny-breaking
section was immediately set up under Major Ralph Tester.^34 Several
members of the Research Section moved over to the 'Testery'. Armed
with Turingery and other hand methods, the Testery read nearly every
message from July to October 1942--thanks to the insecure 12-letter
indicator system, by means of which the German operator obligingly
conveyed the wheel setting to the codebreakers.^35 In October,
however, the indicators were replaced by numbers from the QEP books,
and the Testery, now completely reliant on depths, fell on leaner
times. With the tightening up of German security, depths were
becoming increasingly scarce. The Research Section renewed its
efforts against Tunny, looking for a means of finding wheel settings
that did not depend on depths.^36

In November 1942 Tutte invented a way of discovering the settings of
messages not in depth. This became known as the 'Statistical Method'.
The rub was that at first Tutte's method seemed impractical. It
involved calculations which, if done by hand, would consume a vast
amount of time--probably as much as several hundred years for a
single, long message, Newman once estimated.^37

The necessary calculations were straightforward enough, consisting
basically of comparing two streams made up of dots and crosses, and
counting the number of times that each had a dot, or cross, in the
same position. Today, of course, we turn such work over to electronic
computers. When Tutte shyly explained his method to Newman, Newman
suggested using high-speed electronic counters to mechanise the
process. It was a brilliant idea. Within a surprisingly short time a
factory of monstrous electronic computers dedicated to breaking Tunny
was affording a glimpse of the future.

Electronic counters had been developed in Cambridge before the war.
Used for counting emissions of sub-atomic particles, these had been
designed by C. E. Wynn-Williams, a Cambridge don.^38 Newman knew of
Wynn-Williams' work, and in a moment of inspiration he saw that the
same idea could be applied to the Tunny problem. Within a month of
Tutte's inventing his statistical method Newman began developing the
necessary machine. He worked out the cryptanalytical requirements for
the machine and called in Wynn-Williams to design the electronic
counters. Construction of Newman's machine started in January 1943
and a prototype began operating in June of that year, in the newly
formed Tunny-breaking section called the 'Newmanry'. The prototype
machine was soon dubbed 'Heath Robinson', after the famous cartoonist
who drew overly-ingenious mechanical contrivances.

Tutte's method delivered the settings of the chi wheels. Once the
Newmanry had discovered the settings of the chis by machine, the
contribution that the chis had made to the ciphertext was stripped
away, producing what was called the 'de-chi' of the message. The
de-chi was made by a replica of the Tunny machine, designed by
Flowers' Post Office engineers at Dollis Hill. The de-chi was then
passed to the Testery, where a cryptanalyst would break into it by
'ordinary' pencil-and-paper methods requiring only (as a wartime
document described it) 'the power of instantaneous mental addition of
letters of the Teleprint alphabet'.^39

The reason it was feasible to break the de-chi by hand was that the
staggering motion of the psi-wheels introduced local regularities.
Once the contribution of the chis had been stripped out of the key,
what remained of the key contained distinctive patterns of repeated
letters, e.g. ...GGZZZWDD..., since while the psis stood still they
continued to contribute the same letter. By latching onto these
repetitions, the cryptanalyst could uncover some stretches of this
residual key, and this in turn enabled the settings of the psi-wheels
and the motor-wheels to be deduced. For example, adding the guessed
word 'dringend' ('urgent') to the de-chi near the beginning of the
message might produce 888EE00WW--pure gold, confirming the guess. With
luck, once a break was achieved it could be extended to the left or
right, in this case perhaps by trying on the left 'sehr9' ('very'
followed by a space), and on the right ++M88, the code for a full
stop (see Appendix 1). Once the codebreaker had a short stretch of
the key that the psi-wheels had contributed, the wheel settings could
usually be obtained by comparing the key to the known wheel patterns.
When all the wheel-settings were known, the ciphertext was keyed into
one of the Testery's replica Tunny machines, and the German plaintext
would emerge.

In order to illustrate the basic ideas of Tutte's method for finding
the settings of the chi wheels, let us assume that we have an
intercepted ciphertext 10,000 characters long. This ciphertext is
punched on a tape (we call this the 'message-tape'). An assistant,
who knows the chi-wheel patterns, provides us with a second tape (the
'chi-tape'). This assistant has worked out the machine's entire
chi-stream, beginning at an arbitrarily selected point in the
revolution of the chi-wheels, and stepping through all their possible
joint combinations. (Once the wheels have moved through all the
possible combinations, their capacity for novelty is exhausted, and
should the wheels continue to turn they merely duplicate what has
gone before.) The complete chi-stream is, of course, rather long, but
eventually the assistant does produce a roll of tape with the stream
punched on it. The sequence of 10,000 consecutive characters of
chi-stream that was used to encrypt our message is on this tape
somewhere--our problem is to find it. This sequence is called simply
'the chi' of the message. Tutte's method exploits a fatal weakness in
the design of the Tunny machine, a weakness again stemming from the
staggering motion of the psi-wheels. The central idea of the method
is this: The chi is recognisable on the basis of the ciphertext,
provided the wheel patterns are known. Tutte showed by a clever
mathematical deduction that the delta of the ciphertext and the delta
of the chi would usually correspond slightly. That slightly is the
key to the whole business--any degree of regularity, no matter how
weak, is the cryptanalyst's friend. The slight regularity that Tutte
discovered could be used as a touchstone for finding the chi.
(Readers interested in Tutte's mathematical reasoning will find the
details in Appendix 2: The Tunny encipherment equation and Tutte's 1
+ 2 break-in. At present we will concentrate on how the method is
carried out.)

We select the first 10,000 characters of the chi-tape; we will
compare this stretch of the chi-tape with the message-tape. Tutte
showed that in fact we need examine only the first and the second of
the five horizontal rows punched along the chi-tape, the first and
second impulses (these two rows are the contributions of the first
and second chi-wheels respectively). Accordingly we need consider
only the first and second impulses of the message-tape. This
simplifies considerably the task of comparing the two tapes. Because
Tutte's method focussed on the first and second chi-wheels it was
dubbed the '1+2 break in'.^40

Here is the procedure for comparing the message-tape with the stretch
of chi-tape we have picked. First, we add the first and second
impulses of the message-tape and form the delta of the resulting
sequence of dots and crosses. (For example, if the sequence produced
by adding the two impulses begins x*x..., the delta begins xx... .)
Second, we add the first and second impulses of the 10,000-character
piece of chi-tape, and again form the delta of the result. Next we
lay these two deltas side by side and count how many times they have
dots in the same places and how many times crosses. We add the two
tallies to produce a total score for this particular piece of the
chi-tape. We are looking for a match between the two deltas of around
55%. Tutte showed that this is the order of correspondence that can
be expected when the piece of chi-tape under examination contains the
first and second impulses of the actual chi.

[030109-20]

Stepping the ciphertext through the chi-stream, looking for the
starting position of the chi-wheels.^33

The first score we obtain probably won't be anything special--for we
would be extremely lucky if the first 10,000 characters of chi-stream
that we examined were the chi of the message. So next we shift along
one character in the chi-stream and focus on a new candidate for the
message's chi, the 2nd through to the 10,001st characters on the
chi-tape (see the diagram in the right-hand column). We add, delta,
and count once again. Then we shift along another character,
repeating the process until we have examined all candidates for the
chi. A buoyant score reveals the first and second impulses of the
actual chi (we hope).

Once a winning segment of the chi-tape has been located, its place
within the complete chi-stream tells us the positions of the first
and second chi-wheels at the start of the message. With these
settings in hand, a similar procedure is used to chase the settings
of the other chi-wheels.

[030109-43]
Bill Tutte.^34

As mentioned previously, the cause of the slight regularity that
Tutte latched onto is at bottom the staggering movement of the
psi-wheels--the great weakness of the Tunny machine. While the psis
remained stationary, they continued to contribute the same letter to
the key; and so, since delta-ing tracks change, the delta of the
stream of characters contributed by the psis contained more dots than
crosses (recall that a cross in the delta indicates a change). Tutte
calculated that there would usually be about 70% dot in the delta of
the sum of the contributions of the first two psi-wheels.

The delta of the plaintext also contained more dots than crosses (for
reasons explained in Appendix 2, which included the fact that Tunny
operators habitually repeated certain characters). Tutte investigated
a number of broken messages and discovered to his delight that the
delta of the sum of the first two impulses was as a rule about 60%
dot. Since these statistical regularities in the delta of the psi and
the delta of the plain both involved a predominance of dot over
cross, they tended to reinforce one another. Tutte deduced that their
net effect, in favourable cases, would be the agreement, noted above,
of about 55% between the processed ciphertext and the processed chi.

[030109-42]
This machine, eventually called 'Old Robinson', replaced the original
Heath Robinson (the two were of similar appearance). To the left are
    the two large metal frames called 'bedsteads', which held the
 tape-drive mechanism, the photo-electric readers, and the two tapes
supported by pulleys. One tape contained the ciphertext and the other
held impulses from the chi-wheels of the Tunny machine. To the right
      are the 'combining unit' and the electronic counters.^35
[030109-45]
The Telecommunications Research Establishment (TRE) in Malvern.^36

[030109-14]
Flowers lecturing at the National Physical Laboratory in 1977.^37

[030109-46]
A relay.^38

[030109-47]
Some of the types of electronic valves used in Colossus. On the far
right is a photo-cell from the tape reader.^39


[030109-41]
The Post Office Research Station at Dollis Hill, London. Here Flowers
pioneered digital electronics and built Colossus.^40


[030109-18]
Alan Turing. In 2009, the British government apologised for the way
Britain treated Turing in the years after the war.^41


[030109-48]
The Bombe. Turing's Bombes turned Bletchley Park into a codebreaking
                             factory.^42

[030109-49]
 Colossus. In the foreground is the automatic typewriter for output.
The large frames to the right held two message tapes. As one job was
    being run, the tape for the next job would be loaded onto the
 pulleys, so saving time. Using a switch on the selection panel, the
    operator chose to run either the 'near' or the 'far' tape.^43

[030109-50]
Side view of Colossus VII. The four large boxes on the rear frame are
the power supply units.^44
[030109-51]
Colossus, from a sketch by Flowers.^45
[030109-52]
Flowers' diary. 16 January: 'Made Colossus work'. 18 January:
'Colossus delivered to B.P.'^46

[030109-53]
Flowers' diary, 5 February: 'Colossus did its first job. Car broke
down on way home.'^47

[030109-57]
Colossus V, back view. The racks of valves on the right simulated the
movements of the Tunny machine's wheels.^48

[030109-58]
Some of the controls on Colossus VI.^49
[030109-59]
Donald Michie.^50
[030109-73]
W. W. Chandler.^51
[030109-55]
The D-day landing sites on the beaches of Normandy.^52
[030109-60]
Approaching Omaha beach on D-day.^53 [030109-63-]
The D-day landings.^54
[030109-61]
Sturgeon.^55

Tunny's security depended on the appearance of randomness, and here
was a crack in the appearance. The British seized on it. If, instead
of the psi-wheels either all moving together or all standing still,
the designers had arranged for them to move independently--or even to
move regularly like the chis--then the chink that let Tutte in would
not have existed.

Heath Robinson

Smoke rose from Newman's prototype machine the first time it was
switched on (a large resistor overloaded). Around a vast frame made
of angle-iron wound two long loops of teleprinter tape (see photo).
Resembling an old-fashioned bed standing on end, the frame quickly
became known as the 'bedstead'. The tapes were supported by a system
of pulleys and wooden wheels of diameter about ten inches. Each tape
was driven by a toothed sprocket-wheel which engaged a continuous row
of sprocket-holes along the centre of the tape (see previous diagram
). The tapes were driven by the same drive-shaft and moved in
synchronisation with each other at a maximum speed of 2000 characters
per second. To the amusement and annoyance of Heath Robinson's
operators, tapes would sometimes tear or come unglued, flying off the
bedstead at high speed and breaking into fragments which festooned
the Newmanry.

One tape was the message-tape and the other the chi-tape. In practice
the chi-tape might contain, for example, only the first and second
impulses of the complete chi-stream, resulting in a shorter tape. The
drive mechanism was arranged so that as the tapes ran on the
bedstead, the message-tape stepped through the chi-tape one character
at a time (see previous diagram). Photo-electric readers mounted on
the bedstead converted the hole/no-hole patterns punched on the tapes
into streams of electrical pulses, and these were routed to a
'combining unit'--a logic unit, in modern terminology. The combining
unit did the adding and the delta-ing, and Wynn-Williams' electronic
counters produced the scores. The way the combining was done could be
varied by means of replugging cables, a primitive form of
programming. The combining unit, the bedstead and the photo-electric
readers were made by Post Office engineers at Dollis Hill and the
counters by Wynn-Williams' unit at the Telecommunications Research
Establishment (TRE).^41

Heath Robinson worked, proving in a single stroke that Newman's idea
of attacking Tunny by machine was worth its salt and that Tutte's
method succeeded in practice. However, Heath Robinson suffered from
'intolerable handicaps'.^42 Despite the high speed of the electronic
counters, Heath Robinson was not really fast enough for the
codebreakers' requirements, taking several hours to elucidate a
single message.^43 Moreover, the counters were not fully
reliable--Heath Robinson was prone to deliver different results if set
the same problem twice. Mistakes made in hand-punching the two tapes
were another fertile source of error, the long chi-tape being
especially difficult to prepare. At first, undetected tape errors
prevented Heath Robinson from obtaining any results at all.^44 And
paramount among the difficulties was that the two tapes would get out
of synchronisation with each other as they span, throwing the
calculations out completely. The loss of synchronisation was caused
by the tapes stretching, and also by uneven wear around the sprocket
holes.

The question was how to build a better machine--a question for an
engineer. In a stroke of genius, the electronics expert Thomas
Flowers solved all these problems.

Flowers, neglected pioneer of computing

During the 1930s Flowers pioneered the large-scale use of electronic
valves to control the making and breaking of telephone connections.^
45 He was swimming against the current. Many regarded the idea of
large-scale electronic equipment with scepticism. The common wisdom
was that valves--which, like light bulbs, contained a hot glowing
filament--could never be used satisfactorily in large numbers, for
they were unreliable, and in a large installation too many would fail
in too short a time. However, this opinion was based on experience
with equipment that was switched on and off frequently--radio
receivers, radar, and the like. What Flowers discovered was that, so
long as valves were switched on and left on, they could operate
reliably for very long periods, especially if their 'heaters' were
run on a reduced current.

At that time, telephone switchboard equipment was based on the relay.
A relay is a small, automatic switch. It contains a mechanical
contact-breaker--a moving metal rod that opens and closes an
electrical circuit. The rod is moved from the 'off' position to the
'on' position by a magnetic field. A current in a coil is used to
produce the magnetic field; as soon as the current flows, the field
moves the rod. When the current ceases, a spring pushes the rod back
to the 'off' position. Flowers recognised that equipment based
instead on the electronic valve--whose only moving part is a beam of
electrons--not only had the potential to operate very much faster than
relay-based equipment, but was in fact potentially more reliable,
since valves are not prone to mechanical wear.

In 1934 Flowers wired together an experimental installation
containing three to four thousand valves (by contrast, Wynn-Williams'
electronic counters of 1931 contained only three or four valves).
This equipment was for controlling connections between telephone
exchanges by means of tones, like today's touch-tones (a thousand
telephone lines were controlled, each line having 3-4 valves attached
to its end). Flowers' design was accepted by the Post Office and the
equipment went into limited operation in 1939. Flowers had proved
that an installation containing thousands of valves would operate
very reliably--but this equipment was a far cry from Colossus. The
handful of valves attached to each telephone line formed a simple
unit, operating independently of the other valves in the
installation, whereas in Colossus large numbers of valves worked in
concert.

During the same period before the war Flowers explored the idea of
using valves as high-speed switches. Valves were used originally for
purposes such as amplifying radio signals. The output would vary
continuously in proportion to a continuously varying input, for
example a signal representing speech. Digital computation imposes
different requirements. What is needed for the purpose of
representing the two binary digits, 1 and 0, is not a continuously
varying signal but plain 'on' and 'off' (or 'high' and 'low'). It was
the novel idea of using the valve as a very fast switch, producing
pulses of current (pulse for 1, no pulse for 0) that was the route to
high-speed digital computation. During 1938-9 Flowers worked on an
experimental high-speed electronic data store embodying this idea.
The store was intended to replace relay-based data stores in
telephone exchanges. Flowers' long-term goal was that electronic
equipment should replace all the relay-based systems in telephone
exchanges.

By the time of the outbreak of war with Germany, only a small number
of electrical engineers were familiar with the use of valves as
high-speed digital switches. Thanks to his pre-war research, Flowers
was (as he himself remarked) possibly the only person in Britain who
realized that valves could be used reliably on a large scale for
high-speed digital computing.^46 When Flowers was summoned to
Bletchley Park--ironically, because of his knowledge of relays--he
turned out to be the right man in the right place at the right time.

Turing, working on Enigma, had approached Dollis Hill to build a
relay-based decoding machine to operate in conjunction with the Bombe
(the Bombe itself was also relay-based). Once the Bombe had uncovered
the Enigma settings used to encrypt a particular message, these
settings were to be transferred to the machine requisitioned by
Turing, which would automatically decipher the message and print out
the German plaintext.^47 Dollis Hill sent Flowers to Bletchley Park.
He would soon become one of the great figures of World War II
codebreaking. In the end, the machine Flowers built for Turing was
not used, but Turing was impressed with Flowers, who began thinking
about an electronic Bombe, although he did not get far. When the
teleprinter group at Dollis Hill ran into difficulties with the
design of the Heath Robinson's combining unit, Turing suggested that
Flowers be called in. (Flowers was head of the switching group at
Dollis Hill, located in the same building as the teleprinter group.)
Flowers and his switching group improved the design of the combining
unit and manufactured it.^48

Flowers did not think much of the Robinson, however. The basic design
had been settled before he was called in and he was sceptical as soon
as Morrell, head of the teleprinter group, first told him about it.
The difficulty of keeping two paper tapes in synchronisation at high
speed was a conspicuous weakness. So was the use of a mixture of
valves and relays in the counters, because the relays slowed
everything down: Heath Robinson was built mainly from relays and
contained no more than a couple of dozen valves. Flowers doubted that
the Robinson would work properly and in February 1943 he presented
Newman with the alternative of a fully electronic machine able to
generate the chi-stream (and psi- and motor-streams) internally.^49

Flowers' suggestion was received with 'incredulity' at TRE and
Bletchley Park.^50 It was thought that a machine containing the
number of valves that Flowers was proposing (between one and two
thousand) 'would be too unreliable to do useful work'.^51 In any
case, there was the question of how long the development process
would take--it was felt that the war might be over before Flowers'
machine was finished. Newman pressed ahead with the two-tape machine.
He offered Flowers some encouragement but effectively left him to do
as he wished with his proposal for an all-electronic machine. Once
Heath Robinson was a going concern, Newman placed an order with the
Post Office for a dozen more relay-based two-tape machines (it being
clear, given the quantity and very high importance of Tunny traffic,
that one or two machines would not be anywhere near enough). 
Meanwhile Flowers, on his own initiative and working independently at
Dollis Hill, began building the fully electronic machine that he
could see was necessary. He embarked on Colossus, he said, 'in the
face of scepticism'^52 from Bletchley Park and 'without the
concurrence of BP'.^53 'BP weren't interested until they saw it
[Colossus] working', he recollected.^54 Fortunately, the Director of
the Dollis Hill Research Station, Gordon Radley, had greater faith in
Flowers and his ideas, and placed 'the whole resources of the
laboratories' at Flowers' disposal.^55

Colossus

The prototype Colossus was brought to Bletchley Park in lorries and
reassembled by Flowers' engineers.^56 It had approximately 1600
electronic valves and operated at 5000 characters per second. Later
models, containing approximately 2400 valves, processed five streams
of dot-and-cross simultaneously, in parallel. This boosted the speed
to 25,000 characters per second. Colossus generated the chi-stream
electronically. Only one tape was required, containing the
ciphertext--the synchronisation problem vanished. (Flowers' original
plan was to dispense with the message tape as well and set up the
ciphertext, as well as the wheels, on valves; but he abandoned this
idea when it became clear that messages of 5000 or more characters
would have to be processed.^57)

The arrival of the prototype Colossus caused quite a stir. Flowers
said:

    I don't think they [Newman et al.] really understood what I was
    saying in detail -- I am sure they didn't -- because when the first
    machine was constructed and working, they obviously were taken
    aback. They just couldn't believe it! ... I don't think they
    understood very clearly what I was proposing until they actually
    had the machine.^58

On what date did Colossus first come alive? In his written and verbal
recollections Flowers was always definite that Colossus was working
at Bletchley Park in the early part of December 1943.^59 In three
separate interviews he recalled a key date quite specifically, saying
that Colossus carried out its first trial run at Bletchley Park on 8
December 1943.^60 However, Flowers' personal diary for 1944--not
discovered until after his death--in fact records that Colossus did
not make the journey from Dollis Hill to Bletchley Park until January
1944. On Sunday 16 January Colossus was still in Flowers' lab at
Dollis Hill. His diary entry shows that Colossus was certainly
working on that day. Flowers was busy with the machine from the
morning until late in the evening and he slept at the lab.

Flowers' entry for 18 January reads simply: 'Colossus delivered to
B.P.'. This is confirmed by a memo dated 18 January from Newman to
Travis (declassified only in 2004). Newman wrote 'Colossus arrives
to-day'.^61 Colossus cannot therefore have carried out its first
trial run at Bletchley Park in early December. What did happen on 8
December 1943, the date that stuck so firmly in Flowers' mind?
Perhaps this was indeed the day that Colossus processed its first
test tape at Dollis Hill. 'I seem to recall it was in December', says
Harry Fensom, one of Flowers' engineers.^62

By February 1944 the engineers had got Colossus ready to begin
serious work for the Newmanry. Tutte's statistical method could now
be used at electronic speed. The computer attacked its first message
on Saturday 5 February. Flowers was present. He noted laconically in
his diary, 'Colossus did its first job. Car broke down on way home.'

Colossus immediately doubled the codebreakers' output.^63 The
advantages of Colossus over Robinson were not only its greatly
superior speed and the absence of synchronised tapes, but also its
greater reliability, resulting from Flowers' redesigned counters and
the use of valves in place of relays throughout. It was clear to the
Bletchley Park authorities--whose scepticism was now completely
cured--that more Colossi were required urgently.

Indeed, a crisis had developed, making the work of Newman's section
even more important than before. Since the German introduction of the
QEP system in October 1942, the codebreakers using hand-methods to
crack Tunny messages had been reliant upon depths, and as depths
became rarer during 1943, the number of broken messages reduced to a
trickle.^64 Then things went from bad to worse. In December 1943 the
Germans started to make widespread use of an additional device in the
Tunny machine, whose effect was to make depth-reading impossible (by
allowing letters of the plaintext itself to play a role in the
generation of the key). The hand breakers had been prone to scoff at
the weird contraptions in the Newmanry, but suddenly Newman's
machines were essential to all Tunny work.^65

In March 1944 the authorities demanded four more Colossi. By April
they were demanding twelve.^66 Great pressure was put on Flowers to
deliver the new machines quickly. The instructions he received came
'from the highest level'--the War Cabinet--and he caused consternation
when he said flatly that it was impossible to produce more than one
new machine by 1 June 1944.^67

Flowers had managed to produce the prototype Colossus at Dollis Hill
only because many of his laboratory staff 'did nothing but work, eat,
and sleep for weeks and months on end'.^68 He needed greater
production capacity, and proposed to take over a Post Office factory
in Birmingham. Final assembly and testing of the computers would be
done at his Dollis Hill laboratory. Flowers estimated that once the
factory was in operation he would be able to produce additional
Colossi at the rate of about one per month.^69 He recalled how one
day some Bletchley people came to inspect the work, thinking that
Flowers might be 'dilly-dallying': they returned 'staggered at the
scale of the effort'.^70 Churchill for his part gave Flowers top
priority for everything he needed.^71

By means of repluggable cables and panels of switches, Flowers
deliberately built more flexibility than was strictly necessary into
the logic units of the prototype Colossus. As a result, new methods
could be implemented on Colossus as they were discovered. In February
1944 two members of the Newmanry, Donald Michie and Jack Good, had
quickly found a way of using Colossus to discover the Tunny wheel
patterns.^72 Flowers was told to incorporate a special panel for
breaking wheel patterns in Colossus II.

Colossus II--the first of what Flowers referred to as the 'Mark 2'
Colossi^73--was shipped from Dollis Hill to Bletchley Park on 4 May
1944.^74 The plan was to assemble and test Colossus II at Bletchley
Park rather than Dollis Hill, so saving some precious time.^75
Promised by the first of June, Colossus II was still not working
properly as the final hours of May ticked past. The computer was
plagued by intermittent and mysterious faults.^76 Flowers struggled
to find the problem, but midnight came and went. Exhausted, Flowers
and his team dispersed at 1 am to snatch a few hours sleep.^77 They
left Chandler to work on, since the problem appeared to be in a part
of the computer that he had designed. It was a tough night: around 3
am Chandler noticed that his feet were getting wet.^78 A radiator
pipe along the wall had sprung a leak, sending a dangerous pool of
water towards Colossus.

Flowers returned to find the computer running perfectly. 'Colossus 2
in operation', he noted in his diary.^79 The puddle remained,
however, and the women operators had to don gumboots to insulate
themselves.^80 During the small hours Chandler had finally tracked
down the fault in Colossus (parasitic oscillations in some of the
valves) and had fixed it by wiring in a few extra resistors.^81
Flowers and his 'band of brothers' had met BP's deadline--a deadline
whose significance Flowers can only have guessed at.^82

Less than a week later the Allied invasion of France began. The D-day
landings of June 6 placed huge quantities of men and equipment on the
beaches of Normandy. From the beachheads the Allies pushed their way
into France through the heavy German defences. By mid-July the front
had advanced only 20 or so miles inland, but by September Allied
troops had swept across France and Belgium and were gathering close
to the borders of Germany, on a front extending from Holland in the
north to Switzerland in the south.^83

Since the early months of 1944, Colossus I had been providing an
unparalleled window on German preparations for the Allied invasion.^
84 Decrypts also revealed German appreciations of Allied intentions.
Tunny messages supplied vital confirmation that the German planners
were being taken in by Operation Fortitude, the extensive programme
of deceptive measures designed to suggest that the invasion would
come further north, in the Pas de Calais.^85 In the weeks following
the start of the invasion the Germans tightened Tunny security,
instructing operators to change the patterns of the chi- and
psi-wheels daily instead of monthly. Hand methods for discovering the
new patterns were overwhelmed. With impeccable timing Colossus II's
device for breaking wheel patterns came to the rescue.

Once Flowers' factory in Birmingham was properly up and running, new
Colossi began arriving in the Newmanry at roughly six week intervals.
Eventually three were dedicated to breaking wheel patterns.^86
Flowers was a regular visitor at B.P. throughout the rest of 1944,
overseeing the installation programme for the Mark 2 Colossi.^87 By
the end of the year seven Colossi were in operation. They provided
the codebreakers with the capacity to find all twelve wheel settings
by machine, and this was done in the case of a large proportion of
decrypted messages.^88 There were ten Colossi in operation by the
time of the German surrender in 1945, and an eleventh was almost
ready.

Misconceptions about Colossus

One of the most common misconceptions in the secondary literature is
that Colossus was used against Enigma. Another is that Colossus was
used against not Tunny but Sturgeon--an error promulgated by Brian
Johnson's influential televison series and accompanying book The
Secret War.^89 There are in fact many wild tales about Colossus in
the history books. Georges Ifrah even states that Colossus produced
English plaintext from the German ciphertext!^90 As already
explained, the output of Colossus was a series of counts indicating
the correct wheel settings (or, later, the wheel patterns). Not even
the de-chi was produced by Colossus itself, let alone the
plaintext--and there was certainly no facility for the automatic
translation of German into English.

J. A. N. Lee (in his book Computer Pioneers): 'Newman fully
appreciated the significance of Turing's ideas for the design of
high-speed electronic machines for searching for wheel patterns and
placings on the highest-grade German enciphering machines, and the
result was the invention of the "Colossus"'.^56

[030109-64]
Time magazine reported, in total confusion:

'At Bletchley Park, Alan Turing built a succession of vacuum-tube
machines called Colossus that made mincemeat of Hitler's Enigma
codes' (March 29, 1999).^57

An insidious misconception concerns ownership of the inspiration for
Colossus. Many accounts identify Turing as the key figure in the
designing of Colossus. In a biographical article on Turing, the
computer historian J. A. N. Lee said that Turing's 'influence on the
development of Colossus is well known',^91 and in an article on
Flowers, Lee referred to Colossus as 'the cryptanalytical machine
designed by Alan Turing and others'.^92 Even a book on sale at the
Bletchley Park Museum states that at Bletchley Park 'Turing worked
... on what we now know was computer research' which led to 'the
world's first electronic, programmable computer, "Colossus"'.^93

[030109-62-]
From page 35 of General Report on Tunny.^58

The view that Turing's interest in electronics contributed to the
inspiration for Colossus is indeed common. This claim is enshrined in
codebreaking exhibits in leading museums; and in the Annals of the
History of Computing Lee and Holtzman state that Turing 'conceived of
the construction and usage of high-speed electronic devices; these
ideas were implemented as the "Colossus" machines'.^94 However, the
definitive 1945 General Report on Tunny makes matters perfectly
clear: 'Colossus was entirely the idea of Mr. Flowers' (see the
extract from page 35 in the right-hand column).^95 By 1943
electronics had been Flowers' driving passion for more than a decade
and he needed no help from Turing. Turing was, in any case, away in
the United States during the critical period at the beginning of 1943
when Flowers proposed his idea to Newman and worked out the design of
Colossus on paper. Flowers emphasised in an interview that Turing
'made no contribution' to the design of Colossus.^96 Flowers said: 'I
invented the Colossus. No one else was capable of doing it.'^97

Martin Davis (in The Universal Computer: The Road from Leibniz to
Turing): Some of the methods ... used were playfully called
turingismus indicating their source. But turingismus required the
processing of lots of data and for the decryption be [sic] of any
use, the processing had to be done very quickly. ... In March 1943,
Alan Turing sailed home from a visit of several months in the United
States ... He whiled away the time during his Atlantic passage by
studying [an] RCA catalog, for it had been found that vacuum tubes
could carry out the kind of logical switching previously done by
electric relays. And the tubes were fast ... Vacuum tube circuits had
in fact been used experimentally for telephone switching, and Turing
had made contact with the gifted engineer, T. Flowers, who had
spearheaded this research. Under the direction of Flowers and Newman,
a machine, essentially a physical embodiment of turingismus, was
rapidly brought into being. Dubbed the Colossus and an engineering
marvel, this machine contained 1500 vacuum tubes.^59

In his recent book on the history of computing, Martin Davis offers a
garbled account of Colossus (see right-hand column). Here Davis
conflates Turingery, which he calls 'turingismus', with Tutte's
statistical method. (ismus is a German suffix equivalent to the
English ism. Newmanry codebreaker Michie explains the origin of
Turingery's slang name 'Turingismus': 'three of us (Peter Ericsson,
Peter Hilton and I) coined and used in playful style various
fake-German slang terms for everything under the sun, including
occasionally something encountered in the working environment.
Turingismus was a case of the latter.'^98) Turing's method of wheel
breaking from depths and Tutte's method of wheel setting from
non-depths were distant relatives, in that both used delta-ing. But
there the similarity ended. Turingery, Tutte said, seemed to him
'more artistic than mathematical'; in applying the method you had to
rely on what 'you felt in your bones'.^99 Conflating the two methods,
Davis erroneously concludes that Colossus was a physical embodiment
of Turingery. But as explained above, Turingery was a hand method--it
was Tutte's method that 'required the processing of lots of data'.
Tutte's method, not Turingery, was implemented in Heath Robinson and
Colossus. 'Turingery was not used in either breaking or setting by
any valve machine of any kind', Michie underlined.^100

Postwar

[030109-56]
Sir Winston Churchill.^60
[030109-65]
ENIAC.^61
[030109-66]
GCHQ at Cheltenham.^62
[030109-67]
Architect's model of GCHQ's new doughnut-shaped building at
Cheltenham.^63
[030109-68]
Jack Good.^64
[030109-70]
NSA Headquarters in Maryland.^65
                             [030109-69]
The Headquarters of the Government Code and Cypher School in Berkeley
                  Street, London, in the 1920s.^66

[030109-71]
The entry for 'Colossus' from Bletchley Park's 1944 A Cryptographic
Dictionary, declassified in 2000.^67
                             [030109-72]
  The table of contents of General Report on Tunny, declassified in
                              2000.^68

[030109-74]
King's College, Cambridge, birthplace of the universal Turing machine
and the stored program concept.^69
[030109-75]
A Turing machine: the scanner and memory.^70
[030109-76]
Turing was a founding father of modern computer science.^71
[030109-77]
John von Neumann.^72
[030109-78-]
Turing's Proposed Electronic Calculator.^73
[030109-79]
The first stored-program electronic computer, built by Tom Kilburn
(left) and Freddie Williams (right) in Newman's Computing Machine
Laboratory at the University of Manchester.^74
[030109-80]
The pilot model of Turing's Automatic Computing Engine, the fastest
of the early machines and precursor of the DEUCE computers.^75


If Flowers could have patented the inventions that he contributed to
the assault on Tunny, he would probably have become a very rich man.
As it was, the personal costs that he incurred in the course of
building the Colossi left his bank account overdrawn at the end of
the war. Newman was offered an OBE for his contribution to the defeat
of Germany, but he turned it down, remarking to ex-colleagues from
Bletchley Park that he considered the offer derisory.^101 Tutte
received no public recognition for his vital work. Turing accepted an
OBE, which he kept in his toolbox.

At the end of hostilities, orders were received from Churchill to
break up the Colossi, and all involved with Colossus and the cracking
of Tunny were gagged by the Official Secrets Act. The very existence
of Colossus was to be classified indefinitely. Flowers described his
reactions:

    When after the war ended I was told that the secret of Colossus
    was to be kept indefinitely I was naturally disappointed. I was
    in no doubt, once it was a proven success, that Colossus was an
    historic breakthrough, and that publication would have made my
    name in scientific and engineering circles--a conviction confirmed
    by the reception accorded to ENIAC, the U.S. equivalent made
    public just after the war ended. I had to endure all the acclaim
    given to that enterprise without being able to disclose that I
    had anticipated it. What I lost in personal prestige, and the
    benefits which commonly accrue in such circumstances, can now
    only be imagined. But at the time I accepted the situation
    philosophically and, in the euphoria of a war that was won, lost
    any concern about what might happen in the future.^102

ENIAC, commissioned by the U.S. army in 1943, was designed to
calculate trajectories of artillery shells. Although not operational
until the end of 1945--two years after Colossus first ran--ENIAC is
standardly described as the first electronic digital computer.
Flowers' view of the ENIAC? It was just a number cruncher--Colossus,
with its elaborate facilities for logical operations, was 'much more
of a computer than ENIAC'.^103

The Newmanry's Colossi might have passed into the public domain at
the end of the fighting, to become, like ENIAC, the electronic muscle
of a scientific research facility. The Newmanry's engineers would
quickly have adapted the equipment for peacetime applications. The
story of computing might have unfolded rather differently with such a
momentous push right at the beginning. Churchill's order to destroy
the Colossi was an almighty blow in the face for science--and for
British industry.

In April 1946, codebreaking operations were transferred from
Bletchley Park to buildings in Eastcote in suburban London.^104 At
the time of the move, the old name of the organisation, 'Government
Code and Cypher School', was formally changed to 'Government
Communications Headquarters' (GCHQ).^105 Six years later another move
commenced, and during 1952-54 GCHQ shifted its personnel and
equipment, including its codebreaking machinery, away from the London
area to a large site in Cheltenham.^106 Some machines did survive the
dissolution of the Newmanry. Two Colossi made the move from Bletchley
Park to Eastcote, and then eventually on to Cheltenham.^107 They were
accompanied by two of the replica Tunny machines manufactured at
Dollis Hill.^108 One of the Colossi, known as 'Colossus Blue' at
GCHQ, was dismantled in 1959 after fourteen years of postwar service.
The remaining Colossus is believed to have stopped running in 1960.

During their later years the two Colossi were used extensively for
training. Details of what they were used for prior to this remain
classified. There is a hint of the importance of one new role for
these Newmanry survivors in a letter written by Jack Good:

    I heard that Churchill requested that all Colossi be destroyed
    after the war, but GCHQ decided to keep at least one of them. I
    know of that one because I used it myself. That was the first
    time it was used after the war. I used it for a purpose for which
    NSA [National Security Agency] were planning to build a new
    special-purpose machine. When I showed that the job could be
    carried out on Colossus, NSA decided not to go ahead with their
    plan. That presumably is one reason I am still held in high
    regard in NSA. Golde told me that one of his friends who visits
    NSA told Golde that I am 'regarded as God' there.^109

After Bletchley's own spectacular successes against the German
machines, GCHQ was--not unnaturally--reluctant to use key-generating
cipher machines to protect British high-grade diplomatic traffic.
Instead GCHQ turned to one-time pad. Sender and receiver were issued
with identical key in the form of a roll of teleprinter tape. This
would be used for one message only. One-time pad is highly secure.
The disadvantage is that a complex and highly efficient distribution
network is required to supply users with key. It is probably true
that GCHQ initially underestimated the difficulties of distributing
key.

The GCHQ Colossi assisted in the production of one-time pad.
Ex-Newmanry engineers used some of Flowers' circuitry from Colossus
to build a random noise generator able to produce random teleprinter
characters on a punched tape. This device, code-named 'Donald Duck',
exploited the random way in which electrons are emitted from a hot
cathode. The tapes produced by Donald Duck were potential one-time
pad. The tapes were checked by Colossus, and those that were not
flat-random were weeded out. Newmanry-type tape-copying machines were
used to make copies of tapes that passed the tests, and these were
distributed to GCHQ's clients.

Probably the Colossi had additional postwar applications. They may
have been used to make character counts of enemy cipher traffic,
searching for features that might give the cryptanalysts a purchase.
Perhaps the GCHQ Colossi were even used against reconditioned German
Tunny machines. Many Tunnies were captured by the invading British
armies during the last stages of the war. If the National interest so
dictated, Tunny machines may have been sold to commercial
organisations or foreign powers, and the resulting traffic read by
GCHQ.

Until the 1970s few had any idea that electronic computation had been
used successfully during the Second World War. In 1975, the British
government released a set of captioned photographs of the Colossi
(several of which are reproduced above).^110 By 1983, Flowers had
received clearance to publish an account of the hardware of the first
Colossus.^111 Details of the later Colossi remained secret. So, even
more importantly, did all information about how Flowers' computing
machinery was actually used by the codebreakers. Flowers was told by
the British authorities that 'the technical description of machines
such as COLOSSUS may be disclosed', but that he must not disclose any
information about 'the functions which they performed'.^112 It was
rather like being told that he could give a detailed technical
description of the insides of a radar receiver, but must not say
anything about what the equipment did (in the case of radar, reveal
the location of planes, submarines, etc., by picking up radio waves
bouncing off them). He was also allowed to describe some aspects of
Tunny, but there was a blanket prohibition on saying anything at all
relating to 'the weaknesses which led to our successes'. In fact, a
clandestine censor objected to parts of the account that Flowers
wrote, and he was instructed to remove these prior to publication.^
113

There matters more or less stood until 1996, when the U.S. Government
declassified some wartime documents describing the function of
Colossus. These had been sent to Washington during the war by U.S.
liaison officers stationed at Bletchley Park. The most important
document remained classified, however: the 500 page General Report on
Tunny written at Bletchley Park in 1945 by Jack Good, Donald Michie,
and Geoffrey Timms. Thanks largely to Michie's tireless campaigning,
the report was declassified by the British Government in June 2000,
finally ending the secrecy.

Colossus and the modern computer

As everyone who can operate a personal computer knows, the way to
make the machine perform the task you want--word-processing, say--is to
open the appropriate program stored in the computer's memory. Life
was not always so simple. Colossus did not store programs in its
memory. To set up Colossus for a different job, it was necessary to
modify some of the machine's wiring by hand, using switches and
plugs. The larger ENIAC was also programmed by re-routing cables and
setting switches. The process was a nightmare: it could take the
ENIAC's operators up to three weeks to set up and debug a program.^
114 Colossus, ENIAC, and their like are called 'program-controlled'
computers, in order to distinguish them from the modern
'stored-program' computer.

This basic principle of the modern computer, that is, controlling the
machine's operations by means of a program of coded instructions
stored in the computer's memory, was thought of by Turing in 1936. At
the time, Turing was a shy, eccentric student at Cambridge
University. His 'universal computing machine', as he called it--it
would soon be known simply as the universal Turing machine--emerged
from research that no-one would have guessed could have any practical
application. Turing was working on a problem in mathematical logic,
the so-called 'decision problem', which he learned of from lectures
given by Newman. (For a description of the decision problem and
Turing's approach to it, see 'Computable Numbers: A Guide' in The
Essential Turing.^115) In the course of his attack on this problem,
Turing thought up an abstract digital computing machine which, as he
said, could compute 'all numbers which could naturally be regarded as
computable'.^116 The universal Turing machine consists of a limitless
memory in which both data and instructions are stored, in
symbolically encoded form, and a scanner that moves back and forth
through the memory, symbol by symbol, reading what it finds and
writing further symbols. By inserting different programs into the
memory, the machine can be made to carry out any algorithmic task.
That is why Turing called the machine universal.

Turing's fabulous idea was just this: a single machine of fixed
structure that, by making use of coded instructions stored in memory,
could change itself, chameleon-like, from a machine dedicated to one
task into a machine dedicated to a completely different task--from
calculator to word processor, for example. Nowadays, when many have a
physical realisation of a universal Turing machine in their living
room, this idea of a one-stop-shop computing machine is apt to seem
as obvious as the wheel. But in 1936, when engineers thought in terms
of building different machines for different purposes, the concept of
the stored-program universal computer was revolutionary.

In 1936 the universal Turing machine existed only as an idea. Right
from the start Turing was interested in the possibility of building
such a machine, as to some extent was Newman, but before the war they
knew of no practical way to construct a stored-program computer.^117
It was not until the advent of Colossus that the dream of building an
all-purpose electronic computing machine took hold of them. Flowers
had established decisively and for the first time that large-scale
electronic computing machinery was practicable, and soon after the
end of the war Turing and Newman both embarked on separate projects
to create a universal Turing machine in hardware. Racks of electronic
components from the dismantled Colossi were shipped from Bletchley
Park to Newman's Computing Machine Laboratory at Manchester.
Historians who did not know of Colossus tended to assume quite
wrongly that Turing and Newman inherited their vision of an
electronic computer from the ENIAC group in the U.S.

Even in the midst of the attack on Tunny, Newman was thinking about
the universal Turing machine. He showed Flowers Turing's 1936 paper
about the universal machine, 'On Computable Numbers', with its key
idea of storing symbolically encoded instructions in memory, but
Flowers, not being a mathematical logician, 'didn't really understand
much of it'.^118 There is little doubt that by 1944 Newman had firmly
in mind the possibility of building a universal Turing machine using
electronic technology. It was just a question of waiting until he
'got out'.^119 In February 1946, a few months after his appointment
to the University of Manchester, Newman wrote to the
Hungarian-American mathematician von Neumann (like Newman
considerably influenced by Turing's 1936 paper, and himself playing a
leading role in the post-ENIAC developments taking place in the
U.S.):

    I am ... hoping to embark on a computing machine section here,
    having got very interested in electronic devices of this kind
    during the last two or three years. By about eighteen months ago
    I had decided to try my hand at starting up a machine unit when I
    got out. ... I am of course in close touch with Turing.^120

The implication of Flowers' racks of electronic equipment was obvious
to Turing too. Flowers said that once Colossus was in operation, it
was just a matter of Turing's waiting to see what opportunity might
arise to put the idea of his universal computing machine into
practice. (By the end of the war, Turing had educated himself
thoroughly in electronic engineering: during the later part of the
war he gave a series of evening lectures 'on valve theory'.^121)
Turing's opportunity came along in 1945, when John Womersley, head of
the Mathematics Division of the National Physical Laboratory (NPL) in
London, invited him to design and develop an electronic
stored-program digital computer. Turing's technical report 'Proposed
Electronic Calculator',^122 dating from the end of 1945 and
containing his design for the ACE, was the first relatively complete
specification of an electronic stored-program digital computer.^123
The slightly earlier 'First Draft of a Report on the EDVAC',^124
produced in about May 1945 by von Neumann, was much more abstract,
saying little about programming, hardware details, or electronics.
(The EDVAC, proposed successor to the ENIAC, was to be a
stored-program machine. It was not fully working until 1952.^125)
Harry Huskey, the electronic engineer who subsequently drew up the
first detailed hardware designs for the EDVAC, stated that the
'information in the "First Draft" was of no help'.^126 Turing, in
contrast, supplied detailed circuit designs, full specifications of
hardware units, specimen programs in machine code, and even an
estimate of the cost of building the machine.

Turing asked Flowers to build the ACE, and in March 1946 Flowers said
that a 'minimal ACE' would be ready by August or September of that
year.^127 Unfortunately, however, Dollis Hill was overwhelmed by a
backlog of urgent work on the national telephone system, and it
proved impossible to keep to Flowers' timetable. In the end it was
Newman's team who, in June 1948, won the race to build the first
stored-program computer. The first program, stored on the face of a
cathode ray tube as a pattern of dots, was inserted manually, digit
by digit, using a panel of switches. The news that the Manchester
machine had run what was only a tiny program--just 17 instructions
long--for a mathematically trivial task was 'greeted with hilarity' by
Turing's team working on the much more sophisticated ACE.^128

A pilot model of the ACE ran its first program in May 1950. With an
operating speed of 1 MHz, the pilot model ACE was for some time the
fastest computer in the world. The pilot model was the basis for the
very successful DEUCE computers, which became a cornerstone of the
fledgling British computer industry--confounding the suggestion, made
in 1946 by Sir Charles Darwin, Director of the NPL and grandson of
the great Darwin, that 'it is very possible that ... one machine
would suffice to solve all the problems that are demanded of it from
the whole country'.^129

                                   

Appendix 1: The teleprinter alphabet

CONVENTIONAL NAME    IMPULSE                      MEANING      
                   1  2  3  4  5      IN LETTER SHIFT    IN FIGURE SHIFT
        /          *  *  *  *  *      (no meaning)       (no meaning)
  -----------------------------------------------------------------  
        9          *  *  x  *  *      space              space
        H          *  *  x  *  x      H                  PS
        T          *  *  *  *  x      T                  5
        O          *  *  *  x  x      O                  9
        M          *  *  x  x  x      M                  full stop
        N          *  *  x  x  *      N                  comma
        3          *  *  *  x  *      carriage return    carriage return
        R          *  x  *  x  *      R                  4
        C          *  x  x  x  *      C                  colon
        V          *  x  x  x  x      V                  equals
        G          *  x  *  x  x      G                  @
        L          *  x  *  *  x      L                  close bracket
        P          *  x  x  *  x      P                  0 (zero)
        I          *  x  x  *  x      I                  8
        4          *  x  *  *  *      line feed          line feed
        A          x  x  *  *  *      A                  dash
        U          x  x  x  *  *      U                  7
        Q          x  x  x  *  x      Q                  1
        W          x  x  *  *  x      W                  2
      + or 5       x  x  *  x  x      move to figure     (none)
                                      shift
      - or 8       x  x  x  x  x      (none)             move to letter
                                                         shift
        K          x  x  x  x  *      K                  open bracket
        J          x  x  *  x  *      J                  ring bell
        D          x  *  *  x  *      D                  who are you?
        F          x  *  x  x  *      F                  per cent
        X          x  *  x  x  x      X                  /
        B          x  *  *  x  x      B                  ?
        Z          x  *  *  *  x      Z                  +
        Y          x  *  x  *  x      Y                  6
        S          x  *  x  *  *      S                  apostrophe
        E          x  *  *  *  *      E                  3
                    The teleprinter alphabet.^76

In teleprinter code the letters most frequently used are represented
by the fewest holes in the tape, which is to say by the fewest
crosses, in B.P. notation.^130 For instance E, the commonest letter
of English, is x****, and T, the next most frequent, is ****x. The
table in the right-hand column gives the 5-bit teleprinter code for
each character of the teleprint alphabet.

The left-hand column of the table shows the characters of the
teleprint alphabet as they would have been written down by the
Bletchley codebreakers. For example, the codebreakers wrote '9' to
indicate a space (as in 'to9indicate') and '3' to indicate a carriage
return.

The 'move to figure shift' character (which some at Bletchley wrote
as '+' and some as '5') told the teleprinter to shift from printing
letters to printing figures; and the 'move to letter shift' character
(written '-' or '8') told the machine to shift from printing figures
to printing letters. With the teleprinter in letter mode, the keys
along the top row of the keyboard would print QWERTYUIOP, and in
figure mode the same keys would print 1234567890.

Most of the keyboard characters had different meanings in letter mode
and figure mode. In figure mode the M-key printed a full stop, the
N-key a comma, the C-key a colon, and the A-key a dash, for example.
(Unlike a modern keyboard, the teleprinter did not have separate keys
for punctuation.) The meanings of the other keys in figure mode are
given at the right of the table.

To cause the teleprinter to print 123 WHO, ME? the operator must
first press figure shift and key Q W E to produce the numbers. He or
she then drops into letter mode and keys a space (or vice versa),
followed by W H O. To produce the comma it is necessary to press
figure shift then N. This is followed by letter shift, space, and M
E. A final figure shift followed by B produces the question mark.

                          +QWE-9WHO+N-9ME+B

Often Tunny operators would repeat the figure-shift and letter-shift
characters, sending a comma as ++N-- and a full stop as ++M--, for
example. Following this practice, the operator would key

                       ++QWE--9WHO++N--9ME++B

Presumably the shift characters were repeated to ensure that the
shift had 'taken'. These repetitions were very helpful to the
British, since a correct guess at a punctuation mark could yield six
characters of text (including the trailing 9).

Appendix 2: The Tunny encipherment equation and Tutte's 1 + 2
break-in

First, some notation. P is the plaintext, C is the cipher text, kh is
the stream of letters contributed to the message's key by the
chi-wheels, and Ps is the stream contributed by the psi-wheels. kh + Ps
is the result of adding kh and Ps using the rules of Tunny-addition. DC
is the result of delta-ing the ciphertext, D(kh + Ps) is the result of
delta-ing the stream of characters that results from adding kh and Ps,
and so forth.

Since C is produced by adding the key to P, and the key is produced
by adding kh and Ps, the fundamental encipherment equation for the
Tunny machine is:

                            C = P + kh + Ps

C[1] is written for the first impulse of C (i.e. the first of the
five streams in the teleprint representation of the ciphertext); and
similarly P[1], kh[1] and Ps[1] are the first impulses of P, kh and Ps
respectively. The encipherment equation for the first impulse is:

                      C[1] = P[1] + kh[1 ]+ Ps[1]

Delta-ing each side of this equation gives

                    DC[1] = D(P[1] + kh[1 ]+ Ps[1])

Delta-ing the sum of two or more impulses produces the same result as
first delta-ing each impulse and then summing. So

                    DC[1] = DP[1] + Dkh[1 ]+ DPs[1]

Likewise for the second impulse:

                    DC[2] = DP[2] + Dkh[2 ]+ DPs[2]

Adding the equations for the first and second impulses gives

    DC[1 ]+ DC[2] = DP[1] + DP[2] + Dkh[1 ]+ Dkh[2 ]+ DPs[1 ]+ DPs[2]

which is the same as

 D(C[1 ]+ C[2]) = D(P[1] + P[2]) + D(kh[1 ]+ kh[2])[ ]+ D(Ps[1 ]+ Ps[2])

Because of the staggering motion of the psi-wheels, D(Ps[1 ]+ Ps[2]) 
turns out to be about 70% dot. But adding dot leaves you where you
started: cross plus dot is dot and dot plus dot is dot. It follows
that the addition of D(Ps[1 ]+ Ps[2]) more often than not has no
effect. So

          D(C[1 ]+ C[2]) = D(P[1] + P[2]) + D(kh[1 ]+ kh[2])

is true more often than not.

Tutte also discovered that D(P[1] + P[2]) is approximately 60% dot.
This effect is the result of various factors, for instance the Tunny
operators' habit of repeating certain characters (see Appendix 1),
and contingencies of the way the individual letters are represented
in the underlying teleprinter code--for example, the delta of the sum
of the first and second impulses of the common bigram (or letter
pair) DE is dot, as it is for other common bigrams such as BE, ZE,
ES. So it is true more often than not that

                   D(C[1 ]+ C[2]) = D(kh[1 ]+ kh[2])

Tutte's '1 + 2 break in' is this. D(C[1 ]+ C[2]) is stepped through
the delta-ed sum of the first and second impulses of the entire
stream of characters from the chi-wheels. Generally the
correspondence between D(C[1 ]+ C[2]) and a strip from the delta-ed
chi of the same length will be no better than chance. If, however, D(
C[1 ]+ C[2]) and a strip of delta-ed chi correspond more often than
not, then a candidate has been found for D(kh[1 ]+ kh[2]), and so for
the first two impulses of kh. The greater the correspondence, the
likelier the candidate.^131


References

[1] Bauer, F. L. 2006 'The Tiltman Break', in [10].

[2] Cairncross, J. 1997 The Enigma Spy: The Story of the Man who
Changed the Course of World War Two, London: Century.

[3] Campbell-Kelly, M. 2005 'The ACE and the Shaping of British
Computing', in [9].

[4] Chandler, W. W. 1983 'The Maintenance and Installation of
Colossus', Annals of the History of Computing, vol. 5, pp. 260-2.

[5] Coombs, A. W. M. 1983 'The Making of Colossus', Annals of the
History of Computing, vol. 5, pp. 253-9.

[6] Copeland, B. J. (ed.) 2004 The Essential Turing, Oxford: Oxford
University Press.

[7] Copeland, B. J. 2004 'Computable Numbers: A Guide', in [6].

[8] Copeland, B. J. 2004 'Enigma', in [6].

[9] Copeland, B. J. (ed.) 2005 Alan Turing's Automatic Computing
Engine: The Master Codebreaker's Struggle to Build the Modern
Computer, Oxford: Oxford University Press.

[10] Copeland, B. J. et al. 2010 Colossus: The Secrets of Bletchley
Park's Codebreaking Computers (2nd edition), Oxford: Oxford
University Press.

[11] Davies, D. 1995 'The Lorenz Cipher Machine SZ42', Cryptologia,
vol. 19, pp. 517-39.

[12] Davis, M. 2000 The Universal Computer: The Road from Leibniz to
Turing, New York: Norton.

[13] de Bruyne, N. A., Webster, H. C. 1931 'Note on the Use of a
Thyratron with a Geiger Counter', Proceedings of the Cambridge
Philosophical Society, vol. 27, pp. 113-15.

[14] Enever, E. 1994 Britain's Best Kept Secret: Ultra's Base at
Bletchley Park (2nd edition), Stroud: Alan Sutton.

[15] Erskine, R., Freeman, P. 2003 'Brigadier John Tiltman: One of
Britain's Finest Cryptologists', Cryptologia, vol. 27, pp. 289-318.

[16] Flowers, T. H. 1983 'The Design of Colossus', Annals of the
History of Computing, vol. 5, pp. 239-52.

[17] Flowers, T. H. 2006 'D-Day at Bletchley Park', in [10].

[18] Golden, F. 1999 'Who Built the First Computer?', Time, March 29,
1999, no. 13, p. 82.

[19] Hinsley, F. H. et al. 1981 British Intelligence in the Second
World War, Vol. 2, London: Her Majesty's Stationery Office.

[20] Hinsley, F. H. et al. 1988 British Intelligence in the Second
World War, Vol. 3, Part 2, London: Her Majesty's Stationery Office.

[21] Hinsley, H. 1996 'The Counterfactual History of No Ultra',
Cryptologia, vol. 20, pp. 308-24.

[22] Hull, A. W. 1929 'Hot-cathode Thyratrons', General Electric
Review, vol. 32, pp. 390-99.

[23] Huskey, H. D. 1972 'The Development of Automatic Computing', in
Proceedings of the First USA-JAPAN Computer Conference, Tokyo.

[24] Ifrah, G. 2001 The Universal History of Computing: From the
Abacus to the Quantum Computer, New York: John Wiley.

[25] Johnson, B. 1978 The Secret War, London: British Broadcasting
Corporation.

[26] Lee, J. A. N. 1995 Computer Pioneers, Los Alamitos: IEEE
Computer Society Press.

[27] Lee, J. A. N., Holtzman, G. 1999 '50 Years After Breaking the
Codes', Annals of the History of Computing, vol. 17, pp. 32-43.

[28] McCorduck, P. 1979 Machines Who Think, New York: W. H. Freeman.

[29] Murray, D. (no date) Murray Multiplex: Technical Instructions,
Manual no. 1: General Theory, Croydon: Creed and Coy.

[30] Stern, N. 1981 From ENIAC to UNIVAC: An Appraisal of the
Eckert-Mauchly Computers, Bedford, Mass.: Digital Press.

[31] Turing, A. M. 1936 'On Computable Numbers, with an Application
to the Entscheidungsproblem', Proceedings of the London Mathematical
Society, Series 2, vol. 42 (1936-7), pp. 230-65. Reprinted in [6].

[32] Turing, S. 1959 Alan M. Turing, Cambridge: W. Heffer.

[33] Tutte, W. T. 2006 'My Work at Bletchley Park', in [10].

[34] Weierud, F. 2006 'Bletchley Park's Sturgeon--The Fish That Laid
No Eggs', in [10].

[35] Wynn-Williams, C. E. 1931 'The Use of Thyratrons for High Speed
Automatic Counting of Physical Phenomena', Proceedings of the Royal
Society, Series A, vol. 132, pp. 295-310.

[36] Wynn-Williams, C. E. 1932 'A Thyratron Scale of Two Automatic
Counter', Proceedings of the Royal Society of London, Series A, vol.
136, pp. 312-24.

 

Notes

1 The physical Tunny machine is described in section 11 of General
Report on Tunny, and in Davies [11]. The machine's function and use
is described in sections 11 and 94 of General Report on Tunny.
General Report on Tunny was written at Bletchley Park in 1945 by
Tunny-breakers Jack Good, Donald Michie and Geoffrey Timms; it was
released by the British government in 2000 to the National Archives/
Public Record Office (PRO) at Kew (document reference HW 25/4 (vol.
1), HW 25/5 (vol. 2)). A digital facsimile is available in The Turing
Archive for the History of Computing http://www.AlanTuring.net/
tunny_report.

2 Bletchley's work on Sturgeon is described in Weirud's 'Bletchley
Park's Sturgeon, the Fish that Laid No Eggs' in a previous issue of
this journal.

3 On Thrasher, see section 93 of General Report on Tunny.

4 See Copeland [14], ch. 7.

5 General Report on Tunny, p. 14. General Report on Tunny mentions
that the first messages on the experimental link passed between
Vienna and Athens (p. 297).

6 General Report on Tunny, p. 320.

7 General Report on Tunny, pp. 14, 320, 458.

8 General Report on Tunny, p. 14.

9 General Report on Tunny, p. 14.

10 General Report on Tunny, p. 395.

11 General Report on Tunny, p. 15.

12 General Report on Tunny, p. 15.

13 General Report on Tunny, p. 5.

14 General Report on Tunny, p. 4.

15 General Report on Tunny, p. 5.

16 British message reference number CX/MSS/2499/T14; PRO reference
HW1/1648. Words enclosed in square brackets do not appear in the
original. (Thanks to Ralph Erskine for assistance in locating this
document. An inaccurate version of the intercept appears in Hinsley
[19], pp. 764-5.)

17 Copy of message CX/MSS/2499/T14, PRO document reference HW5/242,
p. 4.

18 'A Postponed German Offensive (Operations ZITADELLE and EULE)'
(anon., Government Code and Cypher School, 7 June 1943; PRO reference
HW13/53), p. 2.

19 Documents from G.C. & C.S. to Churchill, 30 April 1943 (PRO
reference HW1/1648). An earlier decrypt concerning Zitadelle (13
April 1943), and an accompanying note from 'C' to Churchill, are at
HW1/1606.

20 Tape-recorded interview with Harry Hinsley (Sound Archive,
Imperial War Museum, London (reference number 13523)).

21 Cairncross [2], p. 98, Hinsley [21], pp. 322-3, interview with
Hinsley (see above).

22 Hinsley [19], p. 626.

23 Hinsley [19], p. 625.

24 Hinsley [19], p. 627.

25 Hinsley [19], p. 627.

26 Newman in interview with Christopher Evans ('The Pioneers of
Computing: An Oral History of Computing' (London: Science Museum)).

27 Erskine and Freeman [15].

28 For further information on Turing, see Copeland The Essential
Turing and Colossus: The Secrets of Bletchley Park's Codebreaking
Computers.

29 Bauer [1], p. 372.

30 Tutte [33], pp. 359-630.

31 See Copeland The Essential Turing.

32 Good in interview with Pamela McCorduck (McCorduck [28], p. 53).

33 General Report on Tunny, p. 313.

34 General Report on Tunny, p. 28.

35 General Report on Tunny, p. 28.

36 General Report on Tunny, pp. 28, 320-2.

37 Newman in interview with Evans.

38 Wynn-Williams [35], [36]; see also Hull [22], de Bruyne & Webster
[13].

39 General Report on Tunny, p. 22.

40 General Report on Tunny, p. 20.

41 Letter from Harry Fensom to Copeland (4 May 2001).

42 General Report on Tunny, p. 328.

43 Newman in interview with Evans.

44 General Report on Tunny, p. 328.

45 Unless indicated otherwise, material in this chapter relating
directly to Flowers derives from (1) Flowers in interviews with
Copeland, 1996-1998 (2) Flowers in interview with Christopher Evans
in 1977 ('The Pioneers of Computing: an Oral History of Computing',
London: Science Museum).

46 Flowers in interview with Copeland (July 1996).

47 Flowers in interview with Copeland (July 1998).

48 Flowers in interview with Copeland (July 1996); General Report on
Tunny, p. 33.

49 Flowers in interview with Copeland (July 1996); Flowers [16], p.
244.

50 Flowers, T. H. 'Colossus - Origin and Principles', typescript, no
date, p. 3; Coombs in interview with Christopher Evans in 1976 ('The
Pioneers of Computing: An Oral History of Computing' (London: Science
Museum)). 'Incredulity' is Flowers' word.

51 Flowers, 'Colossus - Origin and Principles', p. 3.

52 Flowers in interview with Copeland (July 1996).

53 Ibid.

54 Ibid.

55 Flowers, 'Colossus - Origin and Principles', p. 3.

56 Myers, K. 'Dollis Hill and Station X', in The Turing Archive for
the History of Computing http://www.AlanTuring.net/myers.

57 General Report on Tunny, p. 35.

58 Flowers in interview with Evans ((c) Board of Trustees of the
Science Museum).

59 Flowers [16], p. 245; Flowers in interview with Evans.

60 Flowers in interview with Copeland (July 1996); Flowers in
interview with Darlow Smithson (no date); Flowers in interview with
staff of the Imperial War Museum, London (1998).

61 Newman, M. H. A. 'Report on Progress' (Newmanry, 18 January 1944;
PRO document reference HW14/96), p. 4.

62 Letter from Fensom to Copeland (18 August 2005).

63 General Report on Tunny, p. 35.

64 General Report on Tunny, p. 34.

65 General Report on Tunny, p. 28.

66 General Report on Tunny, p. 35.

67 Flowers [16], p. 246.

68 Flowers [16], p. 245.

69 Flowers [16], p. 246.

70 Flowers in interview with Copeland (July 1996).

71 Note from Donald Michie to Copeland (27 May 2002), reporting a
disclosure by Coombs in the 1960s.

72 General Report on Tunny, p. 461.

73 Flowers' personal diary, 4 May 1944.

74 Ibid.

75 Chandler [4], p. 261.

76 Flowers [16], p. 246.

77 Flowers' personal diary, 31 May 1944.

78 Letter from Chandler to Brian Randell, 24 January 1976;
unpublished manuscript by Gil Hayward '1944 - 1946' (2002).

79 Flowers' personal diary, 1 June 1944.

80 Hayward, '1944 - 1946'.

81 Flowers [16], p. 247.

82 Coombs [5], p. 259.

83 Hinsley [20]: maps 'OVERLORD' (frontispiece) and 'September
position 1944' (facing p. 365).

84 Some crucial decrypts are listed by Hinsley [20], ch. 44 and
appendix 10.

85 Hinsley [20], pp. 47-65.

86 General Report on Tunny, p. 36.

87 Flowers' personal diary for 1944.

88 General Report on Tunny, p. 35.

89 Johnson [25], pp. 339-47.

90 Ifrah [24], p. 218.

91 Lee [26], p. 671.

92 Lee [26], p. 306.

93 Enever [14], pp. 36-7.

94 Lee and Holtzman [27], p. 33.

95 General Report on Tunny, p. 35.

96 Flowers in interview with Copeland (July 1996).

97 Flowers in interview with Copeland (July 1996).

98 Letter from Michie to Copeland (29 July 2001).

99 Tutte [33], p. 360.

100 Letter from Michie to Copeland (28 November 2001).

101 Peter Hilton in interview with Copeland (May 2001).

102 Flowers [17], pp. 82-3.

103 Flowers in interview with Copeland (July 1996).

104 Freeman, P. 'How GCHQ Came to Cheltenham' (undated, GCHQ), p. 8.

105 Ibid.

106 Freeman, 'How GCHQ Came to Cheltenham', p. 30.

107 Unpublished manuscript by Gil Hayward (2002).

108 Ibid.

109 Letter from Jack Good to Henry H. Bauer (2 January 2005).

110 The photographs were released to the Public Record Office (PRO
reference FO 850/234).

111 Flowers [16].

112 Personal files of T. H. Flowers (24 May 1976, 3 September 1981).

113 Personal files of T. H. Flowers (3 September 1981).

114 Campbell-Kelly [3], p. 151.

115 Copeland [7], pp. 45-53.

116 Turing [31], p. 249.

117 Newman in interview with Evans.

118 Flowers in interview with Copeland (July 1996).

119 Letter from Newman to von Neumann (8 February 1946) (in the von
Neumann Archive at the Library of Congress, Washington, D.C.; a
digital facsimile is in The Turing Archive for the History of
Computing http://www.AlanTuring.net/newman_vonneumann_8feb46).

120 Ibid.

121 Turing [32], p. 74.

122 In Copeland [9].

123 A digital facsimile of the original typewritten report is in The
Turing Archive for the History of Computing http://www.AlanTuring.net
/proposed_electronic_calculator.

124 In Stern [30].

125 Huskey [23], p. 702.

126 Letter from Huskey to Copeland (4 February 2002).

127 'Status of the Delay Line Computing Machine at the P.O. Research
Station' (anon., National Physical Laboratory, 7 March 1946; in the
Woodger Papers (catalogue reference M12/105); a digital facsimile is
in The Turing Archive for the History of Computing http://
www.AlanTuring.net/delay_line_status).

128 Michael Woodger in interview with Copeland (June 1998).

129 Darwin, C. 'Automatic Computing Engine (ACE)' (National Physical
Laboratory, 17 April 1946; PRO document reference DSIR 10/275); a
digital facsimile is in The Turing Archive for the History of
Computing http://www.AlanTuring.net/darwin_ace).

130 In Murray [29].

131 This article is a revised and illustrated version of Copeland,
B.J. 'Breaking the Lorenz Schlusselzusatz Traffic', in de Leeuw, K.,
Bergstra, J. (eds) The History of Information Security: A
Comprehensive Handbook (Amsterdam: Elsevier Science, 2007), pp.
447-477.


Illustration credits

  * 1 Photo by Duncan Shaw-Brown
  * 2 Source: General Report on Tunny; Crown copyright, National
    Archives Image Library, Kew. Reproduced from Copeland, B.J.
    Colossus: The Secrets of Bletchley Park's Codebreaking Computers
    (Oxford: Oxford University Press, 2010). Photo enhanced by Dustin
    Barrett and Parker Bright

  * 3 Recreated from damaged archival photographs by Jack Copeland
    and Dustin Barrett. Reproduced from Copeland, B.J. Colossus: The
    Secrets of Bletchley Park's Codebreaking Computers (Oxford:
    Oxford University Press, 2010)
  * 4 Source: Science and Society Picture Library, National Museum of
    Science and Industry, London
  * 5 Reproduced from Copeland, B.J. Colossus: The Secrets of
    Bletchley Park's Codebreaking Computers (Oxford: Oxford
    University Press, 2010), p. 38
  * 6 Reproduced from Copeland, B.J. Colossus: The Secrets of
    Bletchley Park's Codebreaking Computers (Oxford: Oxford
    University Press, 2010), p. 38
  * 7 Map by Dustin Barrett and Jack Copeland. Reproduced from
    Copeland, B.J. Colossus: The Secrets of Bletchley Park's
    Codebreaking Computers (Oxford: Oxford University Press, 2010),
    p. 41
  * 8 Source: Bletchley Park Trust. Photo enhanced by Parker Bright
  * 9 Source: General Report on Tunny; Crown copyright, National
    Archives Image Library, Kew. Reproduced from Copeland, B.J.
    Colossus: The Secrets of Bletchley Park's Codebreaking Computers
    (Oxford: Oxford University Press, 2010). Photo enhanced by Parker
    Bright
  * 10 Recreated from a damaged family photograph by Jack Copeland
    and Dustin Barrett. Reproduced from Copeland, B.J. Colossus: The
    Secrets of Bletchley Park's Codebreaking Computers (Oxford:
    Oxford University Press, 2010)
  * 11 Source: Picture Library, Imperial War Museum, London. Photo
    enhanced by Dustin Barrett
  * 12 Reproduced from Copeland, B.J. Colossus: The Secrets of
    Bletchley Park's Codebreaking Computers (Oxford: Oxford
    University Press, 2010), p. 47
  * 13 Satellite image courtesy of Google Maps
  * 14 Reproduced from Copeland, B.J. Colossus: The Secrets of
    Bletchley Park's Codebreaking Computers (Oxford: Oxford
    University Press, 2010), pp. 5-6
  * 15 Source: Bletchley Park Trust. Photo enhanced by Parker Bright
  * 16 Source: William Newman
  * 17 Source: Barbara Eachus and Government Communications
    Headquarters, Cheltenham. Photo enhanced by Parker Bright
  * 18 Source: Beryl Turing and King's College Library, Cambridge.
    Photo enhanced by Dustin Barrett and Parker Bright
  * 19 Source: William Tutte. Photo enhanced by Dustin Barrett and
    Parker Bright
  * 20 Source: Bletchley Park Trust. Reproduced from Bauer, F.L. 'The
    Tiltman Break', in Copeland, B.J. Colossus: The Secrets of
    Bletchley Park's Codebreaking Computers (Oxford: Oxford
    University Press, 2010), p. 370
  * 21 Reproduced from Bauer, F.L. 'The Tiltman Break', in Copeland,
    B.J. Colossus: The Secrets of Bletchley Park's Codebreaking
    Computers (Oxford: Oxford University Press, 2010), p. 372
  * 22 Source: William Tutte. Photo enhanced by Parker Bright
  * 23 Source: Beryl Turing and King's College Library, Cambridge.
    Photo enhanced by Parker Bright
  * 24 Recreated by Jack Copeland and Dustin Barrett from a wartime
    photograph held by the Picture Library, Imperial War Museum,
    London
  * 25 Reproduced from Copeland, B.J. Colossus: The Secrets of
    Bletchley Park's Codebreaking Computers (Oxford: Oxford
    University Press, 2010), p. 67
  * 26 Source: Beryl Turing and King's College Library, Cambridge.
    Photo enhanced by Jack Copeland, Dustin Barrett and Parker Bright
  * 27 Source: Karin Dawe. Photo enhanced by Parker Bright
  * 28 Source: William Newman
  * 29 Source: General Report on Tunny; Crown copyright, National
    Archives Image Library, Kew. Reproduced from Copeland, B.J.
    Colossus: The Secrets of Bletchley Park's Codebreaking Computers
    (Oxford: Oxford University Press, 2010). Photo enhanced by Dustin
    Barrett and Parker Bright
  * 30 Source: General Report on Tunny; Crown copyright, National
    Archives Image Library, Kew. Reproduced from Copeland, B.J.
    Colossus: The Secrets of Bletchley Park's Codebreaking Computers
    (Oxford: Oxford University Press, 2010). Photo enhanced by Dustin
    Barrett and Parker Bright
  * 31 Source: Government Communication Headquarters, Cheltenham.
    Photo enhanced by Jack Copeland and Dustin Barrett
  * 32 Source: Jerry Roberts. Photo enhanced by Dustin Barrett and
    Parker Bright
  * 33 Reproduced from Copeland, B.J. Colossus: The Secrets of
    Bletchley Park's Codebreaking Computers (Oxford: Oxford
    University Press, 2010), p. 70
  * 34 Source: William Tutte. Photo enhanced by Parker Bright
  * 35 Source: General Report on Tunny; Crown copyright, National
    Archives Image Library, Kew. Reproduced from Copeland, B.J.
    Colossus: The Secrets of Bletchley Park's Codebreaking Computers
    (Oxford: Oxford University Press, 2010). Photo enhanced by Jack
    Copeland, Dustin Barrett and Parker Bright
  * 36 Source: Rowe, A. P. One Story of Radar (Cambridge: Cambridge
    University Press, 1948). Photo enhanced by Parker Bright
  * 37 Source: National Physical Laboratory, Teddington (Crown
    copyright). Photo enhanced by Parker Bright
  * 38 Source: Flowers, T.H. 'Colossus', in Copeland, B.J. Colossus:
    The Secrets of Bletchley Park's Codebreaking Computers (Oxford:
    Oxford University Press, 2010), p. 92
  * 39 Source: General Report on Tunny; Crown copyright, National
    Archives Image Library, Kew. Reproduced from Copeland, B.J.
    Colossus: The Secrets of Bletchley Park's Codebreaking Computers
    (Oxford: Oxford University Press, 2010). Photo enhanced by Jack
    Copeland and Dustin Barrett
  * 40 Photo by Jack Copeland
  * 41 Source: Turing, S. Alan M. Turing (Cambridge: W. Heffer,
    1959). Reproduced by permission of Heffers Bookshop (Cambridge)
  * 42 Source: National Archives and Records Administration, College
    Park, Maryland, USA. Photo enhanced by Jack Copeland and Dustin
    Barrett
  * 43 Source: General Report on Tunny; Crown copyright, National
    Archives Image Library, Kew. Reproduced from Copeland, B.J.
    Colossus: The Secrets of Bletchley Park's Codebreaking Computers
    (Oxford: Oxford University Press, 2010). Photo enhanced by Jack
    Copeland and Dustin Barrett
  * 44 Source: General Report on Tunny; Crown copyright, National
    Archives Image Library, Kew. Reproduced from Copeland, B.J.
    Colossus: The Secrets of Bletchley Park's Codebreaking Computers
    (Oxford: Oxford University Press, 2010). Photo enhanced by Jack
    Copeland and Dustin Barrett
  * 45 Source: Flowers, T.H. 'Colossus', in Copeland, B.J. Colossus:
    The Secrets of Bletchley Park's Codebreaking Computers (Oxford:
    Oxford University Press, 2010), p. 96
  * 46 Image enhanced by Parker Bright
  * 47 Image enhanced by Parker Bright
  * 48 Source: General Report on Tunny; Crown copyright, National
    Archives Image Library, Kew. Reproduced from Copeland, B.J.
    Colossus: The Secrets of Bletchley Park's Codebreaking Computers
    (Oxford: Oxford University Press, 2010). Photo enhanced by Jack
    Copeland and Dustin Barrett
  * 49 Source: General Report on Tunny; Crown copyright, National
    Archives Image Library, Kew. Reproduced from Copeland, B.J.
    Colossus: The Secrets of Bletchley Park's Codebreaking Computers
    (Oxford: Oxford University Press, 2010). Photo enhanced by Jack
    Copeland and Dustin Barrett
  * 50 Source: Donald Michie. Photo enhanced by Jack Copeland, Dustin
    Barrett and Parker Bright
  * 51 Source: Bill Chandler and Brian Randell
  * 52 Source: U.S. Library of Congress. Map modified by Parker
    Bright
  * 53 Source: Army Signal Corps Collection, National Archives and
    Records Administration, College Park, Maryland, USA
  * 54 Photo by Charles Turner
  * 55 Source: Frode Weierud. Reproduced from Copeland, B.J.
    Colossus: The Secrets of Bletchley Park's Codebreaking Computers
    (Oxford: Oxford University Press, 2010)
  * 56 Lee [26], p. 492

  * 57 Golden, F. 'Who Built the First Computer?', Time, March 29,
    1999, no. 13, p. 82. Cover: http://www.time.com/time/covers/
    0,16641,19990329,00.html
  * 58 Crown copyright, National Archives, Kew. http://
    www.AlanTuring.net/tunny_report/
  * 59 Davis [12], pp. 174-175
  * 60 Source: Canada Science and Technology Museum / Musee des
    sciences et de la technologie du Canada. http://www.flickr.com/
    photos/cstm-mstc/; license: CC BY-NC-ND 2.0
  * 61 Source: Collections of the University of Pennsylvania Archive
  * 62 Source: Government Communications Headquarters
  * 63 Source: http://www.flickr.com/photos/senselessviolets/;
    license: CC BY-NC 2.0
  * 64 Source: Jack Good. Photo enhanced by Dustin Barrett and Parker
    Bright
  * 65 Source: National Security Agency. http://www.nsa.gov/about/
    _images/pg_hi_res/nsa_aerial.jpg
  * 66 Source: Government Communications Headquarters
  * 67 Crown copyright, National Archives, Kew
  * 68 Crown copyright, National Archives, Kew
  * 69 Source: Andrew Pearce, (c) Fotogenix.co.uk
  * 70 Source: Copeland, B.J. (ed.) The Essential Turing (Oxford:
    Oxford University Press, 2004), p.7
  * 71 Source: Beryl Turing and King's College Library, Cambridge
  * 72 Source: Archives of the Institute for Advanced Study,
    Princeton; photo by Alan Richards
  * 73 Source: http://www.AlanTuring.net/
    proposed_electronic_calculator
  * 74 Source: School of Computer Science, University of Manchester.
    Photo enhanced by Parker Bright
  * 75 Source: National Physical Laboratory, Teddington (Crown
    copyright). Photo enhanced by Jack Copeland and Dustin Barrett
  * 76 Reproduced from Copeland, B.J. Colossus: The Secrets of
    Bletchley Park's Codebreaking Computers (Oxford: Oxford
    University Press, 2010), p. 349