- Copeland, B. Jack, ed.
Colossus.
Oxford: Oxford University Press, 2006.
ISBN 978-0-19-953680-1.
-
During World War II the British codebreakers at
Bletchley Park
provided intelligence to senior political officials and
military commanders which was vital in winning the
Battle of the Atlantic
and discerning German strategic intentions in the build-up to
the invasion of France and the subsequent campaign in Europe.
Breaking the German codes was just barely on the edge of
possibility with the technology of the time, and required
recruiting a cadre of exceptionally talented and often highly
eccentric individuals and creating tools which laid the
foundations for modern computer technology.
At the end of the war, all of the work of the codebreakers
remained under the seal of secrecy: in Winston Churchill's
history of the war it was never
mentioned. Part of this was due to the inertia of the
state to relinquish its control over information, but also
because the Soviets, emerging as the new adversary, might adopt
some of the same cryptographic techniques used by the Germans and
concealing that they had been compromised might yield valuable
information from intercepts of Soviet communications.
As early as the 1960s, publications in the United States began to
describe the exploits of the codebreakers, and gave the mistaken
impression that U.S. codebreakers were in the vanguard simply
because they were the only ones allowed to talk about their
wartime work. The heavy hand of the Official Secrets Act suppressed
free discussion of the work at Bletchley Park until June 2000, when
the key report, written in 1945, was allowed to be published.
Now it can be told. Fortunately, many of the participants in the work
at Bletchley were young and still around when finally permitted to
discuss their exploits. This volume is largely a collection of their
recollections, many in great technical detail. You will finally understand
precisely which vulnerabilities of the German cryptosystems permitted
them to be broken (as is often the case, it was all-too-clever innovations
by the designers intended to make the encryption “unbreakable”
which provided the door into it for the codebreakers) and how sloppy
key discipline among users facilitated decryption. For example,
it was common to discover two or more messages encrypted with the
same key. Since encryption was done by a binary exclusive or (XOR)
of the bits of the
Baudot teleprinter code,
with that of the key (generated mechanically from a specified
starting position of the code machine's wheels), if you have two messages
encrypted with the same key, you can XOR them together, taking out the
key and leaving you with the XOR of the
plaintext
of the two messages. This, of course, will be gibberish,
but you can then take common words and phrases which occur in
messages and “slide” them along the text, XORing as
you go, to see if the result makes sense. If it does, you've recovered
part of the other message, and by XORing with either message, that
part of the key. This is something one could do in microseconds
today with the simplest of computer programs, but in the day was done
in kiloseconds by clerks looking up the XOR of Baudot codes in
tables one by one (at least until they memorised them, which the
better ones did).
The chapters are written by people with expertise in the topic discussed,
many of whom were there. The people at Bletchley had to
make up the terminology for the unprecedented things they were
doing as they did it. Due to the veil of secrecy dropped over their
work, many of their terms were orphaned. What we call “bits”
they called “pulses”, “binary addition” XOR,
and ones and zeroes of binary notation crosses and dots. It is all
very quaint and delightful, and used in most of these documents.
After reading this book you will understand precisely how
the German codes were broken, what Colossus did, how it was built
and what challenges were overcome in constructing it, and how it
was integrated into a system incorporating large numbers of intuitive
humans able to deliver near-real-time intelligence to decision makers.
The level of detail may be intimidating to some, but for the first
time it's all there. I have never before read any description
of the key flaw in the Lorenz cipher which Colossus exploited and
how it processed messages punched on loops of paper tape to break
into them and recover the key.
The aftermath of Bletchley was interesting. All of the participants
were sworn to secrecy and all of their publications kept under high
security. But the know-how they had developed in electronic computation
was their own, and many of them went to
Manchester
to develop the
pioneering digital computers
developed there. The developers of much of this technology could not speak
of whence it came, and until recent years the history of computing has been
disconnected from its roots.
As a collection of essays, this book is uneven and occasionally repetitive.
But it is authentic, and an essential document for anybody interested in
how codebreaking was done in World War II and how electronic computation
came to be.
March 2013 