Colossus computer





Colossus was the name for any of at least two versions of the world's first
programmable (to a limited extent) digital electronic computer.

It was built by British Post Office at its Dollis Hill Research Station by
Thomas Flowers and crew to a design by Max Newman and associates of
Bletchley Park.

Purpose

It was primarily designed for Cryptanalysis in an attempt to break one of
the Fish cyphers (a Bletchley Park term) used by the German military for its
most secure strategic communications. These were teletype cypher machines in
the spirit of that first proposed by Col Parker Hitt of the US Army around
WWI. The German machines were, essentially, attempts at an electromechanical
implementations of the one-time pad cypher invented by Gilbert Vernam (Bell
Labs) and Joseph Mauborgne (Signal corps, USA) in the US at the end of WWI.
The most important was a teletype based machine built by Lorenz Electric,
the SZ-40 (and later SZ-42) Schlusselzusatz (meaning, more or less,
'auxiliary key').

Another, different, teletype cypher machine was designed and built by
Siemens & Halske, the T-52 Geheimfernschreiber (meaning, more or less,
'secret writer'). Early versions of the Siemens machine (the T-52a and
T-52b) were used to send signals between Germany and Norway over a cable
running through Sweden. The Swedes tapped the cable, copied the traffic, and
Arne Beurling, a Swedish mathematician, broke the cypher. Later production
versions of the T-52 (there were variants through 'e') were harder, even for
Bletchley Park. Some of the T-52 traffic was also sent over Luftwaffe Enigma
networks, and so T-52 traffic was a lower priority for Bletchley Park than
might have otherwise been expected.

The one-time pad requires a random sequence. It is combined with the
plaintext (bit by bit, usually as character by character) resulting in the
cyphertext which is transmitted. On receipt, the same random sequence is
combined with the cyphertext (again character by character), and because the
combining operation is reversible in a particular way (see XOR, for example)
the output is the original plaintext. In the German Fish machines, the
'random' sequence was produced by various electromechnaical arrangements (on
one of them, these were rotors somewhat as in the US SIGABA machine), and
wasn't actually random. Because there were patterns, they could be predicted
if the cryptanalysts were sufficiently clever, and plaintexts recovered. In
the case of the Lorenz machine, Col John Tiltman and Bill Tutte of Bletchley
Park were sufficiently clever.

Origins

The idea for Colossus developed out of a prior project which produced a
special purpose opto-mechanical comparator machine called the Heath
Robinson. The Colossus was intended to be more flexible and faster, and in
the bargain less subject to the vagaries of paper tape stretching when
moving at high speeds; it was decided to make it programmable in a way the
Heath Robinson had not been. The project was headed by the mathematician Max
Newman. It started early in 1943 and the first version of the machine (Mark
1 Colossus) was finished and installed by about January 1944, to be followed
by the improved Mark 2 Colossus in June 1944. Ten Mark 2 Colossus machines
were in use at Bletchley Park by the end of the war.

Design and operation

Since solid state electronics had not yet been invented, the machine used
vacuum tubes and optical devices to read a cyphertext from a paper tape and
then applied a programmable logical function to every character, counting
how often this function returned "true".

Whilst Colossus did feature limited programmability, it was not a true
general purpose computer, not being Turing-complete.

Influence

Colossus was a highly secret device, and had therefore not much influence on
the development of later computers. Nearly all documentation and machinery
was classified immediately after the war, and destroyed in the 1960s. It is
said that Winston Churchill specifically ordered the destruction of the
Colossus machines into 'pieces no bigger than a man's hand' and that Tommy
Flowers personally burned the blueprints in a furnace at Dollis Hill.
Information about Colossus reemerged in the 1970s. Due to this secrecy, it
was not able to be included in the history of computing for many years.

Reconstruction

A copy of one of the Colossus versions has been partly completed by Tony
Sale and is on display in the Bletchley Park Museum in Milton Keynes,
Buckinghamshire.

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Colossus was also the name of a fictional computer that takes over the world
in the 1969 science fiction film "Colossus: the Forbin Project," loosely
based on the novel Colossus by Dennis Feltham Jones. It has been speculated
that Jones named his rogue computer after the "real" Colossus, because of
the secrecy that surrounded the project.