Colossus: The Missing Manual

There has until now been no comprehensive, convenient, and reliable description of the actual capabilities of the Colossus codebreaking machines used at Bletchley Park during World War II, the way they were used, and the jobs they were applied to. This gap in the literature has led to a lack of understanding of the machines’ functionality and hence to exaggerated claims about their capabilities. In this report we remove the Colossi as far as possible from their cryptanalytical context and consider them simply as computational devices. We give an architectural description of the whole family of related machines, including the initial model known as “Heath Robinson”, and a functional description of the major capabilities of the second and final Colossus design. We include detailed examples of how the machines would have been set up to perform a range of typical tasks, and conclude with a discussion of their versatility, examining in particular the question of how useful they would have been once the war had ended. We present several examples of actual Colossus configurations and the historical output they produced, illustrating the cooperation between figures typed automatically by Colossus and text and annotations added by the human operator.