Colossus was the first electronic computer, but news of its existence was kept top secret for 30 years because of the sophistication and sensitivity surrounding the encryption it had helped to break.
Colossus Mark I began operating on 5 February 1944, and was succeeded in June of that year by the Mark II.
The statistics of Colossus are astounding. It occupied the size of a living room (7 ft high by 17 ft wide and 11 ft deep), weighed five tonnes, and used 8kW of power. It incorporated 2,500 valves, 501 of which are thyraton switches, about 100 logic gates and 10,000 resistors connected by 7 km of wiring.
Reading 5000 characters per second (faster than anything ever produced commercially), Colossus found the start wheel positions of Lorenz-encrypted messages to enable the decryption of 63 million characters.