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What part of the Tunny decryption process did Colossus perform?
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Colossus
Funding for this code-breaking machine came from the Ultra project.
Colossus was designed to perform a fundamental stage of the Tunny code-breaking process at electronic speed. Its specific role was to strip a first layer of encryption from the intercepted German messages.1
The output of this process, known as a "de-chi," was still an encrypted message that was then passed to human code breakers. These individuals performed the subsequent work of stripping away the remaining layers of encryption to reveal the original German plaintext. By automating this initial, time-consuming stage, Colossus allowed the code-breaking team to process a significantly higher volume of messages than was possible using manual methods or earlier, less reliable machines like the "Heath Robinson".1