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Why did human code breakers still need to work with Colossus output?
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Colossus
Funding for this code-breaking machine came from the Ultra project.
Human code breakers were essential to the process because Colossus was not designed to perform the entire decryption process on its own. Instead, its primary function was to strip away only the first layer of encryption from the German "Tunny" messages.1
The output produced by Colossusāreferred to as a "de-chi"āremained an encrypted message. Human code breakers were required to take this intermediate output and perform the final steps necessary to strip away the remaining encryption, ultimately revealing the German plaintext. By working in concert with these human collaborators, the Colossus computers were able to provide the Allies with critical intelligence regarding German military intentions, supplies, and unit positions.1