How did Colossus read encrypted German messages from paper tape?

Colossus did not read encrypted German messages from paper tape in the same manner as its predecessor, the Heath Robinson. While the earlier Heath Robinson machine attempted to read two separate paper tapes simultaneously—one containing the message and one containing key data—this process was slow and unreliable due to the difficulty of keeping the tapes synchronized.1

Engineer Tommy Flowers designed Colossus to eliminate this synchronization issue by using only a single paper tape. This tape contained the encrypted German message, while the necessary key data was generated electronically by the computer’s own internal circuits. By reading only one tape, Colossus was able to operate at significantly higher speeds, processing 5,000 characters per second in its initial version and up to 25,000 characters per second in the later Mark II models.1