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Probably a Code?

Is the regularity shown in this encoded message noise or structure?

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More Secret Transmissions

In 'Secret Transmissions', Agent X could send four-digit codes error free. Can you devise an error-correcting system for codes with more than four digits?

Stage 5 Cipher Challenge

Age 16 to 18 Challenge Level:

Our Cipher Challenge is a very difficult sequence of ciphers suitable for keen groups or individuals, maths clubs and very optional homework challenges. Don't try this in the classroom!