When the CIA was building its new headquarters, they commissioned a statue designed to pay homage to the spy agency. 'Krytpos' was both a sculpture and a mystery: the artwork contains encrypted messages, one of which — known as K4 — has never been decrypted. But just this year, as the artist was preparing to auction the solution, two journalists found K4's answer written in plain English in the Smithsonian archives. But have they really solved the K4 mystery?

• Guest: Dr Sam Blake, the Australian mathematician and cryptographer who solved the Zodiac Killer cipher Z340
• Producer: Alex Tighe, Catherine Zengerer