AI cracks Urquhart's 370-year-old cipher — the key was the book itself

Claude Fable 5.1, handed only an open-ended prompt to break an unsolved cipher, appears to have cracked Sir Thomas Urquhart’s Cyphral Distich — a 64-number cryptogram appended to his 1653 Logopandecteision that had resisted decryption since at least an 1899 query in Notes and Queries and earned a spot on Klaus Schmeh’s list of the top 50 unsolved encrypted messages. Running for 44 minutes with no human intervention, the model concluded that generations of analysts failed because they assumed an external cipher alphabet. The real key was the source text itself.

The break hinged on two textual clues: each cipher line holds exactly 32 numbers, mirroring Urquhart’s 32 ‘Proquiritations,’ and an accompanying poem promised the reader would find ‘his own heart’s wishes.’ The decoding rule was that the i-th number selects the i-th Proquiritation, uses the number as a word index, and takes that word’s first letter. The output is a rhyming couplet — a royalist prayer for Charles II, fitting Urquhart’s politics — that self-verifies at 32 letters per line.

The model then reused the same idea, page-indexed rather than paragraph-indexed, on a second and longer Urquhart cryptogram: the 285-number Cyphral Octastich from The Jewel (1652), recovering all but nine letters of an eight-line prayer while flagging transcription ambiguities and the absence of page scans needed to confirm the gaps. The author frames the achievement less as a feat of raw difficulty — the clue is embarrassingly simple in hindsight — than of judgment: where other frontier models had failed for months, Fable 5.1 knew to walk away from intractable puzzles and spot this one as unusually solvable.