AI Model Claude Fable 5.1 Solves 370-Year-Old Urquhart Cipher

The Claude Fable 5.1 AI model solved Sir Thomas Urquhart's Cyphral Distich, a 64-number cryptogram unsolved since 1653, recovering a hidden Royalist prayer for Charles II — a result highlighted by Bruce Schneier as a notable example of LLM cryptanalysis.

Vals AI reports that the Claude Fable 5.1 AI model solved the Cyphral Distich, a 64-number cryptogram from Sir Thomas Urquhart's Logopandecteision that had remained unsolved since 1653 (370 years), completing the decryption in 44 minutes using 176k tokens with no human hints. The key insight was that the cipher's key was the book itself: each number indexes a word in the corresponding Proquiritation, and taking the first letter yields the plaintext 'O GOD UPHOLD KING CHARLS THE SECOND AND MAKE HIM THE SUPREME RULER OF THIS LAND' — a Royalist prayer historically consistent with Urquhart's politics. The model also deciphered the larger 285-number Cyphral Octastich from The Jewel (1652) using page-based word indexing, recovering all but nine letters of a Royalist prayer. The puzzle had been listed among Klaus Schmeh's Top 50 unsolved encrypted messages. Bruce Schneier covered the result on his blog, framing it as a notable demonstration of LLMs applied to classical cryptanalysis, though his post provided limited technical detail beyond the headline. Sources do not disagree on substance; Schneier's post simply offers fewer specifics than the Vals AI/Hacker News report.

• Claude Fable 5.1 solved the Cyphral Distich, a 64-number cryptogram from Sir Thomas Urquhart's Logopandecteision, unsolved since 1653.
• Solve took 44 minutes and 176k tokens, with zero human hints/interjections.
• Cipher key was internal to the book: each number indexes a word in the corresponding one of the 32 Proquiritations; taking first letters yields the plaintext.
• Recovered plaintext: 'O GOD UPHOLD KING CHARLS THE SECOND AND MAKE HIM THE SUPREME RULER OF THIS LAND' — a Royalist prayer for Charles II consistent with Urquhart's politics.
• The model also cracked the 285-number Cyphral Octastich from The Jewel (1652) via page-based word indexing, recovering all but nine letters of a Royalist prayer.
• The puzzle had been listed among Klaus Schmeh's Top 50 unsolved encrypted messages.
• Bruce Schneier highlighted the result on his blog (2026-09-09) as an example of LLM classical cryptanalysis; detailed technical account came via Vals AI on Hacker News (2026-09-13).