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Vals.ai gave Claude Fable 5.1 a single open-ended task: solve the Cyphral Distich, an encrypted couplet published by Sir Thomas Urquhart at the end of his 1653 work Logopandecteision. The cipher had resisted decryption for roughly 373 years.

The model returned a solution in 44 minutes, consuming 176,000 tokens. No human intervened.

The Cipher

The Cyphral Distich is 64 numbers arranged in two lines of 32:

5.3.27.38.32.14.21.8.66.8.70.39.5.9.12.18.2.3.56.5.1.7.3.2.13.19.3.25.9.3.16.6.
25.15.13.6.11.20.5.1.2.12.1.20.20.49.20.20.35.33.4.6.8.35.5.33.5.5.18.10.3.11.32.42.

It was posed as an open problem in Notes and Queries in 1899, appeared in cryptography literature through the 20th century, and was catalogued by historical-cipher researcher Klaus Schmeh among his Top 50 unsolved encrypted messages. Prior attempts applied frequency analysis, substitution, and homophonic substitution. None succeeded.

The Key Was Already There

Fable 5.1 noticed two things human decipherers had missed.

First: the cryptogram appears immediately after Urquhart’s 32 Proquiritations, and Urquhart explicitly draws attention to the number in the surrounding text — “there can no number like that of two and thirty … be pitched upon.”

Second: the accompanying poem promises the reader will find in the cipher “his own heart’s wishes, and the Author’s minde.” The Proquiritations themselves repeatedly close with phrases like “is the desire,” “wish,” or “hope of.”

The rule: for the i-th number in each cipher line, take the i-th Proquiritation, count to that word, and read its first letter. The key was not an external cipher alphabet. It was the book.

The Solution

O GOD UPHOLD KING CHARLS THE SECOND AND
MAKE HIM THE SUPREME RULER OF THIS LAND

Each line contains exactly 32 letters. The couplet rhymes. Urquhart was a committed Royalist; hiding a prayer for Charles II in a dense scholarly text is precisely what you would expect of him.

The Octastich

Urquhart left a second, much larger cryptogram in The Jewel (1652): the Cyphral Octastich, 285 numbers across eight lines, equally unsolved. Fable 5.1 applied the same logic and recovered all but nine letters.

What This Shows

Bruce Schneier noted the result on his security blog: “It’s good at things that involve lots of searching and testing.” That framing is exact. The Cyphral Distich was not cryptographically hard — it was a retrieval and cross-referencing problem requiring patience and a willingness to look for structure inside a 17th-century text rather than outside it. Prior researchers searched for an external cipher key. Fable 5.1 found an internal one.

Vals.ai reports that months of attempts with other frontier models produced no verified decryption before this run. The Cyphral Distich joins a short list of historical ciphers solved with computational assistance — but it appears to be the most long-standing to yield to a large language model working entirely without human guidance.