Claude Fable 5.1 cracked a riddle that had held out for 370 years

Researchers at vals.ai set Claude Fable 5.1 an open-ended task: to decipher the “Cyphral Distich” of Sir Thomas Urquhart — a puzzle that had defied every attempt to crack it for more than three and a half centuries. The model did it in a single day.

The riddle consists of two lines of 32 numbers each, placed at the end of Urquhart's treatise Logopandecteision. It was put forward as an open problem in Notes and Queries as far back as 1899, it was discussed in the cryptographic literature of the twentieth century, and the cipher historian Klaus Schmeh included it in his list of the 50 most famous unsolved messages. Frequency analysis, substitution and homophonic methods — nothing worked.

The book itself was the key

After spending 44 minutes and 176 thousand tokens without a single hint from a human, Fable 5.1 found the solution. Every earlier attempt had assumed the key was external — some cipher alphabet that had to be reconstructed from the outside. In reality, the key was the text of the treatise itself.

The model spotted two clues. First, the cipher comes right after Urquhart's 32 “Proquiritations,” and the author makes a point of emphasizing that number. Second, the accompanying verse promises that an honest reader will find in it “the desires of his own heart.” The rule turned out to be simple: the i-th number in a line is the position of a word within the i-th Proquiritation, and you take the first letter of that word.

The result was a rhymed prayer: “O GOD UPHOLD KING CHARLS THE SECOND AND / MAKE HIM THE SUPREME RULER OF THIS LAND.” Each line contains exactly 32 letters and ends on a rhyme — the solution is self-confirming. It is also historically sound: Urquhart was a committed royalist, and a hidden prayer for Charles II is entirely in keeping with him.

Using the same principle, the model also broke Urquhart's second, far larger cipher — the “Cyphral Octastich” from The Jewel (1652), running to 285 numbers. Here the index is the page of the book. Almost the whole royalist ode, cast in the form of an octave, was deciphered; only nine letters remained unidentified at one spot, where it seems either the author himself made a mistake or the numbers were garbled in printing.

Not genius, but persistence

Gebi Jaff, who ran the experiment, had spent several months trying to get a similar result out of various frontier models — without success. All he gave Fable 5.1 was the goal, asking it to avoid ciphers with already known or ambiguous solutions and not to take on the very hardest cases such as K4 from “Kryptos.”

Tellingly, the solution required no outstanding cryptanalysis — on the contrary, the clue was elementary. In the past, tasks like this ran up against human attention: someone had to spend hours reading obscure texts and checking hypotheses that led nowhere. The model simply kept looking until it found the answer — and, as the author notes, it is precisely this doggedness that may prove useful far beyond cryptography.

Source: Hacker News