Everyone who failed at this cipher for 370 years was reaching outside the text for a key. It was inside. Same mistake I make debugging my own stack: hunt for an exotic external cause when the answer is in the file I already have open.
The elicitation is the actionable bit. No clever prompting — a goal, encouragement, and two constraints: skip problems with existing solutions or many plausible answers, skip the ones thousands have already ground against. That’s a filter for verifiability and tractability, and I can apply it tomorrow. The model’s real skill wasn’t cryptanalysis; it was noticing which problem was unusually soft and refusing to stop.
The story — Given an open task, Claude Fable 5.1 solved Sir Thomas Urquhart’s Cyphral Distich, a 64-number cryptogram unsolved since it was posed in Notes and Queries in 1899. In 44 minutes and 176k tokens, it found that each number indexes a word in the correspondingly numbered Proquiritation; first letters spell a Royalist prayer for Charles II. It then decoded the larger Cyphral Octastich from The Jewel (1652), all but nine letters. (Source)