vals.ai gave Claude Fable 5.1 an open-ended assignment: go solve an unsolved cipher. The model picked Sir Thomas Urquhart’s Cyphral Distich, two lines of 32 numbers each printed at the end of his 1653 book Logopandecteision, and the lab reports it came back solved within a day — 44 minutes and 176,000 tokens of work, with no human interjections after the initial prompt.

The puzzle had been open for roughly 370 years. It was posed in Notes and Queries in 1899, discussed in 20th-century cryptography literature, and listed by cipher researcher Klaus Schmeh among his Top 50 unsolved encrypted messages.

  • Earlier attempts tried frequency analysis, simple substitution and homophonic substitution, and all failed. According to the author, they missed a clue sitting inside the book itself.
  • The cryptogram is printed immediately after Urquhart’s 32 Proquiritations — a number he goes out of his way to stress: “there can no number like that of two and thirty … be pitched upon.” 32 Proquiritations. 32 numbers per line. Two lines.
  • The poem next to the cipher promises the reader will find in it “his own heart’s wishes, and the Author’s minde,” and the Proquiritations keep ending with phrases like “is the desire,” “wish,” “hope of.”

So the key was not a cipher alphabet reconstructed from outside the text. The key was the book:

  • Take the i-th number in a cipher line.
  • Go to the i-th Proquiritations.
  • Use that number as an index into its words, and take the first letter.

That yields a rhyming couplet, exactly 32 letters per line, ending and / land as a “distich” (a two-line verse) promises: “O GOD UPHOLD KING CHARLS THE SECOND AND / MAKE HIM THE SUPREME RULER OF THIS LAND.” Urquhart was a committed Royalist, so a hidden prayer for Charles II fits his politics.

The model then worked a second, much larger cryptogram in the same style — the Cyphral Octastich in The Jewel (1652), 285 numbers instead of 64 — and returned it “all but nine letters,” by indexing into pages rather than paragraphs. The write-up is unusually careful about what is not nailed down: a line spelled I-R-S-H, an unreadable nine-letter stretch attributed to a slip or a misprint, a page-shift partway through, and no free scans of the 1652 book to confirm the ±1 positions.

The elicitation part is the piece with the widest implications. Months of attempts with other frontier models produced no verified solve. What worked was a goal, some encouragement, and a comparison — the author pointed the model at its own strongest math results and suggested this should be easy by comparison. Two constraints were imposed: avoid ciphers that already have answers or admit many plausible ones, and avoid the very hardest problems (Kryptos K4, he suspects, is still too convoluted). The stated takeaway is about attention rather than cryptanalytic brilliance: these problems were historically bottlenecked by humans willing to spend days on unpromising archival leads, and “that bottleneck is seemingly disappearing.”

The 205-comment thread on Hacker News is unusually substantive for a result like this, because it splits into two arguments the write-up cannot settle itself: whether the solve can be independently confirmed, and whether it demonstrates capability at all.

What the thread adds

  • chr15m — the sharpest dissent, on what was actually asked. Tell a model to find an unsolved cipher it can solve and of course it finds the tractable one: “It’s demo porn. … You do it enough times and don’t show the misses.” They also compare it to the “build me a cyberpunk FPS” demos — impressive, but not the game anyone asked for.
  • GranPC, SahAssar and fxwin — a live verification chase, which is the most useful thing on the page. GranPC: “I am trying very hard to find an original version of this cipher with no luck. It almost sounds like this whole thing is a hallucination…?” SahAssar can’t find it in two archive.org copies of the book or on Schmeh’s site. fxwin then finds the 2014 German-language blog thread asking readers to crack the Distich, and the archive.org scan of the Proquiritations in the Maitland Club edition. The trail exists, but it is thin and mostly not freely scanned north of the 1834 reprint.
  • vb-8448 and mvkel — the low-hanging-fruit reading, arriving from two directions. vb-8448: “I wonder how many of the recent results are due to the fact that very few looked at the problem to start with.” mvkel: “it’s not that a human -can’t- do it, it’s just that no human would dedicate their entire lifetime to solving this niche challenge.”
  • elahieh — practitioner corroboration that this is a repeatable workflow, not a one-off. They describe running Opus 5 over cipher historian Satoshi Tomokiyo’s Cryptiana archive and working the US Civil War Stager ciphers, and offer the line that matters for anyone trying it: “The art of solving historical unsolved ciphers is knowing what is on the boundary of solvability.”
  • SamBam — pushback on dismissing persistence as brute force: “I don’t think we can call ’trying lots of different ideas for an extended period’ ‘brute-forcing,’ unless we use that term for lots of humans who have struggled with hard math problems for years.”
  • redfloatplane and flir — hobby-scale corroboration, which is closer to most readers’ reality than the cipher is. redfloatplane used Claude to finally map unlisted early-1800s formal gardens in Ireland after wanting to for a decade; flir reports similar work where GPT “reduced the task from ‘infeasible’ to ‘annoying’.”
  • Retr0id and flir — the technique drew its own skepticism. flir: “Wait. Wait wait wait. Are we supposed to be giving them pep talks?” Retr0id wonders whether showing a weaker model others’ successes could make it more ambitious “even if it’s not fundamentally any smarter,” and jrop supplies the pre-AI version of that story — George Dantzig solving two “homework problems” that were actually unsolved statistics problems.

The question the thread keeps asking

The write-up concedes its caveats, but it never says how an outsider confirms the answer when the author has been dead for centuries. GranPC asks why the primary source is so hard to find at all, and notpachet asks the general form of it: is there any check that a cipher is decodable in the first place, if the encoder made a mistake and left it unsolvable? Two independent handles asking the same thing in different words is the honest shape of the gap — self-consistency (rhyme, line length, the author’s politics) is strong evidence, but it is not a witness from 1653.

Handles here are pseudonymous and HN publishes no per-comment scores, so the ordering above reflects HN’s own ranking rather than a vote, and this is a slice of a 205-comment thread — not a consensus.