Two long-unsolved Enigma messages fall within six days: OpenAI's GPT-6 Astra broke MVUEH unaided, Anthropic's Claude Opus 5 broke FMNGI with human guidance — seven unbroken messages remain

“An AI agent cracked an Enigma message that had resisted codebreakers since 2005 — and it chose that target all by itself.”

Usable claims

• On Friday, September 25, 2026 (calendar-verified), TechCrunch reported (Tim Fernholz, 10:24 AM PDT) that two long-unsolved Enigma messages from 1941 have been broken with AI models — one unaided, by OpenAI's GPT-6 Astra, and one with strong human guidance, by Anthropic's Claude Opus 5 — as documented on cryptanalyst Frode Weierud's Crypto Cellar Research site. Per TechCrunch: "Now, a pair of cryptanalysts say they've cracked two long-unsolved messages encoded by Enigma machines using LLMs built by OpenAI and Anthropic." The MVUEH break, per Weierud's page (updated 24 September 2026 at 20:23 UTC): "On 15 September 2026, Carter Leffen contacted me to request validation of a break of the German Army Enigma message MVUEH of 10 July 1941," a message of which Weierud writes: "Since 2005, the message has resisted all attempts to break it." Weierud writes that "After Carter Leffen sent the MVUEH break details, it was immediately clear he had found the correct key and plaintext," and that "the most astonishing thing about this break is that the GPT–6 Astra did it entirely on its own. Carter Leffen only directed GPT–6 Astra to see if it could break any of the unbroken Enigma messages published on the Crypto Cellar Research web page." Per the page, Astra chose message Nr. 172 MVUEH itself, suspected a relation to the already-broken Nr. 173 SIPVX, "focused on using the repeated place name ROSENOW ROSENOW as a crib," and developed "the necessary Python and C++ software for an Enigma simulator and an Enigma Bombe" before the break succeeded. The page explains why the message had resisted: the ciphertext transcription contained several errors, and "the break revealed that the Enigma's left-hand wheel makes a turnover at the 72nd letter" — a rare event known to complicate a break. Weierud's verdict, quoted in full sentences from the page: "What is clear, though, is that GPT–6 Astra is behaving like a very professional cryptanalyst and archive researcher. What it has achieved in two days would take a human researcher weeks or even months." And: "The AI break of the Enigma message MVUEH is simply amazing, and as an old cryptanalyst, I am still in awe." Per TechCrunch, Leffen "even used Astra to build an interactive website explaining the whole problem" — the site returned a Cloudflare block page this run, so its contents are not described here. The open question, per TechCrunch: "the Astra model's logs include discussion of archived messages in a "private collection" that aren't hosted by Weierud. He still isn't sure if the model accessed them or not, but speculates they may have been shared by a different researcher somewhere online, or that the model was able to access the German government's public archives." Per Weierud's page: "The file references GPT–6 Astra mentions, RS 3–3/20a and RS 3–3/63b, are correct, but they are not available on the Crypto Cellar Research website," and "it is not clear what this is, whether it has succeeded in accessing the Bundesarchiv's digitised collections or whether it has found these files elsewhere." The FMNGI break, per Weierud's page (updated 24 September 2026 at 05:29 UTC): "On 21 September 2026, I received an email from Jack Willis announcing the breaking of another Enigma message, Nr. 285 FMNGI of 31 July 1941" — "only six days after the MVUEH break" — and "While the MVUEH break used OpenAI's GPT–6 Astra, FMNGI used Anthropic's Claude Opus 5," in an attack that "was more directed and relied on strong human guidance": "Jack Willis gave Claude a cryptanalytical workbench written in Go, the necessary historical material and set it free to investigate." Per the page, the final search "happened on 20 September 2026" and "took 13 minutes and 28 seconds on an Apple M2 host," using a 14-letter crib built on SS officer Friedrich Hartjenstein's signature (XHARTJENSTEINX). Per TechCrunch: "Willis provided significantly more guidance to Claude, which was ultimately able to use the known signature of a particular officer's name to break the message." The remaining set, per Weierud's FMNGI page: "Of the close to 1000 Enigma and Truppenschlüssel messages in the German Army collection, only seven Enigma messages remain unbroken, along with one puzzle, Nr. 138 WEUWY, that we know must have identical plaintext to another message from the same day, Nr. 140 WEUWY."