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OpenAI's Astra and Anthropic's Claude Opus 5 Crack Two WWII Enigma Messages Unsolved Since 2005

Two independent researchers used OpenAI's Astra and Anthropic's Claude Opus 5 to decrypt German Army Enigma messages that had resisted cryptanalysis for decades.

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OpenAI Anthropic Astra Claude Opus 5 Enigma cryptanalysis
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Overview

Two cryptanalysts say they have independently used large language models from OpenAI and Anthropic to crack a pair of German Army Enigma messages that had resisted decryption for decades, according to TechCrunch. Crypto Cellar, the cryptology research site run by Frode Weierud that hosts the archive of surviving Enigma messages, confirms the breaks with a banner reading: “BREAKING NEWS! OpenAI GPT-6 Astra breaking an Enigma message that has resisted solution since 2005.”

What We Know

  • Developer Carter Leffen told OpenAI’s newest model, Astra, to search a database of unbroken Enigma messages and decode one, according to TechCrunch. The model, working largely on its own, conducted its own archival research, built a simulator of the Enigma machine, and recovered the plaintext of a message — logged as MVUEH — that had baffled researchers since 2005, the outlet reports.
  • The MVUEH message, sent on July 10, 1941, was received by an SS-Totenkopf Division radio station, according to Crypto Cellar’s case-study page. Astra used the repeated place name “ROSENOW ROSENOW” as a crib to crack the cipher and identified that the message used a different wheel order from other messages sent the same day, which is why it had gone unsolved for so long.
  • Leffen also used Astra to build an interactive website laying out the case study. It gives the recovered German plaintext as “BTTE UM ANGABE DES MARSQWEGES X BEFINDE MIQ IN X ROSENOW ROSENOW X SOFORT FUNKANTWORT X WASCHBBSCH” — rendered in English as “Please specify the route of march. I am in Rosenow, Rosenow. Immediate reply by radio” — along with the recovered machine settings: rotor order II–V–III, ring settings H M F, and ten plugboard pairs, across an 82-letter message.
  • Weierud, described by TechCrunch as “a retired electrical engineer with a lifelong interest in cryptology” who maintains Crypto Cellar’s archive of surviving Enigma messages, validated Leffen’s solution and said it left him in “awe,” according to TechCrunch.
  • “GPT–6 Astra is behaving like a very professional cryptanalyst and archive researcher,” Weierud wrote, per TechCrunch. “What it has achieved in two days would take a human researcher weeks or even months. Personally, I spent several weeks researching the Bundesarchiv files GPT–6 Astra refers to.”
  • Separately, on September 21, cybersecurity executive Jack Willis told Weierud he had used Anthropic’s Claude Opus 5 to break a different unsolved message, logged as FMNGI and dated July 31, 1941, according to TechCrunch and Crypto Cellar’s case-study page. Unlike the Astra break, Willis provided Claude with significantly more guidance, including a cryptanalytical workbench written in Go and historical material, and the model used the known signature of a specific officer’s name to complete the break.
  • The FMNGI break took roughly 13 minutes to search for the correct key on an Apple M2 host, according to Crypto Cellar. The recovered plaintext contained apparent transcription or encipherment errors from the original radio operators, rendering “KOLJNNE” instead of “KOLONNE” and “ZURUEK” instead of “ZURUEQ.”
  • Weierud says just seven Enigma messages now remain unbroken in the collection, along with one additional message whose plaintext is already known but whose underlying key has not been recovered, according to TechCrunch.

What We Don’t Know

  • TechCrunch reports that Astra’s own logs referenced archived messages in a “private collection” not hosted on Weierud’s site, and that Weierud is unsure whether the model actually accessed that material or instead drew on records shared elsewhere online or in German government public archives.
  • Neither Crypto Cellar nor TechCrunch has published a full technical writeup of exactly how many candidate keys or search iterations each model evaluated before converging on a solution; Crypto Cellar’s account of the search process describes it only in general terms as a large-scale computational search across billions of possible machine settings combined with crib-based constraints.
  • It’s not yet clear whether OpenAI or Anthropic plan to discuss these breaks as part of any formal model capability evaluation, or whether either lab was involved in vetting the results beyond the independent validation performed by Weierud.

Analysis

The two breaks illustrate a contrast in how each model was used: Leffen’s instruction to Astra was, in his own framing, close to a single open-ended prompt — find an unsolved message and decode it — while Willis’s use of Claude Opus 5 involved substantially more human-supplied tooling and direction. Both approaches nonetheless converged on the same underlying achievement that a human team once required purpose-built hardware to accomplish during World War II: recovering the daily settings of an Enigma machine from ciphertext alone. With Weierud now counting only seven fully unbroken messages left in the historical collection, along with a single message whose plaintext is known but whose key remains elusive, these two results narrow one of the last remaining gaps in the decades-long effort to complete Turing-era cryptanalysis of the German Army’s wartime traffic.