A message sent to a German military radio station in 1941 has yielded its Enigma key and plaintext. Anthropic’s Claude Opus 5 helped Jack Willis solve the message, known as FMNGI, and cryptography archivist Frode Weierud checked the result against an outgoing copy preserved in the archives.
How they reached that result matters. Willis supplied Claude with a cryptanalysis workbench written in Go and historical material that led to a promising name-based crib, or suspected fragment of plaintext. Weierud describes the attack as strongly guided by a human. It is a documented AI-assisted solution, not evidence that Claude can routinely break Enigma messages on its own.
A second recent break offers a useful contrast. Weierud says OpenAI’s GPT-6 Astra took more initiative in selecting and developing an attack on a different message, MVUEH. Together, the cases show what these models can contribute to historical research and why the conditions behind a successful result matter.
An Archival Message Provides a Check on the Solution
FMNGI was an incoming Enigma message dated July 31, 1941, and recorded by the quartermaster’s radio station of the SS-Totenkopf Division. Its received message form identifies the sender only by a tactical radio callsign. Crypto Cellar Research had published a transcription in 2005, but the message remained unsolved.
In Weierud’s account of the FMNGI break, Willis reported that Claude’s search recovered an Enigma key and this plaintext:
KOLJNNEVONXKORPSNACHSCHUBZURUEKXHARTJENSTEINXHARTJENSTEINX
Enigma messages did not preserve spaces, so interpreting that sequence requires more than inserting word breaks. Weierud reads its opening as an imperfect rendering of Kolonne von Korpsnachschub zurück, followed by the repeated name Hartjenstein. The odd-looking letters are part of the evidence: KOLJNNE contains an error, and Weierud says ZURUEK should have been encoded as ZURUEQ under the operators’ convention of replacing CK with Q.
Weierud could compare the recovered text with an outgoing archival message, No. 205 NF. That provides a check beyond a phrase that merely looks plausible in German. The comparison supports the reported recovery while revealing mistakes made around the message’s original preparation or transmission.
The archival record also calls for a qualification. The outgoing collection, which included material related to FMNGI, had been found and added to Crypto Cellar Research’s message resources in July 2026. This was not the recovery of text for which no corresponding archival record existed anywhere. Weierud says he thought to consult the full plaintext after Willis reported the break; his account of Claude’s attack instead centers on ciphertext transcription, related traffic and a shorter crib.
