An Enigma cipher cracked after 83 years: GPT-6 Astra found the key

The 82-character message had remained undeciphered since 10 July 1941. A German soldier asked for a marching route from Rosenow and demanded an immediate radio reply. The entire text had resisted decoding for 83 years.

Breaking Enigma comes down to brute force: around 159 quintillion daily settings. One property of the machine helps. It never encrypted a letter as itself.

The model found its own clue. Astra searched the archives and came across another radiogram from the same day, already decrypted, mentioning the same Rosenow. Then a classic cryptanalysis technique kicked in: a known fragment of plaintext sharply narrows the number of possible machine settings.

Previous attempts to read this message had failed. Now the model wrote cryptanalysis code, built an Enigma simulator, and ran parallel experiments: 43,016 search batches, 14.8 million tested keys, around 4.29 billion combinations. The compute took roughly 10 hours, spanning 14–15 September 2026.

You can verify it by hand. An independent check is available in the public swarm-ai-research/cipher-break-verification repository: it requires Python 3.10+ with no dependencies, runs with `python3 check.py`, and takes the ciphertext from the archives rather than the codebreaker. Leffen's case-study files can be downloaded from the project page, which also has the key verifier, search code, and a full 134.8 MB audit.

Frode Weierud's CryptoCellar archive has already marked the MVUEH message as decrypted on 14 September 2026. Weierud himself confirmed the key the next day, but differs from the author on attribution: Leffen says he led the research himself, while Weierud attributes the analysis mainly to GPT-6 Astra.

The same archive lists two more unsolved messages from July 1941: LXACA from 5 July and WEUWY from 9 July.