crypto devices, stumbling over M-209 and "US researcher Dennis Ritchie has described a 1970s collaboration with James Reeds and Robert Morris on a ciphertext-only attack on the M-209 that could solve messages of at least 2000–2500 letters.[3] Ritchie relates that, after discussions with the NSA, the authors decided not to publish it, as they were told the principle was applicable to machines then still in use by foreign governments.[3]" https://en.wikipedia.org/wiki/M-209 The paper https://cryptome.org/2015/12/ReedsTheHagelinCipherBellLabs1978.pdf ends with "The program takes about two minutes to produce a solution on a DEC PDP-11/70." No info on the program coding. More info around the story from Richie himself https://www.bell-labs.com/usr/dmr/www/crypt.html