was, however, considered adequate for tactical use and was still used by the US Army during the Korean War. 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. 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. In 2004, German news site Heise Online published a feature about the German efforts to break the M-209. Production and usage The U.S. M-209s were produced at a rate of 400 units per day by Smith Corona Typewriter Company in Groton, NY, starting in 1942. Over 140,000 machines were produced. It gradually replaced the older M-94 tactical cipher. The German SG-41 was supposed to have been a standard tactical cipher machine, but the Germans had only limited supplies of lightweight metals such as magnesium and aluminum, and it was simply too heavy for tactical use. Menzer also worked on two other cipher machines based on Hagelin technology, including a follow-on to the Enigma, the "SG-39", and a simple but fairly strong handheld cipher machine, the "Schlüsselkasten" ("Code Box"). Neither of these machines reached production. Had the Menzer devices been put into service, they would have certainly caused trouble for Allied cryptanalysts, though they were no more uncrackable than the M-209. After the war, Hagelin came up with an improved model of the M-209, designated the "C-52". The C-52 featured a period of up to 2,756,205,443; wheels that could be removed and reinserted in a different order; and a printwheel with a mixed alphabet. However, the C-52 was one of the last generation of the classic cipher machines, as by that time the new digital technology was permitting the development of ciphers that were far more secure. References Further reading J. Reeds, D. Ritchie, R. Morris, "The Hagelin Cipher Machine (M-209): Cryptanalysis from Ciphertext Alone", unpublished technical memorandum, Bell Laboratories, 1978. Submitted to Cryptologia (). External links Dirk Rijmenants' M-209 Simulator for Windows The 1944 M-209 Manual Jerry Proc's page on the M-209 Nick Gessler's page on the M-209 A M-209 simulator written in Python Encryption devices World War II military equipment of the United States Cryptographic hardware United States Army Signals Intelligence Service