Christos T. • June 26, 2024 12:36 AM

The converter M-209 was the medium level cipher system of the US military in the period 1943-45. The US Army used it at Division level (Division-Regiment-Battalion and even up to Corps) also widely used by the USAAF and US Navy.

The regular solution of the M-209 in the period 1943-45 was an impressive achievement for the German side and also the Japanese had some success from late 1944.

Regarding its cryptosecurity the expert on classical cipher systems George Lasry has stated:
(http://scienceblogs.de/klausis-krypto-kolumne/2018/01/21/top-50-cryptogram-solved/)

‘One comment about the security of the M-209. The claim that the Enigma is more secure than the M- 209 is disputable.

1) The best modern ciphertext-only algorithm for Enigma (Ostward and Weierud, 2017) requires no more than 30 letters. My new algorithm for M-209 requires at least 450 letters (Reeds, Morris, and Ritchie needed 1500). So the M-209 is much better protected against ciphertext-only attacks.

2) The Turing Bombe – the best known-plaintext attack against the Enigma needed no more than 15-20 known plaintext letters. The best known-plaintext attacks against the M-209 require at least 50 known plaintext letters.

3) The Unicity Distance for Enigma is about 28, it is 50 for the M-209.

4) The only aspect in which Enigma is more secure than M-209 is about messages in depth (same key). To break Enigma, you needed a few tens of messages in depth. For M-209, two messages in depth are enough. But with good key management discipline, this weakness can be addressed.

Bottom line – if no two messages are sent in depth (full, or partial depth), then the M-209 is much more secure than Enigma’.