1942: German Navy introduced 4-rotor Enigma
Learn more about the Enigma exhibit at the National Cryptologic Museum or read the NSA publication, "Solving the Enigma," by Jennifer Wilcox via the links at the bottom of this page. You can also read about the 4-rotor Enigma via the Cryptomuseum.org site (a virtual museum) - see link below.
Excerpted from NSA:
"Possibly the most well known of all cipher machines is the German Enigma. It became the workhorse of the German military services, used to encrypt tens of thousands of tactical messages throughout World War II. The number of mathematical permutations for every keystroke is astronomical. However, the Enigma is not famous for its outstanding security, but rather for its insecurities. Allied forces were able to read most of the Enigma encrypted messages throughout most of the war as a result of the tireless effort of many Allied cryptologists."
Excerpted from the Cryptomuseum.com site:
"In the U-boats, the Enigma was usually located in the radio room, although in some cases it was carried out to the captain's quarters, e.g. in case of a double encipherment (Sonderschlussel M). Most U-boats even had two Enigma machines available to cope with different keys around the switch-over point at midnight. One Enigma would then be left with the settings of the previous day, whilst the other one was configured with the settings for the new day. As some messages were received with a delay, they could quickly be tried with both keys. "