M-209

In cryptography, the M-209, designated CSP-1500 by the Navy (C-38 by the manufacturer) is a portable, mechanical cipher machine used by the US military primarily in World War II, though it remained in active use through the Korean War. The M-209 was designed by Swedish cryptographer Boris Hagelin in response to a request for such a portable cipher machine, and was an improvement of an earlier machine, the C-36.

The M-209 is about the size of a lunchbox, in its final form measuring 3.25 by 5.5 by 7 inches (83 by 140 by 178 mm). It represented a brilliant achievement for pre-electronic technology. It used a wheel scheme similar to that of a telecipher machine, such as the Lorenz cipher and the Geheimfernschreiber.