Throughout history, there was always a need for secret communication. The growth of radio technology and the lookout for more secure encryption have inspired many inventors to develop encryption devices. So, in the 20th century, Boris Hagelin invented an encryption device called C-38 which was adopted by US Army under the name of M-209. Hagelin’s M-209 was far in front of its time; it could perform encryption and decryption operations, it was relatively small, compact, portable and completely mechanical that needs no electricity or any other power source to operate. One of the disadvantages was that internal settings should be placed each time before use, which was quite a challenge for soldiers in battlefields. In this graduate thesis, the M-209 device is described with its main parts and characteristics. Also, the encryption process, based on Beaufort’s cipher, is elaborated from both a mechanical and mathematical perspective.