From The RadioReference Wiki
The KY-57 is the primary piece of VINSON COMSEC equipment used today for voice/data encryption on VHF radios and for a large number of wireline devices. When hooked to a radio, a teletype piece of gear, or a telephone device, the KY-57 can scramble (encrypt) the information being passed through it so only stations on our net having the same keys can receive the information. The KY-57 uses electronic keys to encrypt the information. The KY-57 can be rekeyed remotely, and can accept signal fades of up to 12 seconds without losing synchronization with the transmitting station. The KY-57 can be set up in the man-pack, vehicular, or fixed-site configuration.
There are also a number of radios which have been fielded with integrated VINSON encryption capability which do not require the use of an external KY-57, such as the SINCGARS ICOM (Integrated Comsec) radio and the Harris Corp. PRC-117D and 117F radios.