KY-57
The Speech Security Equipment (VINSON), TSEC/KY-57, is a portable, tactical cryptographic device in the VINSON family, designed to provide voice encryption for a range of military communication devices such as radio or telephone.

The KY-57 was in use by NATO and its allies towards the end of the cold war.  The device itself was classified as a CCI (controlled cryptographic item) when it was unkeyed. The classification of the device was temporarily raised to the classification of the key when the device was keyed.  It was authorized for TOP SECRET information with the appropriate key.  It is no longer authorized for handling classified information, and it has been de facto, but not officially, declassified.  The details of its technical operation are still classified.  The first unit, serial number 001 is still in operation at the NSA.

The KY-57 can accept signal fades of up to 12 seconds without losing synchronization with the transmitting station. There are storage positions for 6 keys. Keys 1 to 5 are traffic encryption keys (TEK). Key 6 is a key encryption key (KEK) used for over the air rekeying (OTAR) of the other 5 keys. Key 6 must be loaded manually using a fill device such as the AN/CYZ-10.

See also
 NSA encryption systems
 FNBDT
 ANDVT
 SINCGARS
 AN/PRC-77

References

National Security Agency encryption devices