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 SECRET unless it was keyed with TOP SECRET keys, in which case its classification was permanently raised. It was authorized for TOP SECRET information with the appropriate key. It is no longer authorized for handling classified information, and given that there is a wikipedia article on it, it has been de facto declassified but has not been 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.