a method for the encryption , transmission , and subsequent decryption of digital keying data . the method utilizes the data encryption standard and is implemented by means of a pair of apparatus , each of which is selectable to operate as either a master unit or remote unit . each unit contains a set of key encryption keys which are indexed by a common indexing system . the master unit operates upon command from the remote unit to generate a data encryption key and encrypt the data encryption key using a preselected key encryption key . the encrypted data encryption key and an index designator are then downloaded to the remote unit , where the data encryption key is decrypted for subsequent use in the encryption and transmission data . downloading of the encrypted data encryption key enables frequent change of keys without requiring manual entry or storage of keys at the remote unit .