Abstract: A system for securely distributing a communications key from a master unit to a remote unit for use in cryptographic communications between the master and remote units employs first and second secret numbers stored in both the master and remote units, and a random number generated in the master unit which is combined with the first secret number to produce a first intermediate number which is in turn combined with the second secret number to produce a second intermediate number. The second intermediate number is combined with the communications key to produce a transmission number sent with the random number to the remote unit. The remote unit, using the random number, the transmission number, and the first and second secret numbers, is able to reproduce the communications key.