encryption systems typically rely on the distribution of cipher keys between terminals for scrambling and unscrambling transmitted messages . elaborate security precautions are necessary to protect the cipher keys since a compromise of the key could result in a compromise of the transmission . there is disclosed a key distribution method and apparatus which uses a channel from identified terminals to a central key distribution center for the establishment , on a one - session basis , of the key which is to be used for the next session between those terminals . the key establishing link is itself encoded using a cipher key which changes after each usage . provision is made to verify , for each new connection , that a compromise has not priorly occurred .