The authors have developed a system enabling high-speed encrypted file transfer, while conducting secret voice communication using ISDN. The system will simultaneously encrypt and decrypt a one megabyte plain text file on a hard disk and safely transfer the file to the disk of another party in about 2 minutes 50 seconds, between any two personal computers registered at the key management center. The authors developed a protocol for transmitting session keys from the key management center using user-user signaling at the time of calling and making connection, to share session keys between the terminals at which communications are being conducted. Using this protocol adds about 1 second to the normal time spent calling and establishing a connection, and represents a saving in communications costs.>