An experimental system has been developed which illustrates ways cryptography can be applied to certain data-security problems concerning remotely accessible data files. These problems are in two main classes: security of data while in transit over communications lines and security of data while in storage. The system makes use of a combination of software and special hardware to provide enciphering and deciphering of messages between a terminal and a data processor. Not only is the content of messages secured from eaves-droppers but also special means are employed to ensure the validity and integrity of messages, of particular significance for file-updating commands. It is also shown how a more restricted kind of protection is afforded data stored in files.