Excerpts from internet.cypherpunks: 7-Apr-93 Real-time BBS Encryption?? by Eric Hughes@soda.berkele
For PC's, replacing the terminal software is really the best way. There is no effective abstraction of serial port hardware in the PC world. The int 0x14 driver in the BIOS was rampantly defective, and MSDOS does not provide a standard interface.
As a result, almost all comm software on PC's talks to the serial port directly. Now in MS Windows, there is abstraction for ther serial ports, but I don't know how easy it is to insert a device layer.
Actually, there is a rather old (for the PC) abstraction called FOSSIL (Fido Opus Seadog Serial Interface Layer ... or so). It is essentially an extention/replacement for the BIOS int 0x14 driver. It is certainly possible to further extend this for encryption by adding some functions to the interface. The two FOSSILs I know of are X00 and BNU - They can be found in oak.oakland.edu:/pub/msdos/fossil -- David Sward sward+@cmu.edu