A "bump in the stack" encryptor for MS-DOS systems

Most implementations of IP security are deeply entwined in the source of the protocol stack. However, such source code is not readily available for MS-DOS systems. We implemented a version using the packet driver interface. Our module sits between the generic Ethernet driver and the hardware driver; it emulates each to the other. Most of the code is straightforward; in a few places, though, we were forced to compensate for inadequate interface definitions.