Encryption hardware is not available on most computer systems in use today. Despite this fact, there is no well accepted encryption function designed for software implementation - - instead, hardware designs are emulated in software and the resulting performance loss is tolerated. The obvious solution is to design an encryption function for implementation in software. Such an encryption function is presented here -- on a SUN 4/260 it can encrypt at 4 to 8 megabits per second. The combination of modern processor speeds and a faster algorithm make software encryption feasible in applications which previously would have required hardware. This will effectively reduce the cost and increase the availability of cryptographic protection.