Lance J. Hoffman

Although encipherment has often been discussed as a means to protect computer data, its costs are not well established. Five experiments were conducted to measure the cpu time on a CDC 6400 required by additive ciphers programmed both in assembly language and in Fortran: a “null transformation” to measure the time to move data without encipherment; encipherment with a one-word key; encipherment with a 125-word key; double key encipherment; and encipherment using a pseudo random key. The results were analyzed for consistency over 100 runs, and the effects of constant and intermittent errors were considered. Timing rates for assembly language encipherment ranged from 498,800 characters per second for a pseudo random key cipher to 2,092,000 characters per second for a constant one-word key cipher. The latter is almost equivalent to the rate required simply to move data without encipherment. Fortran tests required over four times as much cpu time. This paper introduces the idea of enciphering time coefficient the ratio of enciphering time to the time taken to fetch and store data without encipherment.