This spacing difference of a word is an even number of bits is not a multiple of eight

摘要

This shift method, implemented by means of a microprocessor making use of an instruction of the "bit test" type is provided so as to be applied to shift or rotation operations performed on words consisting of an even number of bits not being a multiple of eight. According to the method, the end bits are duplicated in a crossed and symmetric manner at the locations of the non-significant bits in order to form a word including a number of bits which is a multiple of eight. On this word a bit test is then performed making it possible to position the carry and then to perform the number of shifts desired to the left or to the right, in a symmetric way. This method is particularly well adapted to application to the microcircuit card protected by enciphering, using the DES system for which it makes it possible to reduce very substantially the processing time as well as the size of the coding. Application: microcircuit cards.