Performance Impact of Addressing Modes on Encryption Algorithms

Encryption algorithms commonly use table lookups to perform substitution, which is a confusion primitive. The use of table lookups in this way is especially common in the more recent encryption algorithms, such as the AES finalists like MARS and Twofish, and the AES winner, Rijndael. Workload characterization studies indicate that these algorithms spend a significant fraction of their execution cycles on performing these table lookups, more specifically on effective address calculations. This study considers the five AES finalists (MARS, RC6, Rijndael, Serpent and Twofish) and studies the effect of different addressing modes that can be used to calculate the effective addresses during the table lookups. We report our findings for four different addressing modes and on varying width EPIC processors. The results indicate that speedups exceeding 2x can be obtained when fast addressing modes are used.