Nowadays the need of speed in cipher and decipher operations is more important than in the past. This is due to the diffusion of real time applications, which fact involves the use of cryptography.

Many co-processors for cryptography were studied and presented in the past, but only few works were addressed to the enhancement of the instruction set architecture (ISA) of the embedded processor. This paper presents an extension of the ISA of a 32 bit processor, that aims at speeding up the software implementations of the AES algorithm. After the identification of the most frequently executed and the most time consuming sections of the algorithm, a set of dedicated instructions is designed in order to improve the performances of the cipher operations. We validate our instruction set extension by measuring the speed up for different optimized implementations of AES using an ARM processor simulator, but the enhancements we propose are general enough to be applied to almost all 32 bit processors.