RC6 is very similar to RC5 in structure and could be viewed as interweaving two parallel RC5 encryption processes. However, RC6 does use an extra multiplication operation not present in RC5 and the use of four b/4-bit working registers (see splitting of plaintext blocks in feistel cipher) instead of two b/2-bit registers as in RC5 (b is the block size). Integer multiplication is used to increase the diffusion achieved per round so that fewer rounds are needed and the speed of the cipher can be increased.<br>The reason for using four working registers instead of two is mainly for optimization on 32-bit CPUs. The default block size of RC5 was 64 bits, while the default block size of the AES is 128 bits. RC5 has only two working registers. It uses 32-bit operations when dealing with 64-bit blocks and 64-bit operations when dealing with 128-bit blocks, so four working registers are required to build the AES architecture using only 32-bit operations.