Abstract: Considering the high correlation among adjacent pixels, a "divide and conquer" algorithm was proposed for four adjacent pixels to improve the permutation phase of traditional chaotic encryption schemes. Adjacent pixels were first separated, and then the four groups of separated pixels were permutated using "cat maps" of the four groups having different parameters, followed by reconstruction and diffusion. Simulations results indicated that, when security of the encryption algorithm could be guaranteed, permutation and encryption time decreased.