Recently many chaotic cryptosystems based on the iterative equation have been proposed，but many of them can’t resist the chosen ciphertext attack，chosen plaintext attack and known plaintext attack. In this letter， we analyze the existing problems of cryptography using multiple one-dimensional chaotic maps [14] presented by Pareek et al. and break it. This cryptosystem uses an external secret key of variable length（maximum 128-bits）to obtain the initial condition and number of iterations of four one-dimensional chaotic maps，but this way exists weaknesses of deriving the initial condition by using finite integer problem allowing for attack. So，the external secret key of variable length can be recovered with a little time using the chosen ciphertext attack or chosen plaintext attack.