The encryption and weaknesses of the E.Álvarez cryptosystem are analyzed.A new chaotic cryptosystem based on ergodicity is proposed.The control parameters and initial condition of a chaotic system are selected as the secret key.A bit chain from the chaotic orbit is generated,and the position at which a plaintext block appears in the chain is found.Then record the number of iterations of the chaotic map as the cipher block.Several weaknesses of the E.Álvarez cryptosystem are voided in the new scheme.The security is strengthened.Finally the new cryptosystem is studied experimentally using a logistic map.It is shown that the new cryptosystem satisfies the confusion and diffuse characteristics.Effectiveness of the proposed scheme is demonstrated.