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In this paper, a novel block cryptographic scheme based on a 3-D Chen's chaotic system is proposed, in which permutation by a key-dependent shift approach and substitution by the XOR approach are combined. Two rounds of encryption are performed for the proposed encryption algorithm. The plaintext is first permuted and then substituted block by block in the first round, but is first substituted and then permuted block by block in the second round. As a result, the diffusion property can be achieved. To make the proposed scheme secure enough against known/chosen-plaintext attack, the ciphertext feedback method is adopted in the XOR process. Simulation results show that the proposed scheme is sufficiently sensitive to small changes of the plaintext; furthermore, it is fast enough for practical transmission of large multi-media files over public data communication network.