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A novel symmetric cryptosystem based on turbo codes with nonsystematic constituent codes is proposed. The proposed system introduces an interleaver for each constituent code and secures the interleavers with a secret key. It is verified by simulation results that without affecting the performance of the intended receiver, complete security for the unintended receiver can be achieved. More specifically, while the bit-error rate (BER) of the intended receiver remains the same as that of the insecure turbo code, for the unintended receiver BER is close to 0.5. The implementation issues for key-dependent interleaver design based on linear feedback shift registers are also discussed.