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Utilizing a partially entangled GHZ state, we propose a novel controlled quantum secure direct communication (QSDC) with quantum encryption. Under the supervision and help of the third side, the sender and the receiver can securely share the private quantum entanglement keys used to encrypt and decrypt the secret message. According to the results of checking the eavesdropping on decoy photons, communicators can decide whether the quantum keys are reused in the next round. Not only will eavesdropping inevitably disturb the states of the decoy photons and be detected, but arbitrary transmission errors can also be corrected.

Zhou et al. proposed a quantum encryption scheme based on quantum computation in 2006 [N. Zhou et al., Physica A362 (2006) 305]. Each qubit of the ciphertext is constrained to two pairs of conjugate states. So, its implementation is feasible with the existing technology. But it is inefficient since it entails six key bits to encrypt one message bit, and the resulting ciphertext for one message bit consists of three qubits. In addition, its security cannot be directly reduced to the well-known BB84 protocol. In this paper, we improve it using the technique developed in BB84 protocol. The new scheme entails only two key bits to encrypt one message bit. The resulting ciphertext is just composed of two qubits. It saves about a half cost without the loss of security. Moreover, the new scheme is probabilistic instead of deterministic.