Multi-bit mapping based on constellation rotation in Quantum Noise Stream Cipher

Abstract Based on Y-00 protocol, the Quantum Noise Stream Cipher (QNSC) directly encrypts the data by employing inherent quantum noise, and thus can provide physical layer security in optical fiber communication systems. With increase of bit position of ciphertext, the effect of noise on the higher bit positions of ciphertext decreases. In order to improve the effect of noise on the higher bit positions of ciphertext to protect the plaintext, XOR operation has to be used in QNSC. However, in multi-bit data QNSC system, multi-bit data has to perform XOR operation with additional seed key to protect the multi-bit data. With increase of bit number of plaintext, many additional seed keys have to be used to perform XOR operation. Thus, it is necessary to design a new mapping method without the additional seed key to protect the multi-bit data. In this paper, we propose a multi-bit data mapping method based on constellation rotation (CR) to expand the impact of noise on the higher bit positions of ciphertext to protect data. Without using additional seed key, CR-based QNSC fully expands the random impacts of noise on the multi-bit plaintext and further enhances the security of multi-bit plaintext in QNSC scheme. The CR-based multi-bit mapping method rotates the constellation diagram of plaintext by an angle to code the plaintext alternatively in constellation diagram. Based on IM/DD-OFDM, we conduct QAM/QNSC and PSK/QNSC simulation to test the performance of CR-based multi-bit mapping in optical back-to-back with ASE noise. At the same time, we compare the XOR-based BPSK/QNSC with CR-based BPSK/QNSC in different intensity noises. Results show that the bit error rate of multi-bit plaintext in different bit positions is close to 0.5 for multi-bit plaintext of PSK/QNSC and QAM/QNSC by using CR-based mapping. Thus, the effect of noise on the higher bit positions of plaintext is improved. Eve cannot obtain any information about plaintext. By comparing XOR-based BPSK/QNSC and CR-based BPSK/QNSC in one-bit system, we find that the bit error rate is close to 0.5 in both schemes. The advantage of CR-based QNSC is able to encrypt multi-bit plaintext without extra seed key and hardly affect the transmission performance of the legitimate receiver Bob.