An eavesdropper(Eve)can exploit all the imperfections of a practical quantum key distribution(QKD)system to obtain some information about the secret key,no matter whether these imperfections are from the physical layer or from the post-processing layer.We propose a possible attack on a passive detection QKD system based on the imperfection from the software layer.The analysis shows that Eve can obtain all the information about the key without being discovered.
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