In this paper, we report a new security loophole, which may commonly exist in the transmitters of high-speed QKD systems, but has been overlooked so far despite its seriousness and generality. Therefore, rigorously quantitative evaluation of the imperfections in transmitters are essential to the security certification of QKD systems. 22, 23, 24, 25, 26 Loopholes in transmitters are directly linked to the mismatch of the state preparation between the ideal model and the implementation of QKD protocols. By contrast, researches on loopholes in transmitters have just begun in only a few aspects. 16, 17, 18, 19 Also, the measurement device-independent QKD protocol 20, 21 can circumvent any receiver imperfections in principle. So far, receiver’s security loopholes due to the side channels and countermeasures have been extensively studied for the existing QKD systems. Moreover, those should be acceptable for non-experts. For the QKD technology to be widely adopted, critical requirements are security certification, test-and-measurement method, security criteria for implementation, and countermeasures against the side channels.
In practice, like other cryptographic systems, a QKD system also has potential vulnerability due to mismatches between practical implementation and the theoretical model used for security proofs, which are referred to as side channels. Nevertheless, there remains an obstacle that makes the potential users hesitate to adopt this emerging technology they would not innovate their existing secure communication systems unless convinced that a QKD system at hand is really secure. By employing the ultrafast optical communication devices, high-speed QKD systems stably operated at GHz-clock frequency is realized in the installed fiber networks. 12 For the QKD device itself, high-speed and stable operation is critical. 11 To provide information theoretically secure keys to real applications securely and seamlessly, an efficient key management system and application program interfaces have been developed. Metropolitan QKD networks have been successfully deployed 6, 7, 8, 9, 10 and is going to be a continental scale. Toward its practical realization, tremendous progress has been made during the past decades. Because of growing interest in the confidentiality of those data, this storage network could be one of the killer applications of QKD. Such a system has been exemplified in the literatures 4, 5 as a long-term secure storage network consisting of secret sharing, QKD and authentication schemes to deal with highly confidential data such as personal biomedical data, pharmaceutical, and genetic information. Thanks to this unique feature, referred to as “information theoretic security”, QKD, combined with Vernam’s one-time pad cipher, enables the everlasting protection of confidentiality of data transmission, and hence must be an essential element to construct a long-term security system which cannot be realized only by cryptographic schemes based on computational security. Quantum key distribution (QKD) 1, 2, 3 allows two legitimate parties, Alice and Bob, to establish symmetric keys with the proven security even in the presence of an eavesdropper, Eve, who has unbounded computational power. Our countermeasure is simple, effective and applicable to wide range of high-speed QKD systems, and thus paves the way to realize ultrafast and security-certified commercial QKD systems.
We also provide its countermeasure which does not require significant changes of hardware and can generate keys secure over 100 km fiber transmission. Such correlation violates the assumption of most security theories. We experimentally observe the inter-pulse intensity correlation and modulation pattern-dependent intensity deviation in a practical high-speed QKD system. Here we point out a security loophole at the transmitter of the GHz-clock QKD, which is a common problem in high-speed QKD systems using practical band-width limited devices. However, security loopholes of high-speed systems have not been fully explored yet.
Recently, GHz-clock decoy QKD systems have been realized by employing ultrafast optical communication devices.
Quantum key distribution (QKD) allows two distant parties to share secret keys with the proven security even in the presence of an eavesdropper with unbounded computational power.
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