A striking scheme called ``counterfactual quantum cryptography'' gives a conceptually new approach to accomplish the task of key distribution. It allows two legitimate parties to share a secret even though a particle carrying secret information is not, in fact, transmitted through the quantum channel. Since an eavesdropper cannot directly access the entire quantum system of each signal particle, the protocol seems to provide practical security advantages. However, here we propose an eavesdropping method which works on the scheme in a finite key scenario. We show that, for practical systems only generating a finite number of keys, the eavesdropping can obtain all of the secret information without being detected. We also present a improved protocol as a countermeasure against this attack.
Quantum cryptography allows one to distribute a secret key between two remote parties using the fundamental principles of quantum mechanics. The well-known established paradigm for the quantum key distribution relies on the actual transmission of signal particle through a quantum channel. In this Letter, we show that the task of a secret key distribution can be accomplished even though a particle carrying secret information is not in fact transmitted through the quantum channel. The proposed protocols can be implemented with current technologies and provide practical security advantages by eliminating the possibility that an eavesdropper can directly access the entire quantum system of each signal particle.
Quantum cryptography is a new paradigm of cryptographic information protection, utilizing the laws of quantum mechanics and the corresponding quantum properties of information carriers — photons to overcome the limitations of classical cryptographic protocols and increase the security of cryptographic protocols up to the information-theoretic level. The paper provides a general overview and analysis of modern quantum information security technologies, particularly protocols for quantum key distribution, quantum secure direct communication, and quantum secret sharing. Several other areas of quantum cryptography are briefly considered, such as quantum digital signature, quantum bit commitment, quantum steganography, etc. The advantages and disadvantages of quantum cryptographic protocols, as well as the prospects and challenges of their practical implementation, are analyzed. A brief overview of the laws of quantum physics underlying the security of quantum cryptographic protocols is also provided. Keywords: quantum cryptography, physical foundations of quantum cryptography, qubit, qudit, entangled qubits and qudits, quantum key distribution, quantum secure direct communication, quantum secret sharing.