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arXiv:quant-ph/0111106

Secure communication with a publicly known key

Almut Beige, Berthold‐Georg Englert, Christian Kurtsiefer, Harald Weinfurter

Published Nov 20, 2001• 2 citations
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Abstract

We present a scheme for direct and confidential communication between Alice and Bob, where there is no need for establishing a shared secret key first, and where the key used by Alice even will become known publicly. The communication is based on the exchange of single photons and each and every photon transmits one bit of Alice's message without revealing any information to a potential eavesdropper.

DOI
10.48550/arxiv.quant-ph/0111106

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We present a scheme for direct and confidential communication between Alice and Bob, where there is no need for establishing a shared secret key first, and where the key used by Alice even will become known publicly. The communication is based on the exchange of single photons and each and every photon transmits one bit of Alice's message without revealing any information to a potential eavesdropper.
We propose a cryptographic scheme that is deterministic: Alice sends single photons to Bob, and each and every photon detected supplies one key bit—no photon is wasted. This is in marked contrast to other schemes in which a random process decides whether the next photon sent will contribute to the key or not. The determinism is achieved by preparing the photons in two-qubit states, rather than the one-qubit states used in conventional schemes. In particular, we consider the realistic situation in which one qubit is the photon polarization and the other a spatial alternative. Further, we show how one can exploit the deterministic nature for direct secure communication, that is without the need for establishing a shared key first.
We present a scheme for quantum communication, where a set of EPR pairs, initially shared by the sender Alice and the receiver Bob, functions as a quantum channel. After insuring the safety of the quantum channel, Alice applies local measurement on her particles of the EPR pairs and informs Bob the encoding classical information publicly. According to Alice's classical information and his measurement outcomes on the EPR pairs Bob can infer the secret messages directly. In this scheme, to transmit one bit secret message, the sender Alice only needs to send one bit classical information to the receiver Bob. We also show that this scheme is completely secure if perfect quantum channel is used.
This study shows that secret information can be shared or passed from a sender to a receiver even if not encoded in a secret message. In the protocol designed in this study, no parts of the original secret information ever travel via communication channels between the source and the destination, no encoding/decoding key is ever used. The two communicating partners, Alice and Bob, are endowed with coherent qubits that can be read and set and keep their quantum values over time. Additionally, there exists a central authority that is capable of identifying Alice and Bob to share with each half of entangled qubit pairs. The central authority also performs entanglement swapping. Our protocol relies on the assumption that public information can be protected, an assumption present in all cryptographic protocols. Also any classical communication channel need not be authenticated. As each piece of secret information has a distinct public encoding, the protocol is equivalent to a one-time pad protocol.
We present a quantum secure direct communication scheme achieved by swapping quantum entanglement. In this scheme a set of ordered Einstein-Podolsky-Rosen (EPR) pairs is used as a quantum information channel for sending secret messages directly. After insuring the safety of the quantum channel, the sender Alice encodes the secret messages directly by applying a series local operations on her particles sequence according to their stipulation. Using three EPR pairs, three bits of secret classical information can be faithfully transmitted from Alice to remote Bob without revealing any information to a potential eavesdropper. By both Alice and Bob's GHZ state measurement results, Bob is able to read out the encoded secret messages directly. The protocol is completely secure if perfect quantum channel is used, because there is not a transmission of the qubits carrying the secret message between Alice and Bob in the public channel.
We present a quantum secure direct communication scheme achieved by swapping quantum entanglement. In this scheme a set of ordered Einstein-Podolsky-Rosen (EPR) pairs is used as a quantum information channel for sending secret messages directly. After insuring the safety of the quantum channel, the sender Alice encodes the secret messages directly by applying a series local operations on her particle sequences according to their stipulation. Using three EPR pairs, three bits of secret classical information can be faithfully transmitted from Alice to remote Bob without revealing any information to a potential eavesdropper. By both Alice and Bob's GHZ state measurement results, Bob is able to read out the encoded secret messages directly. The protocol is completely secure if perfect quantum channel is used, because there is not a transmission of the qubits carrying the secret message between Alice and Bob in the public channel.