Quantum Key Distribution Protocols: BB84, E91, And B92
Protocols for Quantum Key Distribution
The foundation of modern encryption, which uses mathematical complexity to secure secret information, is seriously threatened by quantum computing. Current key distribution methods rely on conventional computation and have serious limitations that quantum computing may be able to overcome.
In contrast, Quantum Key Distribution (QKD) provides a secure communication method grounded in the fundamental ideas of quantum mechanics. This is a rapidly expanding and promising area of data and information security. A shared, secret, random key that is known only to Alice (sender) and Bob (receiver), two communicating parties, can produce a key with QKD.
Among the basic concepts of quantum physics used by QKD approaches are the Heisenberg uncertainty principle, quantum entanglement, superposition, and the no-cloning theorem. One important and unique aspect of QKD is its ability to detect the presence of any third party (Eve, the eavesdropper) attempting to learn the key. This capability is predicated on the notion that a quantum system is always perturbed when it is measured, rendering the eavesdropper evident through detectable flaws.
Transmission and key creation stop if the error rate exceeds a threshold. Remember that QKD just generates and distributes this secret key, which is then utilized for message encryption by one-time pad and AES.
Quantum Uncertainty-Based QKD Protocols: BB84 and B92
The measurement-disturbance principle arising from the Heisenberg uncertainty principle is the basis for the "prepare-and-measure" category of Quantum Key Distribution (QKD) techniques, which encompasses a number of them.
Charles Bennett and Gilles Brassard introduced the BB84 protocol in 1984, and it is the most well-known and basic QKD protocol. Single photons and two conjugate pairs of non-orthogonal states, such as the rectilinear basis (vertical/horizontal polarization) and the diagonal basis (45°/135° polarization), are used by BB84 for encoding. Security is guaranteed since it is usually difficult to measure these non-orthogonal states without changing the original state.
Alice randomly selects a bit value (0 or 1) and matching base to encapsulate her photon and send it to Bob. Bob randomly chooses one of the two bases to measure the photon since he doesn't know Alice's pick. They disclose their bases to the public after transmission (sifting), removing any bits whose bases did not match. If there were no interference or eavesdropping, their remaining bit strings should be identical in a perfect channel.
In 1992, Bennett proposed the B92 protocol as an update to BB84. B92 simplifies the process by utilizing only two non-orthogonal quantum states. In order to potentially boost the data exchange rate, the design of B92 aimed to allow variable parameter modification dependent on channel conditions. The IBM Quantum Composer and other real-world systems employ the B92 protocol more simply than the BB84 protocol. The B92 protocol is distinguished by a parameter related to the angle between the non-orthogonal states; adjusting this angle allows trade-offs between quicker data exchange rate and interference resistance.
Quantum Entanglement-Based QKD Protocols: E91
Unlike prepare-and-measure systems, entanglement-based protocols are predicated on the notion that two or more particles are intrinsically linked, independent of their separation, as per quantum entanglement.
Artur Ekert invented the maximally entangled photon pair E91 technique in 1991. Alice, Bob, or another source can create these entangled combinations. Alice and Bob measure with random bases using one photon each from the entangled pair. Because of entanglement, if they use the same basis, their measurements will match.
The main security method of E91 is based on Bell's inequality and the theorem violations. By comparing a subset of their results, Alice and Bob are able to determine these correlations, but Eve destroys them if she attempts to measure the entangled state or intercept a photon. If the Bell test statistic is not maximized, they conclude that Eve has introduced local reality into the system, which violates the fundamental quantum correlations.
The fact that E91's security is "device-independent," or relies on the fundamental properties of quantum entanglement rather than the presumed dependability or perfect calibration of the physical devices used to produce the keys, is one of its primary advantages.
The Steps After the Key Generation Process
Regardless of the underlying quantum principle (uncertainty or entanglement), QKD requires several post-processing steps to finalize the secret key over a sanctioned classical channel.
Sifting: Alice and Bob publicly reveal which states or bases were employed in order to remove the non-matching or inconclusive results, resulting in a raw, correlated key known as the sifted key.
Error Estimation: Alice and Bob then publicly compare a small, random subset of their filtered key bits to get the Quantum Bit Error Rate (QBER). Errors could be caused by eavesdropping or channel defects (such quantum noise). Eve is typically held responsible for all errors. If the QBER exceeds a preset threshold, the procedure is terminated.
Information Reconciliation (Error Correction): If the error rate is acceptable, Alice and Bob use error correction techniques, such as the Cascade protocol, to ensure that their key strings are identical. However, in the process, they often provide Eve some knowledge that is not fully complete.
Privacy Amplification: In this final phase, Eve's remaining partial knowledge from the quantum channel or parity checks during reconciliation is reduced. Since the reconciled key is subjected to a universal hash function to produce a new, shorter final key, Eve has a very slim possibility of discovering the final secret key. See Also: National Quantum Computing Center Receives NPL Ion Trap
Realistic Challenges and Safety
In contrast to conventional public key cryptography, QKD algorithms offer provable security using information theory grounded in quantum mechanical principles.
Despite this theoretical strength, practical application faces numerous challenges. Low secret key rates and transmission distance limitations are caused by issues with channel loss, decoherence, and imperfections in the production, transmission, and measurement of quantum states. Moreover, real-world QKD systems have been demonstrated to be vulnerable to a range of assaults that exploit hardware flaws, such as Trojan-horse attacks and the Photon Number Splitting (PNS) attack, despite the mathematical safety of the underlying protocol.
Because of these security validation problems, the UK National Cyber Security Center (NCSC) and the US National Security Agency (NSA) have recommended moving from QKD to Post-Quantum Cryptography (PQC) for most purposes. Their concerns include high infrastructure costs, the need for specialized hardware, and the difficulty of reliably confirming the hardware of the deployed system, which sometimes lacks the flexibility for basic security fixes or upgrades.
QKD is still evolving swiftly, though, with both commercial devices and extensive research being done to improve security, distance, and effectiveness against actual threats.
