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arXiv:2603.25496

Send the Key in Cleartext: Halving Key Consumption while Preserving Unconditional Security in QKD Authentication

Claudia De Lazzari, Francesco Stocco, Edoardo Signorini, Giacomo Fregona, Fernando Chirici, Damiano Giani, Tommaso Occhipinti, Guglielmo Morgari, Alessandro Zavatta, Davide Bacco

quant-phcs.CRcs.ITSubmitted Mar 26, 2026• Announced Mar 27, 2026• 0 citations
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Abstract

Quantum Key Distribution (QKD) protocols require Information-Theoretically Secure (ITS) authentication of the classical channel to preserve the unconditional security of the distilled key. Standard ITS schemes are based on one-time keys: once a key is used to authenticate a message, it must be discarded. Since QKD requires mutual authentication, two independent one-time keys are typically consumed per round, imposing a non-trivial overhead on the net secure key rate. In this work, we present the authentication-with-response scheme, a novel ITS authentication scheme based on $\varepsilon$-Almost Strongly Universal$_2$ ($\varepsilon$-ASU$_2$) functions, whose IT security can be established in the Universal Composability (UC) framework. The scheme achieves mutual authentication consuming a single one-time key per QKD round, halving key consumption compared to the state-of-the-art.

Related papers

Authentication in Quantum Key Distribution : Security Proof and Universal Hash Functions

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Quantum Key Distribution (QKD) is a secret key agreement technique that consists of two parts: quantum transmission and measurement on a quantum channel, and classical post-processing on a public communication channel. It enjoys provable unconditional security provided that the public communication channel is immutable. Otherwise, QKD is vulnerable to a man-in-the-middle attack. Immutable public communication channels, however, do not exist in practice. So we need to use authentication that implements the properties of an immutable channel as well as possible. One scheme that serves this purpose well is the Wegman-Carter authentication (WCA), which is built upon Almost Strongly Universal2 (ASU2) hashing. This scheme uses a new key in each authentication attempt to select a hash function from an ASU2 family, which is then used to generate the authentication tag for a message. The main focus of this dissertation is on authentication in the context of QKD. We study ASU2 hash functions, security of QKD that employs a computationally secure authentication, and also security of authentication with a partially known key. Specifically, we study the following. First, Universal hash functions and their constructions are reviewed, and as well as a new construction of ASU2 hash functions is presented. Second, security of QKD that employs a specific computationally secure authentication is studied. We present detailed attacks on various practical implementations of QKD that employs this authentication. We also provide countermeasures and prove necessary and sufficient conditions for upgrading the security of the authentication to the level of unconditional security. Third, Universal hash function based multiple authentication is studied. This uses a fixed ASU2 hash function followed by one-time pad encryption, to keep the hash function secret. We show that the one-time pad is necessary in every round for the authentication to be unconditionally secure. Lastly, we study security of the WCA scheme, in the case of a partially known authentication key. Here we prove tight information-theoretic security bounds and also analyse security using witness indistinguishability as used in the Universal Composability framework.

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Authentication Methods for Quantum Key Distribution: Challenges and Perspectives

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In this article, we investigate the issue of authenticating the classical post-processing components of a QKD protocol through an insecure channel. Our analysis starts with state-of-the-art solutions and subsequently introduces an authentication scheme aiming at both Information Theoretic Security and resiliency from particular denial-of-service attacks targeting the authentication procedure. The proposed strategy combines -Almost Strongly Universal hashing for unconditional protection and computationally-secure Message Authentication Codes for DoS detection capability. Specifically, we consider the protocol security in a typical QKD scenario, taking into account partially weak authentication keys. Moreover, we add practical considerations related to authentication parameters and experimentally evaluate the performance of the proposed system in terms of computational complexity and key consumption in a reasonable use case.

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Quantum key distribution (QKD) enables unconditionally secure communication between distinct parties using a quantum channel and an authentic public channel. Reducing the portion of quantum-generated secret keys, that is consumed during the authentication procedure, is of significant importance for improving the performance of QKD systems. In the present work, we develop a lightweight authentication protocol for QKD based on a `ping-pong' scheme of authenticity check for QKD. An important feature of this scheme is that the only one authentication tag is generated and transmitted during each of the QKD post-processing rounds. For the tag generation purpose, we design an unconditionally secure procedure based on the concept of key recycling. The procedure is based on the combination of almost universal2polynomial hashing, XOR universal2Toeplitz hashing, and one-time pad (OTP) encryption. We demonstrate how to minimize both the length of the recycled key and the size of the authentication key, that is required for OTP encryption. As a result, in real case scenarios, the portion of quantum-generated secret keys that is consumed for the authentication purposes is below 1%. Finally, we provide a security analysis of the full quantum key growing process in the framework of universally composable security.

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An Information-Theoretic Secure Group Authentication Scheme for Quantum Key Distribution Networks

Yi Luo, Haokun Mao, Qiong Li +1 more

Information-theoretic secure (ITS) authentication is an essential part of a quantum key distribution (QKD) system. In the QKD network, the authentication scheme based on the ITS QKD keys pre-shared by the QKD system. However, such a scheme is subject to the limitation that consumes a significant amount of authentication keys when the scale and communication requirements of QKD networks grow. To address the issue, we propose an ITS group authentication scheme for QKD networks that consumes considerably fewer keys. In our scheme, multiple QKD network nodes collaborate and complete the authentication as a group. Compared to the pre-shared keys authentication scheme, our scheme reduces the complexity of authentication key consumption from$O\left ({N^{2}d }\right) $to$O\left ({N }\right) $, where$N $is the number of nodes and$d $is the average path length of the network topology. The security of the proposed scheme is analyzed in detail and simulation experiments are conducted to demonstrate our scheme’s effectiveness for varying networks scales and communication requirements.

IEEE Transactions on Communications•May 29, 2023•16 citations

On the robustness of information-theoretic authentication in quantum cryptography

S. N. Molotkov

Abstract In addition to the quantum channel, an auxiliary classical authentic communication channel is required for quantum key distribution. To provide unconditional security in quantum cryptography, which based on the fundamental laws of quantum mechanics, information-theoretic authentication is required, and not authentication based on computational complexity. The paper provides a security proof of information-theoretic authentication in quantum cryptography using ɛ -ASU 2 hash functions. The internal structure of ɛ -ASU 2 hash functions is not used in the proof, so these hash functions can be implemented by a different composition of other hash functions, for example, including the composition ɛ -AXU 2 functions with a reused key and subsequent encryption of the tag with a one-time pad key, which is taken from the previous session of quantum key distribution. It is also shown that information-theoretic authentication preserves composable security of the keys. This authentication method can also be used for key agrrement in in networks with trusted nodes (Molotkov 2022 Laser Phys. Lett. 19 045201–9; Arbekov and Molotkov 2020 Laser Phys. Lett. 17 055202–8) and with quantum key distribution.

Laser Physics Letters•May 19, 2022•11 citations
Quantum Key Distribution (QKD) protocols require Information-Theoretically Secure (ITS) authentication of the classical channel to preserve the unconditional security of the distilled key. Standard ITS schemes are based on one-time keys: once a key is used to authenticate a message, it must be discarded. Since QKD requires mutual authentication, two independent one-time keys are typically consumed per round, imposing a non-trivial overhead on the net secure key rate. In this work, we present the authentication-with-response scheme, a novel ITS authentication scheme based on ε\varepsilonε-Almost Strongly Universal2_22​ (ε\varepsilonε-ASU2_22​) functions, whose IT security can be established in the Universal Composability (UC) framework. The scheme achieves mutual authentication consuming a single one-time key per QKD round, halving key consumption compared to the state-of-the-art.
Quantum Key Distribution (QKD) is a secret key agreement technique that consists of two parts: quantum transmission and measurement on a quantum channel, and classical post-processing on a public communication channel. It enjoys provable unconditional security provided that the public communication channel is immutable. Otherwise, QKD is vulnerable to a man-in-the-middle attack. Immutable public communication channels, however, do not exist in practice. So we need to use authentication that implements the properties of an immutable channel as well as possible. One scheme that serves this purpose well is the Wegman-Carter authentication (WCA), which is built upon Almost Strongly Universal2 (ASU2) hashing. This scheme uses a new key in each authentication attempt to select a hash function from an ASU2 family, which is then used to generate the authentication tag for a message. The main focus of this dissertation is on authentication in the context of QKD. We study ASU2 hash functions, security of QKD that employs a computationally secure authentication, and also security of authentication with a partially known key. Specifically, we study the following. First, Universal hash functions and their constructions are reviewed, and as well as a new construction of ASU2 hash functions is presented. Second, security of QKD that employs a specific computationally secure authentication is studied. We present detailed attacks on various practical implementations of QKD that employs this authentication. We also provide countermeasures and prove necessary and sufficient conditions for upgrading the security of the authentication to the level of unconditional security. Third, Universal hash function based multiple authentication is studied. This uses a fixed ASU2 hash function followed by one-time pad encryption, to keep the hash function secret. We show that the one-time pad is necessary in every round for the authentication to be unconditionally secure. Lastly, we study security of the WCA scheme, in the case of a partially known authentication key. Here we prove tight information-theoretic security bounds and also analyse security using witness indistinguishability as used in the Universal Composability framework.
In this article, we investigate the issue of authenticating the classical post-processing components of a QKD protocol through an insecure channel. Our analysis starts with state-of-the-art solutions and subsequently introduces an authentication scheme aiming at both Information Theoretic Security and resiliency from particular denial-of-service attacks targeting the authentication procedure. The proposed strategy combines -Almost Strongly Universal hashing for unconditional protection and computationally-secure Message Authentication Codes for DoS detection capability. Specifically, we consider the protocol security in a typical QKD scenario, taking into account partially weak authentication keys. Moreover, we add practical considerations related to authentication parameters and experimentally evaluate the performance of the proposed system in terms of computational complexity and key consumption in a reasonable use case.
Quantum Key Distribution (QKD - also referred to as Quantum Cryptography) is a technique for secret key agreement. It has been shown that QKD rigged with Information-Theoretic Secure (ITS) authentication (using secret key) of the classical messages transmitted during the key distribution protocol is also ITS. Note, QKD without any authentication can trivially be broken by man-in-the-middle attacks. Here, we study an authentication method that was originally proposed because of its low key consumption; a two-step authentication that uses a publicly known hash function, followed by a secret strongly universal2 hash function, which is exchanged each round. This two-step authentication is not information-theoretically secure but it was argued that nevertheless it does not compromise the security of QKD. In the current contribution we study intrinsic weaknesses of this approach under the common assumption that the QKD adversary has access to unlimited resources including quantum memories. We consider one implementation of Quantum Cryptographic protocols that use such authentication and demonstrate an attack that fully extract the secret key. Even including the final key from the protocol in the authentication does not rule out the possibility of these attacks. To rectify the situation, we propose a countermeasure that, while not information-theoretically secure, restores the need for very large computing power for the attack to work. Finally, we specify conditions that must be satisfied by the two-step authentication in order to restore information-theoretic security.
Quantum key distribution (QKD) enables unconditionally secure communication between distinct parties using a quantum channel and an authentic public channel. Reducing the portion of quantum-generated secret keys, that is consumed during the authentication procedure, is of significant importance for improving the performance of QKD systems. In the present work, we develop a lightweight authentication protocol for QKD based on a `ping-pong' scheme of authenticity check for QKD. An important feature of this scheme is that the only one authentication tag is generated and transmitted during each of the QKD post-processing rounds. For the tag generation purpose, we design an unconditionally secure procedure based on the concept of key recycling. The procedure is based on the combination of almost universal2polynomial hashing, XOR universal2Toeplitz hashing, and one-time pad (OTP) encryption. We demonstrate how to minimize both the length of the recycled key and the size of the authentication key, that is required for OTP encryption. As a result, in real case scenarios, the portion of quantum-generated secret keys that is consumed for the authentication purposes is below 1%. Finally, we provide a security analysis of the full quantum key growing process in the framework of universally composable security.
Information-theoretic secure (ITS) authentication is an essential part of a quantum key distribution (QKD) system. In the QKD network, the authentication scheme based on the ITS QKD keys pre-shared by the QKD system. However, such a scheme is subject to the limitation that consumes a significant amount of authentication keys when the scale and communication requirements of QKD networks grow. To address the issue, we propose an ITS group authentication scheme for QKD networks that consumes considerably fewer keys. In our scheme, multiple QKD network nodes collaborate and complete the authentication as a group. Compared to the pre-shared keys authentication scheme, our scheme reduces the complexity of authentication key consumption from$O\left ({N^{2}d }\right) tototoO\left ({N }\right) ,where, where,whereN isthenumberofnodesandis the number of nodes andisthenumberofnodesandd $is the average path length of the network topology. The security of the proposed scheme is analyzed in detail and simulation experiments are conducted to demonstrate our scheme’s effectiveness for varying networks scales and communication requirements.
Abstract In addition to the quantum channel, an auxiliary classical authentic communication channel is required for quantum key distribution. To provide unconditional security in quantum cryptography, which based on the fundamental laws of quantum mechanics, information-theoretic authentication is required, and not authentication based on computational complexity. The paper provides a security proof of information-theoretic authentication in quantum cryptography using ɛ -ASU 2 hash functions. The internal structure of ɛ -ASU 2 hash functions is not used in the proof, so these hash functions can be implemented by a different composition of other hash functions, for example, including the composition ɛ -AXU 2 functions with a reused key and subsequent encryption of the tag with a one-time pad key, which is taken from the previous session of quantum key distribution. It is also shown that information-theoretic authentication preserves composable security of the keys. This authentication method can also be used for key agrrement in in networks with trusted nodes (Molotkov 2022 Laser Phys. Lett. 19 045201–9; Arbekov and Molotkov 2020 Laser Phys. Lett. 17 055202–8) and with quantum key distribution.