Security analysis of measurement-device-independent quantum conference key agreement with weak randomness

Abstract

Quantum conference key agreement (QCKA) allows multiple users to distribute secret conference keys over long distances. Measurement-device-independent QCKA (MDI-QCKA) is an effective QCKA scheme, which closes all detection loopholes and greatly enhances QCKA’s security in practical application. However, an eavesdropper (Eve) may compromise the security of practical systems and acquire conference key information by taking advantage of the weak randomness from the imperfect quantum devices. In this article, we analyze the performance of the MDI-QCKA scheme based on the weak randomness model. Our simulation results show that even a small proportion of weak randomness may lead to a noticeable fluctuation in the conference key rate. For the case with finite-key size, we find that the weak randomness damages the performance of MDI-QCKA to different degrees according to the data size of total pulses transmitted. Furthermore, we infer that QCKA based on single-photon interference technology may perform better in resisting weak randomness vulnerabilities. Our work contributes to the practical security analysis of multiparty quantum communication and takes a further step in the development of quantum networks.

1 Introduction

Quantum key distribution (QKD) has become the art of two legitimate parties (Alice and Bob) distributing secret information by virtue of the laws of physics [1]. It has information-theoretical security regardless of the unlimited computational power of an eavesdropper (Eve) [2, 3]. Over the past decades, QKD has developed rapidly and made remarkable progress in terms of theory and practice.

At present, the two-party scheme is the main direction of most theoretical and experimental works. In fact, multi-party quantum communication protocols have also been proposed and studied. The quantum conference key agreement (QCKA) [4–7] is one of the most promising applications that distributes the conference key among multiple parties over a long distance. Particularly, combing the MDI [8] technology via post-selected Greenberger–Horne–Zeilinger (GHZ) entangled states [9, 10], allows measurement-device-independent QCKA (MDI-QCKA) [11] to eliminate all side-channel attacks in detectors and plays a vital role in the construction and development of quantum networks. Furthermore, MDI-QCKA has been studied under different conditions consisting of finite-key size [12], continuous variables [13, 14], four users with the W state [15], and device independence [16, 17]. Recently, based on single-photon interference technology, some new QCKA protocols have also been proposed in which the conference key rate and transmission distance are improved greatly [18–23].

In terms of the practical quantum communication system, Eve may choose quantum devices of state preparation or measurement as the target of her (his) attack, which may result in bit encoding and basis selection being modulated not at random [24–27]. Li et al. proposed a weak randomness model [28–30] to analyze the quantum state preparation vulnerability in the BB84 protocol. Recently, this security analysis technology has been applied in the reference-frame-independent QKD (RFI-QKD) [31], MDI-QKD [32], and sending-or-not-sending twin-field QKD (SNS TF-QKD) [33]. Under the condition of weak randomness, the states prepared consisted of random and non-random parts [28]. States prepared from the non-random part may lead to the leakage of secret key information to Eve. In fact, the weak randomness model is also appropriate for the QCKA for two reasons. First, three communicators (Alice, Bob, and Charlie) are required in the QCKA to perform quantum state preparation operation, which has to be affected by imperfections of quantum devices. Eve may exploit the weak randomness of imperfect devices to intervene in bit encoding and the basis selection process. Second, based on the decoy-state method, signal states or decoy states emitted by Alice, Bob, and Charlie are transmitted in the optical channel. In this case, Eve may perform attenuation operation to maximize the leaked information. Consequently, it is necessary to consider QCKA protocols’ and the practical security of weak randomness.

In this article, we analyze the performance of the decoy-state MDI-QCKA [11] based on the weak randomness model. First, we analyze the potential influence of weak randomness on the conference key rate in the asymptotic case. Second, we analytically derive the formula of the length of the conference key with finite-key size by exploiting the Chernoff bound [34–36]. Utilizing the experimental parameters from Ref. [11], we numerically simulate the secret conference key rate under the condition of weak randomness. The results demonstrate that the performance of MDI-QCKA deteriorates rapidly, even with the fraction of weak randomness. For the case with finite-key size, we notice that weak randomness damages the performance of MDI-QCKA differently for different data sizes of total transmitted pulses, and the impact of weak randomness on the conference key rate may be greater than the data size of the total transmitted pulses. Moreover, we compare the results with our previous work and notice that utilizing single-photon interference technology in the QCKA may enhance resistance to vulnerabilities of weak randomness.

The remainder of the article is organized as follows: in Section 2, we describe a decoy-state MDI-QCKA protocol. In Section 3, we analyze the security of the decoy-state MDI-QCKA protocol based on the weak randomness model in both asymptotic and non-asymptotic cases. In Section 4, we present the numerical simulation results and a discussion. Finally, the conclusion is drawn in Section 5.

2 Protocol description

3 Security analysis with weak randomness

For the security analysis of decoy-state MDI-QCKA under the condition of weak randomness, hidden variables ξ, ζ, and ς from Eve are assumed to determine the states which are prepared by three legitimate members, and Eve should take responsibility for all abovementioned weak randomness imperfections. The probabilities of non-random part states prepared by three legitimate members are p₁, p₂, and p₃. If p_1(2,3) = 1, all conference key information may be leaked to Eve, that is, R = 0. If p₁ = p₂ = p₃ = 0, Eve cannot obtain information theoretically. If 0 < p_1(2,3) < 1, we can apply the weak randomness model to estimate the maximum quantity of information stolen by Eve.

Here, we suppose that the binary set of bits S, T, and G prepared by Alice, Bob, and Charlie, respectively, decide bit encoding and basis selection. |S|, |T|, and |G| represent the order of sets S, T, and G, respectively. Because of the weak randomness imperfection in the practical system, the random number cannot be perfectly prepared, which shows that partial bits belonging to sets S, T, and G are mastered by Eve. S is made up of a non-random part S₁ and a random part S₂. T is made up of a non-random part T₁ and a random part T₂. G is made up of a non-random part G₁ and a random part G₂. This is rational to assume by considering two scenarios. First, Eve may attack the random number generator and obtain partial random number information. Second, the state may be modulated by different laser diodes from three legitimate members, and Eve may distinguish them by observing characteristics such as the spectrum and timing sequence. Here, we define the weak randomness parameter at Alice, at Bob, and at Charlie. However, we cannot ensure that the capability of Eve attacking Alice is identical to her (him) attacking Bob or Charlie. That is, p₁ = p₂ = p₃ is not necessarily satisfied.

Under the condition of weak randomness, Eve attenuates the quantum states from random parts S₂, T₂, and G₂ with a certain probability in the channel. For this case, the non-random part quantum states reach David without attenuation. Here, we assume that bit errors only come from random parts, and that the states of non-random part do not generate bit errors. If Eve performs attenuation and keeps the error rate within a rational range, her (his) presence cannot be detected by three legitimate members. Considering signal loss, the non-random probability in David’s side may be amplified. Consequently, the maximum transmission distance may compromise, the gain of single-photon on the Z basis may decrease, and the bit error rate may increase.

3.1 Decoy-state MDI-QCKA with weak randomness

Based on the weak randomness model, Alice, Bob, and Charlie prepare quantum states as follows:where Eve can obtain information about Alice’s (Bob’s and Charlie’s) system according to the auxiliary quantum state. For Alice’s (Bob’s and Charlie’s) system, if Eve’s auxiliary quantum state is , Eve may acquire the conference key information k_a (k_b, k_c). If Eve’s auxiliary quantum state is , three legitimate members prepare perfect GHZ states ρ_Alice, ρ_Bob, and ρ_Charlie, and Eve cannot distinguish between different quantum states in this case. Compared with MDI-BB84 QKD, the quantum state prepared and transmitted in the MDI-QCKA is a GHZ state. The raw key of Alice needs to be determined as the reference raw key, and the other two parties need to compare and sift their keys. In addition, the definition of successful gain in the MDI-QCKA is similar to that of the MDI-BB84 QKD, and the final bit error rate of the MDI-QCKA is one of the bit error rates calculated by Bob or Charlie.

In the decoy-state MDI-QCKA protocol, three legitimate members may prepare signal and decoy states using the same laser diodes with the same random probabilities p₁, p₂, p₃. Furthermore, signal and decoy states can also be prepared using different laser diodes. For this case, signal and decoy states prepared by three legitimate members can be distinguished with the probabilities p₁, p₂, p₃. If signal states are distinguished, Eve can perform PNS attack [37] without being detected, and Eve performs an attenuate operation in the quantum channel when signal states are not distinguished.

Then, we may estimate the parameters under the condition of weak randomness. The final conference key rate of the decoy-state MDI-QCKA can be given by [11]:where denotes the gain when Alice sends vacuum states while David gets a successful GHZ state measurement event on the Z basis. is the gain of single-photon states on the Z basis. H(x) = −log₂x − (1 − x) log₂(1 − x) is the binary entropy function. Here, the raw conference key of Alice is assumed as the reference raw key. For the case of asymptotic data, we can consider that the phase error rate of the Z basis is equal to the bit error rate of the X basis . () is the overall bit errors between Alice and Bob (Charlie). f is the error correction efficiency and is the overall gain on the Z basis.

The overall gain and bit error rate can be expressed aswhere W = X, Z. is the yield on the X or Z basis when Alice, Bob, and Charlie prepare n-photon states, m-photon states, and k-photon states, respectively, and is the bit error rate corresponding to this case.

The gain of single-photon states on the Z basis can be given bythe gain when Alice sends the vacuum state while David gets a successful GHZ-state measurement event on the Z basis is given by

Considering the condition of weak randomness, the probability of Alice, Bob, and Charlie preparing non-random part quantum states are p₁, p₂, and p₃, respectively. Eve not only attenuates the states in random parts to amplify the non-random proportion at David’s side but also controls the probability of attenuation to ensure that she (he) will not be detected by three members. We assume that Eve only attenuates the random portion of the single photon, and that multi-photons are not actually used to generate secret keys. The probability of signal loss from the random part is given byThe non-random proportion of quantum states reaching David’s side can be expressed asThe gain of the random part single-photon states which can generate conference keys on the Z basis can be expressed aswhere is the gain of the non-random part single-photon states which cannot generate conference keys. The gain of the random part single-photon states when Alice sends the vacuum state while David gets a successful GHZ state measurement event which can generate conference keys on the Z basis can be expressed aswhere is the gain of the non-random part single-photon states when Alice sends the vacuum state while David obtains a successful GHZ state measurement result which cannot generate conference keys.

Considering the attenuation operation from Eve, the value of the bit error rate under the condition of weak randomness can be given by

With the method mentioned in [38, 39], we can calculate the value of and the value of . Then, combining Eqs. 11–13, we can obtain the final conference key rate of the decoy-state MDI-QCKA under the condition of weak randomness.

3.2 Finite-key analysis with weak randomness

Here, we employ the weak randomness model to analyze the security of the decoy-state MDI-QCKA and derive formulas for the lower bound of the successful single-photon gain and the upper bound of the bit error rate. The final length of the conference keys on the Z basis can be given by [11, 12]:where denotes the number of measurement results when Alice sends the vacuum state while David gets a successful GHZ state measurement event on the Z basis. is the lower bound of the successful single-photon counting rate , and is the upper bound of the bit error rate on the X basis. is the amount of the consumed information in the error correction operation. n^Z is the total number of detection events when Alice, Bob, and Charlie prepare states on the Z basis.

Let be the total number of successful detection events obtained by David when Alice, Bob, and Charlie prepare n-photon states, m-photon states, and k-photon states on the Z basis, respectively. For the intensity combination α_aβ_bγ_c (α_a ∈ A, β_b ∈ B, γ_c ∈ C), the expected value of can be expressed aswhere denotes the conditional probability when Alice, Bob, and Charlie prepare n-photon states, m-photon states, and k-photon states on the Z basis, respectively, with the intensity combination α_aβ_bγ_c (α_a ∈ A, β_b ∈ B, γ_c ∈ C). It can be given bywhere denotes the probability when Alice, Bob, and Charlie prepared n-photon states, m-photon states, and k-photon states on the Z basis, respectively, and is the probability when Alice, Bob, and Charlie modulate the intensity α_a, β_b, and γ_c on the Z basis. p_a|n, p_b|m, and p_c|k are the photon number distribution probabilities of Alice, Bob, and Charlie, respectively.

On the basis of Eq. 9, we can also derive the probability of random part signal loss in the case of finite-key size:The non-random proportion of quantum states reaching David’s side can be expressed aswhere N denotes the total data size of transmitted pulses, W = X, Z.

Taking independent events into account, we exploit the Chernoff bound [34–36] to calculate the number of practical measurement events, which can be expressed aswith the probability at least 1 − 2ɛ₁, where , with and .

Furthermore, denotes the total number of the correct measurement events observed by David when Alice, Bob, and Charlie prepared n-photon states, m-photon states, and k-photon states on the X basis, respectively, and denotes the corresponding number of bit errors. denotes the total number of bit errors when Alice, Bob, and Charlie prepared states on the X basis and . For the intensity combination α_aβ_bγ_c (α_a ∈ A, β_b ∈ B, γ_c ∈ C), the expected value of can be given bywhere denotes the conditional probability when Alice, Bob, and Charlie prepare n-photon states, m-photon states, and k-photon states on the X basis, respectively, with the intensity combination α_aβ_bγ_c (α_a ∈ A, β_b ∈ B, γ_c ∈ C). It can be given bywhere denotes the probability when Alice, Bob, and Charlie prepared n-photon states, m-photon states, and k-photon states on the X basis, respectively, and is the probability when Alice, Bob, and Charlie modulate the intensity α_a, β_b, and γ_c on the X basis, respectively.

For the case of finite sample sizes, the relation between the expected value and observed value of under the Chernoff bound [34–36] can be established aswith the probability at least 1 − 2ɛ₂, where , with and .

The number of single-photon detections from a non-random set which cannot generate conference keys on the Z basis can be expressed asThe number of single-photon detections from a random set is supposed to satisfyThe number of detections from the non-random part when Alice sends a vacuum state while David gets a successful GHZ state measurement event on the Z basis can be expressed asThe secure single-photon detection from the random part when Alice sends a vacuum state while David gets a successful GHZ state measurement event on the Z basis is supposed to satisfyThe lower bound of the single-photon detection on the Z basis can be given bywhereThe number of successful single-photon detection when Alice sends a vacuum state while David gets a successful GHZ state measurement event on the Z basis can be given bywhere is the upper bound and is the lower bound of , which can be deduced from Eq. 19 by using the Chernoff bound.

Considering the attenuation operation from Eve, the value of the bit error rate under the condition of weak randomness can be given bywhere denotes the successful single-photon counting rate on the X basis, and it can be given bywhere

Furthermore, the number of bit errors on the X basis is related to the single-photon detection , and it can be expressed aswherewhere is the upper bound and is the lower bound of , which can be deduced from Eq. 22 by using the Chernoff bound.

Adopting the same calculation method of from Eq. 27, we can calculate the number of single-photon detection on the X basis . Next, we calculate the theoretical value of the bit error rate on the X basis

Finally, combining Eqs. 24, 26, 31, we can derive the secret conference key length of the decoy-state MDI-QCKA under the condition of weak randomness.

4 Numerical simulations

In this section, based on the fiber-based channel model and the experimental parameters in Ref. [11], we simulate the performance of the decoy-state MDI-QCKA with the effect of weak randomness in both asymptotic and finite-key size cases.

Here, we define the fiber transmittance of Alice, Bob, and Charlie as , , and , respectively, with . α = 0.2(dB/km) is the loss coefficient of the standard fiber link, L_a,b,c is the fiber length, and p_d = 10⁻⁷ is the dark count rate of David’s detectors. η_d = 90% is the detection efficiency of David. e_d is the optical misalignment-error probability, and the efficiency of error correction is f = 1.16. denotes the conference key rate, and N is the total data size of transmitted pulses. Here, we fix the security bound when considering statistical fluctuations analysis ɛ = ɛ₁ = ɛ₂ = 10⁻¹⁰. The list of numerical experimental parameters is given in Table 1.

In Figure 1, we simulate the performance of the decoy-state MDI-QCKA in the asymptotic case. Here, we assume that three legitimate members are symmetrical in the system. p₁, p₂, p₃ = 0 means that randomness in the quantum-prepared operation is perfect, and p₁, p₂, p₃ = 10^−x(x = 10, 8, 6) means that Eve can master different randomness information. As shown in the results, we can deduce that the lack of randomness on either side will cause information leakage and affect the generation of the conference key. Although Eve only masters one party randomness information for p₁ = 10⁻¹⁰ and p₂, p₃ = 0, the maximum transmission distance dropped evidently from 207 km to 149 km. Particularly, when the parameters of weak randomness p₁, p₂, p₃ = 10⁻⁶, the achievable transmission distance decreases to 132 km.

FIGURE 1

For the case with finite-key size, we simulate the conference key rate with the effects of weak randomness as shown in Figure 2. Here, we fixed the data size of the total transmitted signals N = 10¹⁴. The results show that weak randomness evidently limits the conference key rate and achievable transmission distance. Although Eve only masters one party randomness information for p₁ = 10⁻⁶, 10⁻⁵, 10⁻⁴, and p₂, p₃ = 0, the achievable transmission distance decreases to 26, 42, and 49 km, respectively. When the weak randomness parameters p₁, p₂, p₃ = 10⁻⁶, 10⁻⁵, 10⁻⁴, the achievable transmission distance decreases to 34, 50, and 66 km, respectively.

FIGURE 2

As shown in Figure 3, the curves from right to left are acquired by the data size of transmitted pulses N = 10^x(x = 15, 14, 13, 12). We can see that the greater the data size of the total transmitted pulses, the more obvious the effect of weak randomness. In detail, the achievable transmission distance decreases to 51.63%, 44.25%, 35.05%, and 23.75% when N = 10¹⁵, 10¹⁴, 10¹³, 10¹², respectively. When the data size of the total transmitted signals increases, the number of quantum states attenuated increases. In the practical system, the leaked information may also increase because Eve can take advantage of the relationship between the expected values and observed values of different modulated states.

FIGURE 3

In order to further analyze the impact of weak randomness in the case of finite-key size, we consider the conference key rate for N = 10¹⁴, 10¹³ with different weak randomness parameters p₁, p₂, p₃ = 0, 10^−x(x = 6, 5, 4). As shown in Figure 4, we can notice that the conference key rate lines for N = 10¹³ and N = 10¹⁴ overlap approximately when the weak randomness parameters are p₁, p₂, p₃ ≥ 10⁻⁵. We also deduce that the impact of weak randomness on the conference key rate is stronger than the total data size of transmitted pulses when p₁, p₂, p₃ ≥ 10⁻⁵.

FIGURE 4

Finally, let R(p) be the conference key rate with weak randomness and R(0) be the conference key rate without weak randomness. The relationship between R(p)/R(0) and transmission distance for p₁, p₂, p₃ = 0, 10⁻⁶ and N = 10¹⁴ is shown in Figure 5. Compared with our previous studies on MDI-QKD [32] and SNS TF-QKD [33], we can find that MDI-QCKA is more susceptible to weak randomness. In fact, all three legitimate members need to prepare quantum states in the practical MDI-QCKA system, and it is more difficult to protect random information than the other two. In addition, we notice that SNS TF-QKD can better resist weak random imperfection and realize longer transmission distance; therefore, we suspect that QCKA based on single-photon interference technology may perform better in resisting weak randomness, which deserves further study.

FIGURE 5

From the simulation results presented previously, we can find that the security of MDI-QCKA is sensitive to weak randomness in both asymptotic and finite-key size cases. Furthermore, we find that weak randomness damages the performance of MDI-QCKA to different degrees for different data sizes of the total transmitted signals. Finally, we conclude that QCKA based on the single-photon technology may be better resistant to weak random vulnerabilities.

5 Conclusion

In conclusion, we employ the weak randomness model to analyze the security of the decoy-state MDI-QCKA and study the performance of the decoy-state MDI-QCKA in both the asymptotic case and non-asymptotic case. The simulation results demonstrate that the weak randomness of the practical QCKA system is non-negligible. The conference key rate and achievable transmission distance will be significantly compromised due to the attenuation operation of Eve. Moreover, we find that weak randomness impacts the conference key rate differently for different data sizes of transmitted pulses and the impact may be greater than that of the data size of transmitted pulses in some cases. To avoid such a vulnerability risk in a QCKA system, two approaches can be considered. First, protecting randomness information from leakage. Second, the risk of the side channels at the source should be reduced and distinguishability of the quantum states in all degrees of freedom should be avoided. Compared with our previous work, we infer that QCKA based on single-photon interference technology may have more development prospects in resisting weak randomness, which is also our future research direction.

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