NDUST QUANTA Team Further Improves Quantum Communication Security Protecting the Source from Laser Injection Type Attacks

In a quantum key distribution system, a well-protected and characterized source is needed to ensure its security. However, the source is vulnerable to laser injection type attacks. For this reason, the joint research team has proposed a countermeasure against light injection type attacks - an additional sacrificeable component placed at the exit of the light source. This component is capable of withstanding high power incident light while attenuating it to a safe level that does not alter the rest of the source end, or is destroyed to a permanent high attenuation state, thereby blocking the optical path. It has been demonstrated experimentally that existing fiber optic isolators and circulators can act as such a component.

A related paper, "Protecting fiber-optic quantum key distribution sources against light-injection attacks," was published in PRX Quantum on Oct. 14. The paper was published in PRX Quantum.

The corresponding author of the paper is Associate Researcher Anqi Huang from QUANTA team of School of Computer Science, National University of Defense Technology. The team has achieved several research results in the field of quantum computing, and this publication extends its high-level research capability to quantum information security.

01The Challenge of Laser Injection Attack

Quantum key distribution (QKD) allows the secure establishment of a key between two remote parties over an open channel. Its information-theoretic security is based on quantum physics, rather than any computational complexity. This makes QKD in principle unbreakable even when attacked by super-powerful quantum computers. However, in practice, achieving an unattackable QKD system is a long process due to imperfect devices in the real world.

The two sides of quantum key distribution are divided into a sender and a receiver. Ten years ago, various vulnerabilities were discovered on the receiver side, and with the introduction of measurement device-independent QKD (MDI-QKD) and two-field QKD (TF-QKD) - in which there are no security assumptions about quantum state measurements - all the vulnerabilities on the receiver side were eliminated vulnerabilities.

However, on the sender side (source side), an attacker can learn and even manipulate the characteristics of the source-side components through light injection type attacks, including Trojan horse attacks, laser seeding attacks, intense light damage attacks, and optical power monitoring bypass attacks. Since the altered component characteristics from laser injection type attacks are usually unpredictable, it is difficult to build a security model to counter these active attacks.

Fiber optic isolators or circulators are often used as the last component at the source end and are thought to protect fiber-based QKD systems from attackers injecting light through the quantum channel. Protecting the source-side device by isolating the component seems to be a promising solution. However, unknown attacks on the isolation component may affect the actual isolation level. Therefore, it remains challenging to secure QKD systems in practice in this realistic scenario.

02Success against laser injection type attacks

In this paper, Associate Researcher Angel Wong and co-workers demonstrate that placing an additional sacrificial isolation component at the source-side exit not considered in the security model can effectively counteract light injection type attacks.

It is demonstrated experimentally that when an attacker irradiates the isolator and circulator with a high-powered continuous laser, they maintain 6.4-42.4 dB of isolation despite the fact that the high-powered laser temporarily or permanently reduces their isolation values by 15.2-34.5 dB. Because the isolation components still provide a large isolation under high power attack, other optical components in the QKD source behind the isolation devices can be protected from laser damage attacks.

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Figure 1: Experimental setup for testing the isolator. LD, laser diode; OPM, optical power meter; LT, optical trap; HPL, high power laser. The coupling ratio of the beam splitter (BS) denoted as 95= :5⨉ means that 95% of the light level passes through the opposite port in the BS graphic symbol, while 5% passes through the other port.

Figure 2: Experimental setup for testing the circulator. LD, laser diode; OPM, optical power meter; LT, optical trap; HPL, high power laser.

For the fiber isolator experiments, the team tested four models of fiber isolators used in a real QKD system, as shown in Table 1 for ISO PM 1, ISO PM 2, ISO 4, and ISO 3-1 and ISO 3-2. All isolators are similar in design and operation except that ISO PM 1 and ISO PM 2 are polarization dependent while the other two models are polarization insensitive.

As shown in Table 1, the test samples are susceptible to high power injected laser light, and the isolation is temporarily reduced by 15.2-34.5 dB for a given illumination power. Thus, isolation values of 17.2-42.4 dB remain until the samples are completely damaged, which is below the minimum isolation preset for each sample out of the field.

Table 1: Test results of the isolator. All measurements are at 1550 nm.

As can be seen in Figure 3 (top), all samples show a decrease in isolation at high laser power. The exception is ISO PM 2, for which a 3.4 dB reduction in isolation is observed for its maximum value in the operating power range. however, even for this sample, the measured isolation is within specification when the illumination laser power is in the operating range (ISO PM 2 specifies a minimum isolation of 28 dB). The "breakdown" point in Figure 3 (center) shows that ISO PM 1 and ISO 3-2 were completely damaged at laser powers of 6.7 W and 3.8 W - they exhibited significant insertion loss and isolation. For the other samples, the researchers stopped laser irradiation before they were completely destroyed and observed a permanent drop in isolation of 3.9 dB for ISO PM 2 and a temporary drop in isolation for ISO 3-1 and ISO 4.

It should be noted that before being destroyed, the isolator maintained positive operation, while its isolation value decreased. The insertion loss changes slightly - 0.5-1.1 dB, which results in a loss of at most 22% of the forward transmitted power. Once ISO PM 1 and ISO 3-2 are irreversibly destroyed, they have an insertion loss greater than 80 dB.

Figure 3: Isolator parameters under test

The team then investigated the mechanism of isolation degradation and isolator damage. This is because the injected high power laser emission is rejected from the isolator coupled to the fiber, and the rejected light is absorbed inside the package, leading to local heating. In addition, the isolation decreases rapidly with power after applying laser power higher than the maximum operating value specified for the sample (300 mW). The isolation returns to close to the initial value after cooling.

Similarly, tests of fiber optic circulators were performed. The following graph shows the test results for the three circulators.

Figure 5: Isolation values and maximum surface temperature of the circulators in the test.

03Summary

This study has improved the understanding of unsafe isolation components isolators and circulators in QKD systems for the first time. Specifically, the tests showed that the isolation values provided by the optical isolator and the circulator can be reduced to a minimum of 17.2 and 6.4 dB when a high power laser is injected backwards into the optical isolator and circulator under test.To enhance the protection of the QKD source, an additional sacrificeable isolation component, i.e., an optical isolator or circulator, is required to resist an optical injection type attack. The remaining isolation of this sacrificeable component is sufficient to protect the other components behind it. This study shows that the source side of a QKD system requires an additional layer of protection to ensure security.

About the QUANTA Team

The QUANTA team at the School of Computer Science, National University of Defense Technology has combined the strengths of computer science disciplines into quantum information research, and has achieved a number of research results in quantum hegemony standard research, quantum computing simulation algorithms, quantum computing architecture, quantum computing programming languages, integrated optical quantum chips, and topological superconducting quantum technologies, and has published a number of papers including Science, Science Advances, Nature Photonics, Nature Physics, Physical Review Letters, etc. He has published many papers in international authoritative journals.