Eavesdropping Quantum Key Distribution

The goal of Quantum Key Distribution (QKD) is to ensure you are having secure and private communication with the recipient. The benefit of QKD is that this can be detected and verified.

Adding Eve — The “Eve”sdropper

As in the previous article, we use Alice as the sender and Bob as the receiver. The only time these two are wanting to have secure and private communication is when Alice is sending her encoded qubits to Bob. However, since Eve may want to try to take this information, she comes in the middle to try to figure out Alice’s qubits.

Since quantum measurement is different from classical measurement in the sense that if we measure a qubit it may be changed. The best that Eve can do to measure the qubits is to choose a random basis for each qubit since this is what Alice did when encoding the qubits. Due to the property of quantum measurement, there is a chance that Eve measures a qubit that is different from Alice’s original qubits.

To keep Eve’s interference secret, she has to send her qubits to Bob to make it look as if Alice had sent them to him. After Bob selects his bases to measure his received qubits and converts them to classical bits using the agreed-upon table by him and Alice, they tell each other what their bases were and throw out bits and qubits associated with the bases that didn’t match like normal.

When they then compare their bits that weren’t thrown out, they may likely notice that some of their bits don’t match. This is a result of Eve measuring some of Alice’s qubits on different bases. Because their bits don’t match, Alice and Bob know that their communication was intercepted, meaning that they need to restart the process to ensure that their communication is secure. This can be done by changing their communication channel, making a longer key, or through other methods.

QKD Protocols

In this and the previous article on QKD, I described one of the many QKD protocols. This QKD protocol is called the BB84 Protocol, founded by Bennett and Brassard in 1984.

Two other QKD Protocols:

• B92 Protocol — encodes only 2 states
• E91 Protocol — uses entanglement.

QKD in the Real World

Right now, there are several popular algorithms, such as Grover’s Search Algorithm and Shor’s Algorithm. Unfortunately, these are long-term algorithms that assume we have perfect, fault-tolerant hardware, which is something we don’t have today.

Fortunately, however, QKD can be implemented today. One such company, IDQuantique, has taken the leap to implementing QKD commercially. When utilizing QKD, they, among other companies, have to take into account the components of performing the QKD protocol: the device used, the medium (by air or cable), the distance between senders/receivers, and the cost of implementation. These components can make or break QKD.

According to the National Security Agency (NSA), QKD has many technical limitations, which may lessen the appeal of QKD Protocols for real-world use. These include the following:

• “Quantum key distribution is only a partial solution…QKD does not provide a means to authenticate the QKD transmission source…”
• “Quantum key distribution requires special purpose equipment. QKD is based on physical properties, and its security derives from unique physical layer communications…”
• “Quantum key distribution increases infrastructure costs and insider threat risks.”
• “Securing and validating quantum key distribution is a significant challenge…The specific hardware used to perform QKD can introduce vulnerabilities…”
• “Quantum key distribution increases the risk of denial of service.”

Read into each of these limitations in more depth on the NSA’s website.