The discussion here continues from the discussion of security of quantum key distribution (QKD).

QKD is a method for generating and distributing symmetric cryptographic keys with information-theoretic security based on quantum information theory [ETS18].

In a QKD protocol,

• 👩 Alice and 🧔 Bob attempt to establish a secret symmetric key between themselves.
• 😈 Eve attempts to find out about this secret key.

😈 Eve’s attempt to discover the secret key can be classified into 1️⃣ individual attack (discussed below), 2️⃣ collective attack, and 3️⃣ coherent attack; in 🔼 increasing order of power given to Eve.

Individual attacks are the simplest and most studied class of attacks.

When conducting an individual attack, Eve interacts with each signal (i.e., quantum state) from Alice individually, and is restricted to the same interaction for all Alice’s signals [Sch10, Wol21].

• Some strategies rely on the fact that Eve is able to postpone her interaction with Alice’s signal until after sifting and error correction, or as long as she wants, to obtain the maximum amount of information from Alice’s and Bob’s public interaction.
• In some other strategies, Eve measures the state instantly and uses the information from sifting and error correction later on.

Regardless, Eve has the freedom to choose which unitary operation she applies to a composite system (“composite” because more than one qubit is involved).

Of interest is the amount of information about a single state that Eve gains with an attack. A standard measure of amount of information is mutual information.

Perhaps the most intuitive example of an individual attack is the intercept and resend (I&R) attack [Sch10, Sec. 5.2.1].