I am taking a Quantum Information course and I stumbled onto this problem, which I am unsure about. The situation assumes BB84 protocol, but with Eve doing a cloning operation (let's assume that she's doing it in states $|+>$ and $|->$ which are orthogonal) instead of the measurement. She keeps the cloned qubit. Would it mean that Bob and Alice won't know if the qubit was intercepted?
Also, if Bob and Alice are working in the same basis, would it actually mean that Eve intercepted the qubit without them knowing and has perfect knowledge of the qubit's state?