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The discrete-variable QKD protocols based on BB84 are known to be secureagainst an eavesdropper, Eve, intercepting the flying qubits and performing anyquantum operation on them. However, these protocols may still be vulnerable toside-channel attacks. We investigate the Trojan-Horse side-channel attack whereEve sends her own state into Alice's apparatus and measures the reflected stateto estimate the key. We prove that the separable coherent state is optimal forEve amongst the class of multi-mode Gaussian attack states. We describe howAlice may defend against this by adding thermal noise to the system and give ananalytic expression of the resulting secret key rate. We also provide a boundon the secret key rate in the case where Eve may use any separable state, anddescribe an active defense system based on optical modulators.

Burning the Trojan Horse: Defending against Side-Channel Attacks in QKD