Weak randomness seriously limits the security of quantum key distribution

In usual security proofs of quantum protocols the adversary (Eve) is expected to have full control over any quantum communication between any communicating parties (Alice and Bob). Eve is also expected to have full access to an authenticated classical channel between Alice and Bob. Unconditional security against any attack by Eve can be proved even in the realistic setting of device and channel imperfection. In this paper we show that the security of quantum key distribution protocols is ruined if one allows Eve to possess a very limited access to the random sources used by Alice. Such knowledge should always be expected in realistic experimental conditions via different side channels.

Figure 1

A sketch version of the BB84 protocol. Eve has full access to both the quantum channel (Q.C.) and to authenticated classical channels (A.C.C.) and possesses partial access to random sources of Alice and Bob.