Exploration of Side Channels in our BB84 Freespace Quantum Key
Distribution System

The security of quantum key distribution, (QKD) is based on physical
laws rather than assumptions about computational complexity: An
adversary will necessarily disturb the communication by his
quantum measurement. However, real implementations will be sensitive to
side-channel attacks, i.e. to information losses due to
distinguishabilities in other degrees of freedom, which an adversary can
measure without causing errors.

We are running an implementation of the BB84 protocol
installed on top of two university buildings in downtown Munich. Using
attenuated laser pulses in combination with decoy states we are able to
establish a secret key over a distance of 500 m. Our system is fully remote
controlled and allows for continuous and fast QKD. I will report on the
characterization of this QKD system with respect to side channels of the
transmitter and the receiver and also show some attacks.