  We propose a local scheme to enhance the security of quantum key distribution
in Ekert protocol (E91). Our proposal is a fair sampling test meant to detect
an eavesdropping attempt that would use a biased sample to mimic an apparent
violation of Bell inequalities. The test is local and non disruptive: it can be
unilaterally performed at any time by either Alice or Bob during the production
of the key, and together with the Bell inequality test.
