  We propose a double blinding-attack on entangled-based quantum key
distribution protocols. The principle of the attack is the same as in existing
blinding attack except that instead of blinding the detectors on one side only,
Eve is blinding the detectors of both Alice and Bob. In the BBM92 protocol, the
attack allows Eve to get a full knowledge of the key and remain undetected even
if Alice and Bob are using 100% efficient detectors. The attack can be easily
extended to Ekert protocol, with an efficiency as high as 85.3%.
