Explicit attack on the key in quantum cryptography (BB84 protocol) reaching the theoretical error limit $Q_c\approx11\%$

An explicit attack on the distributed key is presented for the BB84 quantum cryptography protocol. In this attack, the theoretical maximum of the eavesdropper’s (Eve’s) information on the key is reached at the minimum possible error Q at the receiver end. Eve’s maximum information is reached in collective measurements, which are constructed in the explicit form and are performed by Eve at the very end of the protocol. The situation in the critical error range 11% < Q _c < 15% is determined.