Quantum key distribution (QKD) is one of the most active research fields in quantum information science and technology. It can provide a method to generate a shared secret key between two distant parties, and has unconditional security guaranteed by physics laws even if the eavesdropper Eve has unlimited computational power and storage capacity. The unconditional security with the practical devices is an important problem in QKD research. The perfect secrecy of the one-time pad (OTP) and a model combining the OTP and the QKD are first introduced. The development of the unconditional security of the QKD is summarized. The emphasis is put on the quantum attack schemes based on practical systems and the corresponding countmeasures. The recent advances of the measurement device independent QKD (MDI-QKD) scheme are described in detail.