The demands for higher data rates and network decentralized nodes in wireless communication systems urge the need for efficient methods to preserve the security levels. Many of the existing methods to share a private secret key between the connected nodes, tend to have high computational complexity. Furthermore, most physical layer approaches assume TDD systems to use the intrinsic feature of the reciprocal channels. In this paper, we propose a new method for exchanging secret key bits using a private random precoding based in MIMO FDD systems. The principle of this method is to exploit the PMI in a manner that produces low correlation at the intruder. By uniquely relating the secret key bits to the channel precoding matrix in a private version of the universal codebook, a robust key exchange between the transmitter and the receiver is then performed.