A method comprising transmitting, by a first communication node (101), a first communication; receiving, by a relay node (103; 503), the first communication and estimating a first communication channel (Hk; Hk(1)). Transmitting, by a second communication node (102), a second communication; receiving, by the relay node (103; 503), the second communication; and estimating a second communication channel (Gk, Gk(1)); generating a first transmission signal (xk; x1); transmitting, by the relay node (103; 503), the first transmission signal (xk; x1); generating, by the first communication node (101), a first instance of the encryption key (κA) based on a first received signal (rA); generating, by the second communication node (102), a second instance of the encryption key (κB) based on a second received signal (rB), wherein: the transmission signal (xk) is generated such that the first received signal (rA) is equal to the second received signal (rB).