The key feature for secure communication relies in the establishment of cryptographic keys. However, low processing devices like sensor nodes are unable to perform a good random number generator used for the key generation, neither are able to perform complex public key based operations. To address this problem, we consider using the measurements of per-packet signal to generate secret keys. Our generator uses the received packet strength from the other device as one of the sources of randomness. Moreover, we propose to use a bidirectional synchronized transmission of constant bit rate (CBR) between two parties, such that one device stands on the surface, while the other device randomly moves around it. The source of randomness results from the per-packet power received by each device. We also show that is impossible for a third party, which is not located at any of the transceivers' positions or near the standing device, to extract any information from the per-packet signal strength measurement generated by any device. Finally, we present the simulation results of our solution in network simulator NS-2.