Secure Key Management for 5G Physical Layer Security Asim Mazin, Kemal Davaslioglu, and Richard D. Gitlin Department of Electrical Engineering University of South Florida Tampa, Florida 33620, USA Email: asimmazin@mail.usf.edu {kemald, richgitlin}@usf.edu Abstract—Next generation 5G wireless networks pose several important security challenges. One fundamental challenge is key management between the two communicating parties. The goal is to establish a common secret key through an unsecured wireless medium. In this paper, we introduce a new physical layer paradigm for secure key exchange between the legitimate communication parties in the presence of a passive eavesdropper. The proposed method ensures secrecy via pre-equalization and guarantees reliable communications by the use of Low Density Parity Check (LDPC) codes. One of the main findings of this paper is to demonstrate through simulations that the diversity order of the eavesdropper will be zero unless the main and eavesdropping channels are almost correlated, while the probability of key mismatch between the legitimate transmitter and receiver will be low. Simulation results demonstrate that the proposed approach achieves very low secret key mismatch between the legitimate users, while ensuring very high error probability at the eavesdropper. Keywords—Key management, Physical layer security, LDPC, wiretap channel. I. INTRODUCTION The broadcast nature of wireless medium makes wireless transmissions vulnerable to eavesdropping. To ensure that the information is conveyed in a secure way, cryptographic encryption techniques are often employed in the upper layers of the communication protocol stack. For example, symmetric cryptography methods (e.g., Advanced Encryption Standard) employ a common private key that is pre-shared between the source and destination, referred to as Alice and Bob in this paper, to encrypt/decrypt data. In contrast to the symmetric cryptography, asymmetric cryptography methods such as Public key Cryptosystems (PKC) use public and private keys. In today’s mobile communication systems, symmetric cryptography has been used due to its low computational cost compared to PKC. However, if the legitimate parties do not pre-share a common key, then the key needs to be established and conveyed to both parties through a private wireless channel, that may not always exist and is prone to be intercepted by an eavesdropper, referred to as Eve hereafter. For next generation wireless networks, such as 5G wireless, the process of key management (key generation and secure key exchange) will become even more important as the number of nodes increases to a massive scale and nodes become more heterogeneous in their computational capabilities. Also, physical layer security offers a good solution for interoperability between different systems where pre-shared keys may not exist. We envision that physical layer security methods will be used as an additional layer of security to complement traditional cryptographic methods. Recently, physical layer security has gained a lot of attention since it offers enhanced wireless network security by exploiting wireless channel characteristics to generate a secret key between the communication nodes. Using training sequences (probing signals), both parties can measure the channel parameters such as the received signal strength indicator (RSSI) [1]-[4], the channel state information (CSI) [5]-[6], or the power spectral density (PSD) [7] of the probing signals to agree on a secret key. However, the randomness that can be extracted from the channel through the signal processing techniques proposed in [1]-[7] is limited by the randomness in the channel. For stationary or low-mobility users, the channel randomness is very low and the number of uncorrelated bits that can be generated from the channel is very few. Furthermore, the techniques proposed in [1]-[7] are prone to manipulation. An adversary may physically introduce blockage or digitally transmit/not transmit jamming signals to manipulate the distribution of bits. In [8]-[11], precoding matrix indicator (PMI) based key generation methods were proposed, which employ predefined codebooks to generate unique keys for devices with multiple antennas. To increase the key generation rate, a channel independent approach was proposed in [12] for fast secret key extraction. In [12], the receiver with a full-duplex transmission capability jams the one of the two copies of the secret key send by the transmitter. An Artificial Noise Injection (ANI) based physical layer approach was proposed in [13] to secure space-time block codes. ANI symbols are added to the information symbols such that they are aligned at the intended receiver and can be subtracted from the information symbols, while they degrade the unintended receiver performance. However, despite its good performance, it requires the legitimate transmitter to know the instantaneous channel of the eavesdropper that may not be possible in many practical applications. Another drawback of [12]-[13] is that jamming and ANI-based techniques increase the interference in the system and they are not energy efficient. Taking into account the green communication interests for the next generation wireless systems, we propose an energy efficient method that does not require the transmitter or any helper to inject noise into the system, which also prolongs its battery life. Furthermore, the key generation rate of the proposed method is high because it is not limited by the channel randomness. In this paper, we exploit the uniqueness of the main and eavesdropping channels to generate secret keys over wireless channels. A pre-equalization transmit filter that inverts the main channel is employed to decrease the probability of