On Massive MIMO Physical Layer Cryptosystem

In this paper, we present a zero-forcing (ZF) attack on the physical layer cryptography scheme based on massive multiple-input multiple-output (MIMO). The scheme uses singular value decomposition (SVD) precoder. We show that the eavesdropper can decrypt/decode the information data under the same condition as the legitimate receiver. We then study the advantage for decoding by the legitimate user over the eavesdropper in a generalized scheme using an arbitrary precoder at the transmitter. On the negative side, we show that if the eavesdropper uses a number of receive antennas much larger than the number of legitimate user antennas, then there is no advantage, independent of the precoding scheme employed at the transmitter. On the positive side, for the case where the adversary is limited to have the same number of antennas as legitimate users, we give an $\mathcal{O}\left(n^2\right)$ upper bound on the advantage and show that this bound can be approached using an inverse precoder.