  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.
