  We present two attacks on two different versions of physical layer
cryptography schemes based on massive multiple-input multiple-output (MIMO).
Both cryptosystems employ a singular value decomposition (SVD) precoding
technique. For the first one, we show that the eavesdropper (who knows its own
channel and the channel between legitimate users) can decrypt the information
data under the same condition as the legitimate receiver. We study the
signal-to-noise advantage ratio for decoding by the legitimate user over the
eavesdropper in a more generalized scheme when an arbitrary precoder at the
transmitter is employed. 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 $O\left(n^2\right)$ upper bound on the advantage and show that this
bound can be approached using an inverse precoder. For the second cryptosystem,
we show that the required security conditions prevent the legitimate user from
decoding the plaintext uniquely.
