  Current security techniques can be implemented with either secret key
exchange or physical layer wiretap codes. In this work, we investigate an
alternative solution for MIMO wiretap channels. Inspired by the artificial
noise (AN) technique, we propose the unshared secret key (USK) cryptosystem,
where the AN is redesigned as a one-time pad secret key aligned within the null
space between transmitter and legitimate receiver. The proposed USK
cryptosystem is a new physical layer cryptographic scheme, obtained by
combining traditional network layer cryptography and physical layer security.
Unlike previously studied artificial noise techniques, rather than ensuring
non-zero secrecy capacity, the USK is valid for an infinite lattice input
alphabet and guarantees Shannon's ideal secrecy and perfect secrecy, without
the need of secret key exchange. We then show how ideal secrecy can be obtained
for finite lattice constellations with an arbitrarily small outage.
