In this talk we consider the propagation of waves transmitted by ambient noise sources. We discuss a generalized Helmholtz-Kirchhoff identity that derives from Green's identity and Sommerfeld radiation condition. The inspection of this identity makes it possible to design passive imaging methods, i.e., imaging methods using only passive receiver arrays and ambient noise illumination. More surprisingly, it is also possible to design an original passive communication scheme between two passive arrays that uses only ambient noise illumination. The passive transmitter array does not transmit anything but it is a tunable meta-material surface that can modulate its scattering properties and encode a message in the modulation.