  Widely-deployed encryption-based security prevents unauthorized decoding, but
does not ensure undetectability of communication. However, covert, or low
probability of detection/intercept (LPD/LPI) communication is crucial in many
scenarios ranging from covert military operations and the organization of
social unrest, to privacy protection for users of wireless networks. In
addition, encrypted data or even just the transmission of a signal can arouse
suspicion, and even the most theoretically robust encryption can often be
defeated by a determined adversary using non-computational methods such as
side-channel analysis. Various covert communication techniques were developed
to address these concerns, including steganography for finite-alphabet
noiseless applications and spread-spectrum systems for wireless communications.
After reviewing these covert communication systems, this article discusses new
results on the fundamental limits of their capabilities, as well as provides a
vision for the future of such systems.
