Steganographic communication via spread optical noise: A link-level eavesdropping resilient system

We are witnessing a rising concern over communication security and privacy. Conventional cryptography techniques encrypt data into unreadable codes, but still expose the existence of communications through metadata information (e.g., packet timing and size). In contrast, we propose a steganographic communication scheme adopting a novel combination of the intrinsic optical noise and a unique signal spreading technique to hide the existence of optical communications, i.e., optical steganography. We experimentally implement a prototype steganographic communication system, which requires zero cover-signal overhead to enable a stealth communication channel over the existing communication infrastructure. Both the frequency and time-domain characterizations of our prototype implementation verify the feasibility of our approach. We also demonstrate the first practical steganographic communication that provides reliable communication performances between real computers at the application layer (200-300 Mb/s file transfer rate and 25-30 Mb/s Internet data rate) over long-distance optic fibers (25-50 km). Additional bit-error-rate measurements illustrate negligible channel interference between the public and stealth communications (less than 1 dB power penalty). We further quantitatively demonstrate that the eavesdropper's chance of matching system parameters to effectively recover the stealth signal is 2^-10 by random guessing, and that the eavesdropper's ability of detecting the stealth signal hidden in the transmission channel is strictly limited close to a random guess. Our steganographic communication scheme provides an attractive foundation for mitigating eavesdropping at the link level; thus, paving the way for future privacy-enhancing technologies using the physical layer characteristics of communication links.