Signal Processing in the Encrypted Domain

Recent advances in digital signal processing enabled a number of new services in various application domains, ranging from enhanced multimedia content production and distribution, to advanced healthcare systems for continuous health monitoring. At the heart of these services lies the ability to securely manipulate “valuable” digital signals in order to satisfy security requirements such as intellectual property management, authenticity, privacy, and access control. Currently available technological solutions for “secure manipulation of signals” apply cryptographic primitives by building a secure layer on top of existing signal processing modules, able to protect them from leakage of critical information, assuming that the involved parties or devices trust each other. This implies that the cryptographic layer is used only to protect the data against access through unauthorized third parties or to provide authenticity. However, this is often not enough to ensure the security of the application, since the owner of the data may not trust the processing devices, or those actors that are required to manipulate them. It is clear that the availability of signal processing algorithms that work directly on encrypted signals would be of great help for application scenarios where signals must be produced, processed, or exchanged securely. Whereas the development of tools capable of processing encrypted signals may seem a formidable task, some recent, still scattered, studies, spanning from secure embedding and detection of digital watermarks and secure content distribution to compression of encrypted data and access to encrypted databases, have shown that performing signal processing operations in encrypted content is indeed possible. We are delighted to present the first issue of a journal, entirely devoted to signal processing in the encrypted domain. The issue contains both survey papers allowing the reader to become acquainted with this exciting field, and research papers discussing the latest developments. The first part of the special issue contains three survey papers: Fontaine and Galand give an overview of homomorphic encryption, which is one of the key tools for signal processing in the encrypted domain, in their paper “A survey of homomorphic encryption for nonspecialists.” An introduction to the field of securemultiparty computation is provided by the paper “Secure multiparty computation between distrusted networks terminals” by Cheung and Nguyen. Finally, research in the area of signal processing under encryption is surveyed in the paper “Protection and retrieval of encrypted multimedia content: when cryptography meets signal processing” by Erkin et al. The second part of the special issue contains four research papers. Orlandi et al. introduce the notion of oblivious computing with neural networks in the paper “Oblivious neural network computing via homomorphic encryption.” Troncoso-Pastoriza and Perez-Gonzalez present new protocols for zero-knowledge watermark detection in their paper “Efficient zero-knowledge watermark detection with improved robustness to sensitivity attacks.” Prins et al. show in their paper “Anonymous fingerprinting with robust QIMwatermarking techniques” how advanced quantizationindex-modulation watermarking schemes can be used in conjunction with buyer-seller watermarking protocols. Finally, Gschwandtner et al. explore properties of specialized image encryption schemes in their paper “Transmission error and compression robustness of 2D chaotic map image encryption schemes.” Finally, we would like to thank all the authors, as well as all reviewers, for their contribution to this issue. We hope that the readers will enjoy this special issue and that it encourages more colleagues to devote time to this novel and exciting field of research.