Tetsu Ishizue, The University of Electro-Communications; Sara Rampazzi, University of Florida; Takeshi Sugawara, University of Electro-Communications
This paper presents a novel optical-synthesis attack and defense methodology for recaptured image detection systems that rely on infrared (IR) depth sensors. We first introduce SynthIR, which evades detection by independently manipulating the views of the RGB camera and the IR depth sensor across different optical spectra through an inexpensive optical filter that splits and recombines different colors of light. We validate the attack on portrait scenarios by constructing paired RGB images and physical 3D objects that bypass state-of-the-art detection methods, either using low-cost cardboard objects or by generating images geometrically consistent with a target 3D object without requiring precise alignment. The attack further generalizes to multi-frame video scenarios and, when applied to printed images rather than monitors, successfully circumvents both depth-based and image-only recapture detectors. To address the fundamental limitations of these IR-based two-sensor approaches, we propose a new defense pipeline based on dual-pixel image sensors, already deployed in modern smartphone cameras, which capture RGB and stereo images within the same optical spectrum. Our method estimates a depth map from the stereo pair and verifies its consistency with the RGB image, enabling robust recapture detection. Our experimental results demonstrate that the proposed defense detects all recaptured images without falsely rejecting any genuine images.
