A Cryptographic Method for Secure Watermark Detection

We present a semi-public key implementation of quantization index modulation (QIM) watermarking called Secure QIM (SQIM). Given a signal, a watermark detector can learn the presence of an SQIM watermark without learning anything anything else from the detection process. The watermark detector first transforms the signal with a secret transform, unknown to the detector, and then quantizes the transform coefficients with secret quantizers, also unknown to the detector. This is done with the use of homomorphic cryptosystems, where calculations are performed in an encrypted domain. A low-power, trusted, secure module is used at the end of the process and reveals only if the signal was watermarked or not. Even after repeated watermark detections, no more information is revealed than the watermarked status of the signals. The methods we present are for watermark systems with quantizers of stepsize 2.