The intellectual property (IP) business model is vulnerable to a number of potentially devastating obstructions, such as misappropriation and intellectual property fraud. We propose a new method for IP protection (IPP) which facilitates design watermarking at the combinational logic synthesis level. We developed protocols for embedding designer- and/or tool-specific information into a logic network while performing multi-level logic minimization and technology mapping. We demonstrate that the difficulty of erasing author's signature or finding another signature in the synthesized design can be made arbitrarily computationally difficult. We also developed a statistical method which enables us to establish the strength of the proof of authorship. The watermarking method has been tested on a standard set of real-life benchmarks where exceptionally high probability of authorship has been achieved with negligible overhead in solution quality.