Today, although intellectual properties (IP) and their reuse are common, their use is causing design security issues: illegal copying, counterfeiting, and reverse engineering. IP watermarking is an efficient way to detect an unauthorized IP copy or a counterfeit. In this context, many interesting solutions have been proposed. However, few combine the watermarking process with synthesis. This article presents a new solution, i.e. automatic low cost IP watermarking included in the high-level synthesis process. The proposed method differs from those cited in the literature as the marking is not material, but is based on mathematical relationships between numeric values as inputs and outputs at specified times. Some implementation results with Xilinx Virtex-5 FPGA that the proposed solution required a lower area and timing overhead than existing solutions.