A constraint-based watermarking technique using Schmitt Trigger insertion at logic synthesis level

Although the advancement in Intellectual Property (IP) design increases rapidly, it is easy to copy and resell IP cores without noticing its owners. As a consequence, IP Protection (IPP) using watermarking has emerged as the state-of-the-art technique and raises numerous interests from many Integrated Circuit (IC) designers. In this paper, we propose a constraints-based watermarking technique to embed a signature in the design in order to prove the copyright of the IP makers. Our method carefully selects a list of candidate nets for embedding watermark. Then, a bit `1´ in the stream of watermarking bits is defined by adding a pair of improved Schmitt Trigger circuits to the corresponding net. Besides using the ASCII string of the signature, two hash functions, i.e. MD5 and BASE64, are utilized to create a unique stream of watermarked bits. The experimental results show that our proposed method has the ability to preserve the original functionality, performance of the design, and low overhead of the watermarked circuit. Specifically, when the proposed technique is applied into the 720-cell simple CPU IP core, there are 0% and 12% increase in delay and area overhead, respectively. Furthermore, if it is applied into practical circuits with millions cells, the area overhead will be approximately to 0%.