Most blockchains are entirely open-source software. This gives auditors ability to review the security of cryptocurrencies such as Bitcoin. Because only after the source code of the cryptocurrency has been publicly audited by the market, users can transfer their tokens in a secure manner. However, the open-source feature has led to frequent plagiarism of smart contracts and introduces the risk of vulnerability proliferation. Copyright protection for smart contracts has never been investigated thoroughly. Due to performance overhead problems, some traditional watermarking techniques are no longer suitable for smart contracts running in a distributed environment. To reduce the negative impact of watermark implantation on processing performance, we use opaque predicate obfuscation to inject watermarks. Further, we propose a novel layout obfuscation via spaces and tabs to completely eliminate the negative performance impact. We conduct comprehensive experiments to evaluate our scheme. The experimental results demonstrate the effectiveness of our scheme.