Recent advances in cloud computing have led to the advent of Business-to-Business Software as a Service (SaaS) solutions, opening new opportunities for EDA. High-Level Synthesis (HLS) in the cloud is likely to offer great opportunities to hardware design companies. However, these companies are still reluctant to make such a transition, due to the new risks of Behavioral Intellectual Property (BIP) theft that a cloud-based solution presents. In this paper, we introduce a key-based obfuscation approach to protect BIPs during cloud-based HLS. The source-to-source transformations we propose hide functionality and make normal behavior dependent on a series of input keys. In our process, the obfuscation is transient: once an obfuscated BIP is synthesized through HLS by a service provider in the cloud, the obfuscation code can only be removed at Register Transfer Level (RTL) by the design company that owns the correct obfuscation keys. Original functionality is thus restored and design overhead is kept at a minimum. Our method significantly increases the level of security of cloud-based HLS at low performance overhead. The average area overhead after obfuscation and subsequent de-obfuscation with tests performed on ASIC and FPGA is 0.39%, and over 95% of our tests had an area overhead under 5%.