Logic locking has been conceived as a promising proactive defense strategy against intellectual property (IP) piracy, counterfeiting, hardware Trojans, reverse engineering, and overbuilding a.acks. Yet, various a.acks that use a working chip as an oracle have been launched on logic locking to successfully retrieve its secret key, undermining the defense of all existing locking techniques. In this paper, we propose stripped-functionality logic locking (SFLL), which strips some of the functionality of the design and hides it in the form of a secret key(s), thereby rendering on-chip implementation functionally di.erent from the original one. When loaded onto an on-chip memory, the secret keys restore the original functionality of the design. .rough security-aware synthesis that creates a controllable mismatch between the reverse-engineered netlist and original design, SFLL provides a quanti.able and provable resilience trade-o. between all known and anticipated a.acks. We demonstrate the application of SFLL to large designs (>100K gates) using a computer-aided design (CAD) framework that ensures a.aining the desired security level at minimal implementation cost, 8%, 5%, and 0.5% for area, power, and delay, respectively. In addition to theoretical proofs and simulation con.rmation of SFLL's security, we also report results from the silicon implementation of SFLL on an ARM Cortex-M0 microprocessor in 65nm technology.