Reverse engineering of software is the process of recovering higher-level structure and meaning from a lower-level program representation. It can be used for legitimate purposes---e.g., to recover source code that has been lost---but it is often used for nefarious purposes, e.g., to search for security vulnerabilities in binaries or to steal intellectual property. This paper addresses the problem of making it hard to reverse engineering binary programs by making it difficult to disassemble machine code statically. Binaries are obfuscated by changing many control transfers into signals (traps) and inserting dummy control transfers and "junk" instructions after the signals. The resulting code is still a correct program, but even the best current disassemblers are unable to disassemble 40%-60% of the instructions in the program. Furthermore, the disassemblers have a mistaken understanding of over half of the control flow edges. However, the obfuscated program necessarily executes more slowly than the original. Experimental results quantify the degree of obfuscation, stealth of the code, and effects on execution time and code size.