1. A technique whereby the hacker attempts to discover secrets about a program. Some reverse engineering techniques are: strings Dumps all the human-readable strings within a program. In 1999, hackers looked for "strings" within Microsoft's products and found something labeled NSA_KEY. This led the paranoid delusion that the NSA had somehow convinced Microsoft to put a backdoor key into the system. Similarly, early in year 2000, hackers discovered strings like GetPrivateProfileString in the BlackICE Defender personal firewall and made paranoid assumptions (in reality, GetPrivateProfileString is a standard Win32 function). The most commonly used tool for this is the program strings included with UNIX. disassemble Takes the compiled output of a program and retrieves the original assembly language mnemonics, which are easier for humans to read. For example, the byte "0x90" might be converted back into NOOP (no operation) instruction. An example of using this technique to discover code being sent across the wire is at http://www.robertgraham.com/pubs/aol-exploit. The problem with disassembly is that it only makes the object files slightly more readable -- it doesn't reconstruct the full original source code or comments. decompile Decompilation produces high-level source code from an executable. The technique has proven essentially worthless for languages like C/C++, but works well on languages like Java, VisualBasic, and Delphi. It still doesn't obtain the original comments, however. Reverse engineering is often used to: anti-virus Discover how viruses work in order to write more effective signatures against them. cracking serialz Figuring out how copy protection works in order to break it. From Hacking-Lexicon

2. The process of systematically taking apart a chip or application program to discover how it works, with the aim of imitating or duplicating some or all of its functions. From QUECID