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Decompiling and Disassembling Software

Decompilation is the process of transforming a binary executable—that is, a compiled program—into a higher level symbolic language that is easier for humans to understand. Usually this means turning a program executable into source code in a language like C. Most systems for decompiling can't directly convert programs into 100% source code. Instead, they usually provide an "almost there" kind of intermediate representation. Many reverse compilers are actually disassemblers that provide a dump of the machine code that makes a program work.

Probably the best decompiler available to the public is called IDA-Pro. IDA starts with a disassembly of program code and then analyzes program flow, variables, and function calls. IDA is hard to use and requires advanced knowledge of program behavior, but its technical level reflects the true nature of reverse engineering. IDA supplies a complete API for manipulating the program database so that users can perform custom analysis.

Other tools exist as well. A closed-source but free program called REC provides 100% C source code recovery for some kinds of binary executables. Another commercial disassembler is called WDASM. There are several decompilers for Java byte code that render Java source code (a process far less complicated than decompiling machine code for Intel chips). These systems tend to be very accurate, even when simple obfuscation techniques have been applied. There are open-source projects in this space as well, which interested readers can look up. It is always a good idea to keep several decompilers in your toolbox if you are interested in understanding programs.

Decompilers are used extensively in the computer underground to break copy protection schemes. This has given the tools an undeserved black eye. It is interesting to note that computer hacking and software piracy were largely independent in the early days of the computer underground. Hacking developed in UNIX environments, where software was free and source code was available, rendering decompiling somewhat unnecessary. Software piracy, on the other hand, was mainly developed to crack computer games, and hence was confined mainly to Apples, DOS, and Windows, for which source code was usually not available. The virus industry developed along side the piracy movement. In the late 1990s, the hacking and cracking disciplines merged as more network software became available for Windows and hackers learned how to break Windows software. The current focus of decompiling is shifting from cracking copy protection to auditing software for exploitable bugs. The same old tricks are being used again, but in a new environment.

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