A decompiler is a computer program that translates an executable file back into high-level source code.
Decompilers face inherent challenges due to the loss of critical information during the compilation process, such as variable names, comments, and code structure.
Executables containing detailed metadata, such as those used by Java and .NET, are easier to reverse-engineer because they often retain class structures, method signatures, and debugging information.
Some software developers may obfuscate, pack, or encrypt parts of their executable programs, making the decompiled code much harder to interpret.
The first decompilation phase loads and parses the input machine code or intermediate language program's binary file format.
In many cases, it should be able to find the equivalent of the main function of a C program, which is the start of the user written code.
In general, it is best to delay detection of idiomatic sequences if possible, to later stages that are less affected by instruction ordering.
For example, an and instruction implies that the operand is an integer; programs do not use such an operation on floating point values (except in special library code) or on pointers.
However, it is difficult to distinguish many of the possibilities, because of the freedom that machine code or even some high level languages such as C allow with casts and pointer arithmetic.
Just before code generation, it may be desirable to allow an interactive editing of the IR, perhaps using some form of graphical user interface.
These are less readily modified with a simple text editor, although source code refactoring tools may assist with this process.
Finally, incorrect IR may need to be corrected, or changes made to cause the output code to be more readable.
[6] Since the decompilation process involves making multiple such copies, it is generally prohibited without the authorization of the copyright holder.
[7] Additionally, the Digital Millennium Copyright Act (PUBLIC LAW 105–304[8]) has proper exemptions for both Security Testing and Evaluation in §1201(i), and Reverse Engineering in §1201(f).
[9] In Europe, the 1991 Software Directive explicitly provides for a right to decompile in order to achieve interoperability.