reverse engineering c programs

every c program is converted to machine code, if this binary is dis开发者_开发技巧tributed. Since the instruction set of a computer is well known, is it possible to get back the C original program?

You can never get back to the exact same source since there is no meta-data about that saved with the compiled code.

But you can re-create code out from the assembly-code.

Check out this book if you are interested in these things: Reversing: Secrets of Reverse Engineering.

Edit

Some compilers-101 here, if you were to define a compiler with another word and not as technical as "compiler", what would it be?

Answer: Translator

A compiler translates the syntax / phrases you have written into another language a C compiler translates to Assembly or even Machine-code. C# Code is translated to IL and so forth.

The executable you have is just a translation of your original text / syntax and if you want to "reverse it" hence "translate it back" you will most likely not get the same structure as you had at the start.

A more real life example would be if you Translate from English to German and the from German back to English, the sentance structure will most likely be different, other words might be used but the meaning, the context, will most likely not have changed.

The same goes for a compiler / translator if you go from C to ASM, the logic is the same, it's just a different way of reading it ( and of course its optimized ).

It depends on what you mean by original C program. Things like local variable names, comments, etc... are not included in the binary, so there's no way to get the exact same source code as the one used to produce the binary. Tools such as IDA Pro might help you disassemble a binary.

I would guestimate the conversion rate of a really skilled hacker at about 1 kilobyte of machine code per day. At common Western salaries, that puts the price of, say, a 100 KB executable at about $25,000. After spending that much money, all that's gained is a chunk of C code that does exactly what yours does, minus the benefit of comments and whatnot. It is no way competitive with your version, you'll be able to deliver updates and improvements much quicker. Reverse engineering those updates is a non trivial effort as well.

If that price tag doesn't impress you, you can arbitrarily raise the conversion cost by adding more code. Just keep in mind that skilled hackers that can tackle large programs like this have something much better to do. They write their own code.

One of the best works on this topic that I know about is:

Pigs from sausages? Reengineering from assembler to C via FermaT.

The claim is you get back a reasonable C program, even if the original asm code was not written in C! Lots of caveats apply.

The Hex-Rays decompiler (extension to IDA Pro) can do exactly that. It's still fairly recent and upcoming but showing great promise. It takes a little getting used to but can potentially speed up the reversing process. It's not a "silver bullet" - no c decompiler is, but it's a great asset.

The common name for this procedure is "turning hamburger back into cows." It's possible to reverse engineer binary code into a functionally equivalent C program, but whether that C code bears a close resemblance to the original is an open question.

Working on tools that do this is a research activity. That is, it is possible to get something in the easy cases (you won't recover local variables names unless debug symbols are present, for instance). It's nearly impossible in practice for large programs or if the programmer had decided to make it difficult.

There is not a 1:1 mapping between a C program and the ASM/machine code it will produce - one C program can compile to a different result on different compilers or with different settings) and sometimes two different bits of C could produce the same machine code.

You definitely can generate C code from a compiled EXE. You just can't know how similar in structure it will be to the original code - apart from variable/function names being lost, I assume it won't know the original way the code was split amongst many files.

You can try hex-rays.com, it has a really nice decompiler which can decompile assembly code into C with 99% accuracy.