reverse engineering

Download Reverse engineering

Post on 28-May-2015

1.943 views

Category:

Documents

3 download

Embed Size (px)

TRANSCRIPT

  • 1. REVERSEENGINEERINGAn Introduction to Cracking Windows Applications Saswat Padhi 3rd Year UG, CSE Hostel 8, IIT Bombay

2. Presentation for STAB Annual Seminar Weekday 2DISCLAIMER This presentation is intended for educational purposes only. Reverse engineering of copyrighted material is illegal and unethical and in no way, do I encourage this. Playing with malware is not a good idea, unless you takeproper precautionary measures. Always work withinsandboxed environment. For example, in a virtual OS. Malware analysis is advanced stuff. And when you choose tomess with a nasty app, it is expected that you know what youare doing. Only you are responsible for what happens to yoursystem. 3. Presentation for STAB Annual Seminar Weekday 3What is this heavy term? Reverse Engineering simply aimsat understanding a systemthrough analysis of its structure,function and operation. We try to go backwards in thedevelopment cycle: having animplementation, we try to goback to the analysis stage, with ahigh level understanding. With the abstract understanding,we try to modify parts of existingimplementation or implement on Complete disassembly of a Pentax K1000 camera. our own! Image borrowed from bitrebel.com. 4. Presentation for STAB Annual Seminar Weekday 4And why should someone learn this? Malware analysis: Analyzing malware to build anti-malware Bug fixing: Fixing bugs in legacy software Personalization/Customizations Academic/Learning purposes Removing access restrictions Removal of copy protection Compatibility Just for fun! Convey the message Go Open Source! :-P 5. Presentation for STAB Annual Seminar Weekday 5Wait Isnt this illegal!? Public release of information obtained through reversing a proprietary application or sharing the application after modifying it to remove/reduce security is illegal. DMCA allows, reverse engineering applications for achievinginteroperability. Analysis of malware is of course legal! Clean room design is perfectly legal. A team of examiners write a specification for target the software. Several reviews ensure exclusion of any copyrighted materials. A separate team of developers re-implement the software. 6. Presentation for STAB Annual Seminar Weekday 6A bit of history AMD had reverse engineered Intels early processors (and had outperformed all of them!). ReactOS, an open source clone of Windows is still under active development. Its not based on *nix systems at all. Phoenix developed its BIOS chip by reverse engineering IBMs BIOS, with a clean room approach. Phoenix BIOS gave birth to the first IBM-compatible PCs. Wine is reverse engineering Windows for supporting Win API. OpenOffice.org is reverse engineering Microsoft Office for supporting the proprietary file formats. Samba enables file sharing between Windows and non- Windows systems. But they had to reverse engineer Windows file sharing. 7. Presentation for STAB Annual Seminar Weekday 7What we would be discussing todayReverse Engineering is a whole different branch by itself. Wewould only be touching upon a few important fundamentalpoints that would give you the basic feel of it (and hopefullymotivate you to dig further into it)PE Identification: Packers, Identifying the source languageDecompilers; Disassemblers and DebuggersIntroduction to OllyDbgs FeaturesWhere to start? & Where to focus?Patching with OllyDbgPhishing and KeyGening 8. Presentation for STAB Annual Seminar Weekday 8PEiD Identification of PE (Portable Executable) is an important step. It involves identification of compilers, cryptors, packers etc Gives you a starting point to look for a solution. Unpacking needed? De-compilation possible? Crypto libraries used? PEiD is by far the best PE Identification tool (470+ signatures and can be extended with external signatures). If PEiD says Nothing Found, the application might be using a custom packer.PEiD (PEiD) PEiD (OllyDbg) PEiD (KeyGenMe_#6)X 9. Presentation for STAB Annual Seminar Weekday 9Packers ?! Packers compress the compiled program, much like data. They compress either or both code and data sections and can additionally encrypt them.Compiled PackedPackersExecutable Executable Unpacking CODE DATAThe extractor stub Unpackers CODE DATASTUBdecompresses and/ordecrypts the data andcode. Writes them toa separate file or doesit in-memory. OEP: Original Entry PointEntry Point Several packers exist. Most of them are selective about the type of PE they support. 10. Presentation for STAB Annual Seminar Weekday 10Packers ?! Some of the most popular (and thus most wanted targets for unpackers) are ASPack, Enigma, Themida, MPRESS, UPX etc. Most packed PEs are manually customized to avoid usage of direct unpackers. Customized executables might not even be recognized by PE Identifiers! So, might not have any idea of the behavior of the packer. Without direct unpackers, we have to manually step through the executable, and find out the original entry point. (When we step through code in a debugger, the application is actually in execution, so take precaution while trying this with malware). 11. Presentation for STAB Annual Seminar Weekday 11Why should you know the compiler? Java and Python compile sources to byte code instead of machine code, which runs on a virtual machine. Same is the case with managed code like .NET. C#, VB and all .NET languages compile the code to MSIL (MicroSoft Intermediate Language). The meta-data present with the intermediate byte code is enough to make satisfactory de-compilation possible. For example, Java byte code contains names of classes and all members; types and modifiers of fields; signatures of methods. Even names of local variables and line numbers can be saved optionally, by setting specific attributes. 12. Presentation for STAB Annual Seminar Weekday 12Why should you know the compiler? Some of the things de-compilers need to be smart about are: Structured control flow (loops and conditionals) from gotos in byte code. Type inference for locals, especially for generics as they are generated at compile time. This is very different from the compilation with debug info (-g) that can be used with C/C++ compilers. Debug info is just a mapping from binary to source file. If the sources are missing, debugger would not magically show them! Machine code is so much reduced that its almost impossible to grow it back to a high level source code. 13. Presentation for STAB Annual Seminar Weekday 13Some Available Decompilers .NET MSIL Decompiler: Reflector (Freeware till 6.x) KeyGenMe #6Reflector(KeyGenMe #6)UnpackMeReflector(UnpackMe) Java Decompiler: JD (Freeware and Decent, there are dozen others!) SK_CrackMe JD(SK_CrackMe) 14. Presentation for STAB Annual Seminar Weekday 14What if you cant decompile? Well .. The fun begins! Disassemblers and debuggers transform the binary machine code to assembly instructions. Debuggers offer additional features like setting break points, stepping through the code etc OllyDbg is one of the best light-weight debuggers available. IDA Pro is a heavy-weight and paid debugger (but its worth it!). OllyDbg supports external plugins and that makes it an even more powerful tool. IDA Pro has the power of recognizing known library functions by their signatures. It can highly simplify the assembly dump. 15. Presentation for STAB Annual Seminar Weekday 15More about OllyDbg Emphasizes on code analysis. Olly contains description of about 2200 standard C/C++ library and Win32 API functions. It also contains 7800 symbolic constants (grouped into 490 types). Olly can detect nested loops, switches and cascaded ifs. It can also predict register usage. You can add comments at each assembly line. You can assemble your own expressions instantly and re-build the program. (Great for patching!)KeyGenMe #2 OllyDbg(KeyGenMe #2) 16. Presentation for STAB Annual Seminar Weekday 16Finding the needle(s) in a haystack Patience and lateral thinking! Examine All referenced strings Locality of Good Boy or Bad boy Examine All inter-modular calls Get used to Windows API (Olly does provide context sensitive help) I/O calls are a usual starting point for examination msvcrt.printf msvcrt.scanf USER32.GetMessageX USER32.GetDlgItemTextX Use breakpoints at suspicious DLL calls Use memory breakpoint at addresses storing the entered key 17. Presentation for STAB Annual Seminar Weekday 17A (very) simple examplestring username, password, therealpass;Case_1 Olly(Case_1)cout > username;cout > password;threalpass = TOP_SECRET;if(password == therealpass) cout username;}Case_2cin >> password;therealpass = username.substr(username.length() - 4) + -;therealpass += (0 + (username.length()%10));therealpass += (0 + (username.length()/10)); Olly(Case_2)therealpass += - + username.substr(0,4); Case_2_PATCHEDif(password == therealpass) cout