How can I reverse-engineer and replicate the decryption and decompression routine in DataZipCompression.LoadString using dnSpy?

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I'm analyzing a .NET DLL with dnSpy to understand how the DataZipCompression.LoadString method decompresses and decrypts data into an XML string. My goal is to reconstruct the algorithm so that I can implement the decryption and decompression process externally.

Here’s a key code snippet from the DLL:

CSHARP
<Module>.DataZipCompression.LoadString(
    <Module>.?A0xdbfabc02.g_pDataZipCompression, 
    ref basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t>>, 
    ref basic_string<char, std::char_traits<char>, std::allocator<char>>, 
    true
);

Full fragment:

CSHARP
basic_string<char,std::char_traits<char>,std::allocator<char>
> basic_string<char,std::char_traits<char>,std::allocator<char>_u0020>;
                        <Module>.std.basic_string<char,std::char_traits<char>,std::allocator<char>
>.{ctor}(ref basic_string<char,std::char_traits<char>,std::allocator<char>_u0020>);
                        try
                        {
                            <Module>.DataZipCompression.LoadString(<Module>.?A0xdbfabc02.g_pDataZipCompression, ref basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t>_u0020>4, ref basic_string<char,std::char_traits<char>,std::allocator<char>_u0020>, true);
                            byte[] array3 = new byte[<Module>.std.basic_string<char,std::char_traits<char>,std::allocator<char>
> .length(ref basic_string<char,std::char_traits<char>,std::allocator<char>_u0020>)];
                            for (int i = 0; i < array3.Length; i++)
                            {
                                sbyte* ptr3 = <Module>.std.basic_string<char,std::char_traits<char>,std::allocator<char>
> .[](ref basic_string<char,std::char_traits<char>,std::allocator<char>_u0020>, (uint)i);
                                array3[i] = (byte)(*ptr3);
                            }
                            UTF8Encoding utf8Encoding = new UTF8Encoding();
                            string @string = utf8Encoding.GetString(array3);
                            XmlDocument xmlDocument = new XmlDocument();
                            xmlDocument.LoadXml(@string);
                            dataContainerProperties = genericSerializer.FromXmlStringIgnoreParentName(xmlDocument.OuterXml);
                        }
                        catch
                        {
                            <Module>.___CxxCallUnwindDtor(ldftn(std.basic_string<char,std::char_traits<char>,std::allocator<char>
> .{dtor}), (void*)(&basic_string<char,std::char_traits<char>,std::allocator<char>_u0020>));
                            throw;
                        }
                        <Module>.std.basic_string<char,std::char_traits<char>,std::allocator<char>
> .{dtor}(ref basic_string<char,std::char_traits<char>,std::allocator<char>_u0020>);

What I’ve Tried:

Decompiling the DLL with dnSpy to locate the DataZipCompression class and the LoadString method.
Analyzing the method parameters and the flow of data after decompressing.
Tracing data manipulation and understanding how the XML string is produced.

I’m looking for insights or guidance on reconstructing this algorithm for use outside the DLL. Any help or suggestions on dynamic analysis, data pattern recognition, or alternative debugging approaches with Visual Studio, WinDbg, or other tools would be greatly appreciated.

Comments 0

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Answers 1

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1 Answer

Answer by StackOverflow TychoAI Agent • 1 month ago

Below is some guidance on how you might approach this problem—but keep in mind that without official documentation or source code access for the method you’re examining, any reconstruction of the algorithm used by DataZipCompression.LoadString is partly an exercise in reverse engineering, which can be challenging and may have legal or licensing implications if done on proprietary code.

What We Know From the Code

The method takes several parameters, including:
- A pointer (or reference) to what appears to be a blob of compressed/encrypted data.
- Two strings (one wide and one narrow) that are used to receive or compare data.
- A Boolean true that likely controls a particular branch in the code.
Once the method returns, a UTF‑8–encoded XML string appears in memory.
The calling code then builds a byte array from a basic_string of char and converts it to an XML document.

Steps to Investigate Further

Since you have already decompiled the DLL and examined the method signature and parameters, you might consider the following additional approaches:

Dynamic Analysis:
- Set breakpoints and step through: Use a debugger (e.g. Visual Studio or WinDbg) to set breakpoints around the call to LoadString. Observe the memory buffers before and after the call to see which parts change.
- Inspect intermediate buffers: Look for clues such as recognizable headers (for example, “PK” for Zip) or patterns that match common compression signatures (such as those used by Deflate, GZip, or ZLib).
Compare to Known Algorithms:
- Since the method ends up with an XML string, it could well be using a combination of a common compression algorithm (for example, Deflate or GZip) along with symmetric encryption (perhaps using standard .NET cryptography libraries on the native side).
- If you can extract a portion of the encrypted/compressed data (by dumping memory before and after the method), try to match it against outputs from standard de/compression libraries to see if you can trigger a match.
Examine the Native Code Further:
- The naming of types (for example, the usage of std::basic_string) indicates that a native (possibly mixed-mode) implementation might be in play. Tools like IDA Pro, Ghidra, or even the native debugging capabilities of Visual Studio could provide further insight into the inner loop of the method.
- Look for functions called from within LoadString (or nearby in the decompiled binary) which might call native compression or cryptography APIs.
Consider Known Patterns:
- Often, such routines use known algorithms with minor tweaks (for example, XOR-based obfuscation combined with standard compression). Look for any calls to crypto libraries or compression streams in adjacent code or in any imports from the DLL.

Caveats and Next Steps

Legal and Ethical Considerations: Make sure you comply with the license of the DLL and any applicable laws regarding reverse engineering.
Documentation Gaps: Since DataZipCompression.LoadString is not documented in Microsoft C# docs or the standard .NET Framework documentation, there isn’t an official reference to compare against. Replicating its behavior exactly may require extensive dynamic tracing and testing.
Alternative Options: If your goal is to obtain the XML string outside the DLL, consider whether the DLL vendor provides an API or tool for this purpose. In some scenarios, the desired processing (decryption and decompression) might be available as a documented API.

Summary

There isn’t a “one‑size‑fits‑all” answer because the algorithm inside DataZipCompression.LoadString is effectively a black box without source or official documentation. Your strategy—using a combination of static analysis (with dnSpy or equivalent) and dynamic analysis (stepping through execution, dumping intermediate buffers, comparing against known algorithms)—is on the right track.

If you can identify known patterns (such as headers or specific crypto routines), you may be able to write your own C# version of the decryption and decompression routine. Otherwise, further disassembly and native debugging might be required to fully replicate the algorithm.

Remember, this process may be time‑consuming and might require you to become familiar with both managed and unmanaged debugging techniques in the Visual Studio ecosystem.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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