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Desktop RE Writeup: Multi-Stage DLL Decryption

By Shay Mordechai | April 19, 2024

Static analysis of `crack.dll` revealed `IsDebuggerPresent` checks executed via TLS Callbacks before `main`[cite: 10]. By patching the `StartAddress` control flow, I forced the left branch to execute and dump the decrypted payload to disk[cite: 10].

DLL Reconstruction: The dumped `Java.dll` lacked valid PE headers. I used a Known-Plaintext attack (leveraging the standard `MZ` header) to calculate the XOR keys (`0x81` and `0x53`). I wrote a C++ script to run `xorEncryptAndWrite`, reconstructing a valid, executable PE file[cite: 10].

XorForever Decryption: Loading the reconstructed DLL into IDA's Hex-Rays Decompiler revealed an indexing array replacing specific characters. Analytical deduction yielded the final string `3TwB99f`, completing the URL flag[cite: 10].