Opaque

Prerequisites & Requirements

• PE (Portable Executable) Structure:
  • Understanding the layout of a Windows executable, specifically the .data section where global variables and encrypted artifacts are stored.
  • Knowing how memory addresses in a running process relate to the file offsets (Virtual Address vs. File Offset).
• C/C++ Reverse Engineering:
  • Familiarity with C-style string manipulation functions (strlen, strncpy, fgets).
  • Ability to recognize pointer arithmetic and dereferencing in decompiled code ( e.g ., *(_QWORD *)v22).
• Cryptography Basics:
  • Understanding XOR Encryption/Decryption operations (key ^ value = result).
  • Recognizing common obfuscation patterns, such as "Rolling/Rotating Key" algorithms where the key changes or cycles based on the index or a seed.

What will you learn?

• Deobfuscating Pointers:
  • Analyzing routines that "fix" or calculate valid memory addresses at runtime (deobfuscate_pointer).
  • Understanding how malware authors hide the location of shellcode or flags by only generating valid pointers when specific conditions are met.
• Hybrid Analysis (Static + Dynamic):
  • Combining Static Analysis (reading the code in IDA to find the .data section) with Dynamic Analysis (debugging to pluck the ephemeral decryption key from the stack).
• Manual Decryption:
  • Extracting raw binary data (the encrypted flag bytes) from a specific memory address.
  • Using external tools (CyberChef) to replicate the decryption logic (rotating_key_decrypt) offline using the recovered key (0x87878787) and data.

Tools

• IDA Pro:
  • Used for Static Analysis . It allows you to disassemble the binary, view the pseudocode for the user_flag_process function, and locate the encrypted byte array in the .data section (starting at 00007FF62B654050).
• Debugger (Integrated in IDA or x64dbg):
  • Used for Dynamic Analysis .
  • Setting breakpoints ( e.g ., at the xor_decrypt or rotating_key_decrypt calls) allows you to inspect the stack and registers at runtime to reveal the final calculated key (0x87878787) which is otherwise hard to compute statically.
• CyberChef:
  • Used for the final Exploitation .
  • It serves as the "decryption station" where you input the raw hex bytes extracted from the binary and apply the XOR operation with the discovered key to reveal the plaintext flag.
• Detect It Easy (DiE):