Challenge Overview

Target: DinoConfigurator.exe — a Windows .NET WinForms application Goal: Find valid username + password to trigger AES decryption of the flag

Initial Analysis

Used file and strings on Linux to identify it as a .NET assembly. Loaded into ILSpy (VS Code extension) to decompile.

Decompiling the Login Logic

In btnLogin_Click:

Verifier.ValidateUser(username, password);
// If successful — AES-CBC decrypt hardcoded 48-byte array using Password as key

Discovery: Pixel-XOR Verification

Inside Verifier.VerifySecret — not a simple string comparison:

  • Embedded image: matteo_discardi_tdA2J1uJHew_unsplash
  • For each byte in the secret array, the program uses a Random object to pick a pixel coordinate
  • Computes a mask from the pixel’s color channels
  • Compares: secret_byte XOR mask == expected

Critical Finding: Custom LCG (DinoRandom)

The program did not use System.Random. It implemented a custom Linear Congruential Generator with hardcoded constants. Standard library random solvers would produce wrong sequences because the math is different.

We had to manually reimplement DinoRandom to stay in sync with the program’s state.

Cryptographic Reconstruction

  • Algorithm: AES-256-CBC
  • IV: 908194288470cb5d (16 bytes)
  • Key: The recovered password string (32 bytes)
  • Ciphertext: 48-byte array hardcoded in btnLogin

Solution

Wrote a C# solver for the Mono runtime that:

  1. Re-implemented the DinoRandom LCG
  2. Loaded the resource image
  3. Performed inverse XOR to recover each secret byte
  4. Recovered Username (seed: 1337) and Password (seed: 42)
  5. Decrypted AES-CBC — flag: SHC{...}

Key Lessons

  • Static analysis is king: The custom Random class was the pivot point. Missing it would have blocked progress entirely.
  • XOR is reversible: A XOR B = C → A = C XOR B. Always.
  • Resource extraction matters: One bit of image compression difference would break the entire chain.