DownUnderCTF 2025 — zeus (100 pts)

Overview

Analyzing a 64-bit ELF binary to find a flag hidden behind command-line argument validation and XOR decryption.

Analysis

Opening the binary in IDA, main reveals:

• Multiple variables containing XOR-encoded data
• A function named xor using the key Maimaktes1337
• Three conditions that must all be true:
  1. Exactly 3 command-line arguments
  2. Second argument equals "-invocation"
  3. Third argument matches the greeting from the challenge description

Two Approaches

Direct invocation (simplest):

./zeus -invocation 'To Zeus Maimaktes, Zeus who comes when the north wind blows, we offer our praise, we make you welcome!'

Manual decryption: XOR the embedded data array with Maimaktes1337.

Flag

DUCTF{king_of_the_olympian_gods_and_god_of_the_sky}