xorox

June 17, 2025 - 1 min read

tamu2025 - rev Challenge: xorox

Description

This challenge involves reverse engineering a binary to determine the required input that produces the desired output. The solution involves XOR operations and understanding the binary’s constants and register values.

Solution

The following Python script demonstrates the solution:

import struct

# Constants from the binary
constant = [
    0x2a8c7f3acdf36ffb,  # First 8 bytes of the constant
    0x8cc2eef32660caaa,  # Next 8 bytes
    0xefa1fd61d7a3b592,  # Next 8 bytes
    0xa9ddc2d22a90025e   # Last 8 bytes
]

# YMM7 register values from GDB (converted to 4x 64-bit integers)
ymm7 = [
    0x1eca2043bfc01980,
    0xd386a3ba753fbe9f,
    0x87d5cc1688d185ea,
    0xd4aebbb741cf3001
]

def qwords_to_bytes(qwords):
    return b''.join(struct.pack('<Q', q) for q in qwords)

constant_bytes = qwords_to_bytes(constant)
ymm7_bytes = qwords_to_bytes(ymm7)

required_input = bytes(a ^ b for a, b in zip(constant_bytes, ymm7_bytes))

flag = b"gigem" + required_input

print("Raw bytes:", flag)

# Try to decode as ASCII (some bytes may not be printable)
try:
    print("ASCII:", flag.decode('ascii'))
except UnicodeDecodeError:
    print("Contains non-ASCII bytes")

Flag

The flag for this challenge is:

gigem{v3ry_F45t_SIMD_x0r_w1th_2_keys}