solve.py

import struct

def decrypt_transmission(encrypted):
    FLAG_LEN = 30
    XOR_KEY = [0x42, 0x73, 0x21, 0x69, 0x37]
    MAGIC_ADD = 0x2A

    buffer = bytearray(encrypted)

    for i in range(FLAG_LEN):
        buffer[i] ^= i

    for i in range(FLAG_LEN):
        buffer[i] = (buffer[i] - MAGIC_ADD) % 256

    for i in range(0, FLAG_LEN, 2):
        buffer[i], buffer[i + 1] = buffer[i + 1], buffer[i]

    for i in range(FLAG_LEN):
        buffer[i] ^= XOR_KEY[i % 5]

    return buffer.decode('utf-8')

if __name__ == "__main__":
    TARGET = [
        0x5A, 0x3D, 0x5B, 0x9C, 0x98, 0x73, 0xAE, 0x32, 0x25, 0x47,
        0x48, 0x51, 0x6C, 0x71, 0x3A, 0x62, 0xB8, 0x7B, 0x63, 0x57,
        0x25, 0x89, 0x58, 0xBF, 0x78, 0x34, 0x98, 0x71, 0x68, 0x59
    ]

    encrypted = bytearray(TARGET)
    decrypted = decrypt_transmission(encrypted)
    print("Decrypted flag:", decrypted)