Sora

Description

This obnoxious kid with spiky hair keeps telling me his key can open all doors.

Can you generate a key to open this program before he does?

Connect to challenges.auctf.com 30004

Solution

We decompile the binary using Ghidra, and see that it encrypts the input string and then compares it to the key encrypted with the same function. We just reverse the encryption function and give it the encrypted key found in the file.

secret = b"aQLpavpKQcCVpfcg"

key = []

for i in range(len(secret)):
    for c in range(65, 122):
        if (c * 8 + 0x13) % 0x3d + 0x41 == secret[i]:
            key.append(c)
            print(key)

print("".join([chr(c) for c in key]))

The key is try_to_break_meG. This gives us the flag.

Flag: auctf{that_w@s_2_ezy_29302}

Original writeup (https://github.com/apoirrier/CTFs-writeups/blob/master/AUCTF2020/Reversing/sora.md).