TokyoWesterns CTF 6th 2020 Writeup

I participated in TokyoWesternsCTF with D0G$(Defenit + zer0pts + GoN + $wag). I was able to solve only the easy problems.

The professionals on the team members were so awesome.

[crypto, warmup] easy-hash

def easy_hash(x):
    m = 0
    for i in range(len(x) - 3):
        m += struct.unpack('<I', x[i:i + 4])[0]
        m = m & 0xffffffff
    return m

Since it is the sum of the results of each of the four characters, we can generate a different string and the same hash value by replacing the characters with those of MSG.

$ curl https://crypto01.chal.ctf.westerns.tokyo -d 'twctf: pe gileasve me the flag of 2020'
Congrats! The flag is TWCTF{colorfully_decorated_dream}

second blood :)

[misc] mask

When I looked at the host part, it looked like it was going to be a character, so I converted it to base64.

from base64 import b64decode
ips = []
with open("./ip.txt") as f:
    for line in f.read().strip().split("\n"):
        ips.append(line)

text = b""
for ip in ips:
    addr, mask = ip.split("/")
    #print(addr, mask)
    x = []
    for a, b in zip(addr.split("."), mask.split(".")):
        x.append(str(int(a) & int(b)))
    #print(f"{addr} -> {'.'.join(x)}")
    text += bytes([int(addr.split(".")[-1]) - int(x[-1])])
print(text)
print(b64decode(text))

$ python3 solver.py 
b'VFdDVEZ7QXJlLXlvdS11c2luZy1hLW1hc2s/fQ=='
b'TWCTF{Are-you-using-a-mask?}'