2017 WITHCON Prequals - 다산관 제육볶음 - Q10. catdb

cat_db_decryptor.py

from Crypto.Cipher import Blowfish
from struct import unpack

def once(d):
    iv = '\x01\x02\x03\x04\x05\x06\x07\x08'

    key, d = d[:2], d[2:]

    name, d = d[:8], d[8:]
    iv, name = name, Blowfish.new(key, Blowfish.MODE_CBC, iv).decrypt(name)
    print 'fileName:', name

    comm, d = d[:8], d[8:]
    iv, comm = comm, Blowfish.new(key, Blowfish.MODE_CBC, iv).decrypt(comm)
    print 'comment:', comm

    length, d = unpack('<H', d[:2])[0], d[2:]

    data = d
    data = data + '\x00' * (8 - len(data) % 8)
    resu = ''
    for i in range(0, len(data), 8):
        cur = data[i: i + 8]
        iv, cur = cur, Blowfish.new(key, Blowfish.MODE_CBC, iv).decrypt(cur)
        resu += cur
        if '\xFF\xD9\x00' in resu[-16:]:
            break

    offs = len(resu)
    resu, d = resu[:offs], d[offs:]

    with open(name.strip('\0'), 'wb+') as f:
        f.write(resu)

    return d

def main():
    with open('cat.db', 'rb') as f:
        assert 'MEOW' == f.read(4)
        data = f.read()
        while len(data) > 0:
            print '----------------------------------------'
            data = once(data)

if __name__ == '__main__':
    main()