[ACTF新生赛2020]crypto-aes

1.题目

from Cryptodome.Cipher import AES
import os
import gmpy2
from flag import FLAG
from Cryptodome.Util.number import *

def main():
    key=os.urandom(2)*16
    iv=os.urandom(16)
    print(bytes_to_long(key)^bytes_to_long(iv))
    aes=AES.new(key,AES.MODE_CBC,iv)
    enc_flag = aes.encrypt(FLAG)
    print(enc_flag)
if __name__=="__main__":
    main()
91144196586662942563895769614300232343026691029427747065707381728622849079757
b'\x8c-\xcd\xde\xa7\xe9\x7f.b\x8aKs\xf1\xba\xc75\xc4d\x13\x07\xac\xa4&\xd6\x91\xfe\xf3\x14\x10|\xf8p'

2.复现

key是32位bytes 256位bits iv是16位bytes 128位bits，所以异或的结果高16为bytes、128位bits不变，key又是两个bytes重复16次所以，它的高128位bits和低128位bits是一样的
然后再把key异或x就得到了iv

from Crypto.Cipher import AES
from Crypto.Util.number import*
import os
import gmpy2
x=91144196586662942563895769614300232343026691029427747065707381728622849079757
c=b'\x8c-\xcd\xde\xa7\xe9\x7f.b\x8aKs\xf1\xba\xc75\xc4d\x13\x07\xac\xa4&\xd6\x91\xfe\xf3\x14\x10|\xf8p'
x=long_to_bytes(x)
key=x[:16]*2
iv=bytes_to_long(x)^bytes_to_long(key)
iv=long_to_bytes(iv)
aes=AES.new(key,AES.MODE_CBC,iv)#iv,key的类型为bytes
flag=aes.decrypt(c)
print(flag)
# b'actf{W0W_y0u_can_so1v3_AES_now!}'