一个简单的writeup

Problem: [HGAME 2023 week1]a_cup_of_tea

[[xtea]]

思路

找到main函数，即可发现加密算法
注意是加密是减法，不可以用sum = delta*32

EXP


  from ctypes import *
from Crypto.Util import *

enc = [0x2E63829D,0xC14E400F,0x9B39BFB9,0x5A1F8B14,
       0x61886DDE,0x6565C6CF,0x9F064F64,0x236A43F6,0x7D6B]

key = [0x12345678,0x23456789,0x34567890,0x45678901]

def decrypt(v,k):
    v0 = c_uint32(v[0])
    v1 = c_uint32(v[1])
    sum1 = c_uint(0)
    delta = 0x543210DD
    for i in range(32):
        sum1.value -= delta
    for i in range(32):
        v1.value -= (sum1.value + v0.value) ^ (k[2] + (v0.value<<4)) ^ (k[3] + (v0.value>>5))
        v0.value -= (sum1.value + v1.value) ^ (k[0] + (v1.value<<4)) ^ (k[1] + (v1.value>>5))
        sum1.value += delta
    v[0] = v0.value
    v[1] = v1.value
    return v0.value,v1.value
res = [0]*2
for i in range(0,8,2):
    res[0] = enc[i]
    res[1] = enc[i+1]
    decrypt(res,key)
    print(number.long_to_bytes(res[0]).decode()[::-1],end="")
    print(number.long_to_bytes(res[1]).decode()[::-1],end="")

print(number.long_to_bytes(enc[8]).decode()[::-1])

总结

主要考察了tea的魔改逆向分析