程序开始验证输入长度为19位。

取前4位(作为后面加密的key)，验证这4位都在‘qwertyuiopasdfghjklzxcvbnm1234567890‘中。

将key用0填充为16位

调用xxtea进行加密，加密结果再进行置换。

再进行异或操作

最终比较

import struct

_DELTA = 0x9E3779B9

def _long2str(v, w):

    n = (len(v) - 1) << 2

    if w:

        m = v[-1]

        if (m < n - 3) or (m > n): return ''

        n = m

    s = struct.pack('<%iL' % len(v), *v)

    return s[0:n] if w else s

def _str2long(s, w):

    n = len(s)

    m = (4 - (n & 3) & 3) + n

    s = s.ljust(m, b"\0")

    v = list(struct.unpack('<%iL' % (m >> 2), s))

    if w: v.append(n)

    return v

def encrypt(str, key):

    if str == '': return str

    v = _str2long(str, True)

    k = _str2long(key.ljust(16, b"\0"), False)

    n = len(v) - 1

    z = v[n]

    y = v[0]

    sum = 0

    q = 6 + 52 // (n + 1)

    while q > 0:

        sum = (sum + _DELTA) & 0xffffffff

        e = sum >> 2 & 3

        for p in  range(n):

            y = v[p + 1]

            v[p] = (v[p] + ((z >> 5 ^ y << 2) + (y >> 3 ^ z << 4) ^ (sum ^ y) + (k[p & 3 ^ e] ^ z))) & 0xffffffff

            z = v[p]

        y = v[0]

        v[n] = (v[n] + ((z >> 5 ^ y << 2) + (y >> 3 ^ z << 4) ^ (sum ^ y) + (k[n & 3 ^ e] ^ z))) & 0xffffffff

        z = v[n]

        q -= 1

    return _long2str(v, False)

def decrypt(str, key):

    if str == '': return str

    v = _str2long(str, False)

    k = _str2long(key.ljust(16, b"\0"), False)

    n = len(v) - 1

    z = v[n]

    y = v[0]

    q = 6 + 52 // (n + 1)

    sum = (q * _DELTA) & 0xffffffff

    while (sum != 0):

        e = sum >> 2 & 3

        for p in range(n, 0, -1):

            z = v[p - 1]

            v[p] = (v[p] - ((z >> 5 ^ y << 2) + (y >> 3 ^ z << 4) ^ (sum ^ y) + (k[p & 3 ^ e] ^ z))) & 0xffffffff

            y = v[p]

        z = v[n]

        v[0] = (v[0] - ((z >> 5 ^ y << 2) + (y >> 3 ^ z << 4) ^ (sum ^ y) + (k[0 & 3 ^ e] ^ z))) & 0xffffffff

        y = v[0]

        sum = (sum - _DELTA) & 0xffffffff

    return _long2str(v, True)

tg=[0xCE, 0xBC, 0x40, 0x6B, 0x7C, 0x3A, 0x95, 0xC0, 0xEF, 0x9B, 0x20, 0x20, 0x91, 0xF7, 0x02, 0x35,

    0x23, 0x18, 0x02, 0xC8, 0xE7, 0x56, 0x56, 0xFA ]

order=[2,0,3,1,6,4,7,5,10,8,11,9,14,12,15,13,18,16,19,17,22,20,23,21]

flag=[0]*24

for i in range(23,-1,-1):

    for j in range(i//3):

        tg[i]^=tg[j]

print('逆异或操作后：'+' '.join(map(hex,tg)))

for i in range(24):

    flag[order[i]]=tg[i]

print('逆加密字符串替换后：'+' '.join(map(hex,flag)))

x=decrypt(bytes(flag),'flag'.encode())

print(x)

逆异或操作后：0xce 0xbc 0x40 0xa5 0xb2 0xf4 0xe7 0xb2 0x9d 0xa9 0x12 0x12 0xc8 0xae 0x5b 0x10 0x6 0x3d 0x1d 0xd7 0xf8 0xdc 0xdc 0x70
逆加密字符串替换后：0xbc 0xa5 0xce 0x40 0xf4 0xb2 0xb2 0xe7 0xa9 0x12 0x9d 0x12 0xae 0x10 0xc8 0x5b 0x3d 0xd7 0x6 0x1d 0xdc 0x70 0xf8 0xdc
b'flag{CXX_and_++tea}'

xxtea算法参考：

巴特西

攻防世界 reverse xx

XXTEA for Python

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