[Crypto] baby_N1ES - CirQ

xp0int Posted on Mar 12 2018

? Crypto ? ? feistel ?

看题目名字就能猜测和DES有关系，直接看所给的python代码也能看出和现有的DES遵循一个同样的框架，更确切来说，和现行DES都遵循feistel结构，其中要求轮函数F是自反的，即

$$F(x)=y\ \ \Leftrightarrow\ \ F(y)=x$$

这里的$F$就是代码中的`round_add`函数，很容易能写出对应的解密函数，跟加密函数完全相反就可以。

```python
def encrypt(self, plaintext):
    if (len(plaintext) % 16 != 0 or isinstance(plaintext, bytes) == False):
        raise Exception("plaintext must be a multiple of 16 in length")
    res = ''
    for i in range(len(plaintext) / 16):
        block = plaintext[i * 16:(i + 1) * 16]
        L = block[:8]
        R = block[8:]
        for round_cnt in range(32):
            L, R = R, (round_add(L, self.Kn[round_cnt]))
        L, R = R, L
        res += L + R
    return res

def decrypt(self, ciphertext):
	if (len(ciphertext) % 16 != 0 or isinstance(ciphertext, bytes) == False):
        raise Exception("ciphertext must be a multiple of 16 in length")
    res = ''
    for i in range(len(ciphertext) / 16):
        block = ciphertext[i * 16:(i + 1) * 16]
        L = block[:8]
        R = block[8:]
        L, R = R, L
        for round_cnt in range(32):
            L, R = (round_add(R, self.Kn[-1-round_cnt])), L
        res += L + R
    return res
```

代码写在`N1ES`类中，由于使用的是相同的密钥，直接改challenge.py中的代码即可：

```python
from N1ES import N1ES
import base64
key = "wxy191iss00000000000cute"
n1es = N1ES(key)
#flag = "N1CTF{*****************************************}"
#cipher = n1es.encrypt(flag)
#print base64.b64encode(cipher)

cipher = base64.b64decode('HRlgC2ReHW1/WRk2DikfNBo1dl1XZBJrRR9qECMNOjNHDktBJSxcI1hZIz07YjVx')
print n1es.decrypt(cipher)
```

**Flag**
N1CTF{F3istel_n3tw0rk_c4n_b3_ea5i1y_s0lv3d_/--/}

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