Rating: 1.0
# itsy-bitsy:crypto:436pts
The itsy-bitsy spider climbed up the water spout...
nc 2020.redpwnc.tf 31284
[itsy-bitsy.py](itsy-bitsy.py)
# Solution
ncを行うと二つの数字を聞かれて、暗号化されたバイナリテキストが出てくる。
```bash
$ nc 2020.redpwnc.tf 31284
Enter an integer i such that i > 0: 3
Enter an integer j such that j > i > 0: 9
Ciphertext: 0001000111111001011010011100110000000011000001001011110101111101110100100101100001000001101001000110110011111001000001110100101111100001001000001010101010101111110001101000111110011110000100000001110101100101111100011001111010100000011000101111000101101010001010011100111110110100010101110010100010010
```
ソースコードitsy-bitsy.pyは以下のようになっている。
```python:pseudo-key.py
#!/usr/bin/env python3
from Crypto.Random.random import randint
def str_to_bits(s):
bit_str = ''
for c in s:
i = ord(c)
bit_str += bin(i)[2:]
return bit_str
def recv_input():
i = input('Enter an integer i such that i > 0: ')
j = input('Enter an integer j such that j > i > 0: ')
try:
i = int(i)
j = int(j)
if i <= 0 or j <= i:
raise Exception
except:
print('Error! You must adhere to the restrictions!')
exit()
return i,j
def generate_random_bits(lower_bound, upper_bound, number_of_bits):
bit_str = ''
while len(bit_str) < number_of_bits:
r = randint(lower_bound, upper_bound)
bit_str += bin(r)[2:]
return bit_str[:number_of_bits]
def bit_str_xor(bit_str_1, bit_str_2):
xor_res = ''
for i in range(len(bit_str_1)):
bit_1 = bit_str_1[i]
bit_2 = bit_str_2[i]
xor_res += str(int(bit_1) ^ int(bit_2))
return xor_res
def main():
with open('flag.txt','r') as f:
flag = f.read()
for c in flag:
i = ord(c)
assert i in range(2**6,2**7)
flag_bits = str_to_bits(flag)
i,j = recv_input()
lb = 2**i
ub = 2**j - 1
n = len(flag_bits)
random_bits = generate_random_bits(lb,ub,n)
encrypted_bits = bit_str_xor(flag_bits,random_bits)
print(f'Ciphertext: {encrypted_bits}')
if __name__ == '__main__':
main()
```
ソースコードを読むと、flag.txtの中身をバイナリ化しそれをランダムなバイナリでXORしているようだ。
ランダムなバイナリは入力数字の小さい方iと大きい方jで2^iから(2^j)-1までの乱数をバイナリ化したものであるらしい。
一見すると総当たりしかないように思われる。
しかし、適切にiとjの範囲を指定することで先頭ビットが1であることを使い元の文がわかる。
```python
>>> bin(2**1)[2:]
'10'
>>> bin(2**2-1)[2:]
'11'
>>> bin(2**2)[2:]
'100'
>>> bin(2**3-1)[2:]
'111
>>> bin(2**3)[2:]
'1000'
>>> bin(2**4-1)[2:]
'1111'
```
2^1と(2^2)-1はともに一つ飛ばしで1となる。
2^2と(2^3)-1の間の数字はともに二つ飛ばしで1となる。
解析の例としては以下になる(XORなのでビット反転に注意)。
```text
X = Not yet
c = 0011001100001000101000001101010011000011010010110000101001011001110100001001110110100010100101000001101100000000011001001001000111010100101100000111100001011000110100000001010101010000000001011011001111110010100101000100110000101101001111001101010110000001010000110000001011010000000101010011110100010
m = 11001101101100110000111001111111011X1X0X1X1X0X0X1X1X0X0X1X1X0X1X0X1X1X1X0X1X0X1X0X0X1X0X0X1X1X1X1X1X0X0X1X1X1X1X1X0X1X1X0X1X1X1X0X1X1X1X0X0X1X1X1X0X0X1X1X1X0X1X0X1X1X1X1X1X1X1X1X1X1X1X1X1X1X1X0X0X1X0X0X0X1X0X0X1X1X1X1X1X0X1X1X0X0X1X1X0X0X1X0X1X1X1X0X1X1X1X1X1X1X0X1X1X1X0X0X1X1X1X1X1X1X1X1X0X0X1111101
c = 0011001101011000111110001000000010010111010000100010001110111110000000101101100100010011100011100000111010100101101000001010000110110100111100010100110110111001000100010111000111110101000000101001001011100010100110010000110110001010011001101011010101100110110000110110111000100010010111010110100100100
m = 11001101101100110000111001111111011X1X001X1X010X1X110X0X111X0X110X1X111X0X100X1X010X1X000X1X101X1X110X0X111X1X101X0X111X0X111X1X011X1X110X0X111X1X010X1X111X0X100X1X111X1X101X1X101X1X101X1X111X0X001X0X000X1X010X1X111X1X110X1X110X0X111X0X001X0X101X1X001X1X111X1X110X1X111X0X011X1X111X1X101X1X010X1111101
```
これを以下のbit1X1X.pyで自動化する。
```python:bit1X1X.py
import socket
host = "2020.redpwnc.tf"
port = 31284
text = "11001101101100110000111001111111011" + "X"*259 + "1111101"
# "flag{" + X + "}"
text = list(text)
try:
for i in range(1, 300):
print("[{}]".format(i))
client = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
client.connect((host, port))
client.recv(256)
client.send(("{}\n".format(i)).encode('utf-8'))
client.recv(256)
client.send(("{}\n".format(i + 1)).encode('utf-8'))
response = client.recv(1024)
response = response.decode('utf-8')
response = response.replace("Ciphertext: ", "")
for j in range(len(text)):
if j % (i + 1) == 0:
text[j] = (str(int(response[j]) ^ 1))
#print(response)
#print("".join(text))
text = "".join(text)
print(text)
for i in range(0, len(text), 7):
print(chr(int(text[i: i+7], 2)), end="")
print()
except:
print("\n" + "".join(text))
```
実行するとflagが得られる。
```bash
$ python bit1X1X.py
[1]
[2]
[3]
[4]
~~~
[296]
[297]
[298]
[299]
1100110110110011000011100111111101111000101101001111010011100111011111110110011001011100001110101111010011101110110011110111111101111111010111101001011111110010011011111110111110111010111111110100110100011001011011111111011111000011110100110010111100101011111111001111100001101111111010111101001111101
flag{bits_leaking_out_down_the_water_spout}
```
## flag{bits_leaking_out_down_the_water_spout}
