## OTP

#### 06/10/2022

By: HELLOPERSON

**Tags:**crypto HSCTF-2022

#### Problem Description:

the one-time pad (OTP) is an encryption technique that cannot be cracked Wikipedia

We’re given an output, consisting of the bit-length of a randomly generated number, and the flag xored with that number. We’re also given the program used to generate the number:

```
import random
from Crypto.Util.number import bytes_to_long
def secure_seed():
x = 0
# x is a random integer between 0 and 100000000000
for i in range(10000000000):
x += random.randint(0, random.randint(0, 10))
return x
flag = open('flag.txt','rb').read()
flag = bytes_to_long(flag)
random.seed(secure_seed())
l = len(bin(flag)) - 1
print(l)
k = random.getrandbits(l)
flag = flag ^ k # super secure encryption
print(flag)
```

The function `secure_seed()`

is defined weirdly. It adds 10 billion values of x together, where x is a random integer between 0 and a random integer between 0 and 10. Since you’re doing this 10 billion times, there must be some sort of statistical analysis you can do here. Testing this with smaller amounts of x added together, we see that it tends towards 2.5*n, where n is the amount of x you’re adding together. Using the smaller amounts of x we calculated earlier, and graphing the deviations on desmos, we can calculate the deviation of n=10,000,000,000 to be around 250k. Therefore, we can write the following code to brute force the seed that encrypts the flag.

```
import random
n = 250000
x = 25000000000 - n
fleg = 444466166004822947723119817789495250410386698442581656332222628158680136313528100177866881816893557
for i in range(2*n):
x += 1
random.seed(x)
k = random.getrandbits(328)
flag = fleg^k
binary_array = flag.to_bytes(41, "big")
try:
ascii_text = binary_array.decode()
print(ascii_text)
break
except (UnicodeDecodeError):
a = 0
```

Which outputs the flag: `flag{c3ntr4l_l1m1t_th30r3m_15431008597}`