Rating:
## Challenge name: Follow the Currents
### Description:
> go with the flow... Source
>
> P.S: given files: *[enc](./enc)*, *[source.py](./source.py)*
### Solution:
A simple bruteforce challenge! all source code do is generating key with *keystream* function and **XOR** it with flag. Keystream has 2 bytes for initial. We bruteforce all 65536 of them, generate the key and compute the plaintext and check if it contains "actf{".
with open("enc","rb") as f:
ciphertext = f.read()
plain = []
for j in range(65535):
kb = j.to_bytes(2, 'big')
k = keystream(kb)
for i in ciphertext:
plain.append(i ^ next(k))
m = bytes(plain)
if (b"actf{" in m):
print (m)
plain = []
[final exploit.py](./exploit.py)
**Output**
b"x''R@\xc5\x05\xfb/9\xe7\x10\x13L\xc5\xd6\x19 minutes left before the ctf starts so i have no idea what to put here other than the flag which is actf{low_entropy_keystream}"
b'\x88\x86\x02\x15\x8b are like 30 minutes left before the ctf starts so i have no idea what to put here other than the flag which is actf{low_entropy_keystream}'
b'uX\x07\x80^\xec\xe0\x1e\x069\xe7\x10\x13L\xc5\xd6\x19 minutes left before the ctf starts so i have no idea what to put here other than the flag which is actf{low_entropy_keystream}'
b'\x85\xf9"\xc7\x95\t\x84\x97L like 30 minutes left before the ctf starts so i have no idea what to put here other than the flag which is actf{low_entropy_keystream}'
b'there are like 30 minutes left before the ctf starts so i have no idea what to put here other than the flag which is actf{low_entropy_keystream}'
b'\x84\xc9@5\xae\xc5\x05\xfb/9\xe7\x10\x13L\xc5\xd6\x19 minutes left before the ctf starts so i have no idea what to put here other than the flag which is actf{low_entropy_keystream}'
b'y\x17E\xa0{\t\x84\x97L like 30 minutes left before the ctf starts so i have no idea what to put here other than the flag which is actf{low_entropy_keystream}'
b'\x89\xb6`\xe7\xb0\xec\xe0\x1e\x069\xe7\x10\x13L\xc5\xd6\x19 minutes left before the ctf starts so i have no idea what to put here other than the flag which is actf{low_entropy_keystream}'
**The Plaintext**
there are like 30 minutes left before the ctf starts so i have no idea what to put here other than the flag which is actf{low_entropy_keystream}
