Rating: 5.0
# Follow the Currents - angstromCTF 2021
- Category: Crypto
- Points: 70
- Solves: 271
## Description
go with the flow...
Author: lamchcl
## Solution
We are given an encrypted string and the script used to encrypt it.
As we can see from the source, it uses a keystream-like cipher which at the beginning generates two random bytes and creates the following ones with a deterministic function that takes the previous bytes of the key as input. This keystream is xored with the key.
```python
def keystream():
key = os.urandom(2)
index = 0
while 1:
index+=1
if index >= len(key):
key += zlib.crc32(key).to_bytes(4,'big')
yield key[index]
```
The real problem is to find the first two bytes, after that we can deduce the following ones. Since it is only two bytes, we can brute-force them and generate the keystream for every possible combination. With one of them we obtained the string `Flag: 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}`
```python
for p in product(range(256), repeat=2):
key = bytearray(p)
k = keystream(key)
plain = decrypt(k)
plaintext = [chr(c) for c in plain]
if "actf{" in "".join(plaintext):
print("".join(plaintext))
```
**Full script in https://github.com/r00tstici/writeups/blob/master/angstromCTF_2021/follow_the_currents/exploit.py**
