Rating:
# AngstromCTF2021 Folow the Currents
From the code provided we spot that the key is only two bytes. This enables us to do an exhaustive search of the key-space.
```
import os
import zlib
def keystream_decode(key):
index = 0
while 1:
index+=1
if index >= len(key):
key += zlib.crc32(key).to_bytes(4,'big')
yield key[index]
with open("enc","rb") as f:
ciphertext = f.read()
for i in range(65536):
plain = []
k = keystream_decode((i).to_bytes(2, 'big'))
for i in ciphertext:
plain.append(i ^ next(k))
try:
if bytes(plain).decode("utf-8"):
print(bytes(plain))
except:
pass
```
The solution script tries every key and tries to decode the decrypted data as uft-8. If the decoding succeeds it prints the decrypted values as utf-8.
Luckly only one key decrypts the data into uft-8 and we get the flag:
`actf{low_entropy_keystream}`
