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))


Original writeup (https://github.com/r00tstici/writeups/tree/master/angstromCTF_2021/follow_the_currents).