This is actually pretty simple. We are given a multiple times base64 encoded string. We need to decode it to get the flag.
The challenge file is here and to reach the challenge page click here.
Here's a recursive Python function to decode the base64 encoded string multiple times until we get the flag.
import base64
def decode(data):
try:
return decode(base64.b64decode(data))
except:
return data
with open("flag.txt", "r") as f:
data = f.read().strip()
print(decode(data).decode())