So it's a simple un-xor
```
import base64

chaine = "BRQDER1VHDkeVhQ5BRQfFhIJGw=="

def decode():

un_chaine = base64.b64decode(chaine)
flag = ""

for i in un_chaine:
flag += chr((i)^102)
print(flag)

decode()

#crew{3z_x0r_crypto}
```

Do with 0xdeadbeef :")