Decoding issue in python using tkinter

When I run this decode function I only get the first letter as my result for eg: if i try to decode "hello" i only get "h" in my result

def Decode(key,message):
    dec = []
    message = base64.urlsafe_b64decode(message).decode()

    for i in range(len(message)):
        key_c = key[i % len(key)]
        dec.append(chr((256 + ord(message[i]) - ord(key_c)) % 256))
        return "".join(dec)