docs: def compute_wcs(key, challenge):
      """
      Compute an WAMP-CRA authentication signature from an authentication
      challenge and a (derived) key.
  
      :param key: The key derived (via PBKDF2) from the secret.
      :type key: str/bytes
      :param challenge: The authentication challenge to sign.
      :type challenge: str/bytes
  
      :return: The authentication signature.
      :rtype: bytes
      """
      key = key.encode('utf8')
      challenge = challenge.encode('utf8')
      sig = hmac.new(key, challenge, hashlib.sha256).digest()
      return binascii.b2a_base64(sig).strip()