code: protected String urlDecodeSignedRequestToken(String signedRequestToken) {
      verifyParameterPresence("signedRequestToken", signedRequestToken);
      return signedRequestToken.replace("-", "+").replace("_", "/").trim();
    }
docs: Decodes a component of a signed request received from Facebook using FB's special URL-encoding strategy.
  
  @param signedRequestToken
  Token to decode.
  @return The decoded token.