code: public static Symm obtain(InputStream is) throws IOException {
  	  ByteArrayOutputStream baos = new ByteArrayOutputStream();
  	  try {
  		  base64url.decode(is, baos);
  	  } catch (IOException e) {
  		  // don't give clue
  		  throw new IOException("Invalid Key");
  	  }
  	  byte[] bkey = baos.toByteArray();
  	  if(bkey.length<0x88) { // 2048 bit key
  		  throw new IOException("Invalid key");
  	  }
  	  return baseCrypt().obtain(bkey);
    }
docs: Obtain a Symm from 2048 key from a Stream
  
  @param is
  @return
  @throws IOException