One of my professors told me this: In case you have a good idea you can simply write it down, print it out, sign it and send it to yourself via certified paper

One of my professors told me this: In case you have a good idea you can simply write it down, print it out, sign it and send it to yourself via certified paper mail. When you receive the mail you don't open it. You simply put it in a safe place. Then you can use this to prove prior art because its more or less a official document according to the (German) law. Again: I am not a lawyer and this may be totally wrong...