Cryptographic proof of survival

Mediumship is mostly fake, no big deal. I'm inclined to think that all of it is either coincidence or fake, but I'm open to the chance of some of it being real  

People leaving messages behind them will always go unanswered. Even if mediumship was real (I doubt), contact with the deceased proving that afterlife is real would be banned from us.

I don't think the word 'banned' is appropriate. It presumes a particular description of the larger reality. I'd gently suggest that there are other possible descriptions of reality which lead to different ideas of the difficulties involved.

To take a historical example, our knowledge of astronomy and the nature of heavenly bodies such as the stars and planets was pretty much an unknown quantity for most of humankind's history. There may even have been those who declared that such knowledge was banned from us. But what it really took was a combination of increasingly accurate observations, together with appropriate tools in the form of theoretical ideas and mathematics with which to manipulate them. One might even argue that at present, these theoretical, mathematical constructs are rushing on ahead without concrete support in the form of observations. Nevertheless, it wasn't correct to say that knowledge of astronomy was banned.