Comment by Zamicol

A simple proof of possession (pop), signed ahead of time, solves #3.

The message: `{"pay":{"alg":"ES256","msg":"I own this key","tmb":"9PcBWntvjAktwfiPp8WxgOyQOwc1h6Lo1UnB_gkWXKk"},"sig":"eXuV0_HYCM-WnS2CbOnGXdce-9M8AzivCw23Hihtp1h69Ix6HwWCA79FR6cs3Nym2bWJoKajtnIY0xcTnuRnNQ"}`

The public key: `{"alg":"ES256","kid":"Zami Mobile 2","x":"PZpmb3CI_2LTWcxopqjliqohPpmxFmNwKLb52wJgMg-4Xd0hTRKn7OruUMa3LvHmuTA9pHidocLHnEdOcQ04OA","tmb":"9PcBWntvjAktwfiPp8WxgOyQOwc1h6Lo1UnB_gkWXKk"}`