Therefore sha256 hash of encryption key is validated against one which is stored
on server on poll creation.
This one is transfered as X-Croodle-Proof-Key-Knowledge HTTP HEADER.
Prevents an attacker of transmitting data with wrong encryption key, which
would cause decryption errors for legit users.
therefore we have to duplicate it in store:
* encrypted for to serve for clients (encryptedExpirationDate)
* unencrypted for server to check if it's exceeded (serverExpirationDate)
serverExpirationDate should never be send to client
