* splitting large secret keys
* tune hashes on more powerful hardware than thermal throttling laptop
* store to servers
* improve restore progress bar points (update after every hash try)
* Keep secret keys in locked memory until they're encrypted.
  (Raaz makes this possible to do.)
  Would be nice, but not super-important, since gpg secret keys
  are passphrase protected anyway..
* If we retrieved enough shards successfully, but decrypt failed, must
  be a wrong password, so prompt for re-entry and retry with those shards.