Comment by ajamesm

You send a message to [the person who can prove ownership of HN username 'perfmode'].

c.f.: https://keybase.io/docs/kbfs

  Soon, you'll be able to throw data into /keybase/private/yourname,pal@twitter,
  even if that Twitter user hasn't joined Keybase yet.
  Your app will encrypt just for you and then awake and
  rekey in the background when that Twitter user joins
  and announces a key.