docs: def _compute_key(self, id, nbytes):
          """id is 'A' - 'F' for the various keys used by ssh"""
          m = Message()
          m.add_mpint(self.K)
          m.add_bytes(self.H)
          m.add_byte(b(id))
          m.add_bytes(self.session_id)
          out = sofar = sha1(m.asbytes()).digest()
          while len(out) < nbytes:
              m = Message()
              m.add_mpint(self.K)
              m.add_bytes(self.H)
              m.add_bytes(sofar)
              digest = sha1(m.asbytes()).digest()
              out += digest
              sofar += digest
          return out[:nbytes]