The key to sealing Kurama was something Namikaze Minato left behind, out of concern that the seal would weaken over time. Before his death, he entrusted it to Gamarou of Mount Myoboku!
Though called a key, it was actually a scroll containing the user manual for the Kurama seal.
In other words, whoever obtained that scroll would have the power to either reinforce the seal on Kurama or release him entirely!
Chapter 82