If you have a black box that signs things, the information required to make it sign arbitrary things is always going to be extractable, one way or another. PUF

If you have a black box that signs things, the information required to make it sign arbitrary things is always going to be extractable, one way or another. PUFs attempt to prevent this, but I'm not sure it's been done in a provably secure way yet https://en.wikipedia.org/wiki/Physical_unclonable_function https://en.wikipedia.org/wiki/Physical_unclonable_function