[lxgr]

> It does not matter how good your security chip actually is if all I have to do is copy the correct interface.

A security chip actually deserving the name (i.e. a tamper-proof one) can protect a private key even against physical attacks, with the corresponding public key marked as authentic by the manufacturer.

If the interface contains a challenge-response interaction with that private key (and ideally ties that to any further communication with the trusted applications on it), you can't copy/emulate that.