Well it's a pretty simple definition: "exclusive omnipotence over an item". And that power could be challenged but it can't be taken away without it.

By default sovereignty is only achievable if proper defenses can be mounted to defend against thiefs. In a State, it's not the contract that secures the property, but the tanks and the nukes. Internally of course a State has internal laws to treat it's subjects, but this means that the subjects can't be sovereign, so an individual can't own property, since he would need an army to defend it.

However cryptographic property doesn't need an army to defend it. It's already secured by the laws of physics, the supreme law sort to speak. And that is by default more preferable than to have large masses of people being organized for your defense.

So I believe with privacy enhancing technology and cryptographic property, the individual can regain it's sovereignty.