Why not just "own your data"?
Many privacy arguments say your information is your property. That sounds strong, but it has a weakness: property can be sold or signed away, and every app's terms of service ask you to do exactly that. It also does not fit government, which collects information under legal authority rather than by taking your property.
Fiduciary Commons starts somewhere sturdier. Facts cannot be owned. Your birth date and address are facts. What you have is more important than ownership: you are the person those facts describe, so you have the greatest stake in their accuracy and the most legitimate say over how they are used.
A trustee's duty is different from a promise
A trustee holds something on another person's behalf, under a legal duty to act only in that person's interest. An attorney cannot use what you told her against you. A trustee managing an estate cannot spend it on himself. These duties are enforceable in court.
A privacy policy is a promise an institution makes to itself and enforces at its own discretion. A trustee's duty runs to you, and you can enforce it. That is the difference between a protection and a pledge.
Two trustees, one relationship
You are the primary trustee of facts about yourself. Government, which collects those facts because you have no choice, is a secondary trustee. Its duties run first to you. The relationship works both ways: you owe government accurate information for legitimate purposes, such as paying taxes honestly and telling the truth on an application. In return, government owes you four duties.
Loyalty
Use what you provided only for the purpose you provided it. Collect no more than the task needs. Information given for one service does not become raw material for unrelated inquiries.
From the Fourth Amendment's particularity requirementCare
Keep information accurate and protect it. A data breach is not just a technical failure; it is the failure of a duty already owed.
From the reasonableness requirementConfidentiality
No one looks at your information without proper authority for that specific purpose. An unauthorized lookup is itself a search.
From the prohibition on unreasonable searchesTransparency
Tell you what is held, how it is used and who has looked. You cannot defend rights you cannot see.
From the warrant requirementThe boundary that matters most
Government's trust is bounded by your specific relationship with it. The motor vehicle office knows your address because you need a license. That does not give the motor vehicle office, or anyone who can reach its database, authority to use your address for something else. Authority stays inside the relationship that created it. When systems erase those boundaries, a thousand small, legitimate permissions become one general power that nobody granted.