Congress can change the rules and has so there is no social contract or rights of any kind to receive benefits. When I first paid into the system, the retirement age was 65, now it is 67. SS is a pay as you go system so whatever is paid in by you now is automatically paid out to those collecting benefits. Any "surplus" is used by the USG as part of general revenue. The SS Trust Fund receives an IOU for the "surplus" in the form of non-market T-bills. The SS Trust Fund represents an unfunded liability and is part[about 40%] of our current $9 trillion debt. It is held as "intragovernmental holdings."
I understand all that quite well. There's no "fund" there, just a bunch of T-bills. And T-bills are just IOU's. By the same token, the US government "owns" nothing more than raw political power. But I have political power, too. Not as much as the government, but collectively with all the rest of those in my age peer group, we should be able to get back some of our money that we paid in.
As an aside, those who feel the present system is not "fair" or "unjust" are under no obligation to file for it. One doesn't have to collect it. It's sort of like all the do-gooders who want to do good with someone else's money. If someone feels he isn't paying enough taxes, merely send and extra payment in to the US Treasury. They'll cash the check, I promise you.