Cato Paper by Milton Friedman
For those already retired, that would be an obligation—a treasury bill or bond—with a market value equal to the present actuarial value of expected future benefits minus expected future payroll taxes, if any. For everyone else, it would be an obligation due when the individual would have been eligible to receive benefits under the current system. And the maturity value would equal the present value of the benefits the person would have been entitled to, less the present value of the person’s future tax liability, both adjusted for mortality.
This would end Social Security, while ensuring that everyone gets what they payed in. Obviously, the debt would have to be paid off, but it would be worth it.