CBO Paper: CBO's Long-Term Social Security Projections: Changes Since 2018 and Comparisons with the Social Security Trustees' Projections [open pdf - 994KB]
From the Summary: "Each year, the Congressional Budget Office [CBO] updates its projections of the Social Security system's finances to incorporate newly available data and methodological improvements. CBO published its latest long-term budget projections in June 2019. This report compares those with the ones CBO prepared in 2018 and with the latest projections by the Social Security trustees. [...] For the 2018-2093 period, CBO projects larger deficits in Social Security's finances than do the Social Security trustees. That difference is largely explained by CBO's and the trustees' different projections of several key inputs into estimates of the system's finances: the population, components of GDP [gross domestic product] growth, earnings subject to Social Security payroll taxes, and real interest rates (that is, interest rates adjusted to remove the effects of inflation)."
Congressional Budget Office: http://www.cbo.gov/