SSN is a fine ID, but it cannot be used as authorization. I think that a government issued photo ID, such as a passport, is probably the current best bet if you

SSN is a fine ID, but it cannot be used as authorization. I think that a government issued photo ID, such as a passport, is probably the current best bet if you require the other part to save a photocopy of it. Perhaps in this digital age you could have a system where you need to sign in with 2 factor authentication to verify any claims to authorize you. Biometric is a very poor ID. With current technology we can fake fingerprints and iris, the two most commonly used biometrics. They also have the issue that you cannot change them, so if you do get compromised and flag it, then you cannot use that option yourself.