If your future self came back in time to meet you and was unrecognizable, what system would you use so that they could convince you that they were you?: originally appeared on Quora: The best answer to any question. Ask a question, get a great answer. Learn from experts and get insider knowledge. You can follow Quora on Twitter, Facebook, and Google+.
Answer by Lawrence D. Roberts, Legal academic specializing in science and technology policy, space law, and international law.
There is no absolutely foolproof solution, but you can greatly reduce the risk. The key is preparation.
Step 1 - Set up a program on your computer that generates a string of random numbers and prints out the results.
Step 2 - Without looking at the page (and preferably wearing gloves and a full bunny suit to prevent leaving any physical evidence that can be used to identify you), fold and place the printout into an envelope.
Step 3 - Seal the envelope with a unique, tamper-proof seal. To be extra secure, place the envelope in a personally transportable container that is impervious to any state of the art or currently theoretical means of remotely sensing the contents of the box.
Step 4 - Place the box in a safe in a secure location that can not be traced back to you. Lock the safe and use another unique tamper-proof seal.
Step 5 - Tell no one about your preparations.
Step 6 - Before you return to the past, open the safe, remove the box, and take it back with you to the past.
Step 7 - Give the box to your past self and have him open both this box and the one in his safe. If the random number strings match, the likelihood is that future you is who he claims to be.
Step 8 - When future you returns to the future (assuming he or she still exists or is even capable of returning to the universe from which future you came) future you can still not reveal the existence of the preparations (so as to prevent some nefarious person from the future from going back in time to just before future you left, breaking into the safe and stealing the box before making the full jump back in time to mess with present you).
Step 9 - As soon as future you departs, go back to step 1 and begin the process again just in case another future you shows up. Make sure to use a different safe in a new location.
This system will work for any time travel incident that occurs after the initial preparations have been made. The only caveat is that past you would have no way of knowing if future you slipped up at some point in the future (or, as Wu Yiheng pointed out, accurate and effective brain scanning becomes possible and is used on future you) and revealed your preparations for possible time travel.