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    <p>As several others have pointed out:</p> <ol> <li>You must use server time, because client time is vulnerable to manipulation.</li> <li>Checking the time on the server will potentially penalize people with slow network connections, or people that are far away.</li> </ol> <p>The answer to the problem is to use a time synchronization protocol between the client and the server similar to the protocol that NTP uses. Working together, the client and the server determine the amount of delay caused by network latency. This is then factored into the times given to each user.</p> <p>NTP's algorithms are complicated and have been developed over years. But a simple approach is below; I think that the protocol should work, but you may wish to test it.</p> <p>Have the client measure the round-trip time with two successive HTTP XMLRPC pings. Each ping returns a different nonce. The second ping requires the nonce from the first ping, which assures that they are sequential. The puzzle time starts when the second HTTP ping is sent from the client. The server timestamps each request and assumes that the puzzle is displayed 1/2 way between the receipt of the first and the second request.</p> <p>When the puzzle is finished the client pings twice again, following the same protocol as before. The server knows when it receives each request and it knows the time delta. Now take half the time delta and subtract that from when the first ping of the second set is received. That can be safely assumed to be the time that the puzzle was completed.</p>
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    1. COVery interesting solution, but I don't fully understand it. (1) If puzzle time starts when the second ping is sent from the client, then doesn't the puzzle itself need to be sent by the server in response to the first ping? Otherwise the client could peek before initiating the sync. (2) Similarly, I assume the client's answer is sent along with the first of the second set of pings? (3) Lastly, can't the client delay the final ping to exaggerate latency? Am I missing something?
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    2. COAh. Those are really good points. I guess the puzzle needs to be sent in response to the second ping, and we can dispense with the second set and just send a response. But you are correct that the client could delay the time between the two pings to create a false sense of long latency. Hm...
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