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I am wondering which of these two solutions is better for security on the long run. The problem is:

Alice and Bob exchange a secret key/private key. Then they go far away from each other and never have the opportunity to exchange a private key again. They will use the private key to exchange messages intensively (let's say 1 per 20 minutes for years).

Which option is the best secured?

  • Just let the private key as it is, for all the time, with the risk that a brute force attack succeds at some time: example: the private key is 1234
  • Regularly change with a pre-set manner the private key, using today's date: example: 107212320422 on 07/12/2022, 108212320422 on 08/12/2022: thus the keychanges reguarly so brute force attacks could not work, but is the part that is still the same (the 1xx2xx3xx4 template) a vulnerability?

Of course, I am also interested in a better solution than those two :)

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    $\begingroup$ There is WPA-PSK protocol that you might be interested. $\endgroup$
    – kelalaka
    Jan 9 at 18:01
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The classic One Time Pad is actually viable for such a scenario. If they can keep it safe, and ensure they never re-use any part of it, they could just exchange 8TB hard drives full of pad material. With one pad per direction, if messages are 10kiB, that gives them 29 years worth of pad material. They'd have to occasionally copy the pad material to new drives, and would likely need some redundancy in practice, and would have to securely destroy the old drives as they wear out (a shredder is best), but it's perfectly secure otherwise.

The "meet once, exchange data, then never have a trusted channel again" scenario is exactly what One-Time pads are useful for. It's also why they're so rare in real use, because that's a very rare scenario.

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  • $\begingroup$ Thank you for the answer but that does not fit my need, since they don't plan to meet again $\endgroup$ Jan 9 at 20:30
  • $\begingroup$ That's the point of the OTP. If 29 years of 10kiB messages isn't enough, use bigger pads. $\endgroup$ Jan 10 at 13:46
  • $\begingroup$ Yes I understand but I would like to find a solution to generate the key instead of stocking it somewhere before $\endgroup$ Jan 10 at 13:56
  • $\begingroup$ The "never exchange keys again" is the hard part. It's quite easy to use something like the Signal protocol to get a good forward-secure key ratchet, but as stated the puzzle is much more difficult. Real-world protocols tend not to have such a restriction on the contents of communications. $\endgroup$ Jan 11 at 18:39
  • $\begingroup$ What is the signal protocol? $\endgroup$ Jan 11 at 20:05

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