How long do you want these messages to remain secret?

The longer the key you are trying to generate, the longer this takes. Randy is trying to generate one that is ridiculously long. He has pointed out to Avi, in an encrypted e-mail message, that if every particle of matter in the universe could be used to construct one single cosmic supercomputer, and this computer was put to work trying to break a 4096-bit encryption key, it would take longer than the lifespan of the universe. “Using today’s technology,” Avi shot back, “that is true. But what about quantum computers? And what if new mathematical techniques are developed that can simplify the factoring of large numbers?” “How long do you want these messages to remain secret?” Randy asked, in his last message before leaving San Francisco. “Five years? Ten years? Twenty-five years?”

I want them to remain secret for as long as men are capable of evil.

(…) If you want your secrets to remain secret past the end of your life expectancy, then, in order to choose a key length, you have to be a futurist. You have to anticipate how much faster computers will get during this time. You must also be a student of politics. Because if the entire world were to become a police state obsessed with recovering old secrets, then vast resources might be thrown at the problem of factoring large composite numbers.

— Cryptonomicon, Neal Stephenson

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