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Is there a function or system which is time depending in which the effort required to brute force the decryption increases with time? It is easy to break encryption from many years ago because computers are so much faster. Is there a system in which time is a component and as time passes then it gets harder & harder to decrypt? Perhaps there is a physical entropy component which degrades after time? Something akin to disappearing ink, but digital.

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Only quantum based crypto has a chance to fit. Indeed, computational based crypto becomes easier to break as your computing power increases (and computing power isn't going to decrease, unless there is a huge societal collapse). For quantum crypto, maintaining an intercepted (quantum) state intact for a long time is hard, so there is a chance that it could give a positive answer. –  minar Aug 27 '13 at 17:09
@minar : $\:$ See this paper on long-term security. $\;\;\;$ –  Ricky Demer Aug 27 '13 at 19:01
Doesn't seem useful in practice. It's easy to design huge security margins against brute-force into a system. Crypto doesn't get broken by brute-force nowadays. It's either broken by analysis (finding a flaw in the crypto) or circumvented (finding flaws in the software). The only potential brute-force attack that's not easy to prevent is a quantum computer, but we'll solve that in a few years as well. –  CodesInChaos Aug 31 '13 at 15:18
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2 Answers

Any protocol with long-term security becomes harder to break after the protocol execution has finished.

In the Bounded-Storage Model, protocols become harder of break as [information about the
randomizer that's not stored by the adversary] is lost. $\:$ (This point is similar to minar's observation.)

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Thanks. I found a pointer to what I was thinking of: lsv.ens-cachan.fr/Seminaires/?sem=201201171100 –  minar Aug 27 '13 at 19:07
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When the data stream is used to generate the key, then what you end up with is a dynamically generated one-time pad. Here is one design from the 50's only recently declassified:


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Lol! .......... –  Chloe Aug 27 '13 at 19:17
A "dynamically generated one time pad" is nonsense. Your linked image is full of nice-looking graphics, and a bit of nonsense text, but has neither a specification of this system nor anything else usable. –  Paŭlo Ebermann Aug 29 '13 at 7:17
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