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for a project I'm looking to make as an art project a series of encrypted "time capsules" that have deliberately weak encryption, enough that it isn't feasible to crack them now, but could be done with say a month of CPU time in in {10, 20, etc} years (making some assumptions about how computing power will increase).

The plan I'm toying with is to zero pad short keys for AES-128 (assume the amount of padding is known to whoever is trying to decrypt them).

How do I calculate the difficulty of guessing an AES-128 key with N known bits?

Phrased differently, are there more efficient methods for guessing an AES key if we already know the first N bits of the key? Or, if we know N bits, we simply need to guess 2^{128-N} bits?

As a bonus, if you could come up with a formula for estimating the amount of CPU time we would need to brute force a key with N known bits in M years, that would be helpful as well. If you have any other suggestions for what I could do to implement this time capsule idea that would be interesting as well.

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  • $\begingroup$ What if someone reversely searches? What if someone starts from the middle? What if someone randomly searches and hits faster than the average? $\endgroup$
    – kelalaka
    May 12, 2021 at 12:03

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