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Lets say, a 128 bit AES key takes 20 years to break using brute force. So now how do I calculate the time to break a 256-bit AES key by the same method from the given information?

I guess applying the unitary method to calculate for 256 is incorrect.

closed as off-topic by Steffen Ullrich, Xander, Serge Ballesta, Matthew, schroeder Oct 30 '17 at 14:59

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    Multiply by 2^(256-128)? – Ajedi32 Oct 27 '17 at 21:21
  • TL;DR, a bit more than a duodecillion years and a bit shy of 1e40 years. – Stephen Touset Oct 27 '17 at 21:22
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    Multiply by 2^(256-128). – Stephen Touset Oct 27 '17 at 21:34
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    @user162334: multiply by 2^128 the time it takes you to break 128-bit AES. A quick mental calculation tells me that 20 years is about 2^4.3 years, so it should take about 2^(128 + 4.3) = 2^132.3 years. A.k.a. too many years to bother with base 10 conversion. – Luis Casillas Oct 27 '17 at 22:09
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    20*2^128 = 6805647338418769269267492148635364229120 years. The sun will swallow earth in roughly 5000000000 years. – Dog eat cat world Oct 28 '17 at 9:33

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