# Time to break 256-bit AES key if 128-bit AES key could be broken in 20 years using brute force [closed]

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
• Multiply by `2^(256-128)`. – Stephen Touset Oct 27 '17 at 21:34
• @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
• 20*2^128 = 6805647338418769269267492148635364229120 years. The sun will swallow earth in roughly 5000000000 years. – Dog eat cat world Oct 28 '17 at 9:33