How long will an encryption key be in 30 years

I am trying to understand the correlation between encryption key length and processing speeds.
If I assume that the total processing speed of microprocessors (based on clock rate and number of circuits) is doubling roughly every year....and if a symmetric session key needs to be 100 bits long to be considered strong today. How long will a symmetric session key have to be in 10, 20 or even 30 years to be considered strong?
Milkus1Asked:
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ozoCommented:
Assuming processing speed doubling every year, and assuming a symmetric session key needs to be 100 bits long to be considered strong today,
then a symmetric session key would need to be 110 bits long to be considered strong in 10 years,
120 bits in 20 years,
130 bits in 30 years
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Milkus1Author Commented:
Thanks ozo, I now get the concept.
Double the speed requires just doubling the permutations possible.
I  how do I write this as a formula?
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ozoCommented:
Assuming processing speed doubling every T0, and assuming a symmetric session key needs to be N bits long to be considered strong today,
then a symmetric session key would need to be N+T1/T0 bits long to be considered strong in T1.
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Milkus1Author Commented:
Thanks oro...much clearer now.
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