You (like most people) have difficulty grasping how large 2^256 is (or even 2^128 which is the effective security of 256 bit ECDSA keys). The 128 bit or 256 bit seems deceptively small.
As a math literate person I do gasp how huge 2^256 is.
Nobody credible is saying classical computers could brute force keys in thousands of years..... it would be billions of years using all the energy of our sun. That also assumes you have a perfect computer.
And I do agree with this as in TODAY, the math is simple, our most powerfull supercomputers calculates in 30sh PFlops that's about 30x10^15 Flops Time in year = 3600x(24x365+6) = 31557600s and 2^256 ~ 1.14x10^77 so it will take to crack it with the usumption that it will require 100Flops per combination = 1.14x10^79/(31557600x30x10^15) =~ 1.20x10^55 years !
BUT THAT'S NOT THE POINT! My point is if you consider only classical computing in the last 30 years we've moved from KiloFlops to PentaFlops or 10^15Flops in terms of processing power, it is easy to assume that in the next few decades, we can easly achieve 10^30 / 10^40 (we've already gone past the point of cracking 2^128 or 128bits in a few seconds) and it will reach eventually 10^70+. In the 80/90s people (like you) were claiming 56 bit encryption was impossible to crack, and you know what, it takes like
3s and less to break with our current supercomputers!
And this doesn't take into consideration Alghorithm break trought as I mentioned, even the current classic computer with the proper alghorithms can simulate Quantum computers and have similar results in some areas for example......... Now if you add in the mix Quantum computing which will bring computing to a whole other level as the potentiel from a dozen of Qubit and the impact they have is already being proven.
None of those (except QC) would do anything more than switching from a teaspoon to a bucket when trying to empty an ocean.
Wrong as proven above.
a) The private key isn't random enough (insufficient entropy due to flaw in PRNG)
b) ECDSA is cryptographically weakened/broken.
c) It becomes possible to build a QC with the tens of thousands of qubits necessary to implement Shor's algorithm against a 256 bit ECDSA public key (and public key is known).
It's not limited to this as proven above but :
a = Possible as proven with AES thanks to NSA Middeling
b = Possible
c = it will happen in the next decade or the one folowing, considering we've moved from 4 Qubits to 128 in a very short laps of time heck Dwave just released a 512 Qbits Processor and they claim to have a 1000 Qubits in their lab ready to roll
http://www.washingtonpost.com/blogs/the-switch/wp/2014/01/10/this-company-sold-google-a-quantum-computer-heres-how-it-works/Also the Shor Alghorithm is not the most efficient Alghrorithm beyond 600 Qubits in comparaison to Fourier Transform
On one hand factoring and calculation logs and the other the usual linear transform that can be decomposed to I or Unitary Matrix, which Qubits likes.