Other than the 51% hack, the way to hack bitcoin it to hack the math which underlies the currency.

SHA256 is a special type of mathematical function which is really easy to solve in one direction but is very hard to solve in reverse. At least the only known way to solve SHA256 is reverse is very hard.  So hard in fact that our best implementation is to just randomly guess until we get the answer.

If you could hack the math and find an easier solution, then you could hack Bitcoin without the need for more computing power.  Since your algorithm might be 99.99% more efficient than the brute force method, you'd only need a tiny fraction of computing power to overwhelm the network.


It's believed that quantum computers might have the ability to crack these types of one way functions way more efficiently than the brute force method.