In fact, so hard that a classical computer might take a lifetime or more to perform the calculation.
Why was Shor's result so important? Much of modern encryption, such as RSA encryption, is based on the idea that it is very difficult to factor a large integer that is the product of two large prime numbers.7 While it is easy to take two primes and multiply them together to form a big number, doing the inverse from a large number to find its constituting unknown primes is hard. In fact, so hard that a classical computer might take a lifetime or more to perform the calculation. However, on a quantum computer, if it can be made to operate with a large number of qubits, finding prime numbers could be done efficiently (mathematically in polynomial time), putting the basis of much modern encryption used on the internet at risk, If that hard problem can be solved, what other hard problems might become much easier? Actually, quite a few, including hacking bitcoins and other cryptocurrencies.