Quantum Computers the next hot thing to come out has been speculated to be the magic bullet in password cracking.
Do Quantum Computers pose a threat to hashing algorithms such as SHA256 commonly used in Bitcoin and other crypto currencies?
Experts tend to agree the strongest hashing algorithms and strong symmetric key ciphers are still resilient to to quantum computer attacks if sufficient enough key lengths are used.
There can still be discussion that most public Bitcoin keys ciphers are vulnerable to quantum attacks.
All crypto algorithms depend on proof of equivalence to a mathematical problem. Many computationally hard problems can be looked as soft to a large quantum computer.
The solution is to add quantum hard math problems to the ciphers.
There are already several open source projects such as Open Quantum Safe OQS projects to help deal with that.
First Generation quantum computers are being made now that will have 50-qubit machines by January 2020.
A quantum computer with 50-qubit is faster than any existing supercomputer for specific tasks and problems to solve.
Qubits is set to increase 30 percent yearly in the next 20-30 years.
Quantum computers are still at this moment very expensive to use.
Superconducting Quantum computers require refrigerators that can cost up to one million dollars.
Then there is the cost for the quantum chips and other room temperature equipment and other interface devices.
Some quantum computers can cost up to 10-25$ million dollars. And it will require permission to buy one.
Further more the price of quantum computers is not set to drop since the amount of qubits will increase.
They can be compared to older mainframes.
A 50 qubit quantum computer can solve specific problems such as four color map theorem.
A 128 qubit quantum computer can solve more complex 16 step traveling salesman problem in under a second.
A 320 qubit quantum computer can solve 32 step travling sales problem and more.
To crack a DES Data Encryption Standard that as used in the 80ties and early 90ties will require 8000 qubits.
For this trend in 20 years quantum computers can have an effect on computer security.
A quantum computer to break AES-128 will require 20000 qubits and AES-256 will require 40000 qubits.
AES Advanced Encryption Standard.
Second Generation quantum computers can be designed to work at room temperature and replace superconducting flux loops possible based on optical waveguides, photonic crystals and plasmonic devices to perform.
When we can see 100000 qubits processors come out it can be more likely to bruteforce and crack encryption that is impossible to crack today.
Quantum computers themself can also be under attack since they use a normal computer for the interface which can be vulnerable to exploits.