As far as i know, a quantum attack is a brute force attack performed by a quantum computer. source: wiki
Is this very simple definition correct ? or the scope of a quantum computing attack is bigger.
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First of all, a very big NO.
Saying that a Quantum Computer (QC) simply performs a lot of actions (in this case brute forcing) simultaneously to get the results quickly is a misconception. Quantum Computer doesn't brute force over the available set of actions in order to find the answer. QCs doing a lot of things simultaneously is due to the fact that it is in a superposition state of exponentially many states. NOT that the processor is working very very fast on many different things at the same time. You can find more about how it does it is the link provided below. Apart from that, the logic gates are fundamentally different for QC. In classical computer all logic gates except the NOT gate are irreversible while for QC all gates are reversible.
QCs are fundamentally different than our classical computers as they rely on qubits (instead of bits). One qubit can be in 0, 1, or both at once (superposition of both). Two qubits can represent four states simultaneously, while a hundred qubits can represent 1.3 quadrillion quadrillion states. This allows QCs to find pattern in the data set very quickly.
Asymmetric cryptosystems' security depends on unfeasible factoring of very big numbers by classical computers. While for the QCs it is just a matter of time.
Go over my this answer on Quora. It has a full explanation of what you are looking for plus more. Don't really feel like repeating everything.
Old topic but Grovers algorithm should be mentioned. With this algorithm it is possible to search an unsorted database faster than linear which means that with grover it could be possible to have more efficient brute force attacks.
A side note: this is also very interesting in the field of machine learning. For example the storage capacity of neural networks for pattern recognition increases with Grover's quantum neural networks (in analogy to a quantum bit the neuron could fire, not fire or be in a superposition of fire and not fire [as long nobody measures because if you measure/look (or even have the possibility to look) the system loses it's quantum behavior due to decoherence]).
You can play on your own. IBM gave access to their old quantum machines so you could experiment with real quantum technology and not "only" simulated quantum behavior. All you need is an account (which is free) and Python. Take a look ;)