If we consider quantum computers as a means of attack, we are generally thinking about the phenomenal factoring power which can swamp traditional security measures. As we have discussed, there has been a lot of work in post-quantum cryptography to establish robust classical defences, ranging across a number of different approaches using some quite sophisticated maths, be it lattice, Lamport/Merkle sig or (deep breath) supersingular-isogeny-graph-Diffie-Hellman-key-exchange.
Some of these approaches show a lot of promise. However quantum cryptography - building fundamentally unbreakable security measures by using the laws of quantum mechanics - is for me a hugely important strategy. I say fundamentally unbreakable because the act of observation (or eavesdropping, or hacking) is an integral component in any quantum mechanical system. The observer impacts the results, as in the Schrodinger's Cat thought experiment (incidentally the theoretical cat is not both alive and dead at the same time, rather it is in a superposition of the states, a probability function that only resolves upon being observed).
Any attempt to observe/hack/eavesdrop on the quantum state alters the data, and it's impossible to pull the data out and copy it, either. This is demonstrated by the no-cloning theorem.
I've already mentioned Kak's 3-stage protocol, which is entirely quantum, unlike the more commonly used approach of Quantum Key Distribution... but I'd like to go into QKD a bit here, just to demystify it a bit.
The standard terrestrial approach to QKD is essentially to send photons one-by-one down a normal fibre-optic cable. If someone tries to read (hack) the communication, then this act of observation alters the polarity of the photon, and so the recipient becomes aware of the hacker's presence. This is simple QM, it's a basic physical law that can't be bypassed. Of course you wouldn't send a whole message in this way, but it is a perfect method for establishing a shared key.
There is also a satellite approach to QKD that China is developing, which relies on quantum entanglement so that two photons both have the same physical state. Again, anyone eavesdropping disrupts the communication.
... so whilst understandably a lot of focus is on the dangers posed by attacks that exploit the properties of quantum mechanics, it is (in mainstream opinion) sometimes overlooked that those same properties can provide a phenomenal (and as we understand QM, perfect) defence.
[image sourced from this article: https://www.sciencemag.org/news/2017/06/china-s-quantum-satellite-achieves-spooky-action-record-distance ]
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Creating a shared key over fiber? Quantum methods? But what's revolutionary here. These experiments are already 40 years old. This method is not for the average user with a device connected to a wifi point. This is for special organizations. And not at great distances. In addition, there should be a mass of photon amplifiers and other equipment on the track.
And is this just to create a shared key?
It’s easier to find an open communication channel that no one has been controlling for a long time ... no one will track an ordinary paper letter “to the grandfather’s village”, not a single hacker.
Mankind likes methods that require a ton of money and technology. This gives rise to new value. This gives rise to the consumer. This spins the economy. Well, where is the development of cryptography?
But in essence, for us, for ordinary users of cryptography, this method of technology development is similar to building a fortified fortress only for protection against one mosquito, and not for life.
It seems to me that the future is not for technological solutions in this area, but for logical ones.
In technology, humanity has long been “grazing behind”, so far an electric drone can be in autonomous flight for a limited time, measured in hundreds of minutes. Guys, this chemical method of storing electrons is already 3000 years old. The Nobel Prize in Chemistry was given for the molding of electrical power supplies, for a lithium-ion battery. Previously, for this “invention” they would only give a patent. And now the Nobel Prize. Awesome!
Do not feel the lack of development? Today everyone considers development a thinner smartphone case and they are proud of it. But this is the level of molding. Is evolution reversed?
Especially, this degradation became noticeable with the development of digital communications. Instead of coming up with new encryption methods, everyone comes up with an increase in the key length (and candidates for post-quantum systems have keys of phenomenal length), and work in the fields of huge, "astronomical" numbers that do not exist in the universe, which load our skinny smartphones. Aw, people, wake up ... There must be "human", reasonable decisions.