Why POST-Quantum?
Let’s describe what makes a quantum computer a (supposed) threat. Before, intellectual honesty imposes the following:
Remark
Quantum computers are still moving the first steps and are a THEORETICAL threat. No quantum computer at the moment is powerful enough to make economies fall. Or to factor the number 42, for what it matters (and it does not, as 42 has 3 factors!).
But there’s a reason to be concerned and to start taking precautions now. We don’t really know WHEN (or even IF, to be honest) quantum computers will become a practical threat, but we know what will happen if stars align and the quantum threat becomes real before we take shelter. Let’s play a game: Bohb comes from the future with his time machine. In his timeline, quantum computers replaced classical ones, to the point that every kid uses his DOLL-Atom3000 to check the weather on Mars for the weekend. Bohb is making a thesis on anthropology (a futur-ish version of it) and wants to check what would happen if quantum computers were dropped in a society not ready for them. We are his test subjects. Let’s see what Bohb would put in his report.
$\textbf{EVIDENCE 1}$ Digital signatures and certificates can now be forged easily, no online transaction can be claimed secure, no website is to be trusted.
$\textbf{EVIDENCE 2}$ Seems like some people sitting on the riverbank are having their moment. Old communications containing sensitive or secret data are now being decrypted. Many military and intelligence secrets are being leaked to the public. Now even governments aren’t to be trusted (editor’s note: assuming you trusted them before).
Even if we said that we aren’t here to scare, it is possible you are scared now. But there are countermeasures we can apply in order to not be Bohb’s guinea pigs. Those countermeasures all fall under the definition of $\textbf{post-quantum cryptography}$.