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What exactly are the dangers of quantum computing?
It's very simple.
I'm talking about the global, the danger to a lot of people, not to private cases.

All protection protocols, I'm talking about cryptographic methods of protection, built on a principle:
1. Asymmetric cryptography is the first step in any protocol to agree on a common session key for symmetric cryptography.
2. The second step is symmetric cryptography encryption, where secrets are encrypted securely (AES).

Why is a quantum computer dangerous today that will work far tomorrow?

Because all of our encrypted messages are stored.
Details:
- those encryptions that are very interesting - stored many times, it's communication between interesting and big people of our time;
- all other messages are also stored, just in case, they can be interesting, probably.

Now how quantum cheaters will work:
1) they will only crack the first stage of the encryption protocol - only asymmetric cryptography, where the shared session encryption key was encrypted. That's it.
2) They use the resulting key to quietly read the AES cipher, the second step of the encryption protocol.

And now, everything falls into place: AES-256, the symmetric system, is not cracked, and RSA (with any length of key) or ECC (with any length of key), the asymmetric system is cracked without a doubt, even by very weak, first quantum computers.

That's why everyone is so concerned, that's why post quantum asymmetric encryption systems are already needed.

Yes, not all people encrypt good messages, there are so many that lead two lives at once and one of those lives is very bad.
But the bad thing is to read and decide what's bad and what's good will be guys with the same questionable reputation as the first ones.

Here is the real vulnerability of all the key encryption methods: everything secret, sooner or later, becomes known and not secret.

This vulnerability is completely devoid of new keyless encryption systems.