What will happen to existing cyphertext when today's encryption is broken? Even optimistically, I think we can all agree that many encryption algorithms that are widely used in production today will be "broken" sometime in the next decades.

With "broken", I mean that (probably because of quantum computing but maybe because of some black swan) it will be possible to decrypt ciphertext that is generated through those algorithms with a substantially lower effort as opposed to today. And over time, this "attack cost" will only decrease.

Again, no doom scenarios, I am optimistic. Let's assume that every system that uses cryptography will upgrade/switch to "quantum computing proof" cryptography before it becomes reasonably easy to break today's encryption.

However, even in this scenario, can we agree that all "old" ciphertext (that is being generated in PRD all over the world as we speak) will be easily breakable ? Again: "sometime in the next DECADES" ...

If we can agree on this, let's discuss the implications. I list some things that I can think of. Please comment, prove me wrong, add, ...


Encrypted e-mails, Whatsapp/Telegram/... messages, ... that are stored on any server or on any device in general (e.g. a mobile phone confiscated by police) will be easily decryptable. Idem for encrypted files on servers or any devices.
TOR traffic will become as crystal-clear as unencrypted direct traffic. Let's assume a TOR entry node logs everything. Once the encryption attack cost is low enough, those logs can be decrypted and they would show for example: "ip address 1.2.3.4 sent me a request at 1:01 AM May 12th to visit website X through TOR path A B C".
Idem for VPN's. Or for "VPN over TOR" or "TOR over VPN".
Also ISPs who keep logs can dig into all that happened in the past. Idem for example for public wifi hotspots who keep logs (or any "proxy" between the users' device and the ISP).
Passwords / secret keys can be "sniffed" from logs.
Payments through cryptocurrencies that promise anonymity (like Monero) will become just as crystal-clear as payments in non-anonymous currencies like Bitcoin.
Again, we assume that also cryptocurrencies will have switched to quantum computing proof cryptography by when the attack cost became very low, but the history of what happened while using the old algorithms will not magically disappear. Hell, not making it disapear is exactly what blockchain does ....


My guess is that this "event", sometime in the next decades, will unveil proof against a lot of criminals who are safe now thanks to cryptography.
Likewise it will open many doors for people with bad intentions ... Sniffing passwords from "old" logs. Blackmailing people through "old" secrets from "old" ciphertext. Company secrets being stolen. Etc etc.

I am not saying that this is good or bad. I am just saying that imo this will be a very notable event in the next decades and that it's worth discussing about what impact this will have.

Since this is a "question answering" and no discussion website, here you go:
If/when today's widespread encryption algorithms become easily breakable (for example through quantum computing), what will be the implications of the fact that the existing data encrypted through these algorithms becomes public to those in possession of the ciphertext ?
For me it is absolutely not certain at all that many cryptographic primitives will become broken. This was certainly true - to some extend - for older algorithms. But modern cryptography is a different beast than the classical ciphers. We know a lot more about various attack vectors and semantic security. So your initial premise is very speculative.

Symmetric algorithms are already thought to be relative secure against quantum analysis, only halving the key strength (in bits) at best. There are a lot of asymmetric primitives that could be broken using a full fledged quantum computer, but there are certainly post quantum algorithms available. They may not be completely stable or standardized yet, but they are there.

Now you start to talk of an "event" that would happen in the future. Now I speculate that this "event" will never happen. Even if the algorithms we use today get broken, they won't get broken all at once. There are different levels of being "broken" as well, even MD5 - a known broken hash - is still secure in many protocols (password hashing, key derivation, HMAC, you name it).

If ciphertext gets broken it will not happen all at once either. And the impact of the plaintext becoming available will depend completely on the contents of the plaintext. This could be huge for state secrets. However, for the majority of the plaintext: the ciphertext will either not be available to persons that want to abuse the information or the plaintext will have completely lost the importance it once had.

Huge data leaks are happening right now, and the world moves on. We don't need any magic cryptographic breakthrough for that. If you want to know the impact you only need to study what is happening today.