Refreshing! Not wanting to be the "told you so" guy, I've been saying this for at least 2 years now:

> Post-quantum authentication is no longer a problem the Web PKI ecosystem should defer. Long-lived keys (root certificate authorities, code-signing keys, identity systems) are particularly valuable targets, and new technology takes years to gain broad adoption, so the work has to start early.

This is a problem that I have met so many times talking with people: they parrot the "Harvest-Now-Decrypt-Later is the only urgent problem, signatures can wait" mantra, and this piece of misinformation has spread so much that even AI repeats it (because it has been trained on open data, where the overwhelming sentiment has been following this trend), thereby reinforcing the problem. Ask Claude/ChatGPT/Gemini about the problem, and they will invariably tell you that signatures are less urgent because theyr are not subjective to retroactive compromise.

There are two problems here.

The first one is included by the Letsencrypt announcement: the migration path for signatures/certificates is typically longer and more complex than encryption: long-lived certificates, firmware update keys, secure boot certificates, these are all objects that are painful to migrate.

The second one, even more serious in my opinion, is: "retroactive" in respect to what? "Retroactive" presupposes you can observe the trigger (the arrival of a cryptanalytically-relevant quantum computer), but this is precisely the kind of capability an adversary keeps secret, and a quantum forgery is operationally indistinguishable from, e.g., key exfiltration, a library bug, or a classical break. You may see a forged signature, a drained wallet, a failing certificate, and have no way to attribute it to quantum cryptanalysis. The threat is dark: reactive migration against an unobservable trigger is structurally impossible.

This is not to say that Harvest-Now-Decrypt-Later is a less urgent threat, but it's not so asymmetric as people have been believing so far. Glad to see things are changing!

A Post-Quantum Future for Let's Encrypt