“Nothing prevents an opponent of intercepting all crypted communications that go on the internet, just copying them and storing them and keeping them at home… whenever a quantum computer becomes available to them, they can go back, take it all out of storage, and decrypt everything retroactively… There's nothing, absolutely nothing you can do to save the past. All you can hope for is save the future.”
Hanselminutes with Scott Hanselman · Hands-On Engineering Podcasts · October 2026
Asked whether software engineers should panic about quantum computers breaking today's public-key cryptography, Brassard, co-inventor of quantum cryptography, says yes, and that the main reason is "harvest now, decrypt later". Traffic encrypted for decades with the standard methods could become an open book once a quantum computer exists.
Transcript
…post-quantum crypto has a chance to solve, even though we cannot prove it does. But let me come back to should we panic? The main reason to panic is not what I just said. It's much, much worse. By the way, when you ask if we should panic, I should have jumped on it and said, what a good question.
That would have made me feel better at this point. I'm
sorry I didn't say it immediately, but it is. So the reason we should panic, the main reason, is that it's known as the harvest. Now and decrypt later paradigm, by which nothing prevents an opponent of intercepting all crypted communications that go on the internet, just copying them and storing them and keeping them at home. So all this crypted information can be already stored somewhere, probably is actually, in the hands of would-be opponents who are totally unable to decrypt it because they don't know how to break RSC yet. But whenever a quantum computer becomes available to them, they can go back, take it all out of storage, and decrypt everything retroactively. In other words, because RSC and DV Handlin were…
So if it's to paraphrase and make sure that I'm understanding, if post-quantum cryptography as a software-based defense is going to ensure that today's classical communications will remain secure against future quantum computers, while acknowledging that there's a significant window where we…
no, no, no, no, no, no. No, no. Post-quantum crypto, if we switch from the current infrastructure to post-quantum solutions today, and by the way, we take years to do that. But if we switch today, it will not in any way ensure safety in the future. That was
my question. There's no guarantee.
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