Root certificates need expiration dates for the same reason that LetsEncrypt certs need an expiration date: risk of cert compromise and forgery increases over time.

Over a long enough timeline, there will be vulns discovered in so much of the software that guards the CA certs in RAM

> risk of cert compromise and forgery increases over time.

And what if the certificate is compromised before it expires? Right, there's a revocation mechanism for that. So why expire them then if they can be revoked anyway IF they get compromised?

The reason why domain TLS certificates expire is that domains can change owners. It makes sense that it should not be possible for someone to buy a domain for one year, get a non-expiring TLS certificate issued for it, and then have the ability to MitM its traffic if it ever gets bought by someone else later.

Domain certificates are sent as part of the connection handshake, so them expiring is unnoticeable for the end users. However, root certificates rely on the OS getting updates forever, which is unsustainable. Some systems lack the ability to install user-provided root CAs altogether, and some (Android) do allow it but treat them as second-class.

Because the most dangerous secret is one that has been compromised and you don’t know it. This sets a time limit for their usefulness. Sometimes the stories about terrible default choices that are insecure sink in and architects choose a better path.

Also, details about the certs and the standards for them change over time. This makes it easier for the browser venders (via the CA forum) to force cert providers to update over time.

You're talking about it like they change by the force of nature, not because humans change them.

The revocation mechanism is basically just a list of revoked certificates. Without expiration dates, those lists will grow infinitely.

Certificates need expiration dates to be able to garbage collect certificate revocation lists.

Do certificate revocation lists need to keep including certificates that have long since expired? I don't see why root certificates need to expire as long as the certificates signed by those roots all have reasonable expiration windows, unless someone is doing something strange about trusting formerly-valid certificates, or not checking root certificates against revocation lists.

Right, because DNS entries never expire.

Of course they do, they have to. But it's okay for things that are sent to you over the network to expire. It's not okay for things built into your potentially abandoned OS to expire.

> Of course they do, they have to.

Why do they have to?

(This will also tell you why certs in your OS need to expire.)

Because domains change owners.

https://news.ycombinator.com/item?id=47074127

More specifically: because they cannot be revoked, they need to expire. Same with root certs.

The instant we bound encrypted connections with identity we failed. And decades later we're still living with the mistake.

I'm completely serious when we need to abandon the ID verification part of certificates. That's an entirely separate problem from encryption protocol. An encryption protocol needs absolutely no expiration date, it's useful until it's broken, and no one can predict that. Identity should be verified in a separate path.