Soreth @Soreth@awoo.space

so, for those who don't know how TLS works, websites verify their authenticity through a chain of trust, where everyone keeps of copy of the "root" trust certificates on their devices. a handful of organisations are deemed trustworthy enough to vouch for other organisations, and those other organisations vouch for certificates of websites to say that they really are who they say they are

this way, we don't need to change the certificates stored on devices for a while, since they can vouch for other certificates to confirm their trustworthiness

however, sometimes, a root certificate expires and new ones will be put in place

this happened recently for Let's Encrypt, a service that has been offering free TLS certificates for websites who just have to verify ownership of their domain name. there are more advanced certificates which will verify much more than just the domain (for example, big companies will go through a proper validation process so they really are who they say they are) but Let's Encrypt has made basic security available to the masses.

now, recently, Let's Encrypt switched to a different root certificate since the old one expired. it expired yesterday, actually. it has been publicly known this would happen since May, and it means that devices which are around 10+ years old won't have the new root, and hence can't properly connect to Let's Encrypt websites, since they don't trust them.

now, there are lots of reasons why those devices can't connect nowadays anyway, but at least to sidestep the problem, Let's Encrypt is using a compromise approach where it marks certificates with both the new and old root.

so, to clarify, devices fall into three categories right now. the oldest devices know about the old root, and not that it's expired. the newest devices know about the new root and that the old root is expired. but there's a weird middle ground of devices that know that the old root is expired, but don't know of the new root.

the Let's Encrypt workaround of signing with both roots mitigates problems for the oldest devices and the newest devices, but devices in the middle area still break. which is why some websites yesterday broke.

however, today, an issue still impacts some new devices. normally, a certificate signed with an expired root and a non-expired root should just ignore the expired signature and trust the non-expired one. but it turns out that some software is improperly stopping after it sees the expired signature, and marking the entire thing as invalid

people had time to update their devices to include the new certificate since May, but because this newer issue was not foreseen, it wasn't accounted for until now.

now, you might be wondering, then: how many devices fall into the older group? well, older versions of Android do, which makes it a sizeable demographic. so, there's not really a great solution right now that covers all bases. basically, it sucks.