Is it the right time to rant about the cert expiration as a concept? I understand why certs might be revoked. But expire?
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Improvements can be changes to cryptographic algorithms, like "Don't use SHA-1" or to the nuances of the certificate document like "Don't use this X509 feature" or to the CA infrastructure like "Don't issue certificates for names which don't exist".
Shortened certificate lifetimes improve agility by bringing forward that horizon. We can say "Stop doing X by August" tomorrow, and by Christmas 2026 there are no trusted end entity certificates which relied on X. A few years ago that took 3-5 years, at the turn of the century it was more than a decade and we repeatedly paid a price for that.