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I know little about SSL certificates. Could someone tell me why they should expire regularly? Is it for precaution? Is it just because "good practices". What's

I know little about SSL certificates. Could someone tell me why they should expire regularly? Is it for precaution? Is it just because "good practices". What's the difference (from a security point of view) between a certificate that expires in 3 months and one that expires in 3 years?

zorr 5y ago
It's mostly to limit the damage in case of compromise and to encourage automation. Let's Encrypt explains their choice for 90day certificates here: https://letsencrypt.org/2015/11/09/why-90-days.html https://letsencrypt.org/2015/11/09/why-90-days.html edit: With 3 year certificates there is also a greater risk for certificates expiring unexpectedly because the renewal process happens so infrequently. The person with the knowledge on the renewal process might not be at the company anymore by the time the certificate expires.
- cm2187 5y ago
  And in the case of letsencrypt to encourage the automation of the renewal process.
gchamonlive 5y ago
I don't know for the rest, but for letsencrypt you don't even have an option to set the expiration date, as far as I know. You have to make sure certificates generated there are regularly refreshed (every three months). So it might well be an imposition from the Certificate Authority.
brinox 5y ago
Even if your certificate expires after 3 years, the certificate renewal process should be automated. The long validity of 3 years leads many people to not consider this necessary. If the certificate is only valid for three months however, many people will automate the renewal right away, because nobody wants to do this manually every couple weeks. From a security POV, shorter lifetimes require more periodic checks for the server's identity. E. g. a Letsencrypt-issued certificate using the ACME protocol will validate the server really belongs to the given domain more often, which is a nice property I think.
dspillett 5y ago
The longer a certificate is valid for, the longer an attacker has to try compromise it somehow. Relying on certificate revocation instead of short-lived certificates is not an ideal solution. As systems have improved such that regular renewal can be easily done without human intervention (and if done properly, with no less, or in fact possibly more, assurance of security) it makes sense to take advantage of this by reducing certificate lifetimes. Like single-use credit cards, to give an analogy that a non-techie man-on-the-street can latch on to.
tialaramex 5y ago
There are two related but different things going on here. 1. Revocation is broken. The obvious way for revocation to work (your browser checks if the certificates it sees have been revoked) not only has negative privacy implications (so e.g. Firefox does not implement this) it also plain doesn't work. It has "fail-open" behaviour for a bunch of sad but inevitable social reasons. In theory we have viable fixes for this, but in practice we can fix really big problems (e.g. a whole CA is compromised) with a lot of effort and smaller problems (e.g. your IT guy emailed your private keys to a scammer) mostly not at all. Thus, expiry is the hard stop, if your certificate expires in six weeks, the scammer loses any benefit [edited] in six weeks. Problem "solved" or at least mitigated. 2. The Web PKI can only evolve at the rate permitted by certificate expiry. Anything faster will set important stuff on fire, because no matter how hard you try you can't publicise changes enough to ensure people are aware of them. e.g. Everybody needed to stop using SHA-1 certificates, that took several years, when the same happened for MD5 it took so long to be effective that even though it was deprecated before a reliable collision was known there were multiple successful attacks anyway. Under Apple's current 12 month rule, if we need to fix something ASAP, we can either break the world, or we wait until late 2022. But under the historic Five year rule that becomes break the world or don't fix it until 2027. That's a pretty big difference in how agile we can be without telling everybody, "Sorry, we broke the whole Internet, reinstall and try again".
- arsome 5y ago
  To fix #1 on Firefox just set security.OCSP.require = true
  - tialaramex 5y ago
    Sure, however this has the effect of showing any eavesdropper exactly which certificates you're trusting (and when), and thus in most cases which web sites you're visiting. It also tells the Certificate Authorities the same things about sites they've issued certificates to. Now, I assume that the list of people who visit Porn Hub isn't valuable enough to justify the CA (DigiCert again) explicitly monitoring who visits the site and selling it when they already charge PornHub good money for a certificate. But who knows?

zorr 5y ago

It's mostly to limit the damage in case of compromise and to encourage automation. Let's Encrypt explains their choice for 90day certificates here: https://letsencrypt.org/2015/11/09/why-90-days.html https://letsencrypt.org/2015/11/09/why-90-days.html edit: With 3 year certificates there is also a greater risk for certificates expiring unexpectedly because the renewal process happens so infrequently. The person with the knowledge on the renewal process might not be at the company anymore by the time the certificate expires.

cm2187 5y ago
And in the case of letsencrypt to encourage the automation of the renewal process.

gchamonlive 5y ago

I don't know for the rest, but for letsencrypt you don't even have an option to set the expiration date, as far as I know. You have to make sure certificates generated there are regularly refreshed (every three months). So it might well be an imposition from the Certificate Authority.

brinox 5y ago

Even if your certificate expires after 3 years, the certificate renewal process should be automated. The long validity of 3 years leads many people to not consider this necessary. If the certificate is only valid for three months however, many people will automate the renewal right away, because nobody wants to do this manually every couple weeks. From a security POV, shorter lifetimes require more periodic checks for the server's identity. E. g. a Letsencrypt-issued certificate using the ACME protocol will validate the server really belongs to the given domain more often, which is a nice property I think.

dspillett 5y ago

The longer a certificate is valid for, the longer an attacker has to try compromise it somehow. Relying on certificate revocation instead of short-lived certificates is not an ideal solution. As systems have improved such that regular renewal can be easily done without human intervention (and if done properly, with no less, or in fact possibly more, assurance of security) it makes sense to take advantage of this by reducing certificate lifetimes. Like single-use credit cards, to give an analogy that a non-techie man-on-the-street can latch on to.

tialaramex 5y ago

There are two related but different things going on here. 1. Revocation is broken. The obvious way for revocation to work (your browser checks if the certificates it sees have been revoked) not only has negative privacy implications (so e.g. Firefox does not implement this) it also plain doesn't work. It has "fail-open" behaviour for a bunch of sad but inevitable social reasons. In theory we have viable fixes for this, but in practice we can fix really big problems (e.g. a whole CA is compromised) with a lot of effort and smaller problems (e.g. your IT guy emailed your private keys to a scammer) mostly not at all. Thus, expiry is the hard stop, if your certificate expires in six weeks, the scammer loses any benefit [edited] in six weeks. Problem "solved" or at least mitigated. 2. The Web PKI can only evolve at the rate permitted by certificate expiry. Anything faster will set important stuff on fire, because no matter how hard you try you can't publicise changes enough to ensure people are aware of them. e.g. Everybody needed to stop using SHA-1 certificates, that took several years, when the same happened for MD5 it took so long to be effective that even though it was deprecated before a reliable collision was known there were multiple successful attacks anyway. Under Apple's current 12 month rule, if we need to fix something ASAP, we can either break the world, or we wait until late 2022. But under the historic Five year rule that becomes break the world or don't fix it until 2027. That's a pretty big difference in how agile we can be without telling everybody, "Sorry, we broke the whole Internet, reinstall and try again".

arsome 5y ago
To fix #1 on Firefox just set security.OCSP.require = true
- tialaramex 5y ago
  Sure, however this has the effect of showing any eavesdropper exactly which certificates you're trusting (and when), and thus in most cases which web sites you're visiting. It also tells the Certificate Authorities the same things about sites they've issued certificates to. Now, I assume that the list of people who visit Porn Hub isn't valuable enough to justify the CA (DigiCert again) explicitly monitoring who visits the site and selling it when they already charge PornHub good money for a certificate. But who knows?