There’s been some discussion recently about how long an x509 certificate should be valid for if they were issued by a Certificate Authority that is a member of the CA/B Forum.
Currently, the limit is 39 months, or three and a quarter years. This means that operationally, a certificate from a CA must be changed at least every 39 months. The discussion proposed shortening that length to 13 months.
Why Shorten It?
While Let’s Encrypt is lauded for being free, the most impressive aspect of it is that it can be - and is easy - to fully automate. Let’s Encrypt makes CertBot, a piece of software you install on your server that sets up HTTPS for various web servers, and handles renewals, domain validation, etc. Since this is fully automated, the validity period of a certificate is inconsequential - the certificate could be valid for a single day as long as it keeps getting renewed and replaced correctly.
This has a lot of positives. A short lifespan of a certificate means revocation is less of a concern. Revocation in PKI largely doesn’t work in HTTPS simply because that in most* cases, online revocation checking isn’t performed. We have tools coming soon that will help fix that like Must Staple, but those are still a ways off from being widely deployed and adopted. If a certificate is only valid for three months and is mis-issued - this limits the period of time that a mis-issued certificate could be used.
Along with Must Staple and CT, this also helps address the issue of domain squatters buying a domain, getting a long-length certificate for it, and then selling the domain all the while having a valid certificate.
There’s also plenty of good reasons aside from these to shorten a certificate’s length.
Why Not Shorten It?
Shorter certificate lifetimes have several benefits, so what are the reasons not to allow such a thing? We have a proven system to demonstrate that it’s automatable, and for more complex cases, it should be relatively painless to automate, right?
That’s where I have to disagree, and why I’m rather hesitant to support this with the current state of certificate deployment.
I’d like to tell a short story about a certificate I had to manage. It was for an HTTPS endpoint that a 3rd party used to upload data to us. The 3rd party required our endpoint to support HTTPS, and strangely while doing this integration they asked us to securely deliver the x509 certificate to them. When asked why, they said they pin to the certificate that we send them. They required pinning the leaf certificate. This means when we have to change our certificate, we need to coordinate with the 3rd party.
Unfortunately, this 3rd party wasn’t exactly fast to perform these changes. We needed to coordinate days in advance with them, discuss the operations, and they actually counted the hours of work against our support contract.
If this sounds ridiculous - I agree. But, it was the requirement. The 3rd party insisted on doing it - and talking with others they were frustrated by the same requirements. The certificate still needed to be issued by a CA - that is they would not pin against a self-signed certificate, etc. Also, this party had a monopoly on the data we wanted, so we didn’t have much choice there, either.
This is one example of many that I can recount in an environment where renewing a certificate is not easy - or possible - to automate. Other situations involved an overly-complex CCRB where changing the certificate required a lot of operational testing, sign off, approvals, etc. Process can be fixed, but it’s more stubborn than some might realize. Other challenges are technology, like when an HSM is involved. Yes, it’s automatable - but it will take a lot of time for an organization to get there, and HSMs are unforgiving with mistakes.
It’s also worth pointing out that I think a lot of people lose sight of the fact that certificates are used (often!) outside of HTTPS. TLS is a general purpose transport tunnel. You can encrypt all sorts of traffic with it - such as Remote Desktop, SQL Server, VPN, CAPWAP, etc. Some of these circumstances do require or use a certificate from a CA. While a web server might be easy to automate, other things are not.
This would lead to a tripling of certificate replacement work.
I’m not happy with the status quo, either. Certificates should be automatable, they should have a shorter lifespan - but we’re not quite there yet. I would argue that it would take some organizations months, or years of work to support automating their entire infrastructure. Yes, I think it would be a big benefit for organizations to have that anyway.
Going from 39 months to 13 months is over ambitious at this point. I would test the waters of this with a change to 25 months to see how CA’s customers are able to cope with the change. That will also put the writing on the wall that they need to start automation before the 13 month limit is imposed.
It’s hard to balance good security with what works in the real world. I just don’t think the real world is ready at this point for this change. Organizations are already scrambling to keep up with other changes. The TLS 1.2 requirement for PCI vendors already have them working hard.
I do hope we get there one day though.
* “Most” is used generally here - revocation checking behavior differs from environment to environment and the type of certificate, such as EV certificates.