We can't use Let's Encrypt on our production systems right now
I really like Let's Encrypt, the new free and automated non-profit TLS Certificate Authority. Free is hard to beat, especially around here, and automatically issued certificates that don't require tedious interaction with websites are handy. And in general I love people who're striking a blow against the traditional CA racket. Unfortunately, despite all of that, there's basically no prospect of us using LE certificates in production around here.
The problem is not any of the traditional ones you might think of. Browsers trust the LE certificates, and that LE only does basic 'Domain Validation' (DV) certificates is not an issue because those are what we use anyways. And I have no qualms about using a free CA; CAs are in a commodity business and LE is easier to deal with than the alternatives due to their automation. It's not even the short 90-day duration on their certificates (although that's a potential issue).
The problem for us is that Let's Encrypt (currently) has relatively low rate limits, and especially it has a limit of five certificates per domain per week. Even if LE interpreted this very liberally (applying it to just our department's subdomain instead of the entire university), this is probably nowhere near enough for our usage. We have more than five different servers doing TLS ourselves, never mind all of the web servers run by research groups or even individual graduate students. This isn't just an issue of having to carefully schedule asking for certificates (and the resulting certificate renewals); it's also a massive coordination problem among all of the disparate people who could request certificates. As far as I can tell, using LE certificates in production here would mean giving a very large number of people the power to stop us from being able to renew (production) certificates. That's just not a risk we can take, especially since you have to renew LE certificates fairly often.
(Sure, we'd renew well ahead of time and if there were problems we could buy a commercial TLS certificate to replace the LE one. But if we're going to have problems very often we can save ourselves the heartburn and the fire drill by just buying commercial certificates in the first place. The university may not value staff time very highly in general but our time is still worth some actual money, and commercial certificates are cheap.)
I do feel sad about this, as I'd certainly like to be able to use LE certificates in production here (and I'd prefer to use them, especially with automatically handled renewal). But I suspect that a big university is always going to be a corner case that LE's rate limits simply won't deal with. If the university got seriously into 'TLS for all web sites', we're probably talking about at least thousands of separate servers.
(This doesn't mean that I have no use for LE certificates here. But that's another entry.)
Sidebar: my views on multiple names on the same certificate
TLS certificates can be issued with multiple names by using SANs, which means that you can theoretically cut down the number of distinct certificates you need by cramming a bunch of names on to one certificate. LE is especially generous with how many SANs you can attach to one certificate.
My personal dividing line is that I'm only willing to put multiple names into a TLS certificate when all of the names will be used on the same server. If I'm putting fifteen virtual host names into a certificate that will be used on a single web server, that's fine. If I'm jamming fifteen different web servers into one TLS certificate and so I'm going to have fifteen copies of it (and its key) on fifteen hosts, that's not fine. I should get separate certificates, so that the damage is more limited if one of those hosts gets compromised.