As many others, I have been following the launch of Let’s Encrypt. Let’s Encrypt is a new zero-cost X.509 Certificate Authority that supports the Automated Certificate Management Environment (ACME) protocol. ACME allow you to automate creation and retrieval of HTTPS server certificates. As anyone who has maintained a number of HTTPS servers can attest, this process has unfortunately been manual, error-prone and differ between CAs.
On some of my personal domains, such as this blog.josefsson.org, I have been using the CACert authority to sign the HTTPS server certificate. The problem with CACert is that the CACert trust anchors aren’t shipped with sufficient many operating systems and web browsers. The user experience is similar to reaching a self-signed server certificate. For organization-internal servers that you don’t want to trust external parties for, I continue to believe that running your own CA and distributing it to your users is better than using a public CA (compare my XMPP server certificate setup). But for public servers, availability without prior configuration is more important. Therefor I decided that my public HTTPS servers should use a CA/Browser Forum-approved CA with support for ACME, and as long as Let’s Encrypt is trustworthy and zero-cost, they are a good choice.
I was in need of a free software ACME client, and set out to research what’s out there. Unfortunately, I did not find any web pages that listed the available options and compared them. The Let’s Encrypt CA points to the “official” Let’s Encrypt client, written by Jakub Warmuz, James Kasten, Peter Eckersley and several others. The manual contain pointers to two other clients in a seamingly unrelated section. Those clients are letsencrypt-nosudo by Daniel Roesler et al, and simp_le by (again!) Jakub Warmuz. From the letsencrypt.org’s client-dev mailing list I also found letsencrypt.sh by Gerhard Heift and LetsEncryptShell by Jan Mojžíš. Is anyone aware of other ACME clients?
By comparing these clients, I learned what I did not like in them. I wanted something small so that I can audit it. I want something that doesn’t require root access. Preferably, it should be able to run on my laptop, since I wasn’t ready to run something on the servers. Generally, it has to be Secure, which implies something about how it approaches private key handling. The letsencrypt official client can do everything, and has plugin for various server software to automate the ACME negotiation. All the cryptographic operations appear to be hidden inside the client, which usually means it is not flexible. I really did not like how it was designed, it looks like your typical monolithic proof-of-concept design. The simp_le client looked much cleaner, and gave me a good feeling. The letsencrypt.sh client is simple and written in /bin/sh shell script, but it appeared a bit too simplistic. The LetsEncryptShell looked decent, but I wanted something more automated.
What all of these clients did not have, and that letsencrypt-nosudo client had, was the ability to let me do the private-key operations. All the operations are done interactively on the command-line using OpenSSL. This would allow me to put the ACME user private key, and the HTTPS private key, on a YubiKey, using its PIV applet and techniques similar to what I used to create my SSH host CA. While the HTTPS private key has to be available on the HTTPS server (used to setup TLS connections), I wouldn’t want the ACME user private key to be available there. Similarily, I wouldn’t want to have the ACME or the HTTPS private key on my laptop. The letsencrypt-nosudo tool is otherwise more rough around the edges than the more cleaner simp_le client. However the private key handling aspect was the deciding matter for me.
After fixing some hard-coded limitations on RSA key sizes, getting the cert was as simple as following the letsencrypt-nosudo instructions. I’ll follow up with a later post describing how to put the ACME user private key and the HTTPS server certificate private key on a YubiKey and how to use that with letsencrypt-nosudo.
So you can now enjoy browsing my blog over HTTPS! Thank you Let’s Encrypt!