Ten years ago, we printed one of the nerdiest t-shirts we’ve ever made. On the front was the entire PEM encoding of ISRG Root X1 in base64. Back then, it represented a future we were working toward. Today, that same design tells the story of just how far Let’s Encrypt has come. Let’s Encrypt was already issuing publicly trusted certificates in 2016, but ISRG Root X1 itself was still slowly and quietly making its way into browsers and operating systems around the world. For many years, our certificates were trusted through a cross-sign from IdenTrust. ISRG Root X1 itself was added to the major trust stores fairly early on; the slow part was waiting for that update to reach the browsers and devices already out in the world, since many of them only get new trust stores when they’re updated. That took years.