看到 OpenSSL 的官方居然特地寫一篇與 Let's Encrypt 的相容性問題：「Old Let’s Encrypt Root Certificate Expiration and OpenSSL 1.0.2」。
這邊提到的 OpenSSL 1.0.2 很舊了 (在 Ubuntu 16.04 內是 1.0.2g)，理論上大多數的機器應該不太會遇到這個問題。
問題出自 Let's Encrypt 舊的 DST Root CA X3 將在這個月月底過期，這在 Let's Encrypt 的「DST Root CA X3 Expiration (September 2021)」這邊也有提到。
The currently recommended certificate chain as presented to Let’s Encrypt ACME clients when new certificates are issued contains an intermediate certificate (ISRG Root X1) that is signed by an old DST Root CA X3 certificate that expires on 2021-09-30.
理想上只有要任何一條 trust chain 成立，就應該會把這個憑證認為是合法的憑證，但這在 OpenSSL 1.0.2 (以及之前的版本) 不是這樣設計。
舊版的設計是只要有任何一條過期的憑證，就會把憑證認為過期而失效：
Unfortunately this does not apply to OpenSSL 1.0.2 which always prefers the untrusted chain and if that chain contains a path that leads to an expired trusted root certificate (DST Root CA X3), it will be selected for the certificate verification and the expiration will be reported.
OpenSSL 官方給了三個 workaround 可以做，另外我還有想到一個惡搞方式，是可以用其他家免費的憑證... 不過也是得測看看在 OpenSSL 1.0.2 下會不會動。