Let's Encrypt 弄了新的 Root Certificate 與 Intermediate Certificate：「Let's Encrypt's New Root and Intermediate Certificates」。
一方面是本來的 Intermediate Certificate 也快要要過期了，另外一方面是要利用 ECDSA 降低傳輸時的頻寬成本：
On Thursday, September 3rd, 2020, Let’s Encrypt issued six new certificates: one root, four intermediates, and one cross-sign. These new certificates are part of our larger plan to improve privacy on the web, by making ECDSA end-entity certificates widely available, and by making certificates smaller.
本來有 Let's Encrypt Authority {X1,X2,X3,X4} 四組 Intermediate Certificate，都是 RSA 2048 bits。
其中 X1 與 X2 差不多都到期了 (cross-signed 的已經過了，自家 ISRG Root X1 簽的剩不到一個月)，不過這兩組已經沒在用了，這次就不管他了。
而 X3 與 X4 這兩組則是明年到期，會產生出新的 Intermediate Certificate，會叫做 R3 與 R4，跟之前一樣會被自家 ISRG Root X1 簽，以及 IdenTrust DST Root CA X3 簽：
For starters, we’ve issued two new 2048-bit RSA intermediates which we’re calling R3 and R4. These are both issued by ISRG Root X1, and have 5-year lifetimes. They will also be cross-signed by IdenTrust. They’re basically direct replacements for our current X3 and X4, which are expiring in a year. We expect to switch our primary issuance pipeline to use R3 later this year, which won’t have any real effect on issuance or renewal.
然後是本次的重頭戲，會弄出一個新的 Root Certificate，叫做 ISRG Root X2，以及兩個 Intermediate Certificate，叫做 E1 與 E2：
The other new certificates are more interesting. First up, we have the new ISRG Root X2, which has an ECDSA P-384 key instead of RSA, and is valid until 2040. Issued from that, we have two new intermediates, E1 and E2, which are both also ECDSA and are valid for 5 years.
主要的目的就是降低 TLS 連線時的 bandwidth，這次的設計預期可以降低將近 400 bytes：
While a 2048-bit RSA public key is about 256 bytes long, an ECDSA P-384 public key is only about 48 bytes. Similarly, the RSA signature will be another 256 bytes, while the ECDSA signature will only be 96 bytes. Factoring in some additional overhead, that’s a savings of nearly 400 bytes per certificate. Multiply that by how many certificates are in your chain, and how many connections you get in a day, and the bandwidth savings add up fast.
另外一個特別的修改是把名字改短 (把「Let's Encrypt Authority」拿掉)，也是為了省傳輸的成本：
As an aside: since we’re concerned about certificate sizes, we’ve also taken a few other measures to save bytes in our new certificates. We’ve shortened their Subject Common Names from “Let’s Encrypt Authority X3” to just “R3”, relying on the previously-redundant Organization Name field to supply the words “Let’s Encrypt”. We’ve shortened their Authority Information Access Issuer and CRL Distribution Point URLs, and we’ve dropped their CPS and OCSP urls entirely. All of this adds up to another approximately 120 bytes of savings without making any substantive change to the useful information in the certificate.
這個部份讓我想到之前寫的「省頻寬的方法：終極版本...」這篇，裡面提到 AWS 自家的 SSL Certificate 太胖，改用 DigiCert 的反而可以省下不少錢 XDDD
另外也提到了這次 cross-sign 的部份是對 ECDSA Root Certificate 簽 (ISRG Root X2)，而不是對 ECDSA Intermediate Certificate 簽 (E1 與 E2)，主因是不希望多一次切換的轉移期：
In the end, we decided that providing the option of all-ECDSA chains was more important, and so opted to go with the first option, and cross-sign the ISRG Root X2 itself.
這算是蠻重要的進展，看起來各家 client 最近應該都會推出新版支援。
