OpenSSH 與 Dropbear 對 Ed25519 的支援
source link: https://blog.gslin.org/archives/2023/04/06/11129/openssh-%e8%88%87-dropbear-%e5%b0%8d-ed25519-%e7%9a%84%e6%94%af%e6%8f%b4/
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OpenSSH 與 Dropbear 對 Ed25519 的支援
查了一下這兩個 server 端的軟體支援 Ed25519 的時間點。
OpenSSH 是在 2014/01/30 的 6.5 就支援了：
* ssh(1), sshd(8): Add support for Ed25519 as a public key type. Ed25519 is a elliptic curve signature scheme that offers better security than ECDSA and DSA and good performance. It may be used for both user and host keys.
算是相當久以前就支援了。對應到第一個支援的 Debian 版本是 Jessie 使用的 OpenSSH 6.7：「sshd(8) — openssh-server — Debian jessie — Debian Manpages」；第一個支援的 Ubuntu (LTS) 版本是 Trusty (14.04) 用的 OpenSSH 6.6：「openssh source package in Trusty」。
Dropbear 這邊就晚不少，在 2020/06/15 的 2020.79 版本才支援：
- Support ed25519 hostkeys and authorized_keys, many thanks to Vladislav Grishenko. This also replaces curve25519 with a TweetNaCl implementation that reduces code size.
所以對應到第一個支援的 Debian 版本是 Bullseye 的 2020.81：「Debian -- Details of package dropbear in bullseye」；第一個支援 Ubuntu (LTS) 的版本是 Jammy (22.04) 的 2020.81：「Ubuntu – Details of package dropbear in jammy」。
這樣看起來如果就是想要用 Ed25519 的話，變成 server 端的軟體得配合：預設裝的 sshd 應該都是 OpenSSH，如果想要換 Dropbear 的話要看 distribution 內給的版本夠不夠新，或是透過 PPA 之類的方法裝新版。
但大多數採用 BusyBox 的機器應該沒有採用新版 Dropbear (像是 AP 刷機)，這邊還是得使用其他 key format，如果要避開 NIST 有介入的格式，就還是得用 ssh-rsa 了。
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