Ubuntu 22.04 Dislikes RSA Keys

I wrote in a past article about how I’m setting up Hetzner dedicated servers with full disk encryption even if they miss an ikvm and why Debian 10 machines require a RSA key for this.

But since I switched one of my workstations to Ubuntu 22.04 I was unable to login using this RSA key. Running ssh with debug enabled showed the likely culprit:

debug1: Offering public key: /home/user/.ssh/id_rsa
debug1: send_pubkey_test: no mutual signature algorithm

The message sent me on the right track, Ubuntu 22.04 has disabled RSA keys support by default. I’m not arguing with that, I don’t really like using RSA since better alternatives are around so I don’t want to change this default, but still I would like to be able to reboot my Debian 10 servers. So a command line option later I was able to use RSA keys only when I want them:

ssh -o PubkeyAcceptedKeyTypes=+ssh-rsa root@1.2.3.4

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