I am not expert but just want to share information. dropbear works with dbclient, in my home one machine is ssh with other machine through this command "dbclient -i /home/homeassistant/.ssh/homeassistant homeassistant@192.168.1.90 'sudo systemctl hibernate'"
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dropbear make ssh key through this command //home/homeassistant/.ssh# sudo -u homeassistant dropbearkey -f /home/homeassistant/.ssh/homeassistant -t ecdsa -s 256
Using putty is working well with ssh keys and Dropbear. I guess keys are not setup correctly on your end. There are a couple of similar topics on this forum describing the way.
Can you check permissions of ~/.ssh which you created, it has to be 700 or something similar, where only the owner can write. Authorized key folder needs to be readable at least by the owner (eg 400)
did you verify the content of authorized_keys file? Is it correct? Should be a single line. And the permission of the file itself? Theoretically you could copy the file from a different server and just upload it via SCP. And you are sure on client side you use as well the key while opening the SSH session?
On my system key authentication is running fine
Dec 01 09:42:23 DietPiProd dropbear[3693]: [3693] Dec 01 09:42:23 Pubkey auth succeeded for 'root' with ssh-rsa key SHA256: