SSH connection fails after upgrade from 20.04 to 22.04

Question

A team member just upgraded our AWS EC2 instance from 20.04 LTS to 22.04 LTS, and ssh connections broke as a result. Now when I try to open a session (in MobaXterm) I get:

Server refused our key
No supported authentication methods available (server sent: publickey)

I have a public key added to ~/.ssh/authorized_keys on the EC2 box, and I have been using a corresponding .ppk file on my Windows desktop for my private key. This has been working flawlessly, until this "upgrade".

What happened, and how do I go about fixing it?

UPDATE: The issue is that RSA keys are no longer supported. You need keys generated by a more secure algorithm, such as ed25519.

Does this answer your question? Recent update for SSH breaks backward compatibility, how do I downgrade to the previous version? — Artur Meinild, Oct 03 '22 at 14:12
Please note that my answer does not actually downgrade, but reenables the legacy RSA SHA-1 hash algorithm for older connections. — Artur Meinild, Oct 03 '22 at 14:13
Does this answer your question? Ubuntu 22.04 SSH the RSA key isn't working since upgrading from 20.04 — user68186, Oct 03 '22 at 15:07
@user68186 Basically, yes. Using a new stronger key-pair is superior to re-enabling RSA support. — arayq2, Oct 03 '22 at 15:22

score 2 · Accepted Answer · answered Oct 03 '22 at 15:04

In our case, the issue is that we were using keys generated by the RSA algorithm. This is no longer supported. The resolution is to use new keys generated by a more secure algorithm, such as ed25519. (Generate a key pair, add the public key to the ~/.ssh/authorized_keys file on the EC2 instance[*], and use the private key from the remote client.)

I did exactly that, and now I can connect.

I suppose this is documented somewhere, but where? (Sigh.)

[*] An alternate way to get into the EC2 instance (to transfer the public key) is via the AWS Mangement Console.

SSH connection fails after upgrade from 20.04 to 22.04

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