What factor might have affected SSH host authentication?

Question

Recently when connecting with SSH to my Raspberry Pi, I got a host authenticity warning. I am aware of the way SSH authenticates host and what this message means, however I have no clue what might have caused either client or server to suddenly start reporting this error. I have not made any intentional changes, although it is possible that packages either on Pi's side or connecting OSX (homebrew) were updated.

This was the warning displayed at the connection time:

[~]# ssh pi1
The authenticity of host 'pi1 (192.168.1.114)' can't be established.
ECDSA key fingerprint is SHA256:XVL4PwzqDdNrmY+/8DeErSAR2gtkHkXo2U+6if8BE44.
Are you sure you want to continue connecting (yes/no)?

Compared to the ssh output from this post the fingerprint is clearly in different format:

ECDSA key fingerprint is 55:5b:0a:76:40:17:61:3f:58:43:74:3b:54:d7:88:34.

Could a library or packet update cause the authentication mechanism to change recently? If so which? Client-side or server-side?

Answer 1

The way how fingerprint is handled has changed in recent version of openssh (on the client-side), so if you want, you can add configuration option to get the fingerprint using the old MD5 function.

For the first connection, you can try:

ssh -o FingerprintHash=md5 pi1

This will cause ssh to use the old MD5 function to get the fingerprint of the host.

You can also put this in your ssh_config, but there is no use for this in later connections, when the host key had already been verified.