Today I was attempting to update a local repository, when SSH complained about a changed fingerprint, something like the following:

Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
The fingerprint for the ECDSA key sent by the remote host is
Please contact your system administrator.
Add correct host key in /home/user/.ssh/known_hosts to get rid of this message.
Offending ECDSA key in /home/user/.ssh/known_hosts:9
ECDSA host key for has changed and you have requested strict checking.
Host key verification failed.

I checked if the host was changed recently, or the alias through which I connected switched host, or the SSH key changed. But that wasn't the case. Or at least, it wasn't the case recently, and I distinctly remember connecting to the same host two weeks ago.

Now, what happened I don't know yet, but I do know I didn't want to connect until I reviewed the received SSH key fingerprint. I obtained the fingerprint from the administration (who graceously documented it on the wiki)...

... only to realize that the documented fingerprint are MD5 hashes (and in hexadecimal result) whereas the key shown by the SSH command shows it in base64 SHA256 by default.

Luckily, a quick search revealed this superuser post which told me to connect to the host using the FingerprintHash md5 option:

~$ ssh -o FingerprintHash=md5

The result is SSH displaying the MD5 hashed fingerprint which I can now validate against the documented one. Once I validated that the key is the correct one, I accepted the change and continued with my endeavour.

I later discovered (or, more precisely, have strong assumptions) that I had an old elliptic curve key registered in my known_hosts file, which was not used for the communication for quite some time. I recently re-enabled elliptic curve support in OpenSSH (with Gentoo's USE="-bindist") which triggered the validation of the old key.


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