Simple definition
An SSH host key identifies the remote SSH server and helps clients detect connection interception.
Purpose and use
An SSH host key lets a client recognize the server it is connecting to.
How does it work?
The public key or host certificate is checked under client policy, for example against known hosts or a trusted authority.
Example and verification
When it changes, compare the fingerprint through an independent channel and confirm an authorized server change.
Points to watch
Automatically trusting a replacement prevents distinguishing legitimate replacement from interception. It is not the user’s authentication key.
In short
A host key identifies the server; a user key authenticates the client.