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OpenSSH has a system of certificates that it can use for authentication, under the following algorithm names:
They are described in this document.
PuTTY could usefully support using them for authentication. While the changes to the SSH protocol are trivial, the necessary modifications to the PuTTY private key file and to PuTTYgen might be a little more complicated, and host certificates would have to be integrated into PuTTY's host-key checking mechanisms.