In a context of proprietary client boxes and servers, I want to limit authentication to ECDSA keys. Historically, "RSAAuthentication yes" was used to enable authentication with RSA and maybe DSA keys. Does it mean "key authentication enable" or strictly "RSA key authentication enable"? Should I set it to false?
The option names are not part of the SSH protocol; they are specific to a given implementation. I suppose you are talking about OpenSSH.
As per the documentation:
RSAAuthentication Specifies whether pure RSA authentication is allowed. The default is “yes”. This option applies to protocol version 1 only.
So this does not apply to your case, since you want to use ECDSA and ECDSA keys are supported only with version 2 of the SSH protocol.
Whether clients may authenticate with keys (instead of, say, passwords) is set with the PubkeyAuthentication option. The allowed key types does not seem to be configurable, though. A client key will be accepted if it matches the public key stored in the
.ssh/authorized_keys file of the target account; thus, if you want to restrict key authentication to ECDSA, you should arrange for only ECDSA public keys to appear in such files.