Why hostkeys-00 at openssh.com is following user authentication?

[Alon Bar-Lev]

Hello,

This hostkeys extension is great, reading[1]:
"""
OpenSSH supports a protocol extension allowing a server to inform a
client of all its protocol v.2 host keys after user-authentication has
completed.
"""

I wonder, why should user authentication be completed before this
functionality is available? This means that ssh-keyscan tool (for
example) cannot take advantage of the functionality.

The only reason I could think of (apart of sshd internal
implementation ordering) is to avoid cpu load attack of anonymous
clients forcing server to perform expensive operations.

However, a similar attack can be achieved by multiple connections and/or re-kex.

In sshd.c I see hostkeys-00 at openssh.com sent before calling
do_authenticated(), so I even more curios.

Thanks!
Alon Bar-Lev.

[1] https://github.com/openssh/openssh-portable/blob/master/PROTOCOL#L284