Forward only specific identities

[Tim Spriggs]

Hi OpenSSH peeps!

  I have looked around a few man pages and the usual sources of
information but I can't seem to find a way to only forward specific
identities to some hosts. What I would really like to have is a way to
only forward the identity that gave me a successful auth:

% ls ~/.ssh | grep .pub
id_ecdsa.pub
id_ed25519.pub
id_rsa.pub
% cat .ssh/config
Host example.com:
  IdentitiesOnly=yes
  IdentityFile=/home/tspriggs/.ssh/id_rsa.pub

Host another-example.com:
  IdentitiesOnly=yes
  IdentityFile=/home/tspriggs/.ssh/id_ecdsa.pub

# This would be super cool:
Host *
  OnlyForwardAuthedKey=yes

% ssh tspriggs at example.com
example.com % ssh-agent -L
ssh-rsa ...
example.com % ssh tspriggs at another-example.com
Permission denied (publickey)
example.com % logout
Connection to example.com closed.

% ssh tspriggs at another-example.com
another-example.com % ...

Cheers,
-Tim