This document tries to find a way to get rid of authorized_keys file.

Certificates may be used for user (auth user to serv) or host authentication
(auth servers to users).

Certificate has a public key, identity information, zero or more principal
(user or host) names and a set of options signed by CA key.

Generate a user cerificate:
	ssh-keygen -f ~/.ssh/key1
	ssh-keygen -s CA.key -I key_id ~/.ssh/key1.pub
	Output will be in ~/.ssh/key1-cert.pub

Host certificates:
	ssh-keygen -f ~/ssh/host1
	ssh-keygen -s CA.key -I key_id -h ~/.ssh/host1.pub
	Output will be in ~/.ssh/host1-cert.pub

Lets see how we generate CA.key. Seems that is a normal key, but, in
authorized_keys file is marked with option cert-authority.


Plan:
Create a user (A).
Create a CA key.
Add it to authorized_keys file and mark it as cert-authority.
Generate a key for a connection user (B) and sign it with the CA key.
Add the cert to B's ~/.ssh/
Try to connect with user B to userA@host.

Seems we can specify "-z serial" to ssh-keygen. But ssh-keygen crashes.

Questions:
- How to revoke a key?
- Seems the key_id is not exported in environment. :(