SSH Login doesn't work using a key Without Password?

You were suffering from the following failure:

Agent admitted failure to sign using the key.

This is an unfortunately non-diagnostic message. There are (at least) two classes of issues it could address:

 

The key is not loaded

For most issues, this means that your ssh-agent doesn't have any ssh keys loaded that are accepted for your account on the target server. In this case, as noted by @Networker's answer to this question, the solution is rather simple: add the key:

ssh-add

If the key is in a non-default location, you'll need to tell that to ssh-add:

ssh-add /path/to/key

 

The agent cannot understand the key

This was GNOME bug 754028, resolved in Seahorse 3.29.90 (stable 3.30 released 2018-09-03, included in Ubuntu 18.10, Fedora 29, and probably Red Hat/CentOS 9). Seahorse before 3.29.90 (and therefore GNOME Keyring) could neither create nor add new key types like ed25519 and keys generated with ssh-keygen -o -a 100 (as suggested by the Secure Secure Shell tutorial).

Diagnosis of this problem:

• ssh myserver fails with "ssh Agent admitted failure"
• SSH_AUTH_SOCK= ssh myserver works just fine
• Conclusion: gnome-keyring can't deal with complex keys

As I just found a viable workaround for this bug, and it doesn't seem to be published anywhere (except the comment I just added to an Ubuntu bug), I'll put it here.

Workaround: Launch a new ssh-agent using the same socket as the one from gnome-keyring:

ssh-agent -a $SSH_AUTH_SOCK

This launches a new instance of ssh-agent (overwriting GNOME's less capable instance), so it won't have any keys in it (despite what seahorse says, since that's tied to the old agent). You'll have to add them via ssh-add as noted in the The key is not loaded section above.

You'll have to run this every time you log in (or manually add it to your startup scripts). If you want to preserve the old socket, run mv $SSH_AUTH_SOCK $SSH_AUTH_SOCK.broken first.

I have solved the problem simply by running this command on the local machine ( after generating the key):

$ ssh-add