I copy/paste the content of the following URL here to ease reading
http://superuser.com/questions/1046556/cant-add-ssh-keys-to-gnupg-agent-anymore.
I had several old ssh keys in my ~/.gnupg/sshcontrol so I removed all lines in
this file and rebooted my computer. Now, I can't add ssh keys to gpg agent anymore:
$ cat ~/.gnupg/gpg-agent.conf
enable-ssh-support
$ gpg-connect-agent --verbose /bye
gpg-connect-agent: closing connection to agent
$ gpg-connect-agent updatestartuptty /bye
OK
$ ssh-add -l
The agent has no identities.
$ ssh-add ~/.ssh/id_rsa
Enter passphrase for /home/cassou/.ssh/id_rsa:
Identity added: /home/cassou/.ssh/id_rsa (/home/cassou/.ssh/id_rsa)
$ ssh-add -l
The agent has no identities.