It might happen that after updating your mac OS to Sierra, you no longer can login through SSH or use Git.
This is frustrating, but there is a fix. Run
You might see something like
Identity added: /Users/alex/.ssh/id_rsa (/Users/alex/.ssh/id_rsa)
Which means the key you had previously was added successfully to list of SSH known keys. But, it is known that after reboot everything will go away. So to fix that also, edit your bash_profile file
Or, if you have zsh, then
Add to the end
ssh-add -A 2>/dev/null;
Done, no more problems for SSH.