Unable to add ssh key using ssh-add

Question

I have the following file in ~/.ssh that has my public SSH key:

• authorized_keys

I then run the following and add my passphrase and get no errors or warnings:

$ ssh-add ~/.ssh/authorized_keys

When I try to list the fingerprints I get:

$ ssh-add -l
> The agent has no identities.

When I re-run ssh-add and follow up with echo $? it returns an exit code of 1 meaning a failure occurred. This documentation says that code 1 means the error was caused by:

Invalid identification string of SSH-protocol

What does this mean exactly, and how do I resolve it?

Answer 1

ssh-add loads private keys, not public keys. From man ssh-add:

DESCRIPTION
     ssh-add adds private key identities to the authentication agent, ssh-agent(1).  When run without arguments, it adds
     the files ~/.ssh/id_rsa, ~/.ssh/id_dsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 and ~/.ssh/identity.  After loading a
     private key, ssh-add will try to load corresponding certificate information from the filename obtained by appending
     -cert.pub to the name of the private key file.  Alternative file names can be given on the command line.

Try to run it without arguments.