ssh-keygen

ssh-keygen         # generates rsa key pair by default (-t rsa optional)

id_rsa (private key) access

With a user's id_rsa: copy it locally, fix perms, log in:

chmod 600 id_rsa
ssh -i id_rsa user@ip

sshpass

Quick one-liner login with password:

sshpass -p 'password' ssh user@IP

authorized_keys

Confirm the public key is present in ~/.ssh/authorized_keys on the target, compare against the matching id_rsa.pub.