Enable pem key based ssh linux

On the remote machine (machine to be sshed) execute:

ssh-keygen -t rsa
Press <Enter> key twice. One for public/private key file path and other for passphrase.

If required you can enter the passphrase.

By default, two keys will be created as follows:
$HOME/.ssh/id_rsa (private) and $HOME/.ssh/id_rsa.pub (public)

Create a pem key with the following command:
openssl rsa -in ~/.ssh/id_rsa -outform pem > id_rsa.pem

Edit the /etc/ssh/sshd_config file change as the following:

RSAAuthentication yes

PubkeyAuthentication yes

then restart ssh service : service sshd restart

cat ~/.ssh/id_rsa.pub >> ~/.ssh/authorized_keys

Copy the pem file to the machine from where you want to ssh the remote machine.
chmod 400 id_rsa.pem

Then ssh with:
ssh -i id_rsa.pem <remote m/c IP>

Create public key from .pem

Execute the following command:
ssh-keygen -y -f /tmp/key.pem > key.pub