Install SSH server

apt-get install ssh

Running ssh-keygen as root will install the host keys, ssh-keygen as <user> the keypair into directory ~/.ssh. The private key file, e.g. ~/.ssh/id_rsa needs to be copied to all clients which need access to the server, the public key file, e.g. ~/.ssh/id_rsa.pub, needs to be added to the authorized_keys file of the server to allow access to the server. In addition, you should adjust the access rights for the different files and directories. As <user>, enter the following commands

On the client:

ssh-keygen
ssh -vvv -p <port#> <servername> // check the ssh connection
sftp -oPort=<port#> <servername> // transfer the public key file
sftp > put client_id_rsa.pub

On the server:

sudo cat client_id_rsa.pub >> ~/.ssh/authorized_keys

Make sure permissions and ownership are set correctly:

sudo chmod 700 ~/.ssh
sudo chmod 600 ~/.ssh/*
sudo chown -R user:user ~/.ssh

Check the ssh access:

ssh <servername> whoami // needs to return the username under which the ssh access was established, e.g. user1
ssh -l root <servername> whoami // needs to return "root", as this establishes the ssh access as root, not as user1
ssh -vvv -p <port#> -l root <servername> whoami // same as above, but use different port number with full debug verbose output

Important

You might need to include the user in sshd.conf to authorize access.

SSH access with keypair