39.6 SSH Authentication Mechanisms

Now the actual authentication takes place, which, in its simplest form, consists of entering a password as mentioned above. The goal of SSH was to introduce a secure software that is also easy to use. Because it is meant to replace rsh and rlogin, SSH must also be able to provide an authentication method appropriate for daily use. SSH accomplishes this by way of another key pair, which is generated by the user. The SSH package provides a helper program for this: ssh-keygen. After entering ssh-keygen -t rsa or ssh-keygen -t dsa, the key pair is generated and you are prompted for the base filename in which to store the keys.

Confirm the default setting and answer the request for a passphrase. Even if the software suggests an empty passphrase, a text from 10 to 30 characters is recommended for the procedure described here. Do not use short and simple words or phrases. Confirm by repeating the passphrase. Subsequently, you will see where the private and public keys are stored, in this example, the files id_rsa and id_rsa.pub.

Use ssh-keygen -p -t rsa or ssh-keygen -p -t dsa to change your old passphrase. Copy the public key component (id_rsa.pub in the example) to the remote machine and save it to ~/.ssh/authorized_keys. You will be asked to authenticate yourself with your passphrase the next time you establish a connection. If this does not occur, verify the location and contents of these files.

In the long run, this procedure is more troublesome than giving your password each time. Therefore, the SSH package provides another tool, ssh-agent, which retains the private keys for the duration of an X session. The entire X session is started as a child process of ssh-agent. The easiest way to do this is to set the variable usessh at the beginning of the .xsession file to yes and log in via a display manager, such as KDM or XDM. Alternatively, enter ssh-agent startx.

Now you can use ssh or scp as usual. If you have distributed your public key as described above, you are no longer prompted for your password. Take care of terminating your X session or locking it with a password protection application, such as xlock.

All the relevant changes that resulted from the introduction of version 2 of the SSH protocol are also documented in the file /usr/share/doc/packages/openssh/README.SuSE.