SSH server local key

SSH server configuration - Authentication with RSA keys

Introduction

If you’d like to increase the authentication process you can use authentication by private/public key.

• Generate new private / public keys on your own computer
• Put the public key on the remote SSH server
• Only the person with the private key can be authenticate on the server

For instance, this is how hosting company such as OVH can log on your system.

Security improvement: remove password authentication

When the key authentication is working you can remove the default access by login / password. Then, only people with a valid private/public key pair can log in.

That way, there is no way for brute-force attacks to be successful, so your system is more secure.

Declare the public key on the server

You have to:

• log in to your SSH server with the user that’s gonna use this key
• Go to user's home directory
• Create a .ssh folder (if there was none before).

cd ~
mkdir .ssh
cd .ssh

Add the new public key to the list of allowed keys:

vim authorized_keys

Prefix your key with:

• RSA: ssh-rsa
• DSA: ssh-dss

Then paste the public key in one line - the public key mustn't be change or separated in 2 lines!

# Example: 
ssh-rsa AAAAB3NzaC1yc2EA[...]Lg5whU0zMuYE5IZu8ZudnP6ds= myname@example.com
ssh-dss AAAAB3NzaC1yc2EA[...]Lg5whU0zMuYE5IZu8ZudnP6ds= myname@example.com

Adjust file rights, the authorized_keys file must be write/readable only by that user

chmod 640 authorized_keys
cd ..
chmod 700 .ssh

Configuration changes

vim /etc/ssh/sshd_config

Allow empty password

PermitEmptyPassword yes	        # allow empty password in favor of RSA keys

Do not allow password authentication

Once you've checked that the key authentication is working you can disable the standard authentication with password. :) With that option enabled all brute-force attacks will failed.

!!BE CAREFUL !!

Once that setting is enable there is no rolling back from it. You might get kicked for good from your own server.

PermitEmptyPassword yes	                # That must be set to YES
PasswordAuthentication no	        # Do not allow standard login + password anymore. Only key authentication is allowed.

Restart SSH server

/etc/init.d/ssh restart

References

Source:

• Public / private key theory: http://en.wikipedia.org/wiki/Public_Key_Cryptography
• http://www.howtoforge.com/ssh_key_based_logins_putty