Archive for August, 2010

Setting up SSH-RSA with Putty - No Password necessary

Monday, August 2nd, 2010


SSH Protocol

SSH (Secure Shell) is a network protocol that provides secure access to a computer (mostly Unix based).  When you want to connect to a remote Unix server, SSH is one way of accessing the server. SSH is very powerful by combining both security of the data transmitted over network and accessibility to the remote system. SSH protocol works between two computers by a client-server architecture. When a client computer connects to the server, the server requires the client to authenticate itself. There are different ways a client can authenticate itself to the server. A typical authentication mode will be to enter a password when logging into a remote system. In this howto we can explore another mode of authentication in which server doesn’t require a password to be entered by the user. This mode will be very useful if you are connecting to a remote system frequently and dont want to enter the password everytime.

Before we see the steps, just to give a background on the components involved:


When you need to connect to a remote computer via SSH, that computer should have a SSH server running on it. All Unix based distributions ( Linux, Mac OSX etc.,) includes a ssh server. For Windows based systems Cygwin can be used as an SSH server.


Assuming your remote computer has an SSH server running on it, to connect to that computer you would need a SSH client on the local computer. On Unix based systems, SSH clients are available as command line utilities. For Windows based systems, putty is an excellent client. Check here for more information about putty.


  1. We start the configuration at the client windows computer. Download the latest version of Putty.exe and Puttygen.exe from here. Using the Puttygen tool we have to generate an authentication key. This key will serve as a substitute for the password that will be entered during login.
  2. Start puttygen.exe by double clicking on the executable. The following window opens up.
  3. Leave the default ‘SSH-2 RSA’ selection and click on the ‘Generate’ button. The following window opens. Move mouse randomly over the empty space below the progress bar to create some randomness in the generated key.
  4. Don’t enter any key phrase. Click on ‘Save private Key’ button. Click ‘Yes’ on the window asking for confirmation for saving the key without a password.
  5. Save the key file to a safe location (Let us assume you will be saving it as C:\Personal\SSHKey\Laptop.ppk).
  6. Now you can close the Puttygen window.
  7. Open the Laptop.ppk file in a notepad. Copy the four lines under ‘Public-Lines’ section to windows clipboard.
  8. Now open putty and connect to the remote system using the user id you want to use for future no password connections. (Let us assume you will connect to the remote machine using user name ‘ubu’. This time when you login, you have to provide the password at the prompt. Future logins won’t require this password.
  9. Under the logged in user’s  home directory there will be .ssh directory, under that create a new  file called authorized_keys using a text editor such as vi. (In our case the file will be created under /home/ubu/.ssh/authorized_keys).
  10. Type the word ” ssh-rsa ” (including  spaces on both ends of the word) and paste the 4 lines copied from step 7. Remove the carriage return at end of each line, merging four lines into one single line. Be careful not to delete any characters while doing that.  Final output should like the following window.
  11. Now we have configured SSH server, its time to test our setup.
  12. On the local system, open Putty, enter the ip address details of the remote system.
  13. Now from the left navigation, select Connection -> Data. Enter ‘ubu’ as ‘Auto-login username’ on the right panel.
  14. Again from the left navigation menu, scroll down and select Connection -> SSH -> Auth. Enter the path of the saved private key file ( In our case C:\Personal\SSHKey\Laptop.ppk ). Leave other defaults as such and press open button.
  15. Now the putty connects to the remote SSH server and there won’t be any password prompt here after :-).