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SSH Public-Private Keys

27th December 2003 Ian Wells

Introduction

This is an introduction to using Public-Private Keys as applied to SSH. It has been written in response to questions regarding my CVS How-To.

It currently covers the first three steps needed to use Public-Private keys

It also has sections on

The following has been tested using a Mitel SME Server, and Windows XP& RedHat 8.0 clients.

Some sections use PuTTY to connect to the server, it is a free implementation of SSH for Win32 platforms, with extensive online documentation. See Tom Carroll's HowTo for details of how to use PuTTY.

Generate Public-Private Key Pairs

This section describes three ways to generate the public-private key pairs, and specifically a SSH protocol 2 RSA key pair.

  1. On Windows using PuTTY
  2. On SME Server using OpenSSH
  3. On RedHat 8.0 using OpenSSH

On Windows using PuTTY

PuTTYgen PuTTYgen can be used to generate the keys, which is described in detail in Chapter 8.2

  1. Select SSH2 RSA.
  2. Press Generate, and generate the key by moving the mouse.
  3. Change the Key comment field to something meaningful such as your name.
  4. Enter a passphrase which is used to encrypt the key.
  5. Save the public key.
  6. Save the private key.
  7. To save the public key in the OpenSSH2 format.
    Select all of the text as shown, and copy to the clipboard
    (CTRL-C, or CTRL-Insert or Right-Click Copy).
    Then paste the OpenSSH2 version to a file, eg putty.pub.

On SME Server

It is possible to use the SME Server itself to generate the public-private key pairs. This is done by using ssh-keygen.
By default the pair is written to $HOME/.ssh/id_rsa and $HOME/.ssh/id_rsa.pub

bash-2.05a$ ssh-keygen -t rsa

Generating public/private rsa key pair.
Enter file in which to save the key (/home/e-smith/files/users/dummy/.ssh/id_rsa): [Enter to accept default]
Created directory '/home/e-smith/files/users/dummy/.ssh'.
Enter passphrase (empty for no passphrase): [Your passphrase]
Enter same passphrase again: [Your passphrase]
Your identification has been saved in /home/e-smith/files/users/dummy/.ssh/id_rsa.
Your public key has been saved in /home/e-smith/files/users/dummy/.ssh/id_rsa.pub.
The key fingerprint is:
aa:bb:cc:dd:ee:ff:aa:bb:cc:dd:ee:ff:aa:bb:cc:dd dummy@gatekeeper

On RedHat 8.0

To generate the public-private key pair on a RedHat 8.0 box is identical to the SME Server, as both use OpenSSH.
In this example I have specified the comment field. 

$ ssh-keygen -t rsa -C dummy@rh8

Generating public/private rsa key pair.
Enter file in which to save the key (/home/dummy/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):   
Enter same passphrase again:                  
Your identification has been saved in /home/dummy/.ssh/id_rsa.
Your public key has been saved in /home/dummy/.ssh/id_rsa.pub.
The key fingerprint is:
aa:bb:cc:dd:ee:ff:aa:bb:cc:dd:ee:ff:aa:bb:cc:dd dummy@rh8

Installing the Public Key onto the Server

To make use of the Public-Private Key pair the Public keys must be put onto the server that will be accessed. The Public Key file and its contents do not need to be kept secret.

The Public Key needs to be saved in the file $HOME/.ssh/authorized_keys . For the user dummy on an SME Server it would be:

/home/e-smith/files/users/dummy/.ssh/authorized_keys

The directory .ssh should have permissions 700 and the file authorized_keys should have permissions either 644 or 600. If the directory does not exist it can be created as follows, which presumes that you are logged in as yourself, not root.

bash-2.05a$ cd
bash-2.05a$ mkdir .ssh
bash-2.05a$ chmod 700 .ssh
bash-2.05a$ ls -al 
drwx------    2 dummy     dummy         4096 Feb 18 11:23 .ssh

The authorized_keys file can store multiple public keys, one on each line, so the following examples always append to the end of the file.

Public key from OpenSSH

Copying the Public Key generated by ssh-keygen, $HOME/.ssh/id_rsa.pub
This example presumes that the key was generated on the server, or has been copied into the $HOME/.ssh directory.

bash-2.05a$ cat id_rsa.pub >> authorized_keys
bash-2.05a$ ls -l
-rw-r--r--    1 dummy     dummy          225 Feb 18 11:26 authorized_keys

Public key from PuTTYgen

There are two main ways of doing this, they each require that the "Public key for pasting into authorized_keys file" is copied to the clipboard (see Step 7 on the figure above). For more details read the PuTTY manual.

One method is to paste the key into a file (putty.pub in the example below).
Then copy this file into the $HOME/.ssh directory on the server. This file can then be appended to the authorized_keys file. 

bash-2.05a$ cat putty.pub >> authorized_keys

The other method is simpler but more complicated to explain.

Installing the Private Key onto the Clients

The Private Key needs to be accessible from the clients that you use. The Private Key file should be kept secure, and should be protected by a pass phrase.

Private Key files cannot be directly shared between PuTTY and OpenSSH, however PuTTYgen can convert the files.

Converting the OpenSSH Private Key to work with PuTTY

  1. Either press Load, or from the Conversions menu select Import Key.
  2. Change the Files of Type to All Files
  3. Select the Private Key, id_rsa, that was created with ssh-keygen.
  4. Enter the passphrase to load the key.
  5. The Key comment may be changed at this time.
  6. Save the private key.

Converting the PuTTY Private Key to work with OpenSSH

  1. Press Load and select the Private Key that was created with PuTTYgen.
  2. Enter the passphrase to load the key.
  3. From the Conversions menu select export OpenSSH key
  4. Save the private key.

Using public keys for SSH authentication

In these examples the server is the SME Server, and the clients will be Windows and RedHat 8.0

In use with PuTTY

To start using PuTTY with SME Server see Tom Carroll's HowTo

To use the Private Key go to the PuTTY Configuration and select the Connection - SSH - Auth.
Then in the Private key file for authentication: enter the path to the private key file, or find it using Browse.

When starting the PuTTY terminal the following is shown. You will need to enter the passphrase, if one was set.

Using username "dummy".
Authenticating with public key "dummy@homepc"      Using the private key generated on my windows machine
Passphrase for key "dummy@homepc":                
Welcome to the Mitel Networks SME Server.

Using username "dummy".
Authenticating with public key "dummy@gatekeeper" Using the private key generated on my SME Server
Passphrase for key "dummy@gatekeeper":
Welcome to the Mitel Networks SME Server.

In use with OpenSSH

This brief example shows the start of a SSH session where the username is specified using the -l option.

[dummy@homepc dummy]$ ssh -l dummy gatekeeper
Enter passphrase for key '/home/dummy/.ssh/id_rsa':
Last login: Tue Feb 18 11:38:43 2003 from somewhere
Welcome to the Mitel Networks SME Server.
bash-2.05a$

Further Information

OpenSSH Files

Files used by OpenSSH, full details can be found from the ssh-keygen (1) and sshd (8) manual pages.

$HOME/.ssh/id_rsa
This is the private key, it contains the protocol version 2 RSA authentication identity of the user. This file should not be readable by anyone but the user.

$HOME/.ssh/id_rsa.pub
Contains the protocol version 2 RSA public key for authentication. The contents of this file should be added to the authorized_keys file on all machines where the user wishes to log in using public key authentication. There is no need to keep the contents of this file secret.

$HOME/.ssh/authorized_keys
Lists the public keys that can be used to log into the user's account. This file must be readable by root. It is recommended that it not be accessible by others.

Each line of the file contains one key (empty lines and lines starting with a `#' are ignored as comments). Each protocol version 2 public key consists of: options, key-type, base64 encoded key, comment. The comment field is not used for anything (but may be convenient for the user to identify the key).

Changing the passphrase

OpenSSH
You can change the passphrase at any time by using the -p option of ssh-keygen.

bash-2.05a$ ssh-keygen -p
Enter file in which the key is (/home/e-smith/files/users/dummy/.ssh/id_rsa): [Enter to accept default key file]
Enter old passphrase: [Old passphrase]
Key has comment '/home/e-smith/files/users/dummy/.ssh/id_rsa'
Enter new passphrase (empty for no passphrase): [New passphrase]
Enter same passphrase again: [New passphrase]
Your identification has been saved with the new passphrase.

PuTTY
To change the passphrase of a PuTTY key use PuTTYgen.

  1. Load the Private Key
  2. Type the new passphrase into Key Passphrase and Confirm Passphrase
  3. Save the Private Key

Copyright © 2003 Ian Wells : The original can be found from http://www.wellsi.com/sme : Please send additions & corrections to me.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Text and no Back-Cover Text. A copy of the GNU Free Documentation License is available from the Free Software Foundation at http://www.fsf.org/copyleft/fdl.html.