Public Key Authentication

From Luke Jackson

OpenSSH is a FREE version of the SSH connectivity tools that technical users of the Internet rely on. For more information on OpenSSH check out their website OpenSSH

Contents 1 Introduction 2 Requirements 3 Generate Key Pair (Linux) 4 Generate Key Pair (Windows) 5 Configure SSHD 6 Configure User 7 Configure Client (Linux) 8 Configure Client (Windows)

Introduction

Users of telnet, rlogin, and ftp may not realize that their password is transmitted across the Internet unencrypted, but it is. OpenSSH encrypts all traffic (including passwords) to effectively eliminate eavesdropping, connection hijacking, and other attacks. Additionally, OpenSSH provides secure tunneling capabilities and several authentication methods, and supports all SSH protocol versions.

Another great feature about OpenSSH is the Public Key Authentication mechanism. With this feature enabled you are able to securely authenticate with servers with out typing in a password.

Requirements

• Linux/Unix OS
• openssh
• openssh-server
• openssh-clients

This tutorial will assume that you already have the above installed and operating correctly.

Generate Key Pair (Linux)

First I will walk you through creating a key pair on a Linux/Unix OS.

Connect to the server of which you would like to enable Public Key Authentication on. Insure you are in the home directory of the account you use to logon to the server. To confirm this your prompt should look something like this [root@server.ext ~]#.

Locate the command ssh-keygen:

Usage: ssh-keygen [options]
Options:
  -b bits     Number of bits in the key to create.
  -c          Change comment in private and public key files.
  -e          Convert OpenSSH to IETF SECSH key file.
  -f filename Filename of the key file.
  -g          Use generic DNS resource record format.
  -i          Convert IETF SECSH to OpenSSH key file.
  -l          Show fingerprint of key file.
  -p          Change passphrase of private key file.
  -q          Quiet.
  -y          Read private key file and print public key.
  -t type     Specify type of key to create.
  -B          Show bubblebabble digest of key file.
  -C comment  Provide new comment.
  -N phrase   Provide new passphrase.
  -P phrase   Provide old passphrase.
  -r hostname Print DNS resource record.
  -G file     Generate candidates for DH-GEX moduli
  -T file     Screen candidates for DH-GEX moduli

In this tutorial I will use a key type of RSA you are welcome to explore the different types but for my needs RSA is more than capable.

Create your key pair by executing the following command:

ssh-keygen -f <filename> -t rsa

You will be prompted to enter a passphrase THIS IS IMPORTANT!!! If you forget your passphrase the keypair is lost forever please choose a passphrase you will not forget!

[root@server.ext ~]# ssh-keygen -f <filename> -t rsa
Generating public/private rsa key pair.
Enter passphrase (empty for no passphrase): ***********
Enter same passphrase again: ***********
Your identification has been saved in <filename>.
Your public key has been saved in <filename>.pub.
The key fingerprint is:
**:**:**:**:**:**:**:**:**:**:**:**:**:**:**:** root@server.ext

You should now have 2 files in your current directory:

• <filename> - Private Key
• <filename>.pub - Public Key

Generate Key Pair (Windows)

It is also possible to do this on a Windows compatible OS using Putty.

Configure SSHD

Open the file /etc/ssh/sshd_config and browse to the section displayed below:

# Authentication:

#LoginGraceTime 2m
#PermitRootLogin yes
#StrictModes yes
#MaxAuthTries 6

RSAAuthentication yes
PubkeyAuthentication yes
AuthorizedKeysFile      .ssh/authorized_keys

Ensure that the last 3 parameters are set like above and then save your changes.

Configure User

Since root is the most powerful user it will be used in this tutorial. if you wish to enable authentication for another user follow these steps and replace root with the desired username.

Return back to your home directory and check to see if you have a file named authorized_keys in sub-directory called .ssh.

If you don't no problem just create them like so:

mkdir .ssh
cd .ssh
touch authorized_keys

Now add the public key to the authorized_keys file. This may seem easy but it is the most common place for error. If you miss copy one character it will not authenticate you.

To prevent any errros cat the contents of your public key (<filename>.pub) to the authorized_keys file.

cat ../<filename>.pub > authorized_keys

Should you be in a situation where you have to copy and paste the key from one server to another insure that the key file looks identical to the original. This will save you many hours of troubleshooting as there is no log entry telling you the public key was copied incorrectly.

Ensure the ownership is set to root and the permissions are 644 with the commands below:

chown -R root:root ~/.ssh
chmod -R 644 ~/.ssh

Configure Client (Linux)

Open a shell and ensure you are in your home directory.

Again if the (.ssh) directory does not exist create it using the command below:

mkdir .ssh

Copy the private key to the client using the commands below:

cd .ssh
scp root@server.ext:.ssh/<filename> identity

Ensure the ownership is set to root and the permissions are 644 with the commands below:

chown -R root:root ~/.ssh
chmod -R 644 ~/.ssh

Find the command ssh-agent:

Usage: ssh-agent [options] [command [args ...]]
Options:
  -c          Generate C-shell commands on stdout.
  -s          Generate Bourne shell commands on stdout.
  -k          Kill the current agent.
  -d          Debug mode.
  -a socket   Bind agent socket to given name.
  -t life     Default identity lifetime (seconds).

You will need to bind the ssh-agent to your current shell with the command below:

ssh-agent $SHELL

Next locate the command ssh-add:

Usage: ssh-add [options]
Options:
  -l          List fingerprints of all identities.
  -L          List public key parameters of all identities.
  -d          Delete identity.
  -D          Delete all identities.
  -x          Lock agent.
  -X          Unlock agent.
  -t life     Set lifetime (in seconds) when adding identities.
  -c          Require confirmation to sign using identities

By default ssh-add will look in the identity file for you private keys.

Add you private key to the agent with the command below:

ssh-add

You should be prompted for your passphrase similar to the output below:

[root@client.ext .ssh]# ssh-add 
Enter passphrase for /root/.ssh/identity: 
Identity added: /root/.ssh/identity (/root/.ssh/identity)

Now you are ready to connect to the remote server with your private key.

[root@client.ext .ssh]# ssh root@server.ext
Last login: Sun Oct  8 07:45:16 2006 from 127.0.0.1
[root@server.ext ~]#

Configure Client (Windows)

It is also possible to do this on a Windows compatible OS using PuTTY and Pagent.