SSH Public Key Log In

Using Public Key Cryptography and SSH Tools for Smooth Log In

To log in to an SSH server without the needing to enter a password each time can be done in different ways. One way is to to configure your server to allow certain users (or certain users from certain hosts/networks/ip addresses [or ranges]) but another preferred method I am going to demonstrate involves encryption keys and the Diffi-Helmen Key Exchange.

(See also PKI)

Primarily one needs an encryption key to verify his or her identity. There are different algorithms that exist by which a key can be created (I won’t go into them here), but a popular and often recommended one is the RSA algorithm which I will use in my example.

On a POSIX (Unix/Unix Like) system with an SSH server in a typical configuration, this is the process to create an rsa encryption key:

> ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/user/.ssh/id_rsa):
Created directory '/home/user/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/user/.ssh/id_rsa.
Your public key has been saved in /home/user/.ssh/id_rsa.pub.
The key fingerprint is:
a9:9d:f8:df:63:64:68:11:00:13:f1:bb:2d:76:5d:5b
user@gnu-linux

Once our key has been created (client side), we need to copy our public key into the proper directory on the SSH server. DO NOT DISTRIBUTE YOUR PRIVATE KEY, KEEP IT SAFE! When performing public key authentication, by default the server checks in the users home directory ($HOME) for a directory named ‘.ssh’. Inside that directory the server is looking for a file named ‘authorized_keys’. This file is where we place a copy of our public key, on a single new line.

The public key can be distributed automatically with the ssh-copy-id utility.

> ssh-copy-id
Usage: /usr/bin/ssh-copy-id [-h|-?|-n] [-i [identity_file]] [-p port] [[-o ] ...] [user@]hostname
>
> ssh-copy-id user@remote.host

To do it manually:

---In the home directory on the client machine---
> cat .ssh/id_rsa.pub > pubkey
> scp pubkey user@remote.host:
---output omitted---
> ssh user@remote.host
---output omitted---
---On server in home directory---
> cat pubkey >> .ssh/authorized_keys
> exit

Okay. This takes care of half of the puzzle. This will allow our server to use our rsa key to verify our identity, but (assuming a passphrase was entered when prompted above) we will still need to enter the passphrase for this key each time we attempt to log into the server. What we can do to get around this is utilize the ssh-agent which is part of the SSH client.

Our key can be added to the ssh-agent one time and future requests for access to SSH servers will be handled by the agent. When our rsa key was created, we opted to store it in the default location. The ssh-agent will likewise by default look in the default storage location for our encryption keys. So all one needs to do to load their key into the ssh-agent is issue the ssh-add command – Windows users can setup a profile in Putty to accomplish the same result.

(putty how to)

> ssh-add
Enter passphrase for /home/user/.ssh/id_rsa: 
Identity added: /home/user/.ssh/id_rsa (/home/user/.ssh/id_rsa)

Now with our key loaded into the ssh-agent, we can access any SSH server where our public key is authorized without needing to enter our passphrase.