This guide assumes that you are working with an Unix based (including OS X) operating system.
If you are using windows, you might want to have a look at putty a terminal program and WinSCP a secure copy client.

Conditions for access to the AeroCom data server:

• A short project description of the planned analysis is send to AeroCom contact (michael.schulz@met.no)
• An account is opened upon sending a request for an account to (Michael) michael.schulz@met.no and (Anna) annac@met.no and (Jan) jan.griesfeller@met.no
• The project description is made available to AeroCom participants via this wiki page
• Results from analysis are reported to AeroCom workshops
• Publication-Coauthorship is offered to model and data author. See data policy

At first you will get a username and a password for this wiki. To get access to the user server aerocom-users.met.no, you have to send us a ssh public key since password authentication is disabled for security reasons on the user server. You can connect to the user server via ssh, but also use scp and rsync for data transfers. For more details on AeroCom database, please read about data submission procedure.

First, we need to check for existing ssh keys on your computer. Open up Terminal and run:

$ cd ~/.ssh
$ ls
# Lists the files in your .ssh directory

Check the directory listing to see if you have a file named Id_rsa.pub. If you don't, go to step 2. If you already have an existing keypair, skip to step 3.

To generate a new SSH key, enter the code below. We want the default settings so when asked to enter a file in which to save the key, just press enter.

 $ ssh-keygen -t rsa -C "my comment"
Generating public/private rsa key pair.
...

Enter the path to the file that will hold the key: By default, the file name $HOME/.ssh/id_rsa, which represents an RSA v2 key, appears in parentheses.

 Enter file in which to save the key (/home/user/.ssh/id_rsa): <return> 

Enter a passphrase for using your key: The passphrase you enter will be used for encrypting your private key. A good passphrase should be alphanumeric having 10-30 character length. You can also use a null passphrase however this can cause a security loophole.

Enter passphrase (empty for no passphrase): <Type the passphrase> 

Re-enter the passphrase to confirm it: Type your passphrase once again to confirm it.

Enter same passphrase again: <Type the passphrase>
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:
0b:fa:3c:b8:73:71:bf:58:57:eb:2a:2b:8c:2f:4e:37 user@myLocalHost

In the folder ~/.ssh you will find file(s) ending with .pub. Please send us the one you just created e.g. Id_rsa.pub. And no other file

This page was partly stolen from wikipedia and github.
If you want to know how key authentication works, please read this article about public key cryptography.