Gold Source CD Key Security

http://support.steampowered.com/cgi-bin/steampowered.cfg/php/enduser/std_adp.php?p_faqid=101

Steam uses a very good encryption system that takes your CD key and calculates a public key, which it compares to the public key that the authorization system generates. Your actual CD key is not sent over the Internet and can't be snooped by anyone unless they have access to your computer.

If you'd like to make your CD key particularly safe, you can use the registry editor to remove your key from the registry. Once you do this, your CD key will be stored only on the Steam Authorization Servers, not locally on your computer. However, if you no longer have your original CD jewel case with the CD key printed on it, it is VERY IMPORTANT that you copy and save this key before removing it from the registry, otherwise you risk having to buy a new copy of the game.

1. Click the Start button, then click "Run" and type "regedit". Hit Return.
2. Browse to HKEY_CURRENT_USER\Software\Valve\Half-Life\Settings and look for "Key."
3. Copy that number down.
4. To remove the key, double-click on "Key" and delete the numbers in the window.
5. Exit from regedit.