Hack 46 Distribute Your CA to Clients

Be sure all of your clients trust your new Certificate Authority.

Once you have created a Certificate Authority (CA) [Hack #45], any certificates that are signed by your CA will be trusted by any program that trusts your CA. To establish this trust, you need to distribute your CA's certificate to each program that needs to trust it. This could include email programs, IPSec installations, or web browsers.

Since SSL uses public key cryptography, there is no need to keep the certificate a secret. You can simply install it on a web server and download it to your clients over plain old HTTP. While the instructions for installing a CA cert are different for every program, this hack will show you a quick and easy way to install your CA on web browsers.

There are two possible formats that browsers will accept for new CA certs: pem and der. You can generate a der from your existing pem with a single openssl command:

$ openssl x509 -in demoCA/cacert.pem -outform DER -out cacert.der

Also, add the following line to the conf/mime.types file in your Apache installation:

application/x-x509-ca-cert      der pem crt

Now restart Apache for the change to take effect. You should now be able to place both the cacert.der and demoCA/cacert.pem files anywhere on your web server and have clients install the new cert by simply clicking on either link.

Early versions of Netscape expected pem format, but recent versions will accept either. Internet Explorer is just the opposite (early IE would accept only der format, but recent versions will take both). Other browsers will generally accept either format.

You will get a dialog box in your browser when downloading the new Certificate Authority, asking if you'd like to continue. Accept the certificate, and that's all there is to it. Now SSL certs that are signed by your CA will be accepted without warning the user.

Keep in mind that Certificate Authorities aren't to be taken lightly. If you accept a new CA in your browser, you had better trust it completely a mischievous CA manager could sign all sorts of certs that you should never trust, but your browser would never complain (since you claimed to trust the CA when you imported it). Be very careful about who you extend your trust to when using SSL-enabled browsers. It's worth looking around in the CA cache that ships with your browser to see exactly who you trust by default.

For example, did you know that AOL/Time Warner has its own CA? How about GTE? Or VISA? CA certs for all of these entities (and many others) ship with Netscape 7.0 for Linux, and are all trusted authorities for web sites, email, and application add-ons by default. Keep this in mind when browsing to SSL-enabled sites: if any of the default authorities have signed online content, your browser will trust it without requiring operator acknowledgment.

If you value your browser's security (and, by extension, the security of your client machine), then make it a point to review your trusted CA relationships.

Rob Flickenger (Linux Server Hacks)