Paranoid Penguin - Securing Your WLAN with WPA and FreeRADIUS, Part II
For EAP-TLS, you need at least two certificates besides your CA certificate, a server certificate for your FreeRADIUS server and one client certificate for each wireless client on your network. Creating certificates is a three-step process:
Generate a signing request, that is, an unsigned certificate.
Sign the signing request with your CA key.
Copy the signed certificate to the host on which it will be used.
Let's start by creating a server certificate signing request using OpenSSL's req command:
$ openssl req -new -nodes -keyout server_key.pem -out server_req.pem -days 730 -config ./openssl.cnf
This command creates the files server_req.pem, which contains the actual request—an unsigned certificate—and server_key.pem, its passphrase-less private key. First, though, you are prompted for your organization's Country Code, State and so on, much of which can use the default values you tweaked in openssl.conf. Pay special attention, however, to Common Name. When prompted for this, type the fully qualified domain name of your server, for example, server.wiremonkeys.org.
Next, let's use our CA key to sign the request by using OpenSSL's ca command:
$ openssl ca -config ./openssl.cnf \ -policy policy_anything -out server_cert.pem \ -extensions xpserver_ext -extfile ./xpextensions \ -infiles ./server_req.pem
This command reads the file server_req.pem and, after prompting for your CA key's passphrase, saves a signed version of it plus its corresponding private key to the file server_cert.pem. Notice the -extensions and -extfile options—this is why earlier we created the file xpextensions.
Open your signed certificate with the text editor of your choice and delete everything before the line -----BEGIN CERTIFICATE-----. Concatenate it and your key into a single file, like this:
$ cat server_key.pem server.cert.pem > \ server_keycert.pem
Now we've got a server certificate with a key that we can copy over to our FreeRADIUS server. Its private key isn't password-protected, however, so be sure to delete any extraneous copies after you've got it in place.
Now we need to create a client certificate signing request. The OpenSSL command to do this is similar to that used to create server certificates:
$ openssl req -new -keyout client_key.pem \ -out client_req.pem -days 730 -config ./openssl.cnf
As you can see, we're writing our signing request and key to the files client_req.pem and client_key, respectively. Unlike with the server signing requests, however, we're omitting the -nodes option. Therefore, when you run this command, you are prompted for a passphrase with which the certificate's private key can be encrypted.
Next we sign the client certificate's signing request:
$ openssl ca -config ./openssl.cnf \ -policy policy_anything -out client_cert.pem \ -extensions xpclient_ext -extfile ./xpextensions \ -infiles ./client_req.pem
Again, this is similar to the equivalent command for our server, except this time the -extensions command references a different entry in xpextensions. Also, if your clients run Linux, you should delete the extraneous stuff in the certificate, like you did with server_cert.pem. You then either can leave the certificate and key files separate or concatenate them. From there, copy your client certificate file(s) to your Linux client system.
If your certificate is to be used by a Windows XP client, you have one more step to take. You need to convert the certificate file(s) to a PKCS12-format file, with this command:
openssl pkcs12 -export -in client_cert.pem \ -inkey client_key.pem -out client_cert.p12 -clcerts
You are prompted for client_key.pem's passphrase and then for a new passphrase for the new file; you can use the same password as before if you like. You may be tempted simply to press Enter instead, especially given that the WPA supplicant in Windows XP works only when you store its certificates without passphrases. It's very, very bad practice, however, to move private keys around networks unprotected, so I strongly recommend that you not remove the passphrase until after this file is copied safely over to your Windows XP client.
Lest you be tempted to take this opportunity to bash Microsoft, I must note that both Xsupplicant and wpa_supplicant on Linux require you to either use a blank passphrase or store the passphrase in clear text in a configuration file. This is contrary to good certificate-handling wisdom. I hope we some day see WPA supplicants intelligent enough to prompt the user for its certificate passphrase on startup.
The resulting file, in this example client_cert.p12, contains both your signed certificate and its private key. Copy it to your Windows XP client system.
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