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Generating an SSL Certificate with Apache+mod_ssl


Introduction

This document is intended to be a quick guide to generating and installing an SSL certificate on an Apache web server with the mod_ssl module. While this is not an overly difficult process, it does involve running several long commands with numerous options. This document should be all that you need to walk you through the process of generating the certificate and installing it in your web server.

This document does not attempt to discuss compiling or installing Apache and mod_ssl. For detailed instructions on that topic, please see "Building Apache with mod_ssl and Other Modules". This document also does not attempt to discuss detailed configuration of SSL hosts in Apache. Details will be provided on setting up a basic SSL virtual host that should work in nearly all standard circumstances. Ralf Engelschall, the author of mod_ssl, maintains excellent documentation at http://www.modssl.org/docs. For information on more advanced configurations or special situations, please refer to the full documentation. In addition, the openssl toolkit provides fairly extensive man pages, which are also available in HTML format at http://www.openssl.org/docs/


Brief SSL Primer

This section will serve as a very brief introduction to SSL, the Secure Socket Layer. Cryptography is a very extensive topic which literally fills volumes of texts. The following is an extremely simplified view of how SSL is implemented and what part the certificate plays in the entire process. There may be some small inaccuracies in an effort to present the information in the easiest possible format.

Normal web traffic is sent unencrypted over the Internet. That is, anyone with access to the right tools can snoop all of that traffic. Obviously, this can lead to problems, especially where security and privacy is necessary, such as in credit card data and bank transactions. The Secure Socket Layer is used to encrypt the data stream between the web server and the web client (the browser).

SSL makes use of what is known as asymmetric cryptography, commonly referred to as public key cryptography (PKI). With public key cryptography, two keys are created, one public, one private. Anything encrypted with either key can only be decrypted with its corresponding key. Thus if a message or data stream were encrypted with the server's private key, it can be decrypted only using its corresponding public key, ensuring that the data only could have come from the server.

If SSL utilizes public key cryptography to encrypt the data stream traveling over the Internet, why is a certificate necessary? The technical answer to that question is that a certificate is not really necessary--the data is secure and cannot easily be decrypted by a third party. However, certificates do serve a crucial role in the communication process. The certificate, signed by a trusted Certificate Authority (CA), ensures that the certificate holder is really who he claims to be. Without a trusted signed certificate, your data may be encrypted, however, the party you are communicating with may not be whom you think. Without certificates, impersonation attacks would be much more common.


Generating a Private Key and CSR

The openssl toolkit is used to generate an RSA Private Key and CSR (Certificate Signing Request). It can also be used to generate self-signed certificates which can be used for testing purposes or internal usage. The utility used to do all of these tasks is known simply as openssl. It should be installed in the /usr/local/ssl/bin directory. You may want to add this directory to your PATH, or copy or link the openssl utility to a directory that is already in your PATH so that you do not have to type the full path to the executable. The examples below will assume that openssl is in a location that is accessible to you without using the full path to the command.

The first step is to create your RSA Private Key. This key is a 1024 bit RSA key which is encrypted using Triple-DES and stored in a PEM format so that it is readable as ASCII text. We will use several files as random seed enhancers which will help to make the key more secure. Text files that have been compressed with a utility such as gzip are good choices. The key is generated using the following command, where file1:file2:etc represents the random compressed files.

$ openssl genrsa -des3 -rand file1:file2:file3:file4:file5 -out server.key 1024

The command will prompt you for a pass-phrase and then store the key in the file server.key. It is critical that the pass-phrase be secure and not forgotten. If either the key is lost, or the pass-phrase is forgotten, the certificate will be useless! It cannot be stressed enough how important the private key is to the certificate. If the private key and pass-phrase are compromised, the certificate will have to be revoked, costing you the price of the certificate all over again if you have paid an authority for the certificate. It may be a wise idea to back this file up to secure media, such as tape or diskette.

One unfortunate side-effect of the pass-phrased private key is that Apache will ask for the pass-phrase each time the web server is started. Obviously this is not necessarily convenient as someone will not always be around to type in the pass-phrase, such as after a reboot or crash. mod_ssl includes the ability to use an external program in place of the built-in pass-phrase dialog, however, this is not necessarily the most secure option either. It is possible to remove the Triple-DES encryption from the key, thereby no longer needing to type in a pass-phrase. If the private key is no longer encrypted, it is critical that this file only be readable by the root user! If your system is ever compromised and a third party obtains your unencrypted private key, the corresponding certificate will need to be revoked. With that being said, use the following command to remove the pass-phrase from the key:

$ openssl rsa -in server.key -out server.pem

Once the private key is generated a Certificate Signing Request can be generated. The CSR is then used in one of two ways. Ideally, the CSR will be sent to a Certificate Authority, such as Thawte or Verisign who will verify the identity of the requestor and issue a signed certificate. The second option is to self-sign the CSR, which will be demonstrated in the next section.

During the generation of the CSR, you will be prompted for several pieces of information. These are the X.509 attributes of the certificate. One of the prompts will be for "Common Name (e.g., YOUR name)". It is important that this field be filled in with the fully qualified domain name of the server to be protected by SSL. If the website to be protected will be https://www.server.com, then enter www.server.com at this prompt. The command to generate the CSR is as follows:

$ openssl req -new -key server.key -out server.csr

A sample CSR generation session is shown below, with sample responses shown in bold:

$ openssl req -new -key server.key -out server.csr
Using configuration from /usr/local/ssl/openssl.cnf
Enter PEM pass phrase:Enter pass phrase here

You are about to be asked to enter information that will be incorporated into your certificate request.

What you are about to enter is what is called a Distinguished Name or a DN. There are quite a few fields but you can leave some blank.

For some fields there will be a default value, If you enter '.', the field will be left blank.

-----

Country Name (2 letter code) [AU]:US
State or Province Name (full name) [Some-State]:New Hampshire
Locality Name (eg, city) []:Nashua
Organization Name (eg, company) [Internet Widgits Pty Ltd]:Domain.com, Inc.
Organizational Unit Name (eg, section) []:.

Common Name (eg, YOUR name) []:www.domain.com
Email Address []:webmaster@domain.com


Please enter the following 'extra' attributes to be sent with your certificate request
A challenge password []:
An optional company name []:


Generating a Self-Signed Certificate

At this point you will need to generate a self-signed certificate because you either don't plan on having your certificate signed by a CA, or you wish to test your new SSL implementation while the CA is signing your certificate. In my experience dealing with Thawte, it can take up to a week or more before receiving your signed certificate. The time it takes to receive the certificate will vary based on how quickly they receive your required documentation. This temporary certificate will generate an error in the client browser to the effect that the signing certificate authority is unknown and not trusted.

To generate a temporary certificate which is good for 60 days, issue the following command:

$ openssl x509 -req -days 60 -in server.csr -signkey server.key -out server.crt


Installing the Private Key and Certificate

When Apache with mod_ssl is installed, it creates several directories in the Apache config directory. The location of this directory will differ depending on how Apache was compiled. If using my instructions on compiling Apache, the config directory is /usr/local/apache/etc. The directories mod_ssl creates include ssl.crt, ssl.csr, and ssl.key. These are good locations to store server certificates, CSRs, and private keys, respectively. If there will be multiple SSL enabled hosts on one server, it may be good practice to name the files with the fully qualified domain name of the SSL enabled host.

When adding SSL enabled virtualhosts to the web server, I prefer to keep all of the SSL virtualhosts in a separate file. This insures that all SSL hosts can be easily found in one location and helps to keep the httpd.conf file from growing too large. The SSL virtualhosts will be kept in a file called ssl.conf. In order for Apache to recognize and parse this file, it must be included in the httpd.conf file with the following directive:

Include /usr/local/apache/etc/ssl.conf


Configuring SSL Enabled Virtual Hosts

Extensive examples of SSL configurations for a virtualhost are included as part of the /usr/local/apache/etc/httpd.conf.default file installed with mod_ssl. Please refer to this file and to the mod_ssl documentation for more detailed information on configuration options. A basic SSL enabled virtualhost will appear as follows in the ssl.conf file:

# SSL Virtual Hosts
<IfDefine SSL>

<VirtualHost _default_:443>
ServerAdmin webmaster@domain.com
DocumentRoot /usr/local/apache/share/htdocs
ServerName www.domain.com
ScriptAlias /cgi-bin/ /usr/local/apache/share/htdocs/cgi-bin/
SSLEngine on
SSLCertificateFile /usr/local/apache/etc/ssl.crt/server.crt
SSLCertificateKeyFile /usr/local/apache/etc/ssl.key/server.pem
SetEnvIf User-Agent ".*MSIE.*" nokeepalive ssl-unclean-shutdown
CustomLog /usr/local/apache/var/log/ssl_request_log \"%t %h %{SSL_PROTOCOL}x %{SSL_CIPHER}x \"%r\" %b" </VirtualHost>

</IfDefine>

This will create an SSL virtualhost named www.domain.com, which is accessed via port 443 (the standard port for https) on the default IP address of the web server. It is possible to add as many additional virtualhosts as there are IP addresses that the web server listens to. Simply add additional virtualhost blocks inside of the <IfDefine SSL> and </IfDefine> tags. Due to the nature of the SSL encryption of the HTTP traffic, it is NOT possible to have name-based (HTTP1.1) SSL virtual hosts. To create a new SSL virtualhost on a different IP address, simply replace _default_ with the IP address of the virtualhost.

After adding the virtualhost to the ssl.conf file, Apache must be killed and restarted in order for it to recognize the new virtualhost. Unfortunately, this is one of the rare instances where a simple HUP signal will not work. After restarting the server, depending on whether the encrypted or unencrypted key was used, Apache will prompt you for the pass-phrase(s) of the SSL virtualhost(s). Enter the pass-phrase(s) and the web server will start.

Now, point your favorite browser to the new virtualhost you just created, remembering to use https:// instead of http://, and you should be greeted with a warning dialog if you are using the self-signed certificate. Acknowledge the dialog and the page will continue to load, protected by SSL. The status bar of your browser should be graced by the 'lock' icon, which signifies the page is protected via SSL. This is all there is to it!



 

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