The following is a Drupal 6 sample module showing how to encrypt a MySQL database column. Note that there are several cavaets: 1. This was only tested with 6.8; it might not work with other versions. 2. The sample table used only contains one row for simplicity; modifying the database code for your application is left up to you. 3. The secret key is saved in the module file; you will probably want to use a more secure method. 4. And, of course, ensuring that your MySQL installation and traffic are secure is up to you.