A system and method allows a user to authenticate a communication from a computer system, a computer system to authenticate a user, or both. When a user requests a web page from the web site, customization information that is recognizable to the user is provided to allow the user to authenticate the web site. A signed, encrypted persistent file stored on the user's computer system or installed on a different computer system, or a trusted computing subsystem allows the web site to authenticate the user. If the user is using a system that will not allow that user to be authenticated, the user may instruct the system to continue providing information without the user's customization information. The system and method may be used to allow the user to authenticate an e-mail message or its source, and Flash movies or other computer code may be used if the user's e-mail client does not employ cookies.