Security systems for protecting assets are described, including password-based security systems that can provide different levels of access responsive to entry of a primary or secondary password. In some versions, user-configurable security rules can provide customized responses to entry of primary or secondary passwords, including feigned or limited access, security alerts, etc. Passwords comprising overt and covert components can be used to provide enhanced security and improved user control over system response. Improved security systems involving transactions between multiple parties are also considered, with options for user-customized security rules including primary and secondary passwords, and reverse challenge and response methods. Systems for Limited Use Credentials are also disclosed to reduce the risk of identity theft.