User authentication is the process of verifying a user's identity and granting access to resources. It commonly involves a username and password but is vulnerable. Strong authentication uses two or more factors, such as something you have (e.g. card) and something you know (e.g. PIN), making impersonation and repudiation more difficult. Common strong authentication methods include smart cards, digital certificates, and biometrics. Organizations select authentication based on required security level, complexity of techniques, user impact, and cost.