Enterprise mobile security company Duo Security has raised $5 million from Google Ventures, True Ventures, and Resonant Venture Partners. This brings the startup's total funding to $6 million. Duo Security essentially provides a two-factor authentication service to companies who want to protect employee phones, with not just one but two secure log-ins. The company says that deploying two-factor authentication has traditionally been expensive and time-consuming with hardware requirements and other protocols. Duo Security lowers this barrier and expedites deployment with self-service enrollment. Via mobile apps for iOS, Android, BlackBerry and other platform, Duo Security allows for secure logins and transactions. Users can generate secure passcodes without the hassle of hardware tokens. Here's how it works: Once the app is downloaded, users type in their usernames and passwords as usual. If primary authentication succeeds, they are offered a second choice of authentication method. Duo says it seamlessly connects to servers and company infrastructure as well. And the startup says one of its differentiating features is usability. Both from the deployment and employee user interface standpoint, Duo has worked to make its product easy to use. In two years, Duo Security has added over 500 customers with users across 80 countries for their two-factor authentication service, including Tumblr, University of Michigan, Central Ohio Primary Care, and Toyota. The new funding will be used to expand Duo's team, mobile security research, and for authentication platform development.