ANN ARBOR ? Network Security provider Duo Security Inc. announced Wednesday that it has raised a Series C funding round of $30 million that will be used to invest in engineering and international expansion. Duo Security has now raised $48 million from investors.

This new round was led by a new investor, Menlo Park, Calif.-based Redpoint Ventures, and joined by previous investors: San Francisco-based Benchmark Capital, Mountain View, Calif.-based Google Ventures, Palo Alto, Calif.-based Radar Partners; and San Francisco-based True Ventures.

?We?re delighted to welcome Redpoint Ventures to the Duo team,? said Duo Security CEO Dug Song. ?This round of funding validates the work we?re doing and will help us continue to reinvent easy and effective solutions in an industry overloaded with expensive, broken systems.?

Duo includes as customers Facebook, Paramount Pictures, Random House, Twitter, Yelp, TripAdvisor and NASA.

In 2010, Song founded Duo Security. The company uses two-factor authentication to offer highly secure, Web-based authentication, hence its name.