Qualys, a cloud security provider headquartered in Redwood City, Calif., began trading Friday on the NASDAQ, debuting at $12 a share. Trading under the symbol “QLYS,” Qualys offered 6.7 million shares, while stockholders sold an additional 875,000. The stock traded flat for much of the day, but was up 18 percent in after-hours trading. Qualys, founded in 1999 by CEO Philippe Courtot, generated $76.2 million in revenue last year. The IPO is the latest in a string of security companies that have gone public in recent months.