Twitter users will have to enter a six-digit code texted to their mobile phone each time they log into the microblogging service. Twitter launched the service after a string of account hijackings last month. The Syrian Electronic Army has taken credit for breaking into the accounts of the Associated Press, the Financial Times, National Public Radio and The Guardian. The group says it targets news media that are sympathetic to Syria’s rebels. Forrester Research: “This isn’t about Twitter; it’s about your own security and protecting your own brand””]