When he eagerly joined the mass street protests that followed Iran’s tainted June 12 presidential elections, Ibrahim Sharifi, 24, hoped only to add his voice to the hundreds of thousands of demonstrators demanding that the government nullify the results. He never imagined that he would eventually have a far greater impact, as the only person willing to speak publicly about the brutal treatment he was subjected to in prison, including rape and torture. Mr. Sharifi, who recounted his ordeal to the opposition leader and former presidential candidate Mehdi Karroubi, and then released a video account last month on opposition Web...