"Sony knows how - but not who - hacked into its PlayStation Network last month in possibly the biggest Internet security breach in history. And despite beefing up its system to prevent another cyberattack like the one that exposed the personal information of 100 million users, Sony can't guarantee that it won't happen again. In a letter sent this week to members of Congress, the Japanese tech giant said it boosted security measures through added firewalls, new policy changes and increased monitoring of its PlayStation and Qriocity systems. Yet it still may not be enough, said the letter signed by Kazuo Hirai, head of Sony Computer Entertainment, the company's games unit."