Google executives have also said privately that the company had been far more transparent about the intrusions than any of the more than two dozen other companies that were compromised, the vast majority of which have not acknowledged the attacks.

Google continues to use the Gaia system, now known as Single Sign-On. Hours after announcing the intrusions, Google said it would activate a new layer of encryption for Gmail service. The company also tightened the security of its data centers and further secured the communications links between its services and the computers of its users.

Several technical experts said that because Google had quickly learned of the theft of the software, it was unclear what the consequences of the theft had been. One of the most alarming possibilities is that the attackers might have intended to insert a Trojan horse — a secret back door — into the Gaia program and install it in dozens of Google’s global data centers to establish clandestine entry points. But the independent security specialists emphasized that such an undertaking would have been remarkably difficult, particularly because Google’s security specialists had been alerted to the theft of the program.

However, having access to the original programmer’s instructions, or source code, could also provide technically skilled hackers with knowledge about subtle security vulnerabilities in the Gaia code that may have eluded Google’s engineers.

“If you can get to the software repository where the bugs are housed before they are patched, that’s the pot of gold at the end of the rainbow,” said George Kurtz, chief technology officer for McAfee Inc., a software security company that was one of the companies that analyzed the illicit software used in the intrusions at Google and at other companies last year.

Rodney Joffe, a vice president at Neustar, a developer of Internet infrastructure services, said, “It’s obviously a real issue if you can understand how the system works.” Understanding the algorithms on which the software is based might be of great value to an attacker looking for weak points in the system, he said.

When Google first announced the thefts, the company said it had evidence that the intrusions had come from China. The attacks have been traced to computers at two campuses in China, but investigators acknowledge that the true origin may have been concealed, a quintessential problem of cyberattacks.