Moreover, it was usually clear where a conventional attack had originated; the trajectory of a missile could be tracked by radar or satellite. Mr. Obama himself noted last week the difficulty of tracing a cyberattack, and thus of deterring it — or retaliating with confidence. Earlier efforts to get Mr. Xi and other senior Chinese leaders to address cyberattacks have largely failed. Mr. Obama spent a considerable amount of time on the issue during a summit meeting with Mr. Xi at Sunnylands, a California estate, in 2013. But even after that session, the Chinese denied that their military was involved in attacks, and portrayed themselves as victims of attacks from the United States. It was not an entirely spurious claim: Classified documents released by Edward J. Snowden showed a complex effort by the National Security Agency to get into the systems of a Chinese telecommunications giant, Huawei, though the United States maintained that the effort was for national security surveillance, not for the theft of intellectual property. The recent Chinese movement on cybersecurity can be traced to several events, officials say. The Office of Personnel Management breach, which went undetected for roughly a year, was traced to Chinese sources, and one official said evidence had been presented to Chinese officials. In August, Susan E. Rice, Mr. Obama’s national security adviser, took a trip to Beijing to meet with Mr. Xi and other officials, and used it to increase pressure on China, suggesting that newly devised economic sanctions could be imposed. Mr. Obama referred to that possibility in two recent speeches, suggesting that he would hold off only if there was progress with Mr. Xi. Last week, a high-level Communist Party envoy, Meng Jianzhu, who is responsible for state security, came to Washington and met with Ms. Rice, several American intelligence officials and the director of the F.B.I., James B. Comey. That session focused on coming up with some kind of agreement, however vaguely worded, that Mr. Obama and Mr. Xi could announce on Friday. For the United States, agreements limiting cyberweapons are also problematic. The country is spending billions of dollars on new generations of weapons, and in at least one famous case, the cyberattacks on Iran’s nuclear enrichment site at Natanz, it has used them. American cyberwarriors would be concerned about any rules that limited their ability in peacetime to place “beacons” or “implants” in foreign computer networks; these are pieces of code that monitor how foreign computer systems work, and they can be vital in determining how to launch a covert or wartime attack. The Chinese have littered American networks with similar technology, often to the consternation of the Pentagon and intelligence agencies.
Ahead of President Xi Jingping's state visit to the the United States, the two countries are hoping to agree the basics for the equivalent of an arms-control deal for cyberspace. The New York Times reports that the focus of the deal will be to stop infrastructure attacks, but will not cover intellectual property or ID theft, or essentailly any of the current "normal" cyber-espionage attacks.