A lot has been said about PRISM, the recently discovered top-secret NSA program allowing federal agencies to, allegedly, directly tap into the servers of nine major Internet companies.

Agency officials, members of Congress and the Obama administration have come forward and admitted to the program's existence. They also say that it's entirely legal and well within constitutional limits.

Here's why they say that.

The Fourth Amendment requires law enforcement and federal agencies to acquire a warrant to search the property, including data, of any given U.S. citizen. But, thanks to the combination of the Patriot Act and the Foreign Intelligence Surveillance Act (FISA), that Fourth Amendment right to a warrant doesn't extend to data leaving and entering the country, to foreigners in the country or to any intelligence gathering which could involve, as The Atlantic notes, the activities of foreign powers.

And there within lies the rub: foreign intelligence gathering currently defined by law is notoriously vague

Finally, a good deal of these intelligence gathering operations, and the legal reasonings behind them, have largely been confined to secret courts. Politicians, judges and lawyers involved with such operations, and their legal interpretations, are barred by law from discussing them. Consequently, the public isn't aware of these intelligence gathering operations until an intelligence agent, politician or a person with knowledge of these top-secret programs leak these details to the press.

But the Obama administration is actively fighting a closed-door war against such leaks, making it increasingly difficult for the press to apply outside scrutiny to such programs.

What We Know About PRISM:

PRISM began in 2007 under the Bush administration as a cybersecurity program to monitor the use of major U.S.-based websites by foreigners. The first company to be involved with the program, knowingly or not, was Microsoft. Under the Obama administration, the program rapidly expanded to include nine major tech companies.

PRISM is capable of collecting email, chat logs, photos, stored data, VoIP conversations, file transfers, log-ins, social networking details and videos.

To date, there have been 77,000 intelligence reports which have cited PRISM as a source. The program is so popular among intelligence agencies that it's reportedly cited in one in seven reports, according to CNET.

PRISM has access to data moving through Microsoft, Yahoo, AOL, Facebook, Google, Apple, PalTalk, YouTube, and Skype, but 98 percent of the information gathered by PRISM is based on data from Yahoo, Google and Microsoft.

The nine major tech companies outlined in the initial PRISM story by The Washington Post may not have knowingly participated in the PRISM program. Each company has issued flat denials of cooperating and giving the NSA and FBI privileged access to their servers. While The Washington Post has backtracked its claim that each company knowingly participated in the program, the companies involved may be instructed by law to actively deny any knowledge of the program.

Select members of Congress knew of PRISM's existence, but were prevented from discussing it. Some politicians, including President Obama, are actively defending the program, arguing PRISM is necessary for national defense. Obama claims PRISM isn't applied to U.S. citizens or people living in the U.S.

Data belonging to Americans, however, has been intercepted by PRISM. Two former U.S. Intelligence officials tell NBC News such data was collected mistakenly. When this happens, they say, the data is put into a report, which is then given to a secret national security court before being destroyed.