For years, Germany's foreign intelligence service (BND) has been siphoning off data from Frankfurt-based De-Cix (Deutsche Commercial Internet Exchange), the world's largest internet exchange point. Data streams from various internet providers meet there to be passed onto their respective destinations.

The BND claims the data it's been channeling from De-Cix is paramount to Germany's national security interests, but De-Cix counters that the data is being siphoned comes from a purely domestic internet exchange point and that as an international agency, the BND is forbidden to use surveillance measures pertaining to domestic communication without concrete suspicion.

Read more: In the shadows of international law

Germany's telecommunications law only allows the surveillance of international, cross-border communication data.

"The BND looked for the biggest pool in which it can fish," De-Cix's lawyer, Sven-Erik Heun, told the court. De-Cix is capable of processing just over six terabytes of information per second, which is roughly two high-definition feature films or 2.6 billion single-spaced typed pages per second.

De-Cix wants to put an end to the practice and filed a lawsuit against the spy agency — and by extension the Interior Ministry, which oversees the BND and its strategic signals intelligence — at the Federal Administrative Court in Leipzig. De-Cix claims the BND is methodically collecting entire data transcripts, thereby ignoring the statutory quantitative limit of 20 percent. De-Cix accused the government of breaking the law and of lacking the technical understanding to oversee the BND.

"We consider ourselves under obligation to our customers to work towards a situation in which strategic surveillance of their telecommunications only takes place in a legal manner," De-Cix said in a statement. "We consider ourselves under obligation to our customers to work towards a situation in which strategic surveillance of their telecommunications only takes place in a legal manner."

The government has shot back, saying De-Cix appears to be out of its depth regarding the complexity of the issue.

De-Cix says it no longer wants to be complicit in the BND's dealings

The court case: Why now?

De-Cix no longer wants to be complicit as an unwitting participant in the BND's dealings, even more so against the backdrop of the NSA parliamentary inquiry which established that between 2004 and 2008, raw data was siphoned off from De-Cix and forwarded to the NSA. At the time, the BND said information on German citizens had been filtered out, but internal documents showed that at least 5 percent of German citizens' communications data could not be filtered.

For the BND, the De-Cix hub is crucial in terms of access to the data that passes through there from Russia, China, Africa and the Middle East. That hub gives the BND a clear location advantage and makes it attractive for international partners.

The court in Leipzig was not certain to issue a ruling on Wednesday. In the event that De-Cix loses the case, the company has said it would appeal at Germany's Constitutional Court in Karlsruhe.