A system intended to scan emails for child pornography and spam helped Yahoo satisfy a secret court order requiring it to search for messages containing a computer “signature” tied to the communications of a state-sponsored terrorist organization, several people familiar with the matter said on Wednesday.
Two government officials who spoke on the condition of anonymity said the Justice Department obtained an individualized order from a judge of the Foreign Intelligence Surveillance Court last year. Yahoo was barred from disclosing the matter.
To comply, Yahoo customized an existing scanning system for all incoming email traffic, which also looks for malware, according to one of the officials and to a third person familiar with Yahoo’s response, who also spoke on the condition of anonymity.
With some modifications, the system stored and made available to the Federal Bureau of Investigation a copy of any messages it found that contained the digital signature. The collection is no longer taking place, those two people said.
The order was unusual because it involved the systematic scanning of all Yahoo users’ emails rather than individual accounts; several other tech companies said they had not encountered such a demand.
News of the order has opened a new chapter in a public debate over the trade-offs between security needs and privacy rights that has cast a spotlight on the sometimes cooperative, sometimes antagonistic relationship between Silicon Valley companies and the United States government.
It comes six months after a standoff between the F.B.I. and Apple, in which the government obtained a federal magistrate’s order to force the company to help it unlock an encrypted iPhone from one of the attackers in the December mass shooting in San Bernardino, Calif. The F.B.I. gave up the fight with Apple after it found a way into the iPhone without the company’s help.
By contrast, Yahoo cooperated with the Foreign Intelligence Surveillance Court order, although the technical burden on the company appears to have been significantly lighter than the one the F.B.I. placed on Apple.