BARCELONA — When Microsoft, the world’s largest software maker, bought Skype in May 2011 for $8.5 billion, it acquired not only the technology behind the world’s dominant Internet voice and video service, but a connection with more than 250 million active users.

But perhaps what Microsoft did not anticipate when it made the purchase was that it would inherit the delicate privacy aspects of Skype’s business, including its billions of encrypted, peer-to-peer Internet conversations.

Those conversations, and the access Microsoft grants to them, are now the focus of a lobbying campaign by 50 digital rights groups and dozens of individuals.

In a letter sent in January, the group asked Microsoft to disclose what data it collected from Skype users and whether that data was passed on — whether to potential advertisers or to law enforcement agencies conducting criminal investigations.