Methods, systems, and apparatus include computer programs encoded on a computer-readable storage medium for providing content. A method includes: receiving a first request for filling a slot on a first publisher page, the request including an encrypted publisher cookie; providing content responsive to the request; upon receipt of an indication that a user interacted with the content, creating and providing a content serving system cookie; linking the received encrypted publisher cookie with the content serving system cookie; receiving a second, different request for content in association with rendering a second, different publisher page, the second, different request including an encrypted publisher cookie associated with the second different publisher page and the previously provided content serving system cookie; linking the received encrypted publisher cookie associated with the second, different publisher page with the encrypted publisher cookie associated with the first publisher page; and using the linkings to determine content to deliver.