1. A method comprising: receiving a media identifier and an impression identifier from a media device, the media identifier being indicative of media presented at the media device; receiving the impression identifier in association with first user information from a first database proprietor as a result of the first database proprietor obtaining a first identifier encrypted with a first encryption key by the media device and sent from the media device, the first identifier identifying at least one of the media device or a user of the media device; receiving the impression identifier in association with second user information from a second database proprietor as a result of the second database proprietor obtaining a second identifier encrypted with a second encryption key by the media device and sent from the media device, the second identifier identifying at least one of the media device or the user of the media device, the second identifier being different than the first identifier; identifying the first user information and the second user information as associated with a same user based on the impression identifier; and associating the first user information and the second user information with the media identifier.