Methods, systems, and apparatus, including computer programs encoded on a computer-readable storage medium, and including a method for providing content. The method comprises receiving a first login associated with a first anonymous identifier and first device, creating an associated first private-public key pair, storing a first private key locally in the first device, and publishing a first public key. The method further comprises receiving a second login from a second different device, creating a second private-public key pair, storing a second private key, publishing the second public key, creating a secret key using the first public key, and associating a second anonymous identifier with the secret key. The method further comprises subsequently receiving a login the first device, creating the secret key using the second public key, associating the first anonymous identifier with the secret key, receiving a request for content from either device, and providing content using the association.