In one embodiment, a computing device of a social-networking system may receive from a computing device of a first user of the social-networking system information acquired by the computing device of the first user from a tag associated with a second user of the social-networking system. The tag may encode a time stamp, a signature, and a social action on the social-networking system. The computing device of the social-networking system may verify the signature and, if the signature is verified, then initiate the social action.