A system may protect against Sybil attacks on a peer-to-peer (P2P) network based on each one the nodes in the P2P network being identified by a corresponding certificate. In particular, a node may receive a license key, where the license key is evidence of a purchased product license. The node may transmit a message included in the license key to a certificate authority. The node may receive a certificate from the certificate authority in response to authentication of the message. The node may be identified in the P2P network with a node identifier included in the certificate.