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Online social networks (OSNs) such as Facebook and MySpace are flourishing because more and more people are using OSNs to share their interests with friends. Because security and privacy issues on OSNs are major concerns, we propose a security framework for simultaneously authenticating multiple users to improve the efficiency and security of peer-to-peer (P2P)-based OSNs. In the proposed framework, three batch authentication protocols are proposed, adopting the one-way hash function, ElGamal proxy encryption, and certificates as the underlying cryptosystems. The hash-based authentication protocol requires lower computational cost and is suitable for resource-limited devices. The proxy-based protocol is based on asymmetric encryption and can be used to exchange more information among users. The certificate-based protocol guarantees nonrepudiation of transactions by signatures. Without a centralized authentication server, the proposed framework can therefore facilitate the extension of an OSN with batched verifications. In this paper, we formally prove that the proposed batch authentication protocols are secure against both passive adversaries and impersonator attacks, can offer implicit key authentication, and require fewer messages to authenticate multiple users. We also show that our protocols can meet important security requirements, including mutual authentication, reputation, community authenticity, nonrepudiation, and flexibility. With these effective security features, our framework is appropriate for use in P2P-based OSNs.