The metaverse transforms how individuals interact, work, and engage in virtual environments, creating new opportunities in gaming, education, e-commerce, and social networking. At the core of this experience are avatars (i.e., digital representations of users that serve as their proxies in virtual spaces). Ensuring secure and privacy-preserving communication between avatars presents critical challenges, particularly in protecting metaverse user privacy by preventing the linking of avatars to users' social and professional lives. Adopting anonymous schemes such as ring signature schemes requires access to all public keys within the anonymity set, which is impractical in the metaverse. In this paper, we proposeSPARTA(Secure andPrivacy-preserving protocol withRole separation andTrustworthiness forAvatars in the metaverse), a protocol that enables avatar authentication and maintains avatar unlinkability. By leveraging mercurial signatures, our approach allows metaverse users to generate multiple unlinkable avatars without requiring repeated registration with the metaverse service provider, thereby enabling seamless role separation. Additionally, by using a time-based hash chain, only avatars in possession of a reputation token from the time-based hash chain can submit their feedback on a smart contract based on their interactions. Given the soundness property of zero-knowledge proof and the origin-hiding property of mercurial signatures, we formally prove that${\sf SPARTA}$achieves mutual authentication, avatar unlinkability, and penalization enforcement. Additionally, we analyze the performance overheads introduced by its cryptographic primitives and compare${\sf SPARTA}$with existing metaverse authentication protocols. Furthermore, we implement the protocol using socket programming. This implementation simulates real-time message exchanges between protocol entities, resulting in an end-to-end latency of 105 ms. Compared to existing metaverse authentication frameworks,${\sf SPARTA}$provides unlinkable avatar authentication that achieves mutual authentication, role separation, and data sovereignty without reliance on an online trusted third party. The concurrent implementation between two Raspberry Pi devices demonstrates the scalability of${\sf SPARTA}$, achieving a total completion time of 17.338 seconds for 1000 concurrent authentications, corresponding to a throughput of approximately 57.7 authentications per second, confirming its practicality for large-scale metaverse environments.

The metaverse is a virtual world that mirrors real life, allowing users to engage in activities and access services without the constraints of time and space. In the metaverse, users can create one or more avatars that reflect their personal preferences, enabling them to participate in activities that match their tastes and needs. To protect users’ freedom and anonymity, it is imperative for metaverse platforms to support the creation ofunlinkableavatars. This ensures that the different avatars a user creates cannot be connected, keeping their virtual identities separate and reducing the risk of retaliation for their actions. In this paper, we propose an unlinkable avatar authentication scheme, UAVA, which leverages cryptographic group signatures to enable metaverse users to create and certify their avatars without interaction with service providers. These certified avatars can then be anonymously authenticated, ensuring unlinkability between multiple avatars belonging to the same user. UAVA maintains anonymity between users and their avatars, while allowing service providers to trace malicious avatars back to their users. We formally define and prove the security properties of UAVA, and implement the protocol using socket programming, and report on its cryptographic overheads. We also evaluate its cryptographic overhead and compare it to related protocols in terms of efficiency, security, and scalability.