# PriSign, A Privacy-Preserving Single Sign-On System for Cloud Environments
## 2 Related Work and Comparison
Their scheme is an improvement of the scheme in [8] to achieve proxy re-verification of services by introducing a trusted central verifier (CV). In case the designated verifier is unavailable when verifying user access tokens, the CV can authorize a proxy verifier to complete the verification of access tokens on behalf of the original verifier. Unfortunately, the setting of a CV does not improve the stability of verification services; moreover, their scheme does not support token-hiding and fine-grained access control. The closest parallel to ASSO is the attribute-based electronic ticket (e-ticket) system [25; 26; 27]. In such systems, users can buy tickets from issuers using the attribute-based credentials requested from the CA, and then use the tickets to access multiple services. [PERSON] et al. [25] first proposed a privacy-preserving e-ticket system, using attribute-based credentials derived from Boneh-Boyen Signature (BBS) [28] and efficient set membership proof and range proof [29]. Next, [PERSON] et al. [26] utilized structure-preserving signatures on equivalence class [30] and dynamically malleable signatures [31] to construct efficient and strong privacy protection in a transferable attribute-based ticket scheme. [PERSON] et al. [27] proposed an electronic ticketing system to address the problem of existing electronic ticketing systems being challenging to deploy in resource-constrained devices and unable to prevent ticket sharing among unauthorized devices. Similar schemes [25; 26; 27] have achieved user anonymity and fine-grained access control; however, unlike our scheme, none of them support token-hiding and designating verifiers. Table 1 summarizes a detailed comparison between PriSign and related ASSO schemes [4; 5; 6; 7; 8; 9]. Comparisons are conducted in terms of unlinkability, designated verifiers, proxy re-verification, threshold keys issuance, token-hiding, traceability, and fine-grained access control. _Unlinkability_ means that an ASSO scheme can protect users' identities from being linked by malicious verifiers, even if they collude. _Designated verifiers_ mean that an ASSO scheme allows users to designate a verifier for access tokens without any non-designated verifiers being able to determine the correctness of tokens. _Proxy re-verification_ means that an ASSO scheme allows a proxy verifier to perform token verification services on behalf of the original verifier. _Threshold key issuance_ means that an ASSO scheme allows multiple CVs (or policymakers) to generate verification keys for designated verifiers or proxy verifiers using a threshold approach. _Token-hiding_ means that an ASSO scheme hides user service requests from all non-designated verifiers. _Traceability_ means that an ASSO scheme can de-anonymize a user's identity. _Fine-grained access control_ means that an ASSO system supports users' obtaining different types of tickets according to attribute disclosure policies. Table 1 indicates that among all schemes in our comparison, only PriSign supports all desirable features, making it a preferred solution in practice.