# DCAGS-IoT: Dynamic Cross-Domain Authentication Scheme Using Group Signature in IoT
## 4 Analysis of Proposed Protocol  
### Security Attribute Analysis  
(1) Anonymity  
The scheme is based on the group signature scheme. Any group member in a trust domain can sign a message on behalf of the entire group in an anonymous manner, and the receiver does not know that the signature is signed by the group member in the group. Like other digital signatures, group signatures are publicly verifiable and can be verified using only a single group public key. Given a group signature, it is impossible for anyone other than the group administrator to know the identity of the actual signer. (2) Resist replay attack  
The validity of the interactive message is guaranteed by the timestamp. After the message receiver receives the interactive message, it first checks whether it is valid, and then performs subsequent operations. Since the timestamp cannot be tampered with, if the attacker reuses the intercepted message, the verification will fail due to the invalid timestamp, so replay attacks can be effectively prevented. Traceability  
In the event of an argument, a group manager can open a signature to determine the identity of the actual signer, and the signer cannot prevent the opening of the signature, so it is traceable. Privacy protection  
The scheme uses a group signature scheme to hide user identity information, and does not use their real identity when interacting with other devices, and the privacy of the participants will be protected in the subsequent process. In the process of data sharing, no entity will disclose the identity information of the participants. The verifier only knows the trust domain to which the message sender belongs instead of the original identity information. The group administrator can trace the disputed membership, but the blockchain keeps information consistent, so it can effectively protect privacy. Avoid single point of failure  
The heterogeneous inter-domain scheme adopts a decentralized storage architecture. The blockchain structure composed of GM in each trust domain replaces the location of a trusted third party, ensures the consistency of information storage, builds inter-domain trust, and completes cross-domain authentication, thus effectively solving the single point of failure problem. In addition, the dynamic cross-domain authentication scheme based on group signature proposed in this paper and other existing cross-domain authentication schemes can avoid single point of failure, efficiency, privacy protection, anonymity, traceability, and other aspects. Comparisons were made, as shown in Table 2. The authentication scheme based on the group signature proposed by [PERSON] et al. [20] could effectively solve the security problem of user identity authentication in a heterogeneous cloud environment. [PERSON] et al. [26] proposed a two-way anonymous traceability group authentication protocol in IoV. The RSU in the group can anonymously trace the identity of malicious vehicles and use the blockchain to quickly revoke their identity. However, the efficiency of the schemes in [20; 26] was restricted by the number of group members. [PERSON] et al. [8] proposed a complete cross-domain authentication scheme based on blockchain. Participants from different trust domains can directly access the chain code in the blockchain, reducing the computational burden of the verification server. [PERSON] et al. [14] proposed a cross-domain identity authentication scheme based on the identity cryptosystem on the consortium chain based on the IBC identity cryptosystem, aiming at the problem of cross-domain identity authentication when users access network services in different trust domains. However, the schemes in [8; 14] are easy to leak user privacy. [PERSON] et al. [24] proposed a pairless authentication and key management scheme for dynamic cross-domain authentication that achieved low latency and high reliability of vehicle-to-RSU transmission and ensured that vehicle privacy was not leaked, but did not achieve the traceability function.