**6. Conclusions**

The group signature scheme is an electronic signature scheme with which a signer can prove that he/she is a member of a certain group without revealing his/her own identity, and which allows the authenticator to make a judgment on whether the signature is written by the same person or not, but which does not allow the authenticator to know the identity of the signer. A number of previous studies flexibly applied group signature schemes to various applications.

Meanwhile, the proposed algorithm in this study was developed in a way that it can be adopted in previous research [26–32] pertaining to smart grids.

Thus, two anonymous signature schemes in a smart grid environment were proposed in this study: a scheme where the anonymous signer issues a token to let the opener identify him/her only for the designated signature, and another scheme which requires the signer's consent for identification. In the former, the signer generates the token along with his/her signature using a short-term secret key, whereas, in the latter, the token is generated using a long-term secret key only when the signer agrees to disclose his/her identity after entering the signature. Although there is a possibility of compromising the security a little when the latter scheme is adopted, the burden of the signer having to issue and keep the token all the time can be lightened, improving the convenience of the scheme.

**Author Contributions:** Conceptualization, S.E. and J.-H.H. Data curation, S.E. and J.-H.H. Formal analysis, S.E. Methodology, J.-H.H. Project administration, J.-H.H. Resources, S.E. and J.-H.H. Visualization, S.E. and J.-H.H. Writing—original draft, S.E. and J.-H.H. Writing—review and editing, J.-H.H.

**Funding:** This work was supported by the National Research Foundation of Korea (NRF) gran<sup>t</sup> funded by the Korea governmen<sup>t</sup> (MSIT) (No. 2017R1C1B5077157).

**Acknowledgments:** The first draft of this paper was presented at The International Conference on Big data, IoT, and Cloud Computing (BIC 2017) [31,33], 22–24 August 2017, Republic of Korea.

**Conflicts of Interest:** The authors declare no conflict of interest.
