Forward-secure Multi-user Aggregate Signatures based on zk-SNARKs

As a solution to mitigate the key exposure problems in the digital signature, forward security has been proposed. The forward security guarantees the integrity of the messages generated in the past despite leaks of a current time period secret key by evolving a secret key on each time period. However, there is no forward secure signature scheme whose all metrics have constant complexities. Furthermore, existing works do not support multi-user aggregation of signatures. In this paper, we propose a forward secure aggregate signature scheme utilizing recursive zk-SNARKs (zero knowledge Succinct Non-interactive ARguments of Knowledge), whose all metrics including size and time have $O(1)$. The proposed forward secure signature scheme can aggregate signatures generated by not only a single user but also multiple users. The security of the proposed scheme is formally proven under zero-knowledge assumption and random oracle model

Privacy-preserving Identity Management System

Recently, a self-sovereign identity model has been researched actively as an alternative to the existing identity models such as a centralized identity model, federated identity model, and user-centric model. The self-sovereign identity model allows a user to have complete control of his identity. Meanwhile, the core component of the self-sovereign identity model is data minimization. The data minimization signifies that the extent of the exposure of user private identity should be minimized. As a solution to data minimization, zero-knowledge proofs can be grafted to the self-sovereign identity model. Specifically, zero-knowledge Succinct Non-interactive ARgument of Knowledges(zk-SNARKs) enables proving the truth of the statement on an arbitrary relation. In this paper, we propose a privacy-preserving self-sovereign identity model based on zk-SNARKs to allow any type of data minimization beyond the selective disclosure and range proof. The security of proposed model is formally proven under the security of the zero-knowledge proof and the unforgeability of the signature in the random oracle model. Furthermore, we optimize the proving time by checking the correctness of the commitment outside of the proof relation for practical use. The resulting scheme improves proving time for hash computation (to verify a commitment input) from 0.5 s to about 0.1 ms on a 32-bit input

SIMS : Self Sovereign Identity Management System with Preserving Privacy in Blockchain

Blockchain, which is a useful tool for providing data integrity, has emerged as an alternative to centralized servers. Concentrating on the integrity of the blockchain, many applications have been developed. Specifically, a blockchain can be utilized in proving the user\u27s identity using its strong integrity. However, since all data in the blockchain is publicly available, it can cause privacy problems if the user\u27s identity is stored in the blockchain unencrypted. Although the encryption of the private information can diminish privacy problems in the blockchain, it is difficult to transparently utilize encrypted user information in the blockchain. To provide integrity and privacy of user information simultaneously in the blockchain, we propose a SIMS (Self-Sovereign Identity Management System) framework based on a zk-SNARK (zero-knowledge Succinct Non-interactive ARgument of Knowledge). In our proposed SIMS, the user information is employed in a privacy-preserving way due to the zero-knowledge property of the zk-SNARK. We construct a SIMS scheme and prove its security. We describe applications of SIMS and demonstrate its practicality through efficient implementations