A group signature scheme in which signers are able to designate openers by specifying access structures over openers' attributes was introduced at CANDAR 2021, which is called GSdT. In this paper, we present a construction of GSdT from only symmetric-key primitives; pseudorandom functions, hash functions and commitments. Due to the features, our GSdT is expected to be secure against computational power of quantum computers. We first introduce syntax and security definitions in the static group model. Then, in our construction, the key ingredient is a non-interactive zero-knowledge proof of knowledge system that is constructed from the primitives in the “MPC-in-the-head” paradigm, owing the technique that was developed by Katz, Kolesnikov and Wang (ACM-CCS 2018). Our approach starts with their group signature scheme, but non-trivially extends the Merkle tree so that signers can treat (all-AND) boolean formulas as the access structures. According to our estimation, the signing time is less than 3.0 sec and the signature size is less than 0.5 MB in a scenario that the numbers of group members and attributes are 27and 23, respectively, and security to be attained is 128 bit quantum security.

Galindo and Garcia introduced a light-wight identity-based signature (IBS) based on Schnorr signature in AFRICACRYPT 2009. Despite the simple construction and the ef-ficiency of Galindo-Garcia IBS, to the best of our knowledge, their followers do not appear except the IBS scheme by Fukumitsu and Hasegawa in CANDAR'17. Although their IBS achieves the tight security, the efficiency is worse than Galindo-Garcia IBS. Their result suggests that Galindo-Garcia IBS utilizes some special properties of Schnorr signature, and we have to deeply observe the structures of both Galindo-Garcia IBS and Schnorr signature to construct their followers. In this paper, we aim to generalize Galindo-Garcia IBS and to search necessary properties of building cryptographic primitives. As a result, we introduce a generic construction of an IBS from a sigma protocol, and find necessary properties to prove the security of the resulting IBS.

We propose a new secure online storage scheme based on an open P2P network without a central server. In the proposed scheme, attackers cannot detect target user data in the online storage because the user data is divided into some parts by the secret sharing, and they are distributed to P2P nodes via the anonymous communication. The proposed scheme can prevent the direct attack on the target user node because metadata for the reconstruction also hidden in the online storage, hence nothing remains in the user node. Even if the state of the P2P network varies over time between the storing and restoring operation, the proposed scheme ensures that the user can identify the target nodes which stored his metadata by utilizing the Blockchain technology with only memorable secure information for user authentication. Furthermore, a malicious node to attack on others can be detected and ruled out by the mutual monitoring among the nodes and the majority decision rule.

Password managers protect users' passwords by using a master password or a security token. The security of them using the master password is weakened if users use weak master passwords. The usability of them using the security token is low since users always need the token to log in. In this paper, we propose a new framework of password managers which has a high security and a high usability by employing the secret sharing and a personal servers for a user.