Decentralized applications (DApps), which are innovative blockchain-powered software systems designed to serve as the fundamental building blocks for the next generation of Internet services, have witnessed exponential growth in recent years. This paper thoroughly compares and analyzes two blockchain-based decentralized storage networks (DSNs), which are crucial foundations for DApp and blockchain ecosystems. The study examines their respective mechanisms for data persistence, strategies for enforcing data retention, and token economics. In addition to delving into technical details, the suitability of each storage solution for decentralized application development is assessed, taking into consideration network performance, storage costs, and existing use cases. By evaluating these factors, the paper aims to provide insights into the effectiveness of these technologies in supporting the desirable properties of truly decentralized blockchain applications. In conclusion, the findings of this research are discussed and synthesized, offering valuable perspectives on the capabilities of these technologies. It sheds light on their potential to facilitate the development of DApps and provides an understanding of the ongoing trends in blockchain development.

Abstract In identity-based (ID-based) cryptosystems, a local registration authority (LRA) is responsible for authentication of users while the key generation center (KGC) is responsible for computing and sending the private keys to users and therefore, a secure channel is required. For privacy-oriented applications, it is important to keep in secret whether the private key corresponding to a certain identity has been requested. All of the existing ID-based key issuing schemes have not addressed this anonymity issue.

Abstract. Ring signature is a group-oriented signature in which the signer can spontaneously form a group and generate a signature such that the verifier is convinced the signature was generated by one member of the group and yet does not know who actually signed. Linkable ring signature is a variant such that two signatures can be linked if and only if they were signed by the same person. Recently, the first short linkable ring signature has been proposed.

Abstract Delegation is commonly used in organizations to transfer some permission by one user to another user. However, most existing delegation schemes do not support supervision, which allows the delegators to retain control over how the delegated permission can be exercised. In this paper, we describe how to support efficient authorization in delegation with supervision using proxy signature techniques

Abstract Using Cloud Storage, users can remotely store their data and enjoy the on-demand high quality applications and services from a shared pool of configurable computing resources, without the burden of local data storage and maintenance. However, the fact that users no longer have physical possession of the outsourced data makes the data integrity protection in Cloud Computing a formidable task, especially for users with constrained computing resources.

One concern in using cloud storage is that the sensitive data should be confidential to the servers which are outside the trust domain of data owners. Another issue is that the user may want to preserve his/her anonymity in the sharing or accessing of the data (such as in Web 2.0 applications).

Abstract Most existing large-scale networked systems on the Internet such as peer-to-peer systems are vulnerable to Sybil attacks where a single adversary can introduce many bogus identities. One promising defense of Sybil attacks is to perform social-network based admission control to bound the number of Sybil identities admitted.

Abstract We present Gatekeeper, a decentralized protocol that performs Sybil-resilient node admission control based on a social network. Gatekeeper can admit most honest nodes while limiting the number of Sybils admitted per attack edge to O (log k), where k is the number of attack edges. Our result improves over SybilLimit [3] by a factor of log n in the face of O (1) attack edges.

Ring signatures allow a user to sign anonymously on behalf of a group of spontaneously conscripted members. Two ring signatures are linked if they are issued by the same signer. We introduce the notion of Escrowed Linkability of ring signatures, such that only a Linking Authority can link two ring signatures; otherwise two ring signatures remain unlinkable to anyone.

A proxy re-encryption (PRE) scheme supports the delegation of decryption rights via a proxy, who makes the ciphertexts decryptable by the delegatee. PRE is useful in various applications such as encrypted email forwarding. In this paper, we introduce a more generalized notion of conditional proxy broadcast re-encryption (CPBRE).

Spyware refers to programs that steal the user information stored in the user's computer and transmit this information via the Internet to a designated home server without the user being aware of this transmission. Existing anti-spyware solutions are not generic and flexible. These solutions either check for the existence of known spyware or try to block the transmission of the private information at the packet level.

Identity-based (ID-based) cryptosystems eliminate the need for validity checking of the certificates and the need for registering for a certificate before getting the public key. These two features are desirable especially for the efficiency and the real spontaneity of ring signature, where a user can anonymously sign a message on behalf of a group of spontaneously conscripted users including the actual signer.

Peer-to-peer (P2P) systems allow users to share resources with little centralized control. Malicious users can abuse the system by contributing polluted resources. Moreover, selfish users may just connect for their own benefits without donating any resources. The concepts of reputation and currency give possible approaches to address these problems. However, to implement these ideas is non-trivial, due to the non-existence of a single trusted party.

Abstract: Recently, Au et al.[Au et al. 2007] pointed out a seemingly neglected security concern for certificateless public key encryption (CL-PKE) scheme, where a malicious key generation center (KGC) can compromise the confidentiality of the messages by embedding extra trapdoors in the system parameter. Although some schemes are secure against such an attack, they require random oracles to prove the security.

The notion of concurrent signatures was recently introduced by Chen, Kudla and Paterson. In concurrent signature schemes, two entities can produce two signatures that are not binding, until an extra piece of information (namely the keystone) is released by one of the parties. Subsequently, it was noted that the concurrent signature scheme proposed in the seminal paper cannot provide perfect ambiguity. Then, the notion of perfect concurrent signatures was introduced.

We present a new framework to design secure two-party computation protocols for exponentiation over integers and over ZQ where Q is a publicly-known prime. Using our framework, we realize efficient protocols in the semi-honest setting. Assuming the base is non-zero, and the exponent is at most Q/2 for the ZQ case, our protocols consist of at most 5 rounds (each party sending 5 messages) and the total communication consists of a small constant number (≤ 18) of encrypted/encoded elements in Z Q.

Abstract The notion of identity-based encryption (IBE) was proposed as an economical alternative to public-key infrastructures. IBE is also a useful building block in various cryptographic primitives such as searchable encryption. A generalization of IBE is attribute-based encryption (ABE). A major application of ABE is fine-grained cryptographic access control of data. Research on these topics is still actively continuing.

We introduce the notion of security-mediated certificateless (SMC) cryptography. This allows more lightweight versions of mediated cryptography while maintaining the ability for instantaneous revocation of keys. Moreover, our solutions avoid key escrow, which has been used in all previous mediated cryptography algorithms. We provide a model of security against a fully-adaptive chosen ciphertext attacker, who may be a rogue key generation centre or any coalition of rogue users.

In PKC 2006, Chow, Boyd and González Neito introduced the notion of security mediated certificateless (SMC) cryptography. SMC cryptography equips certificateless cryptography with instantaneous revocation. They presented a formal security model with two constructions for SMC encryption. This paper studies SMC signatures. We first present a security analysis of a previous attempt by Ju et al. in constructing a SMC signature scheme.