Collusion-Resistant Multicast Key Distribution Based on Homomorphic One-Way Function Trees

Providing security services for multicast, such as traffic integrity, authentication, and confidentiality, requires securely distributing a group key to group receivers. In the literature, this problem is called multicast key distribution (MKD). A famous MKD protocol—one-way function tree (OFT)—has been found vulnerable to collusion attacks. Solutions to prevent these attacks have been proposed, but at the cost of a higher communication overhead than the original protocol. In this paper, we prove falsity of a recently-proposed necessary and sufficient condition for a collusion attack on the OFT protocol to exist by a counterexample and give a new necessary and sufficient condition for nonexistence of any type of collusion attack on it. We instantiate the general notion of OFT to obtain a particular type of cryptographic construction named homomorphic one-way function tree (HOFT).We propose two structure-preserving graph operations on HOFTs, tree product and tree blinding. One elegant quality possessed by HOFTs is that handling (adding, removing, or changing) leaf nodes in a HOFT can be achieved by using tree product without compromising its structure. We provide algorithms for handling leaf nodes in a HOFT. Employing HOFTs and related algorithms, we put forward a collusion-resistant MKD protocol without losing any communication efficiency compared to the original OFT protocol. We also prove the security of our MKD protocol in a symbolic security model

VeriVoting: A decentralized, verifiable and privacy-preserving scheme for weighted voting

Decentralization, verifiability, and privacy-preserving are three fundamental properties of modern e-voting. In this paper, we conduct extensive investigations into them and present a novel e-voting scheme, VeriVoting, which is the first to satisfy these properties. More specifically, decentralization is realized through blockchain technology and the distribution of decryption power among competing entities, such as candidates. Furthermore, verifiability is satisfied when the public verifies the ballots and decryption keys. And finally, bidirectional unlinkability is achieved to help preserve privacy by decoupling voter identity from ballot content. Following the ideas above, we first leverage linear homomorphic encryption schemes and non-interactive zero-knowledge argument systems to construct a voting primitive, SemiVoting, which meets decentralization, decryption-key verifiability, and ballot privacy. To further achieve ballot ciphertext verifiability and anonymity, we extend this primitive with blockchain and verifiable computation to finally arrive at VeriVoting. Through security analysis and per-formance evaluations, VeriVoting offers a new trade-off between security and efficiency that differs from all previous e-voting schemes and provides a radically novel practical ap-proach to large-scale elections