Payment Channel Hub (PCH) is a promising solution to the scalability issue of first-generation blockchains or cryptocurrencies such as Bitcoin. It supports off-chain payments between a sender and a receiver through an intermediary (called the tumbler). Relationship anonymity and value privacy are desirable features of privacy-preserving PCHs, which prevent the tumbler from identifying the sender and receiver pairs as well as the payment amounts. To our knowledge, all existing Bitcoin-compatible PCH constructions that guarantee relationship anonymity allow only a (predefined) fixed payment amount. Thus, to achieve payments with different amounts, they would require either multiple PCH systems or running one PCH system multiple times. Neither of these solutions would be deemed practical.In this paper, we propose the first Bitcoin-compatible PCH that achieves relationship anonymity and supports variable amounts for payment. To achieve this, we have several layers of technical constructions, each of which could be of independent interest to the community. First, we propose BlindChannel, a novel bi-directional payment channel protocol for privacy-preserving payments, where one of the channel parties is unable to see the channel balances. Then, we further propose BlindHub, a three-party (sender, tumbler, receiver) protocol for private conditional payments, where the tumbler pays to the receiver only if the sender pays to the tumbler. The appealing additional feature of BlindHub is that the tumbler cannot link the sender and the receiver while supporting a variable payment amount. To construct BlindHub, we also introduce two new cryptographic primitives as building blocks, namely Blind Adaptor Signature (BAS), and Flexible Blind Conditional Signature (FBCS). BAS is an adaptor signature protocol built on top of a blind signature scheme. FBCS is a new cryptographic notion enabling us to provide an atomic and privacy-preserving PCH. Lastly, we instantiate both BlindChannel and BlindHub protocols and present implementation results to show their practicality.

In this work, we present the first leakage-resilient identity-based encryption (LR-IBE) scheme that features (almost) tight security under the multi-challenge setting, in which the adversary could receive multiple challenging ciphertexts from multiple users. Meanwhile, our construction enjoys security against chosen-ciphertext attack (CCA) under the Matrix Decisional Diffie-Hellman (MDDH) assumption. Apart from admitting the continual leakage of secret key, we provide strong security via additionally allowing the continual leakage of the master secret key.

We propose MoNet, the first bi-directional payment channel network with unlimited lifetime for Monero. It is fully compatible with Monero without requiring any modification of the current Monero blockchain. MoNet preserves transaction fungibility, i.e., transactions over MoNet and Monero are indistinguishable, and guarantees anonymity of Monero and MoNet users by avoiding any potential privacy leakage introduced by the new payment channel network. We also propose a new crypto primitive, named Verifiable Consecutive One-way Function (VCOF). It allows one to generate a sequence of statement-witness pairs in a consecutive and verifiable way, and these statement-witness pairs are one-way, namely it is easy to compute a statement-witness pair by knowing any of the pre-generated pairs, but hard in an opposite flow. By using VCOF, a signer can produce a series of consecutive adaptor signatures CAS. We further propose the generic construction of consecutive adaptor signature as an important building block of MoNet. We develop a proof-of-concept implementation for MoNet, and our evaluation shows that MoNet can reach the same transaction throughput as Lightning Network, the payment channel network for Bitcoin. Moreover, we provide a security analysis of MoNet under the Universal Composable (UC) security framework.

The US Standard (EC)DSA is currently almost the most popular digital signature scheme. Chinese and Russian governments also proposed their counterparts: SM2 and GOST R 34.10 (GOST). Nowadays, there are already many industrial applications supporting SM2 and GOST digital signatures. Unfortunately, the existing analyses for SM2 and GOST are rather limited when compared to ECDSA. This paper focuses on the security of SM2 and GOST from the viewpoints of RKA security (related-key attack) and sKRKA security (strong known related key attack). RKA captures the real attacks of tampering and fault injection in hardware-stored secret keys. sKRKA, a recently proposed security model modified from RKA, captures the real attacks in the BIP-32 HD wallet and the stealth address used in Monero. It was proved that ECDSA is insecure in the RKA model (ICISC 2015) and but secure in the sKRKA model (NSS 2019). In this work, we proved that GOST is insecure in both RKA and skRKA models, but SM2 is secure in both RKA and sKRKA models. This result well differentiates the security of ECDSA, SM2 and GOST, and demonstrates that Chinese SM2 is capable to construct secure cryptocurrency systems using BIP-32 HD wallet or stealth address, as secure as ECDSA, but outperforms ECDSA in resisting tampering or fault injection attacks.