CRCT: Compact Ring Confidential Transactions Based on Sum Arguments

Ring Confidential Transactions (RingCT) is a classic cryptographic protocol for anonymous transactions on blockchains, currently used in the popular anonymous cryptocurrency Monero. The proof size of RingCT transactions is linearly related to the ring size, which limits the use of larger ring sizes due to the significant communication overhead it incurs. However, reducing the ring size also leads to decreased anonymity. Therefore, in recent years, many studies have focused on optimizing the proof sizes for RingCT, with the latest known solutions reducing the proof size to be logarithmic with the ring size. In this paper, we propose a new compact RingCT protocol (CRCT) for smaller proof sizes. To this end, we first design three extended schemes of the Sum Argument (CRYPTO’21), which are used to generate logarithmic-sized proofs for three distinct zero-knowledge arguments, respectively. We then introduce a new zero-knowledge proof scheme called the Difference Argument. It is used to prove that one has the knowledge of two secret values, with their difference being public. Based on these schemes, we construct our CRCT protocol, whose proof size is independent of the ring size and logarithmic with the number of source accounts. We provide concrete constructions and security proofs for the proposed cryptographic schemes. The experimental results demonstrate that CRCT exhibits significant advantages in computational efficiency and proof size over existing solutions when dealing with large ring sizes and moderate numbers of source accounts.