Determining the Number of Confirmation Blocks in a Two-Level Blockchain with Proof-of-Proof Consensus Protocol for Different Consensus Types in Mainchain/Sidechain to Prevent Double Spend Attack. I. PoS in Mainchain and PoW in Sidechain

The article considers the issues of the secure operation of a two-level blockchain with a complex mixed consensus protocol, i.e., Proof-of-Stake in the main blockchain (mainchain) and Proof-of-Work in the secondary one (sidechain). This two-level blockchain is constructed on the principle of the Proof-of-Proof protocol, where the safety of the sidechain is ensured by the stability of the mainchain, by referring the mainchain blocks to the sidechain blocks using special transactions. Such a structure allows faster block creation in the sidechain and, accordingly, faster processing of transactions without the loss of security and without increasing the volume of the block. Such a two-level blockchain is of the greatest interest for the creation of a cascade system of state registers, which guarantees the security against the substitution and forgery of documents. The main results of the article are explicit analytical expressions for estimates of probability of double spend attack on a sidechain in a two-level blockchain, under the condition of presence of an adversary in the sidechain and the mainchain.