PHANTOM, GHOSTDAG

PHANTOM, GHOSTDAG:
https://eprint.iacr.org/2018/104.pdf
Two Scalable BlockDAG protocols 
Yonatan Sompolinsky and Aviv Zohar School of Engineering and Computer Science, The Hebrew University of Jerusalem, Israel {yoni sompo,avivz}@cs.huji.ac.il ✦

Abstract
 In 2008 Satoshi Nakamoto invented the basis for blockchain based distributed ledgers. The core concept of this system is an open and anonymous network of nodes, or miners, which together maintain a public ledger of transactions. The ledger takes the form of a chain of blocks, the blockchain, where each block is a batch of new transactions collected from users. One primary problem with Satoshi’s blockchain is its highly limited scalability. The security of Satoshi’s longest chain rule, more generally known as the Bitcoin protocol, requires that all honest nodes be aware of each other’s blocks very soon after the block’s creation. To this end, the throughput of the system is artificially suppressed so that each block fully propagates before the next one is created, and that very few “orphan blocks” that fork the chain be created spontaneously. In this paper we present PHANTOM, a PoW based protocol for a permissionless ledger that generalizes Nakamoto’s blockchain to a direct acyclic graph of blocks (blockDAG). PHANTOM includes a parameter k that controls the level of tolerance of the protocol to blocks that were created concurrently, which can be set to accommodate higher throughput. It thus avoids the security-scalability tradeoff which Satoshi’s protocol suffers from. PHANTOM uses a greedy algorithm on the blockDAG to distinguish between blocks mined properly by honest nodes and those that created by non-cooperating nodes who chose to deviate from the mining protocol. Using this distinction, PHANTOM provides a robust total order on the blockDAG in a way that is eventually agreed upon by all honest nodes.