On the security of Bitcoin and DAG-based performance solutions

The Bitcoin protocol has been proposed as a decentralized payment system in which any participant can verify their transactions. However, it performs significantly worse than its centralized counterparts, and many modifications have been proposed since its inception with the aim of improving performance. In this thesis, we explore a category of protocols that use an alternative chain selection rule or use a directed acyclic graph of blocks instead of a blockchain, with a focus on their security guarantees.

We begin by describing the Bitcoin protocol and presenting an overview of its security proofs over time, with the aim of understanding the proof methods and the resulting performance limitations. Then, we describe GHOST, which uses the GHOST rule instead of the longest chain rule of Bitcoin and serves as a stepping stone for DAG-based protocols such as PHANTOM, GHOSTDAG, SPECTRE, and Conflux. We also present an attempt for an alternative proof of the security of GHOST. Finally, we describe these DAG-based protocols and present their security guarantees.