Optimistic Fast Confirmation While Tolerating Malicious Majority in Blockchains

The robustness of a blockchain against the adversary is often characterized by the maximum fraction (fmax) of adversarial power that it can tolerate. While most existing blockchains can only tolerate ${f_{\max }} < \frac{1}{2}$ or lower, there are some blockchain systems that are able to tolerate a malicious majority, namely ${f_{\max }} \geq \frac{1}{2}$. A key price paid by such blockchains, however, is their large confirmation latency. This work aims to significantly reduce the confirmation latency in such blockchains, under the common case where the actual fraction f of adversarial power is relatively small. To this end, we propose a novel blockchain called Flint. Flint tolerates ${f_{\max }} \geq \frac{1}{2}$ and can give optimistic execution (i.e., fast confirmation) whenever f is relatively small. Our experiments show that the fast confirmation in Flint only takes a few minutes, as compared to several hours of confirmation latency in prior works.