Bitcoin and blockchain systems brought us to the brink of a technological revolution: these systems allow us to bypass the need for centralized trusted entities to run protocols on a large scale. However, the decentralized nature of these systems brings unique challenges, including user authentication. While cryptography provides strong solutions to this problem, it often relies on the ability of users to reliably store a large amount of secret information. What happens if one user’s key is lost or stolen? Blockchain systems lack traditional fallback mechanisms that allow one to recover from such an event. In this proposal, we initiate the study of two-factor authentication mechanisms over blockchain and distributed systems as a solution for this problem, and we explore solutions based on established cryptographic primitives. We aim for the design of new cryptographic schemes that are efficient enough to be practically applicable and for a comprehensive understanding of their security guarantees.