In light of the emerging threat of powerful quantum computers appearing in the near future, we investigate the potential attacks on
Bitcoin available to a quantum-capable adversary. In particular, we illustrate how Shor’s quantum algorithm can be used to forge ECDSA based
signatures, allowing attackers to hijack transactions. We then propose
a simple commit–delay–reveal protocol, which allows users to securely
move their funds from non-quantum-resistant outputs to those adhering
to a quantum-resistant digital signature scheme. In a previous paper [34]
we presented a similar scheme with a long fixed delay. Here we improve
on our previous work, by allowing each user to choose their preferred
delay – long for a low risk of attack, or short if a higher risk is acceptable
to that user. As before, our scheme requires modifications to the Bitcoin
protocol, but once again these can be implemented as a soft fork.