  Current solutions to quantum vulnerabilities of widely used cryptographic
schemes involve migrating users to post-quantum schemes before quantum attacks
become feasible. This work deals with protecting quantum procrastinators: users
that failed to migrate to post-quantum cryptography in time.
  To address this problem in the context of digital signatures, we introduce a
technique called signature lifting, that allows us to lift a deployed
pre-quantum signature scheme satisfying a certain property to a post-quantum
signature scheme that uses the same keys. Informally, the said property is that
a post-quantum one-way function is used "somewhere along the way" to derive the
public-key from the secret-key. Our constructions of signature lifting relies
heavily on the post-quantum digital signature scheme Picnic (Chase et al.,
CCS'17).
  Our main case-study is cryptocurrencies, where this property holds in two
scenarios: when the public-key is generated via a key-derivation function or
when the public-key hash is posted instead of the public-key itself. We propose
a modification, based on signature lifting, that can be applied in many
cryptocurrencies for securely spending pre-quantum coins in presence of quantum
adversaries. Our construction improves upon existing constructions in two major
ways: it is not limited to pre-quantum coins whose ECDSA public-key has been
kept secret (and in particular, it handles all coins that are stored in
addresses generated by HD wallets), and it does not require access to
post-quantum coins or using side payments to pay for posting the transaction.
