  Quantum computation represents a threat to many cryptographic protocols in
operation today. It has been estimated that by 2035, there will exist a quantum
computer capable of breaking the vital cryptographic scheme RSA2048. Blockchain
technologies rely on cryptographic protocols for many of their essential
sub-routines. Some of these protocols, but not all, are open to quantum
attacks. Here we analyze the major blockchain-based cryptocurrencies deployed
today -- including Bitcoin, Ethereum, Litecoin and ZCash, and determine their
risk exposure to quantum attacks. We finish with a comparative analysis of the
studied cryptocurrencies and their underlying blockchain technologies and their
relative levels of vulnerability to quantum attacks.
