A Coinbase-led study said zero-knowledge privacy systems used by Railgun, Privacy Pools, Aleo and Aztec are mathematically immune to quantum attacks. The researchers said Bitcoin remains more exposed because about 6.9 million BTC sit in addresses with revealed public keys, including about 1.7 million old pay-to-public-key outputs. The study said blockchains should prepare for large-scale fault-tolerant quantum computers, but the threat is not imminent.