As a layperson, I'm curious if a cryptocurrency could fork to a quantum-resistant algorithm without people losing access to their wallets. Is it possible to generate a quantum-resistant public key from the wallet's private key (that was used before the fork)?

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    $\begingroup$ Ah, I just realized that, since the vulnerable public keys will still be accessible to criminals, generating a quantum-resistant public key is pointless. Nonetheless, I'm interested in the answer. $\endgroup$
    – aleclarson
    Mar 30 at 14:29
  • $\begingroup$ There are (probably) ways to prove possession of the private key as needed for signatures that are post-quantum even for key formats as used by RSA. However for them to make sense you need a sort of adress which as I understand it tends to be the public key - allowing an adversary to use the quantum attack to recover the private key. $\endgroup$
    – SEJPM
    Mar 30 at 14:49
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    $\begingroup$ Are you asking the question in the title, or the one in the body ? The title is answerable, by no as you found by yourself, under the standard assumption the RSA public key is public, with a little on-topic technicality (with usual definitions of RSA, it's not possible to unambiguously find the RSA private key from the public key, but so few choices are common that it's typically possible to enumerate all the likely private keys). The question in the body is vague (what exactly is a fork?), and deals with organizational aspects of cryptocurrencies, this is off-topic. $\endgroup$
    – fgrieu
    Mar 30 at 14:50
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    $\begingroup$ Actually, why would you need to associate the two different keys? Wouldn't be sufficient (assuming BitCoin supported a postquantum algorithm (hash based signatures, Dilithium/Falcon, whatever) as well as ECDSA; then what someone could do a generate an address based on a postquantum algorithm, and issue transactions that transfer all their bitcoins from their old (ECDSA-based) address to their new one. Or, am I missing something? $\endgroup$
    – poncho
    Mar 31 at 3:02
  • $\begingroup$ @poncho If a QC attack was hijacking transactions, Bitcoin would have no choice but to hard fork, so as to restore funds to their original owners and prevent future attacks on unprotected wallets. (See §3.2 in royalsocietypublishing.org/doi/pdf/10.1098/rsos.180410) $\endgroup$
    – aleclarson
    Mar 31 at 15:33

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