I've been thinking about ways to take advantage of multiple asymmetric encryption schemes so that each key could be a backup to the others in case of cryptanalysis of one given scheme. Those public key schemes could each secure one share of a symmetric key splitting scheme (as answered in Is there a multiple asymmetric encryption algorithm, which requires all private keys to reveal the secret?).

Something similar could also apply to digital signatures (just requiring all signatures to be valid) or key sharing schemes (just inputting multiple shared secrets into one KDF).

Is there precedent for that?

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    $\begingroup$ I think Poncho's recent answer covers more than you need on this question Post quantum hybrid model and its security that cover classical and post-quantum adversaries, too. $\endgroup$
    – kelalaka
    Sep 20, 2022 at 21:08
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    $\begingroup$ Also see this article "A hybrid is a combination of a classical and a post-quantum scheme. For instance, we can combine Kyber512 with X25519 to create a single Kyber512X key agreement. The advantage of a hybrid is that the data remains secure against non-quantum attackers even if Kyber512 turns out broken." $\endgroup$
    – knaccc
    Sep 20, 2022 at 21:12
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    $\begingroup$ This is the general gist of all discussions about PQC algorithms at the time. There isn't just a precedent, it's the main line for most. For instance, there have been discussions how we should adjust X509 certificates to host multiple keys. $\endgroup$
    – Maarten Bodewes
    Sep 20, 2022 at 23:29


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