I'm looking for a way for a single individual to sign or otherwise provide proof of ownership of two private keys from keypairs $A$ & $B$.
Simultaneous ownership is important. Alice can't do part of the proof with $A$, then pass the proof to Bob to finish with $B$. The proof requires both keys in a single step. E.g. Signing a message with $A$ then $B$ does not work, as this work could be split between two actors.
Both $A$ & $B$ keypairs are normal, independent keypairs. I.e. this isn't about splitting a single private key into multiple parts to be reconstructed.
Any hints are appreciated, though eventually I'd want to implement this using RSA or ECDSA.
Update: Some further boundaries:
- This doesn't have to be a single step, or a single signature. As long as it can't be colluded on by two parties without exposing their private keys, any process is acceptable.