Oddbean new post about login | logout How does someone on the receiving end decode which keys were used to sign the event after the fact?
"MuSig allows multiple users each with their own private key to create a combined public key that’s indistinguishable from any other schnorr pubkey, including being the same size as a single-user pubkey" via https://bitcoinops.org/en/topics/musig/
You can't "decode", but you can do `nak key combine` and pass in two pubkeys and it check if the result matches the pubkey in the cooperatively-signed event. The problem is that the two pubkeys you pass in require the 02/03 prefix byte that we have stupidly removed from npubs.