Publish your private keys and be considered responsible for what is signed with them, simple really :)
I am not a fan of blockchains but this is a rare case where they could be used to prove that the message is authentic by publishing the hash of the message (including the signature) in it. If the DKIM private key was published after the message was added to the blockchain then the message was clearly legitimate.
Your claim about "x64 assembler" is dubious.