OK. Challenge accepted. Alice and Bob both use asymmetric keys, and use one of a thousand different methods to exchange their public keys (hey, they can even g

OK. Challenge accepted. Alice and Bob both use asymmetric keys, and use one of a thousand different methods to exchange their public keys (hey, they can even go in the Merkle tree). Alice signs a transaction using her private key and publishes it. Bob can now use Alice's public key to verify the signature on the transaction. No private/secret key exposure. Asymmetric encryption is a fairly well explored field, which is what makes the above both easy to reason about and secure.