Multisig means a payment needs approval from more than one key. The protocol already supports it; but it lacks the ability to change the addresses pre-configured to participate in an existing multisig. This milestone also opens the door to explore different options like using a decoy key, one that unlocks only a fake balance. This option is weak against a determined attacker, so the more robust direction is a partial-custody safeguard, where a trusted service holds one of the keys needed to approve transactions.
