[HexagonalKitten]

If I send you a signed message from a trusted key, your "audit trail" is to save that message. Super simple. And if I have a message ID in each you could even show that there aren't any missed messages, if you wanted.

A blockchain is just an incredibly inefficient way to store those messages, giving them an explicit ordering in case they don't have internal IDs, and to establish ordering between users. All of that can be done with a simple 'index server' that doles out ordered message IDs, without the blocks, the mining, etc.

Banks don't even need the currency. Banks just settle their debts in their local currency and they trust enforcement to the courts and bailiffs. This is what Ripple had right, in the beginning. They only need a messaging layer.