An interesting priority for Bitcoin cash is an accurate Zero-confirmation system, the importance of this was underlined at the recent Satoshi's vision conference. The concept of cash requires that a transaction can be verified and confirmed within a 3 second timeframe. If adoption is to become widespread, Bitcoin cash must be more versatile and as simple as fiat. So lets outline at 10,000 feet what a potential Zero-confirmation system might look like. The anti-fragile nature of Bitcoin is held together by incentives, if Bitcoin cash attempts to use altruism as a method to pursue Zero-conf, history has demonstrated that it will fail.

Lets begin with quorums, since these represent a weak form of consensus validation. Quorums are oddly numbered clusters of nodes which work in a cooperative setting such that a majority of the participants will reach a consensus upon a truth, if the majority of the nodes are dishonest, the nodes will agree upon the dishonest outcome. Quorums suffer from a "membership problem", as adversaries who can rapidly deploy consensus nodes can vote to disrupt valid outcomes at a low cost. This is why quorums and federated quorums are only suitable to work within a permissioned environment. The permissioned paradigm however, is orthogonal to the economic design parameters of Bitcoin. It is required that new participants can enter the arena of mining in order to combat the force of centralization.

The membership problem can be mitigated by using a past proof of work. Past proofs can be issued by a mining node writing a public key into the coinbase output upon mining a block. That public key can in turn as an input, write N child pubkeys into a transaction placed within the same block. The signature of the child keys can then be used by a node as an authorization mechanism to join a Zero confirmation validation pool. The child keys are mostly a convenience since all child nodes must be considered a single entity. This partially solves the membership problem without making the system permissioned. Any miner can enter the validation pool, but now adversarial nodes come at a cost.

So what is the primary concern of the Zero-confirmation validators? Resolving transactions with the highest certainty within the shortest timespan. Consensus needs to converge quickly without cross referencing the entire validation pool. It turns out that there exists a nice way to break up validation and route transactions using a distributed hash table. A DHT is an overlay routing network which maps nodes to an identity space of n bits. In its stardard form, it means each node in the DHT is mapped uniquely to an identity. The identity space which is of interest is the identity space of all possible transactions; the bitspace of SHA256. It is possible to segment the bitspace into groups, this can be done simply by truncating bits away from the first n bits of the identity space. A hash table segmented into 255 possible groups would simply use the first 8 bits the identity. This DHT routing structure has been researched and is called a ZRing [ 1 ][ 2 ]

Remember the quorum? Quorums can be placed within each group of the segmented DHT to validate the order of a subsection of all transactions. But If a group quorum was taken over, the group could allow double spends for a large number of transactions. Actually it is worse, an attacker could plan double spends by brute forcing transaction hashes that only routed to a bitspace in the DHT that they planned to control in the future. In order to circumvent problematic groups, the DHT group join process would consist of proving membership via the past proof of work and signing a staking transaction for an amount X that serves as a bet that the quorum will not double spend any transactions in the next block. The outputs of the transaction go to the validator nodes outside of that particular quorum group and would require > 50% threshold signature amount has been signed by the remaining validator pool. Groups would renew transactions every block interval. The sybil attack resistance is then the entire validation pool resistance minus the groupsize of the ZRing.