eCash

Most people who read the Bitcoin whitepaper focus on its explanation of Proof-of-Work (PoW) and how blocks are chained together. That’s understandable, as it’s the core innovation. But buried in plain sight is a lesser-known line that hints at something quite important: the notion that nodes need to agree on a transaction being the first seen long before being confirmed in a block.
In Section 2, Satoshi writes:
“The payee needs proof that at the time of each transaction, the majority of nodes agreed it was the first received.” 
The paper then goes on to describe how this proof is ultimately provided by mining and block confirmations over the course of minutes and hours. But he didn’t describe what kind of proof a payee should expect to accept instant payments, or how nodes would coordinate to agree on the order of transactions in real time. That’s a different kind of consensus which needs to happen live on the network. Satoshi didn’t define how this would work. But by stating this need in the paper, he acknowledged a necessity in Bitcoin that still exists today, even on other PoW blockchains.

The First Seen Rule
In early Bitcoin, there was a simple node policy to handle conflicting transactions, known as the “first seen rule”. When two transactions tried to spend the same input (i.e., a double spend), nodes would accept and relay the first one they saw and reject any conflicting ones that arrived later. This works under the assumption that honest nodes mostly see the same transaction first, making it a cheap way to filter out double spends. But in practice, this is not a reliable solution. An attacker could exploit network latency or partitioning to ensure different parts of the network saw different transactions first, making it impossible for the network as a whole to agree on which of them is the legitimate one. This makes it possible to double-spend coins in 0-confirmation payment environments.
The first seen rule is local, subjective, and easily manipulated. In short, it lacks global coordination. That’s why it’s not sufficient for secure instant payments. It is important for solutions to work even in adversarial conditions, so there was always a need for a solid way to come to a network-wide agreement on which transaction came first, not just what one node happened to see first.
Hybrid Consensus
With the breakthrough invention of the Avalanche consensus mechanism in 2018, this need could finally be met in an elegant way. This new protocol allows nodes to rapidly reach an agreement on which transactions came first using randomized polling and probabilistic consensus. This gives users near-instant assurance that their payment is accepted and safe from double spends, even before mining takes place.
The eCash project is the only PoW network that is actually implementing this protocol today and one of only two projects that leverage Avalanche consensus at all—the other being the AVAX proof-of-stake (PoS) blockchain. By integrating Avalanche with its core Nakamoto PoW, eCash enables near-instant consensus on transaction order and validity. This improves user experience, strengthens security, enables scaling, and completes Bitcoin’s payment use-case.
The idea of improving the payment experience through faster transaction finality has been part of Bitcoin ABC’s vision since the beginning of the project in 2017. Recognizing that waiting for block confirmations limits usability, especially in real-world payments, the team introduced the concept of Pre-Consensus as a mechanism for nodes to agree on transaction validity and order before mining occurs. This lays the foundation for a more responsive peer-to-peer cash system while aligning directly with Satoshi’s insight.
With the advent of the Avalanche whitepaper, Bitcoin ABC immediately recognized that it provided the capabilities needed to realize this vision. By integrating it into eCash, the team created a hybrid consensus model that fuses Nakamoto PoW with Avalanche coordination. But unlike typical Masternode-based hybrids, Avalanche on eCash doesn’t delegate power to privileged nodes, nor are its stake proofs part of the transaction validation process. For an eCash node to join the Avalanche quorum, it must first bootstrap using PoW to get an objective state of the network. This PoW bootstrapping allows nodes to join the Avalanche-validating quorum in a trustless decentralized manner – a key advantage over purely Proof-of-Stake systems that require trusted bootstrapping. The integration is optional and enhances eCash’s PoW itself through probabilistic agreement among all participating nodes.
This model is best described as Avalanche-enhanced Proof-of-Work (APoW). It maintains the trustless nature of mining while adding real-time coordination to improve security, user experience, and scalability. With APoW, eCash achieves what many chains have attempted: a fast, secure, and decentralized payment layer that remains faithful to Bitcoin’s original principles while pushing its capabilities forward.
The Two Pillars of Instant Finality
Avalanche on eCash operates through two core layers: Pre-Consensus and Post-Consensus. Pre-Consensus allows the network to agree on transaction ordering in real time, while Post-Consensus ensures that blocks align with what was previously agreed upon. The two work together: Pre-Consensus provides fast coordination, but it is Post-Consensus that gives the network the power to enforce those decisions by rejecting blocks containing transactions that conflict with those already finalized by Pre-Consensus. Without Post-Consensus, there’s no mechanism to uphold what was finalized on the Pre-Consensus layer.
Avalanche Post-Consensus works by having nodes finalize the order and validity of blocks after they’ve been mined. Once a block is created through PoW, Avalanche is used to poll other nodes and reach an agreement that this block is the correct one to build on. This adds an extra layer of certainty beyond the Nakamoto consensus, helping prevent chain reorgs giving eCash 51% attack protection. It also acts as the foundation for Pre-Consensus. It doesn't replace mining but strengthens the confidence in each block's place in the chain.
Pre-Consensus takes this idea further by applying the same mechanism to each transaction before they even enter a block. Instead of waiting ten minutes for a miner to confirm which transaction came first by including it in a block, nodes can already coordinate and agree on transaction ordering using Avalanche polling. This means that if two conflicting transactions are broadcast, the network can quickly converge on which one to accept as valid, long before a block is found. It brings resolution within 2 seconds at the mempool level, providing near-instant feedback and protecting against double spends in real-time.
A practical example for end-users is deposit times on exchanges and multicoin wallets. Currently, eCash (XEC) users enjoy having deposits credited with just 1 confirmation. Compared to Bitcoin (BTC), which requires about 4 confirmations before exchanges feel safe enough to credit deposits, eCash’s Post-Consensus finality already surpasses transaction settlement speeds of most chains. This competitive speed of about 10 minutes will soon be surpassed with eCash’s record-breaking 2-second finality time once Pre-Consensus goes live. This finality speed is unheard of for fully trustless PoW blockchains.
Scaling eCash with Avalanche

Pre-Consensus isn’t just about instant transaction finality. It’s a foundational shift that enables eCash to scale far beyond traditional Bitcoin architecture. At the heart of this is how Avalanche handles the global state. Unlike Nakamoto Consensus, which relies on block production to resolve conflicts, Avalanche allows nodes to coordinate on state transitions far more rapidly.
One of its most powerful implications is how it enables real-time processing. In the case of block generation, nodes use Avalanche to coordinate on which transactions to include in a block before a newly mined block is broadcast, so they’re already aligned on what a valid block should contain and how it would look like. This means that by the time a miner finds a new block, most nodes already agree on 99% of the block’s contents. There’s no need for every node to process the block in the tiny time window after it's mined, as it enables processing the block during the entire 10-minute block finding process. This drastically reduces latency and computational overhead, making it possible to handle much larger blocks without sacrificing network responsiveness.
In effect, Avalanche allows eCash to decouple consensus from mining speed. It transforms block production into a finalization step rather than a coordination bottleneck. This architecture lays the groundwork for scaling to gigabyte-sized or even terabyte-sized blocks in the future. Enabling massive transaction throughput while keeping processing efficient and confirmation times low.
Sometimes, visionary ideas take years to materialize. What Satoshi hinted at in Section 2 of the Bitcoin whitepaper—a need for proof that the network agreed on a transaction’s order before confirmation—is now being realized on eCash. This is a continuation of Bitcoin’s rules, made possible by another groundbreaking invention that is Avalanche. As we revisit the whitepaper with fresh eyes, we find that some of its most powerful ideas have always lingered in plain sight and have never been fully explored until now.
Originally Published: https://e.cash/blog/satoshis-hint