orWhoever knows the secret can use it to redeem the output.
Routed Payment Channels (Lightning Network)
閃電網絡允許任何人在無須信任中間節點的情況下，通過管道向其他人路由一筆支付。這項功能由於需要用到隔離見證，目前只是在測試網使用。
The Lightning Network is a proposed routed network of bidirectional payment channels connected end-to-end. A network like this can allow any participant to route a payment from channel to channel without trusting any ofthe intermediaries. The Lightning Network was first described by Joseph Poon and Thadeus Dryja in February 2015, building on the concept of payment channels as proposed and elaborated upon by many others.
"Lightning Network" refers to a specific design for a routed payment channel network, which has now been implemented by at least five different open source teams. The independent implementations are coordinated by a set of interoperability standards described in the Basics of Lightning Technology (BOLT) paper.
Prototype implementations of the Lightning Network have been released by several teams. For now, these implementations can only be run on testnet because they use segwit, which is not activated on the main bitcoin blockchain (mainnet).
The Lightning Network is one possible way of implementing routed payment channels. There are several other designs that aim to achieve similar goals, such as Teechan and Tumblebit.
Basic Lightning Network Example
Let’s see how this works.
In this example, we have five participants: Alice, Bob, Carol, Diana, and Eric. These five participants have opened payment channels with each other, in pairs. Alice has a payment channel with Bob. Bob is connected to Carol, Carol to Diana, and Diana to Eric. For simplicity let’s assume each channel is funded with 2 bitcoin by each participant, for a total capacity of 4 bitcoin in each channel.
A series of bidirectional payment channels linked to form a Lightning Network that can route a payment from Alice to Eric shows five participants in a Lightning Network, connected by bidirectional payment channels that can be linked to makea payment from Alice to Eric (Routed Payment Channels (Lightning Network)).
Figure 9. Aseries of bidirectional payment channels linked to form a Lightning Network that can route a payment from Alice to Eric
Alice wants to pay Eric 1 bitcoin.However, Alice is not connected to Eric by a payment channel. Creating a payment channel requires a funding transaction, which must be committed to the bitcoin blockchain. Alice does not want to open a new payment channel and commit more of her funds. Is there a way to pay Eric, indirectly?
Step-by-step payment routing through a Lightning Network shows the step-by-step process of routing a payment from Alice to Eric, through a series of HTLC commitments on the payment channels connecting the participants.
Figure 10.Step-by-step payment routing through a Lightning Network
Alice is running a Lightning Network(LN) node that is keeping track of her payment channel to Bob and has the ability to discover routes between payment channels. Alice’s LN node also has the ability to connect over the internet to Eric’s LN node. Eric’s LN node creates a secret R using a random number generator. Eric’s node does not reveal this secret to anyone. Instead, Eric’s node calculates a hash H of the secret R and transmits this hash to Alice’snode (see Step-by-step payment routing through a Lightning Network step 1).Eric計算H，併發給Alice。然後額外的費用會用於補償中間節點。
Now Alice’s LN node constructs a route between Alice’s LN node and Eric’s LN node. The routing algorithm used will be examined in more detail later, but for now let’s assume that Alice’s node can find an efficient route.
Alice’s node then constructs an HTLC, payable to the hash H, with a 10-block refund timeout (current block + 10), for an amount of 1.003 bitcoin (see Step-by-step payment routing through a Lightning Network step 2). The extra 0.003 will be used to compensate the intermediate nodes for their participation in this payment route. Alice offers this HTLC to Bob, deducting 1.003 bitcoin from her channel balance with Bob and committing it to the HTLC. The HTLC has the following meaning:"Alice is committing 1.003 of her channel balance to be paid to Bob if Bob knows the secret, or refunded back to Alice’s balance if 10 blocks elapse."The channel balance between Alice and Bob is now expressed by commitment transactions with three outputs: 2 bitcoin balance to Bob, 0.997 bitcoin balance to Alice, 1.003 bitcoin committed in Alice’s HTLC. Alice’s balance is reduced by the amount committed to the HTLC.
Bob now has a commitment that if he is able to get the secret R within the next 10 blocks, he can claim the 1.003 locked by Alice. With this commitment in hand, Bob’s node constructs an HTLC on his payment channel with Carol. Bob’s HTLC commits 1.002 bitcoin to hash H for 9 blocks, which Carol can redeem if she has secret R (see Step-by-step payment routing through a Lightning Network step 3). Bob knows that if Carol can claim his HTLC, she has to produce R. If Bob has R in nine blocks, he can use it to claim Alice’s HTLC to him. He also makes 0.001 bitcoin for committing his channel balance for nine blocks. If Carol is unable to claim his HTLC and he is unable to claim Alice’s HTLC, everything reverts back to the prior channel balances and no one is at a loss. The channel balance between Bob and Carol is now: 2 to Carol, 0.998 to Bob, 1.002 committed by Bob to the HTLC.
Carol now has a commitment that if she gets R within the next nine blocks, she can claim 1.002 bitcoin locked by Bob. Now she can make an HTLC commitment on her channel with Diana. She commits an HTLC of 1.001 bitcoin to hash H, for eight blocks, which Diana can redeem if she has secret R (see Step-by-step payment routing through a Lightning Network step 4). From Carol’s perspective,if this works she is 0.001 bitcoin better off and if it doesn’t she loses nothing. Her HTLC to Diana is only viable if R is revealed, at which point she can claim the HTLC from Bob. The channel balance between Carol and Diana is now: 2 to Diana, 0.999 to Carol, 1.001 committed by Carol to the HTLC.
Finally, Diana can offer an HTLC to Eric, committing 1 bitcoin for seven blocks to hash H (see Step-by-step payment routing through a Lightning Network step 5). The channel balance between Diana and Eric is now: 2 to Eric, 1 to Diana, 1 committed by Diana tothe HTLC.
However, at this hop in the route, Eric hassecret R. He can therefore claim the HTLC offered byDiana. He sends R to Diana and claims the 1 bitcoin, adding it to his channel balance (see Step-by-step payment routing through a Lightning Networkstep 6). The channel balance is now: 1 to Diana, 3 to Eric.
Now, Diana has secret R. Therefore, she can now claim the HTLC from Carol. Diana transmits R to Carol and adds the 1.001 bitcoin to her channel balance(see Step-by-step payment routing through a Lightning Network step 7). Now the channel balance between Carol and Diana is: 0.999 to Carol, 3.001 to Diana. Diana has"earned" 0.001 for participating in this payment route.
Flowing back through the route, the secret R allows each participant to claim the outstanding HTLCs. Carol claims 1.002 from Bob, setting the balance on their channel to: 0.998 to Bob, 3.002 to Carol (see Step-by-step payment routing through a Lightning Network step 8). Finally, Bob claims the HTLC from Alice (see Step-by-step payment routing through a Lightning Network step 9). Their channel balance is updated as: 0.997 to Alice, 3.003 to Bob.
通過層層級聯，Alice實現了對Eric的支付，而不需要額外開通通道。對於中間節點，還能賺到一些費用。
Alice has paid Eric 1 bitcoin without opening a channel to Eric. None of the intermediate parties in the payment route had to trust each other.For the short-term commitment of their funds in the channel they are able to earn a small fee, with the only risk being a small delay in refund if the channel was closed or the routed payment failed.
Lightning Network Transport and Routing
All communications between LN nodes are encrypted point-to-point. In addition, nodes have a long-term public key that they use as an identifier and to authenticate each other.
Whenever a node wishes to send a payment to another node, it must first construct a paththrough the network by connecting payment channels with sufficient capacity. Nodes advertise routing information, including what channels they have open, how much capacity each channel has, and what fees they charge to route payments. The routing information can be shared in a variety of ways and different routing protocols are likely to emerge as Lightning Network technology advances. Some Lightning Network implementations use the IRC protocol as a convenient mechanism for nodes to announce routing information. Another implementation of route discovery uses a P2P model where nodes propagate channel announcements to their peers, in a "flooding" model, similar to how bitcoin propagates transactions. Future plans include a proposal called Flare, which is a hybrid routing model with local node"neighborhoods" and longer-range beacon nodes.
In our previous example, Alice’s node uses one of these route discovery mechanisms to find one or more paths connecting her node to Eric’s node. Once Alice’s node has constructed a path,she will initialize that path through the network, by propagating a series of encrypted and nested instructions to connect each of the adjacent payment channels.
Importantly, this path is only known to Alice’s node. All other participants in the payment route see only the adjacent nodes. From Carol’s perspective, this looks like a payment from Bob to Diana.Carol does not know that Bob is actually relaying a payment from Alice. She also doesn’t know that Diana will be relaying a payment to Eric.
This is a critical feature of the Lightning Network, because it ensures privacy of payments and makes it very difficult to apply surveillance, censorship, or blacklists. But how does Alice establish this payment path, without revealing anything to the intermediary nodes?
The Lightning Network implements an onion-routed protocol based on a scheme called Sphinx. This routing protocol ensures that a payment sender can construct and communicate a path through the Lightning Network such that:
Intermediate nodes can verify and decrypt their portion of route information and find the next hop.
Other than the previous and next hops, they cannot learn about any other nodes that are part of the path.
They cannot identify the length of the payment path, or their own position in that path.
Each part of the path is encrypted in such a way that a network-level attacker cannot associate the packets from different parts of the path to each other.
Unlike Tor (an onion-routed anonymization protocol on the internet), there are no "exit nodes" that can be placed under surveillance. The payments do not need to be transmitted to the bitcoin blockchain; the nodes just update channel balances.
Using this onion-routed protocol, Alice wraps each element of the path in a layer of encryption, starting with the endand working backward. She encrypts a message to Eric with Eric’s public key.This message is wrapped in a message encrypted to Diana, identifying Eric asthe next recipient. The message to Diana is wrapped in a message encrypted to Carol’s public key and identifying Diana as the next recipient. The message to Carol is encrypted to Bob’s key. Thus, Alice has constructed this encrypted multilayer "onion" of messages. She sends this to Bob, who can only decrypt and unwrap the outer layer. Inside, Bob finds a message addressed to Carol that he can forward to Carol but cannot decipher himself. Following the path, the messages get forwarded, decrypted, forwarded, etc., all the way to Eric. Each participant knows only the previous and next node in each hop.
Each element of the path contains information on the HTLC that must be extended to the next hop, the amount that is being sent, the fee to include, and the CLTV locktime (in blocks) expiration of the HTLC. As the route information propagates, the nodes make HTLC commitments forward to the next hop.
中間節點路徑限制到20跳，並用隨機數據填充。只有最終的接收方才知道沒有下一跳，於是對每一箇中間節點，看上去都還有20跳。
At this point, you might be wondering how it is possible that the nodes do not know the length of the path and their position in that path. After all, they receive a message and forward it to the next hop. Doesn’t it get shorter, allowing them to deduce the path size and their position? To prevent this, the path is always fixed at 20 hops and padded with random data. Each node sees the next hop and a fixed-length encrypted message to forward. Only the final recipient sees that there is no next hop.To everyone else it seems as if there are always 20 more hops to go.
Lightning Network Benefits
A Lightning Network is a second-layer routing technology. It can be applied to any blockchain that supports some basic capabilities, such as multisignature transactions, timelocks, and basicsmart contracts.
If a Lightning Network is layered on top of the bitcoin network, the bitcoin network can gain a significant increase in capacity, privacy, granularity, and speed, without sacrificing the principles of trustless operation without intermediaries:
Privacy
Lightning Network payments are much more private than payments on the bitcoin blockchain,as they are not public. While participants in a route can see payments propagated across their channels, they do not know the sender or recipient.
Fungibility
A Lightning Network makes it much more difficult to apply surveillance and blacklists on bitcoin, increasing the fungibility of the currency.
Speed
Bitcoin transactions using Lightning Network are settled in milliseconds, rather than minutes, as HTLCs are cleared without committing transactions to a block.採用閃電網絡，比特幣交易可以在毫秒級完成。
Granularity
A LightningNetwork can enable payments at least as small as the bitcoin "dust"limit, perhaps even smaller. Some proposals allow for subsatoshi increments.
Capacity
A Lightning Network increases the capacity of the bitcoin system by several orders of magnitude. There is no practical upper bound to the number of payments per second that can be routed over a Lightning Network, as it depends only on the capacity and speed of each node.在閃電網絡中，每秒可支持的交易，幾乎沒有上限。
Trustless Operation
A Lightning Network uses bitcoin transactions between nodes that operate as peers without trusting each other. Thus, a Lightning Network preserves the principles of the bitcoin system, while expanding its operating parameters significantly.
Of course, as mentioned previously, the Lightning Network protocol is not the only way to implement routed payment channels. Other proposed systems include Tumblebit and Teechan. At this time,however, the Lightning Network has already been deployed on testnet. Several different teams have developed competing implementations of LN and are working toward a common interoperability standard (called BOLT). It is likely that Lightning Network will be the first routed payment channel network to be deployed in production.
閃電網絡目前已經在測試網絡部署，目前有不同的實施方案。
Conclusion
We have examined just a few of the emerging applications that can be built using the bitcoin blockchain as a trust platform. These applications expand the scope of bitcoin beyond payments and beyond financial instruments, to encompass many other applications where trust is critical. By decentralizing the basis of trust, the bitcoin blockchain is a platform that will spawn many revolutionary applications in a wide variety of industries.
歡迎大家關注我的新微信公眾號，“刻意學習區塊鏈”，我會把我所有關於區塊鏈和比特幣學習解析的文章，彙總在上面便於檢索，這是ScalersTalk成長持續論的一個分叉。 搜索“刻意學習區塊鏈”或者長按掃二維碼關注。
持 續 力
★★★★★
ScalersTalk成長會是由Scalers發起的社群生態體系，專注1000天以上的“N階持續行動理論體系與能力構建”，以“從英語初階到同聲傳譯全棧解決方案”為特色，以“持續輸入輸出訓練實踐拓寬認知邊界”為導向。
新 浪 微 博 l
@Scalers網 站 l ScalersTalk.com
開 放 社 群 l 100小時訓練QQ群 456036104
暢 銷 書 籍 l 《刻意學習》火熱銷售中
★★★★★
2018年成長會申請說明
▼
本文原文：http://www.scalerstalk.com/1365-MasterBTC12
閱讀更多 Scalers 的文章
