LotMint

Blockchain Returning to Decentralization with Decentralized Clock, forked from dedis/cothority project.

Abstract

We present LotMint, a permissionless blockchain, with a purposely low set bar for Proof-of-Work (PoW) difficulty. Our objective is for personal computers, cloud virtual machines or containers, even mobile devices, and hopefully future IoT devices, to become the main, widely distributed, collectively much securer, fairer, more reliable and economically sustainable mining workforce for blockchains. An immediate question arises: how to prevent the permissionless network from flooding of block dissemination traffic by a massive number of profit enthusiastic miners? We propose a novel notion of Decentralized Time (DT) as global and logical time which can be agreed as a network consensus. Our construction of DT practically uses distributed private clocks of the participation nodes. A node with DT upon creating or hearing a block can see how unluckily long or luckily short time the block has taken to have been mined plus traveled through the network. They can “time throttle” a potentially large number of unluckily mined/travelled blocks. The luckier blocks passing through the time throttle are treated as time-tie forks with a volume being “throttle diameter” adjustably controlled not to congest the network. With the number of time-tie forks being computationally manageable, it is then easy to break-tie selecting a winner, and even orderly queuing a plural number of them for a “multi-core” efficient way of using the winners. We provide succinct and evident analyses of necessary properties for the LotMint blockchain including: decentralization, energy saving, safety, liveness, robustness, fairness, anti-denial-of-service, anti-sybil, anti-censorship, scale-out transaction processing throughput and sped-up payment confirmation time.

The LotMint Blockchain

The LotMint blockchain has made a number of changes to ByzCoin. Below we describe the LotMint blockchain with changes from ByzCoin being explained, and important security properties analyzed in enumeration.

1. The first and the most important change is on the block mining mechanism. In LotMint, block mining shall use the decentralized clock enabled time throttle mechanism

that we have described in Section 4. Each new epoch begins with a mining competition where the reference block RB that the miners follow is the latest won time block TB. The definition of TB shall be described in Item 9 of our enumeration description. A mining success output, which we shall follow ByzCoin’s (and Bitcoin-NG’s) naming of their “KeyBlocks”, to name it a “KeyBlock Transaction”, and denote it by KB TX. Notice that treating PoW mining blocks as transactions to disseminate in the network is novel in BFT+blockchain technologies. Thanks to our DT time-throttle control, the number of KB TXs in dissemination can be well controlled not to congest the network.

2. After elapsing of ∆ interval of time (see Section 3.4 for the meaning of ∆) from the earliest written self-claimed broadcast time in one of the competing KB TXs, the network shall become with absence of KB TX. The current BFT leader shall propose a ByzCoin CoSi tree to try to have all competing KB TXs to be BFT quorum approved to enter the blockchain. Upon reaching the BFT quorum consensus, the root of this CoSi tree becomes the new reference block RB for to be referenced by a new round of mining competition.

3. If the current BFT leader is detected to be censoring some transactions, including some KB TXs, the BFT leader is said to have made a “censorship-error” omission. Upon detecting a censorship-error omission, other BFT trustees shall follow the current CoSi tree to have the censorship omitted TXs and/or KB TXs to be BFT quorum approved to enter the blockchain. This “censorship-error” correction is shown by the “A SubLeader Led Authority” sub-CoSi-tree in Figure 2. A punitive de-incentive scheme shall apply to the BFT leader who is BFT quorum agreed to have factually made a censorship-error by the very existence of a sub-CoSi-tree. In case of a number of trustees competing in censorship error correction, the deterministic de-forking method of ByzCoin (Figure 5 of [15]) can break forks.

4. If the current leader is detected to be proposing a CoSi to contain any message with a safety error, other than a censorship-error case, this leader is said to have conducted a “safety-error” attack. Upon trustees detecting a safety-error attack, a BFT change of leader event shall take place. This safety-error correction procedure is the same as that ByzCoin’s change-of-leader event does.

5. The diameter of the time throttle, i.e., Φ defined in Section 3.4, may be in consensus algorithm adjusted in real time. If the number of time-tie KB TXs forks is too small/big, then the consensus algorithm can adjust to enlarge/shrink the diameter Φ. Also, an epoch shall complete upon reaching a pre-determined desirable and sufficient number of time-tie KB TXs forks.

6. Time-tie KB TXs are BFT approved by a quorum of trustees in the BFT consortium. In Section 2, we have intuitively attributed a BFT approval of KB TX to it being “lucky”. It is now clear that such a KB TX is lucky in terms of having large deviations, not only having passed through the time throttle, but also having found lucky routes to the quorum of trustees. We further believe that, with the large deviation theory, the size of a “lucky quorum” can also be a consensus algorithm adjustable variable, not necessary to be some 2/3 fractions of the consortium size as in the case of pBFT. We believe that a simple majority can be a reasonable setting to achieve a quorum.

7. In ByzCoin, KeyBlock mining is in a separate chain, which, because a KeyBlock contains no transactions, seemingly to us is not very clear why other not-won miners shall have an eagerness to propagate a winner’s KeyBlock. To increase the blockchain liveness as Bitcoin made a seminal contribution to BFT protocols, we shall use an incentivization rewarding scheme, e.g., that of Bitcoin-NG [17], to divide mining incentive of the current epoch to reward the next epoch mining winner(s). The incentive reward should also include the coin-base minted coins in those not-won KB TXs to encourage miners to eagerly propagate these blocks. This is very important not only for the blockchain’s transaction quality of service viewed by the users, but also for liveness of the blockchain, as in the case of Bitcoin.

8. We have flattened the much eased PoW mining opportunities evenly distributed to the entire duration of an epoch. Each root of a CoSi tree is viewed by the miners a new reference block RB to reference mining new blocks. All lucky KB TXs which references any RB anywhere in an epoch are time-tie forks, even though they may have forked different RBs.

9. An epoch shall end upon reaching a pre-determined, and sufficient number of timetie KB TXs having entered BFT quorum approved CoSi trees. The final CoSi tree in the epoch should be a CoSi on the KB TX which is deterministically de-forked from all quorum approved time-tie KB TXs in the current epoch, using the deterministic de-forking method of ByzCoin (see Figure 5 of [15]). This CoSi tree defines the time block TB for the next epoch, and it should contain a Bitcoin like block timestamp, to function defining a new GC Cycle(TB) of the global and logic clock.

10. With the DT time-throttle method’s embracing mining time-tie forks, the deterministic de-forking method of ByzCoin can select a plural number of BFT leaders with an ordered winning sequence. They can be thought of as “multi-core” CPUs for strengthening the BFT consortium as additional trustees, e.g., to replace safety-error attacking leader(s). This way of strengthening the BFT trustees consortium can improve fairness, safety, liveness and robustness qualities of the blockchain, in addition to energy conservation.

11. With much lowered cost for being a miner, a LotMint miner may like to widely distribute itself over the network in replicas with a plural number of public-key based mining addresses being publicized by its won KB TX. Then, such a distributed miner, upon finding self being DoS attacked due to having disclosed an IP address, may change to a replica to continue working. The order of using replicas may follow the natural order of the public-key replicas.

It is our belief that the transaction processing scalability and the confirmation time properties of the LotMint blockchain, are similar to those of ByzCoin.