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ETHEREUM: A SECURE DECENTRALISED GENERALISED TRANSACTION LEDGER BERLIN VERSION 16 block difficulty from section 10). PoW is the proof-of-work function which evaluates to an array with the first item being the mixHash and the second item being a pseudo- random number cryptographically dependent on H and d. The underlying algorithm is called Ethash and is described below. 11.5.1. Ethash. Ethash is the PoW algorithm for Ethereum 1.0. It is the latest version of Dagger-Hashimoto, intro- duced by Buterin [2013b] and Dryja [2014], although it can no longer appropriately be called that since many of the original features of both algorithms were drastically changed with R&D from February 2015 until May 4 2015 (Jentzsch [2015]). The general route that the algorithm takes is as follows: There exists a seed which can be computed for each block by scanning through the block headers up until that point. From the seed, one can compute a pseudorandom cache, Jcacheinit bytes in initial size. Light clients store the cache. From the cache, we can generate a dataset, Jdatasetinit bytes in initial size, with the property that each item in the dataset depends on only a small number of items from the cache. Full clients and miners store the dataset. The dataset grows linearly with time. Mining involves grabbing random slices of the dataset and hashing them together. Verification can be done with low memory by using the cache to regenerate the specific pieces of the dataset that you need, so you only need to store the cache. The large dataset is updated once every Jepoch blocks, so the vast majority of a miner’s effort will be reading the dataset, not making changes to it. The mentioned parameters as well as the algorithm is explained in detail in Appendix J. 12. Implementing Contracts There are several patterns of contracts engineering that allow particular useful behaviours; two of these that we will briefly discuss are data feeds and random numbers. 12.1. Data Feeds. A data feed contract is one which pro- vides a single service: it gives access to information from the external world within Ethereum. The accuracy and timeliness of this information is not guaranteed and it is the task of a secondary contract author—the contract that utilises the data feed—to determine how much trust can be placed in any single data feed. The general pattern involves a single contract within Ethereum which, when given a message call, replies with some timely information concerning an external phenome- non. An example might be the local temperature of New York City. This would be implemented as a contract that returned that value of some known point in storage. Of course this point in storage must be maintained with the correct such temperature, and thus the second part of the pattern would be for an external server to run an Ethereum node, and immediately on discovery of a new block, creates a new valid transaction, sent to the contract, updating said value in storage. The contract’s code would accept such updates only from the identity contained on said server. 12.2. Random Numbers. Providing random numbers within a deterministic system is, naturally, an impossible task. However, we can approximate with pseudo-random numbers by utilising data which is generally unknowable at the time of transacting. Such data might include the block’s hash, the block’s timestamp and the block’s benefi- ciary address. In order to make it hard for malicious miners to control those values, one should use the BLOCKHASH operation in order to use hashes of the previous 256 blocks as pseudo-random numbers. For a series of such numbers, a trivial solution would be to add some constant amount and hashing the result. 13. Future Directions The state database won’t be forced to maintain all past state trie structures into the future. It should maintain an age for each node and eventually discard nodes that are neither recent enough nor checkpoints. Checkpoints, or a set of nodes in the database that allow a particular block’s state trie to be traversed, could be used to place a maximum limit on the amount of computation needed in order to retrieve any state throughout the blockchain. Blockchain consolidation could be used in order to re- duce the amount of blocks a client would need to download to act as a full, mining, node. A compressed archive of the trie structure at given points in time (perhaps one in every 10,000th block) could be maintained by the peer network, effectively recasting the genesis block. This would reduce the amount to be downloaded to a single archive plus a hard maximum limit of blocks. Finally, blockchain compression could perhaps be con- ducted: nodes in state trie that haven’t sent/received a transaction in some constant amount of blocks could be thrown out, reducing both Ether-leakage and the growth of the state database. 13.1. Scalability. Scalability remains an eternal concern. With a generalised state transition function, it becomes dif- ficult to partition and parallelise transactions to apply the divide-and-conquer strategy. Unaddressed, the dynamic value-range of the system remains essentially fixed and as the average transaction value increases, the less valuable of them become ignored, being economically pointless to in- clude in the main ledger. However, several strategies exist that may potentially be exploited to provide a considerably more scalable protocol. Some form of hierarchical structure, achieved by either consolidating smaller lighter-weight chains into the main block or building the main block through the incremen- tal combination and adhesion (through proof-of-work) of smaller transaction sets may allow parallelisation of trans- action combination and block-building. Parallelism could also come from a prioritised set of parallel blockchains, consolidating each block and with duplicate or invalid transactions thrown out accordingly. Finally, verifiable computation, if made generally avail- able and efficient enough, may provide a route to allow the proof-of-work to be the verification of final state. 14. Conclusion We have introduced, discussed and formally defined the protocol of Ethereum. Through this protocol the reader may implement a node on the Ethereum network and join others in a decentralised secure social operating system. Contracts may be authored in order to algorithmically specify and autonomously enforce rules of interaction.PDF Image | ETHEREUM: A SECURE DECENTRALISED GENERALISED TRANSACTION
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