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A faster, more efficient cryptocurrency



MIT researchers have developed a new cryptocurrency that drastically reduces data users need to join the network and verify transactions – up to 99 percent compared to today's popular cryptocurrencies.

Cryptocurrencies, such as the popular Bitcoin, are networks built on the blockchain, and a financial ledger formatted in a sequence of individual blocks, each containing transaction data. These networks are decentralized, meaning there are no banks or organizations to manage funds and balances, so users join forces to store and verify transactions

But decentralization leads to a scalability problem. To join a cryptocurrency, new users must download and store all transaction data from hundreds of thousands of individual blocks. They must also store these data to use the service and help verify transactions.

In a paper presented at the Network and Distributed System Security Symposium next month, MIT researchers introduced Vault, a cryptocurrency that allows users to join the network by downloading only a fraction of the total transaction data. It also incorporates techniques that delete empty accounts that take up space, and allows for verification using only the most recent transaction data that is shared and shared across the network, minimizing an individual user's data storage and processing requirements

In experiments, Vault reduced the bandwidth for joining its network by 99 percent compared to Bitcoin and 90 percent compared to Ethereum, which is considered one of today's most efficient cryptocurrencies. Importantly, Vault still ensures that all nodes validate all transactions, providing tight security equal to their existing counterparts.

"There are many cryptocurrencies, but they are hitting bottlenecks related to joining the system as a new user and to storage. The broad goal here is to enable cryptocurrencies to scale well for more and more users, "says co-author Derek Leung, and a graduate student in the Computer Science and Artificial Intelligence Laboratory (CSAIL).

Joining Leung on the paper are CSAIL researchers Yossi Gilad and Nickolai Zeldovich, who is also a professor in the Department of Electrical Engineering and Computer Science (EECS); and recent alumnus Adam Suhl '1

8.

Vaulting over blocks

Each block in a cryptocurrency network contains a timestamp, its location in the blockchain, and a fixed-length string of numbers and letters, and "hash," that's basically the block's identification. Each new block contains the hash of the previous block in the blockchain. Blocks in Vault also contain up to 10,000 transactions – or 10 megabytes of data – that must be all verified by users.

New users join cryptocurrency networks, or "bootstrap," by downloading all past transaction data to ensure they are not hacking the blocks without detection. re secure and up to date. To join Bitcoin last year, for example, and the user would download 500,000 blocks totaling about 150 gigabytes. Users must also store all account balances to help verify new users and ensure that users have enough funds to complete transactions.

The researchers build their system on a new cryptocurrency network called Algorand – invented by Silvio Micali, Ford Professor of Engineering at MIT – that's secure, decentralized, and more scalable than other cryptocurrencies

With traditional cryptocurrencies, users compete to solve equations that validate blocks, with the first to solve the equations receiving funds. As the network scales, this slows transaction processing times. Algorand uses a "proof-of-stake" concept to more efficiently verify blocks and better enable new users to join. For each block, a representative verification "committee" is selected. Users with more money – or stake – in the network have a higher probability of being selected.

But each block holds some key information to validate the certificate immediately ahead of it, meaning new users must start with the first block in the chain, along with its certificate , and sequentially validate each one in order, which can be time-consuming. To speed things up, the researchers give each new certificate verification information based on a block of a few hundred or 1,000 blocks behind it – called a "breadcrumb." When a new user joins, they match the breadcrumb of an early block to a breadcrumb 1,000 blocks ahead. That breadcrumb can be matched to another breadcrumb 1,000 blocks ahead, and so on.

"The paper title is a pun," Leung says. "A vault is a place where you can store money, but the blockchain also allows you to" overwhelm "blocks when joining a network. When I'm bootstrapping, I only need a block from way in the past to verify a block way in the future. I can skip over all blocks in between, which saves us a lot of bandwidth. "

Divide and discard

To reduce data storage requirements, researchers designed Vault with a novel" sharding " . The technique divides the transaction data into smaller portions – or shards – that it shares across the network, so individual users only have to process small amounts of data to verify transactions.

To implement sharing in a secure way, Vault uses a well- known data structure called a binary Merkle tree. In the binary trees, a single top node breaks into two child nodes, and those two nodes each break into two child nodes, and so on.

In Merkle trees, the top node contains a single hash, called a root hash. But the tree is built from the bottom, up. The tree combines every pair of children hashes along the bottom to form their parent hash. It replicates the process of the tree, assigning a parent node from each pair of child nodes, until it combines everything into the root hash. In cryptocurrencies, the top node contains a hash of a single block. Each bottom node contains a hash that signifies the balance information about one account involved in one transaction in the block.

To verify any one transaction, the network combines the two child nodes to get the parent node hash. It repairs that process working the tree. If the final combined hash matches the root hash of the block, the transaction can be verified. But with traditional cryptocurrencies, users must store the entire tree structure

With Vault, researchers divide the Merkle tree into separate shards assigned to separate groups of users. Each user account only stores the balances of accounts in its assigned shard as well as root hashes. The trick is that all users store one layer of nodes that cuts across Merkle tree. When a user needs to verify a transaction from outside their shard, they trace a path to that common layer. From this common layer, they can determine the balance of the account outside their shard, and continue validation normally.

"Every shard of the network is responsible for storing a smaller slice of a large data structure, to verify transactions from all other parts of the network, "said Leung.

Additionally, the researchers designed a novel scheme that recognizes and discards the user's assigned shard accounts that have zero balances for a certain length of time. Other cryptocurrencies keep all empty accounts, which increase data storage requirements while serving no real purpose, as they do not need verification. When users store account data in Vault, they ignore those old, empty accounts


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