Chain Program Engine
Starting in 2014, the crypto space shifted from a focus on UTxO-ledger blockchains, such as Bitcoin, to account-ledger blockchains, such as Ethereum. This transition was driven in large part by the development of the Ethereum Virtual Machine (EVM) and model of smart contract execution chosen by the Ethereum team.
Thankfully, considering the benefits of a UTxO ledger model over an account model for scalability and traceability, the model popularized by Ethereum is not the sole model for the complex ledger logic atop a blockchain. As we discuss (here), extended-UTxOs currently being implemented into the Topl Blockchain represent another such model. The key limit of extended-UTxOs lies in the fact that multi-step computations must be carried out across multiple blocks. To offer a second method for ledger logic on the Topl Blockchain, a Chain Program Engine is currently under development to provide greater flexibility to Topl's users.
Topl's Chain Program Engine differs from Ethereum style smart contracts in a few key ways:
    In order to affect state (carry out a transaction), a chain program must always be provided a signature, whether a simple signature or proxy signature, to execute. As many security vulnerabilities exploited in Ethereum smart contracts stem from these contracts being able to execute "autonomously", the requirement for a signature may considerably reduce the likelihood of such exploits.
    In line with the rest of Topl's ledger, chain programs function within the framework of UTxOs with special boxes, called code boxes, state boxes, and execution boxes employed. Not only does this approach enable users to more easily (with fewer resources) verify chain programs than smart contracts, but it also substantially reduces the so-called miner-extractable value (MEV).
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