Execution Environment

The Citrea VM is a fully Ethereum Virtual Machine (EVM) equivalent VM running on Bitcoin and using $cBTC as its native token. EVM, as a concept, is arguably the most battle-tested and mature VM in the cryptocurrency ecosystem. It is a deterministic, stack-based virtual machine, renowned for its ability to efficiently execute smart contracts in a secure and isolated environment.

Citrea implements its own EVM, referred to as a "zkEVM". A zkEVM is a special EVM implementation that makes the full VM implementation provable. Citrea zkEVM is classified as Type 2, which means full equivalency and a scalable and trustless proof system as it is based on zk-STARKs. Citrea zkEVM is built using RISC Zero.

Different characteristics of Citrea

Being a Type 2 zkEVM, Citrea has some different characteristics compared to Ethereum and other EVM blockchains:

• Native currency is Bitcoin: Citrea uses $cBTC for gas fees and transfers in the network, which is the trust-minimized 1-1 backed Bitcoin asset. It is not an ERC-20 - it is the native asset with 18 decimals (not 8!).

• Block time & gas limit: Citrea has a block time of 2 seconds with a block gas limit of 10 million.

• L1 Fee: As Citrea posts state diffs to Bitcoin, there is a fundamental DA cost - this is reflected as L1 Fee to the users. L1 Fee is deducted from the address' balance at the end of a transaction, and it is calculated with the formula of l1FeeRate * diffSize. If there is not enough balance to pay the L1 Fee, the transaction will fail.

  • L1 Fee Rate: Wei per Byte cost of the state diff.

  • Diff Size: The size of the state diff in bytes.

• EVM Version & Tweaks: Citrea supports Pectra version of EVM with some tweaks:

  • EIP-4844: EIP-4844 is not supported on Citrea, along with KZG precompile. If the address 0x0A is called it will return as an EOA address.

  • EIP-2935: EIP-2935 is not supported on Citrea, which means that BLOCKHASH still returns the latest 256 blockhashes (instead of 8192).

• Additional Precompiles: Compared to Pectra version, Citrea offers two additional precompiles: secp256r1 and Schnorr. You can read more about them here.

• Merkle Tree: Citrea uses Jellyfish Merkle Tree for its state instead of Patricia Merkle Tree.

• System Smart Contracts: There are special system smart contracts of Citrea that are optimized for system level operations: BitcoinLightClient, Bridge, and FeeVault. You can read more about them here.