Preimages

Preimages

Precompiles

EVM's escape hatch

Agenda

This is not a cryptography presentation

  • What are Precompiles (with mainnet examples)
  • How do they work?
  • Use outside Mainnet
    • L2 use and Alt L1 use
  • Future developments

Three Kinds of "Precompiles"

  • Precompiles
    • Tasks you could do with the EVM, but are too expensive/slow
  • System Contracts
    • Tasks and side effects you cannot do with the EVM
  • Predeployed Contracts
    • Contracts that are part of the initial state

Ethereum Mainnet Precompiles

  • 0x01 - ecRecover
  • 0x02 - SHA2-256
  • 0x03 - RIPEMD-160
  • 0x04 - identity
  • 0x05 - modexp (Byzantium)
  • 0x06, 0x07, 0x08 - ecAdd, ecMul, and ecPairing on alt_bn128 (Byzantium)
  • 0x09 - Blake2B F Function (Istanbul)
  • 0x0A - KZG Point Evaluation (Cancun)

evm.codes

EVM View

  • Precompiles look like any other contract
    • Variable amount of data passed in via calldata
    • Consume gas based on the input
    • Returns data via the return buffer or to output memory

Deep example - ECRecover

  • Input
    • 0x00 - 0x1f - commit hash
    • 0x20 - 0x3f - v (27 or 28, pre recovery id)
    • 0x40 - 0x5f - r (x-value for secp256k1)
    • 0x60 - 0x7f - s (as per secp256k1)
  • return
    • 0x00 - 0x1f - lower 20 bytes of 256-bit public key
  • Gas cost
    • 3000 no matter what

Design issues in Precompiles

  • All boundary conditions must be specified
  • Gas should scale with effort
    • Execution
      • Algorithms can hide problems
    • Input
      • Variable input should always be charged
  • Costs should account for the worst case
    • People will troll the chain

How it's Implemented

  • Besu - org.hyperledger.besu.evm.precompile package
  • Geth - core/vm/contracts.go
  • Nethermind - Nethermind.EVM.Precompiles namespace
  • Reth - REVM Precompiles Crates

Implementation Strategies

  • Implement with client software
    • For simple and well understood precompiles only
  • Implement with an external library
    • Either as source or as binary
    • pro: Client Devs don't need to understand the math
    • con: Different libraries may have different bugs

System Contracts

"System contracts are a pathway to many capabilities some consider to be unaligned"
- Darth Genesis

System Contract Uses

  • Access L1/L2 Bridging
    • Arbitrum, Optimism, ZK chains
  • Access Foreign host chain services
  • Advanced Services (coming soon)
    • Fhenix (FHE), Ritual (AI Model Execution)

Typical L2 System Contract Uses

  • L1/L2 communications
  • Treasury / Fee Vault Management
  • Security / Admin tasks
  • Chain Info queries (rather than new 0x30-0x4f opcodes)

rollup.codes

Notable Design Choices in L2 Contracts

  • Use of Solidity ABI for Precompiled Access
    • Maybe we need an ERC for this
  • Mixed API designs
    • Multiple contracts
    • One giant contract
  • Mixed Permanence
    • Some Proxied
    • Some Fixed

Notable Design Choices in L2 Contracts (Cont.)

  • Mixed Implementation Strategies
    • Direct implementation in Node Software
    • Solidity implemented predeploys and events controlling L2Node actions
  • Mixed Contract Address Deployments
    • Consecutive addresses (low or prefixed)
    • CREATE2 driven

System Contract — Foreign Host Chain Services

  • AltL1 Token Access
    • Moonbean, Aurora, Hedera all have proxies to their AltL1 Token Systems
  • AltL1 Account Tools
  • AltL1 Foreign Features
    • ICP, Polkadot voting, etc.
  • zk features
    • zkSync Feature Simulation

Security and System Contracts

  • Precompiles don't share Ethereum's Memory Model
    • i.e., Contract Owned Storage
  • DELEGATECALLS can impersonate SENDERs via callback
  • Best to Ban Delegatecalls into precompiles
    • Check that ADDRESS of contract is what your deployment was
  • Also, ensure all actions are revertible
    • All solidity approaches get this for free

Precompile Futures

  • There is resistance to adding new mainnet precompiles
    • BLS - 9 separate functions
  • RollCall is standardizing L2 Precompiles
    • ECDSA(secp256r1) verification
  • EVMMAX (modular math extensions) may reduce the demand
    • Aspirationally to be w/in 2x gas cost

Progressive Precompiles

  • New quasi-proposal to "canonize" well-known contracts

  • How to handle Gas is unresolved

  • Needs better math support (example: EVMMAX)

  • Mixed Execution Example: EIP-4788 - Canonical EVM code exists

Q&A

Select a repo