Ethereum Chain Spec Format

This is a format to describe any Ethereum-like chain. It is derived from the genesis.json format but includes parameters to change and configure the consensus algorithm, to specify infrastructure information, to specify boot nodes and to specify any built-in contracts together with their cost.


It is JSON, with the top level being an object with six keys:

  • name: A string value specifying the chain name. e.g. "Frontier/Homestead", "Morden", "Olympic".
  • forkName: An optional string value specifying a sub-identifier, in case two different chains have equivalent genesis blocks.
  • engine: A enum value specifying the consensus engine. e.g. "Ethash", "Null".
  • params: An object specifying various attributes of the consensus engine, allowing configuration.
  • genesis: An object specifying the header of the genesis block.
  • nodes: An array of strings, each one a node address in enode format.
  • accounts: An object specifying accounts of the genesis block. This includes builtin contracts and premines.

Subformat: engine

There are two valid engines, Ethash and Null.

  • Null: Nonoperative engine.
  • Ethash: Sealing engine used by ethereum.
    • params: Engine specific params.
      • minimumDifficulty: Integer specifying the minimum difficulty a block may have.
      • gasLimitBoundDivisor: Integer specifying the according value in the Yellow Paper.
      • difficultyBoundDivisor: Integer specifying the according value in the Yellow Paper.
      • durationLimit: Integer specifying the boundary point at which difficulty is increased.
      • blockReward: Integer specifying the reward given for authoring a block.
      • registrar: 0x-prefixed, 40-nibble datum of the address of the registrar contract on this chain.

Subformat: params

Different consensus engines may allow different keys in the params object, however there exist a few common to all:

  • accountStartNonce: Integer specifying what nonce all newly created accounts should have.
  • frontierCompatibilityModeLimit: Integer specifying the number of the block that Frontier-compatibility mode finishes and Homestead mode begins.
  • maximumExtraDataSize: Integer specifying the maximum size in bytes of the extra_data field of the header.
  • minGasLimit: Integer specifying the minimum amount of gas a block may be limited at.
  • networkID: Integer specifying the index of this chain on the network.

Note: all integers are 0x-prefixed, hex-encoded strings.

Subformat: genesis

The keys in genesis specify the according fields in the chain's genesis block. All are hex-encoded, 0x prefixed. The fields required are:

  • seal
  • difficulty
  • author
  • timestamp
  • parentHash
  • extraData
  • gasLimit

Subformat: accounts

The accounts object maps from addresses (40-nibble strings without the 0x prefix) to objects, each with a number of allowed keys:

  • balance: Integer to specify the balance of the account at genesis in wei.
  • nonce: Integer to specify the nonce of the account at genesis.
  • code: 0x-prefixed hex specifying the code of the account at genesis.
  • storage: Object mapping hex-encoded integers for the account's storage at genesis.
  • builtin: Alternative to code, used to specify that the account's code is natively implemented. Value is an object with further fields:
    • "name": The name of the builtin code to execute as a string. e.g. "identity", "ecrecover".
    • "pricing": Enum to specify the cost of calling this contract.
      • "linear": Specify a linear cost to calling this contract. Value is an object with two fields: base which is the basic cost in Wei and is always paid; and word which is the cost per word of input, rounded up.


This is the Morden ECS JSON file

    "name": "Morden",
    "engine": {
        "Ethash": {
            "params": {
                "tieBreakingGas": false,
                "gasLimitBoundDivisor": "0x0400",
                "minimumDifficulty": "0x020000",
                "difficultyBoundDivisor": "0x0800",
                "durationLimit": "0x0d",
                "blockReward": "0x4563918244F40000",
                "registrar" : "0xc6d9d2cd449a754c494264e1809c50e34d64562b"
    "params": {
        "accountStartNonce": "0x0100000",
        "frontierCompatibilityModeLimit": "0x789b0",
        "maximumExtraDataSize": "0x20",
        "tieBreakingGas": false,
        "minGasLimit": "0x1388",
        "networkID" : "0x2"
    "genesis": {
        "seal": {
            "ethereum": {
                "nonce": "0x0000000000000042",
                "mixHash": "0x0000000000000000000000000000000000000000000000000000000000000000"
        "difficulty": "0x20000",
        "author": "0x0000000000000000000000000000000000000000",
        "timestamp": "0x00",
        "parentHash": "0x0000000000000000000000000000000000000000000000000000000000000000",
        "extraData": "0x",
        "gasLimit": "0x2fefd8"
    "nodes": [
    "accounts": {
        "0000000000000000000000000000000000000001": { "balance": "1", "nonce": "1048576", "builtin": { "name": "ecrecover", "pricing": { "linear": { "base": 3000, "word": 0 } } } },
        "0000000000000000000000000000000000000002": { "balance": "1", "nonce": "1048576", "builtin": { "name": "sha256", "pricing": { "linear": { "base": 60, "word": 12 } } } },
        "0000000000000000000000000000000000000003": { "balance": "1", "nonce": "1048576", "builtin": { "name": "ripemd160", "pricing": { "linear": { "base": 600, "word": 120 } } } },
        "0000000000000000000000000000000000000004": { "balance": "1", "nonce": "1048576", "builtin": { "name": "identity", "pricing": { "linear": { "base": 15, "word": 3 } } } },
        "102e61f5d8f9bc71d0ad4a084df4e65e05ce0e1c": { "balance": "1606938044258990275541962092341162602522202993782792835301376", "nonce": "1048576" }

Note: the builtin accounts enable usage of Solidity language. Running without them included in the chain definition file may result in unexpected behavior.