The content hash registry hardcoded into go-ethereum — maps key hashes (contract code hashes, domain name hashes) to content hashes. Part of the three-contract
Historical Significance
HashReg was a foundational piece of Ethereum's earliest content-addressing infrastructure. By linking contract code hashes to documentation hashes on-chain, it enabled the NatSpec system — an ambitious early attempt at making smart contract interactions human-readable. This pattern of content-addressed on-chain metadata registration predates and influenced later systems like IPFS pinning registries and ENS content records.
Context
NatSpec (Natural Specification) was part of Gavin Wood's original Ethereum design — a system where smart contract functions could be annotated with human-readable descriptions that wallets would display to users before transaction signing. To make this work on mainnet, three contracts needed to be deployed: GlobalRegistrar (name→address), HashReg (code hash→content hash), and UrlHint (content hash→URL). All three were deployed within 18 blocks of each other on September 24, 2015, ~55 days after Frontier launch.
Contract Information
Key Facts
Description
Deployed on September 24, 2015 (block 282,885), just 5 blocks after the GlobalRegistrar, HashReg is the content hash registry that maps key hashes to content hashes on Ethereum mainnet. It was deployed by the same go-ethereum core developer who set up the entire Frontier registrar infrastructure in a rapid multi-contract deployment session.
HashReg served a specific role in go-ethereum's NatSpec documentation system: when a developer compiled and deployed a Solidity contract, the compiler could generate a JSON document containing human-readable descriptions of each function. The hash of this documentation was then registered in HashReg, linked to the contract's code hash. This allowed any geth user running a transaction against a verified contract to receive a plain-English explanation of what the transaction would do — before signing it.
The contract's address was hardcoded into go-ethereum as HashRegAddr = "0x23bf622b5a65f6060d855fca401133ded3520620" // frontier in common/registrar/registrar.go. It works in conjunction with GlobalRegistrar (which maps the name "HashReg" to this address so go-ethereum can find it on-chain) and UrlHint (which maps content hashes to URLs where the actual documentation files can be retrieved).
The contract exposes two core functions called via ABI signatures: setowner() (establishing ownership before registration) and register(uint256,uint256) (mapping a key hash to a content hash). No source code was verified on Etherscan, but the contract's bytecode was compiled from Solidity source embedded in the go-ethereum codebase.
Source Verified
Bytecode hardcoded verbatim in go-ethereum v1.0.0 common/registrar/contracts.go (HashRegCode). Source from HashRegSrc in same file. Authored by Gav Wood, Ethereum Foundation. Exact 152-byte runtime match confirmed.
Historian Categories
Heuristic Analysis
The following characteristics were detected through bytecode analysis and may not be accurate.
Frontier Era
The initial release of Ethereum. A bare-bones implementation for technical users.
Bytecode Overview
Verified Source Available
Source verified through compiler archaeology and exact bytecode matching.
View Verification ProofShow source code (Solidity)
// SPDX-License-Identifier: UNLICENSED
// Reconstructed source of HashReg (0x23bf622b5a65f6060d855fca401133ded3520620)
// Originally authored by Gav Wood, Ethereum Foundation.
// Source preserved verbatim from go-ethereum v1.0.0 common/registrar/contracts.go (HashRegSrc)
// Compiler: unknown early Solidity (pre-release, Frontier era, ~Aug 7 2015)
// The bytecode was hardcoded directly in go-ethereum — no separate compilation needed.
contract HashReg {
function setowner() {
if (owner == 0) {
owner = msg.sender;
}
}
function register(uint256 _key, uint256 _content) {
if (msg.sender == owner) {
content[_key] = _content;
}
}
address owner;
mapping (uint256 => uint256) content;
}