erc173-ownership

## Abstract

This specification defines standard functions for owning or controlling a contract. 

An implementation allows reading the current owner (`owner() returns (address)`) and transferring ownership (`transferOwnership(address newOwner)`) along with a standardized event for when ownership is changed (`OwnershipTransferred(address indexed previousOwner, address indexed newOwner)`).

## Motivation

Many smart contracts require that they be owned or controlled in some way. For example to withdraw funds or perform administrative actions. It is so common that the contract interface used to handle contract ownership should be standardized to allow compatibility with user interfaces and contracts that manage contracts.

Here are some examples of kinds of contracts and applications that can benefit from this standard:
1. Exchanges that buy/sell/auction ethereum contracts. This is only widely possible if there is a standard for getting the owner of a contract and transferring ownership.
2. Contract wallets that hold the ownership of contracts and that can transfer the ownership of contracts.
3. Contract registries. It makes sense for some registries to only allow the owners of contracts to add/remove their contracts. A standard must exist for these contract registries to verify that a contract is being submitted by the owner of it before accepting it.
4. User interfaces that show and transfer ownership of contracts.

## Specification

Every ERC-173 compliant contract must implement the `ERC173` interface. Contracts should also implement `ERC165` for the ERC-173 interface.

```solidity

/// @title ERC-173 Contract Ownership Standard
///  Note: the ERC-165 identifier for this interface is 0x7f5828d0
interface ERC173 /* is ERC165 */ {
    /// @dev This emits when ownership of a contract changes.    
    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

    /// @notice Get the address of the owner    
    /// @return The address of the owner.
    function owner() view external returns(address);
	
    /// @notice Set the address of the new owner of the contract
    /// @dev Set _newOwner to address(0) to renounce any ownership.
    /// @param _newOwner The address of the new owner of the contract    
    function transferOwnership(address _newOwner) external;	
}

interface ERC165 {
    /// @notice Query if a contract implements an interface
    /// @param interfaceID The interface identifier, as specified in ERC-165
    /// @dev Interface identification is specified in ERC-165. 
    /// @return `true` if the contract implements `interfaceID` and
    ///  `interfaceID` is not 0xffffffff, `false` otherwise
    function supportsInterface(bytes4 interfaceID) external view returns (bool);
}
```

The `owner()` function may be implemented as `pure` or `view`.

The `transferOwnership(address _newOwner)` function may be implemented as `public` or `external`.

To renounce any ownership of a contract set `_newOwner` to the zero address: `transferOwnership(address(0))`. If this is done then a contract is no longer owned by anybody.

The OwnershipTransferred event should be emitted when a contract is created.

## Rationale

Key factors influencing the standard: 
- Keeping the number of functions in the interface to a minimum to prevent contract bloat.
- Backwards compatibility with existing contracts.
- Simplicity
- Gas efficient

Several ownership schemes were considered. The scheme chosen in this standard was chosen because of its simplicity, low gas cost and backwards compatibility with existing contracts.

Here are other schemes that were considered:
1. **Associating an Ethereum Name Service (ENS) domain name with a contract.** A contract's `owner()` function could look up the owner address of a particular ENS name and use that as the owning address of the contract. Using this scheme a contract could be transferred by transferring the ownership of the ENS domain name to a different address. Short comings to this approach are that it is not backwards compatible with existing contracts and requires gas to make external calls to ENS related contracts to get the owner address.
2. **Associating an ERC721-based non-fungible token (NFT) with a contract.** Ownership of a contract could be tied to the ownership of an NFT. The benefit of this approach is that the existing ERC721-based infrastructure could be used to sell/buy/auction contracts. Short comings to this approach are additional complexity and infrastructure required. A contract could be associated with a particular NFT but the NFT would not track that it had ownership of a contract unless it was programmed to track contracts. In addition handling ownership of contracts this way is not backwards compatible.

This standard does not exclude the above ownership schemes or other schemes from also being implemented in the same contract. For example a contract could implement this standard and also implement the other schemes so that ownership could be managed and transferred in multiple ways. This standard does provide a simple ownership scheme that is backwards compatible, is light-weight and simple to implement, and can be widely adopted and depended on.

This standard can be (and has been) extended by other standards to add additional ownership functionality. 

## Security Considerations

If the address returned by `owner()` is an externally owned account then its private key must not be lost or compromised.

## Backwards Compatibility

Many existing contracts already implement this standard.

## Copyright

Copyright and related rights waived via [CC0](../LICENSE.md).