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EVM Smart Contract Packaging Specification

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Ethereum Packaging Specification

Specification

Definitions

The following types are used within this specification.

Contract Name

A string matching the regular expression [_a-zA-Z][_a-zA-Z0-9]*

Package Name

A string matching the regular expression [a-zA-Z][-_a-zA-Z0-9]*

IPFS URI

An URI in the format ipfs://<multihash>[/<path>]

Chain Definition via BIP122 URI

An URI in the format blockchain://<chain_id>/block/<block_hash>

  • chain_id is the unprefixed genesis hash for the chain.
  • block_hash is the hash of a block on the chain.

A chain is considered to match a chain definition if the the genesis block hash matches the chain_id and the block defined by block_hash can be found on that chain. It is possible for multiple chains to match a single URI, in which case all chains are considered valid matches

Resources

Use Cases

The following use cases were considered during the creation of this specification.

  1. owned: A package which contains contracts which are not meant to be used by themselves but rather as base contracts to provide functionality to other contracts through inheritance.
  2. transferable: A package which has a single dependency.
  3. standard-token: A package which contains a reusable contract.
  4. safe-math-lib: A package which contains deployed instance of one of the package contracts.
  5. piper-coin: A package which contains a deployed instance of a reusable contract from a dependency.
  6. escrow: A package which contains a deployed instance of a local contract which is linked against a deployed instance of a local library.
  7. wallet: A package with a deployed instance of a local contract which is linked against a deployed instance of a library from a dependency.

Each use case builds incrementally on the previous one.

Stand Alone Package with an Inheritable Contract

For the first example we'll look at a package which only contains contracts which serve as base contracts for other contracts to inherit from but do not provide any real useful functionality on their own. The common owned pattern is a example for this use case.

pragma solidity ^0.4.0;

contract owned {
    address owner;

    function owned() {
        owner = msg.sender;
    }

    modifier onlyowner { if (msg.sender != owner) throw; _; }
}

For this example we will assume this file is located in the solidity source file ./contracts/owned.sol

The owned package contains a single solidity source source file which is intended to be used as a base contract for other contracts to be inherited from. The package does not define any pre-deployed addresses for the owned contract.

The smallest Release Lockfile for this package looks like this:

{
  "lockfile_version": "1",
  "version": "1.0.0",
  "package_name": "owned",
  "sources": {
    "./contracts/owned.sol": "ipfs://QmUjYUcX9kLv2FQH8nwc3RLLXtU3Yv5XFpvEjFcAKXB6xD"
  }
}

A Release Lockfile which includes more than the minimum information would look like this.

{
  "lockfile_version": "1",
  "version": "1.0.0",
  "package_name": "owned",
  "package_meta": {
    "license": "MIT",
    "authors": [
      "Piper Merriam <[email protected]>"
    ],
    "description": "Reusable contracts which implement a priviledged 'owner' model for authorization",
    "keywords": [
      "authorization"
    ],
    "links": {
      "documentation": "ipfs://QmQiqrwqdav5bV8mtv4PqGksGcDWo43f7PAZYwhJqNEv2j"
    }
  },
  "sources": {
    "./contracts/owned.sol": "ipfs://QmUjYUcX9kLv2FQH8nwc3RLLXtU3Yv5XFpvEjFcAKXB6xD"
  }
}

This fully fleshed out Release Lockfile is meant to demonstrate various pieces of optional data that can be included. However, for the remainder of our examples we will be using minimalistic lockfiles to keep our examples as succinct as possible.

Package with an Inheritable Contract and a Dependency

Now that we've seen what a simple package looks like, lets see how to dependencies are declared.

The next package will implement the transferable pattern and will depend on our owned package for the authorization mechanism to ensure that only the contract owner may transfer ownership. The transferable package will contain a single solidity source file ./contracts/transferable.sol.

pragma solidity ^0.4.0;

import {owned} from "owned/contracts/owned.sol";

contract transferable is owned {
	event OwnerChanged(address indexed prevOwner, address indexed newOwner);

    function transferOwner(address newOwner) public onlyowner returns (bool) {
		OwnerChanged(owner, newOwner);
		owner = newOwner;
		return true;
    }
}

The EPM spec is designed to provide as high a guarantee as possible that builds are deterministic and reproducable. To ensure that each package you install gets the exact dependencies it needs, all dependencies are declared as content addressed URIs. This ensures that when a package manager fetches a dependency it always gets the right one.

The IPFS URI for the previous owned Release Lockfile turns out to be ipfs://QmXDf2GP67otcF2gjWUxFt4AzFkfwGiuzfexhGuotGTLJH which is what we will use in our transferable package to declare the dependency. The Release Lockfile for our package looks like the following.

{
  "lockfile_version": "1",
  "version": "1.0.0",
  "package_name": "transferable",
  "sources": {
    "./contracts/transferable.sol": "ipfs://QmZ6Zg1iEejuJ18LFczowe7dyaxXm4KC4xTgnCkqwJZmAp"
  },
  "build_dependencies": {
	"owned": "ipfs://QmXDf2GP67otcF2gjWUxFt4AzFkfwGiuzfexhGuotGTLJH"
  }
}

It will be up to the package management software to determine how the owned dependency actually gets installed as well as handling any import remappings necessary to make the import statement work.

Stand Alone Package with a Reusable Contract

In this next example we'll look at a package which contains a reusable contract. This means that the package provides a contract which can be on its own in some manner. For this example we will be creating a package which includes a reusable standard ERC20 token contract.

The source code for these contracts was pulled from the SingularDTV github repository. Thanks to them for a very well written contract.

This package will contain two solidity source files.

Given that these source files are relatively large they will not be included here within the guide but can be found in the ./examples/standard-token/ directory within this repository.

Since this package includes a contract which may be used as-is, our Release Lockfile is going to contain additional information from our previous examples, specifically, the contract_types section. Since we expect people to compile this contract theirselves we won't need to include any of the contract bytecode, but it will be useful to include the contract ABI and Natspec information. Our lockfile will look something like the following. The contract ABI and NatSpec sections have been truncated to improve legibility. The full Release Lockfile can be found here

{
  "lockfile_version": "1",
  "version": "1.0.0",
  "package_name": "standard-token",
  "sources": {
    "./contracts/AbstractToken.sol": "ipfs://QmQMXDprXxCunfQjA42LXZtzL6YMP8XTuGDB6AjHzpYHgk",
    "./contracts/StandardToken.sol": "ipfs://QmNLr7DzmiaQvk25C8bADBnh9bF5V3JfbwHS49kyoGGEHz"
  },
  "contract_types": {
    "StandardToken": {
      "abi": [...],
      "natspec": {
        "author": "Stefan George - <[email protected]>",
        "title": "Standard token contract",
        "methods": {
          "allowance(address,address)": {
            "details": "Returns number of allowed tokens for given address.",
            "params": {
              "_owner": "Address of token owner.",
              "_spender": "Address of token spender."
            }
          },
          ...
        }
      }
    }
  }
}

While it is not required to include the contract ABI and NatSpec information, it does provide those using this package with they data they would need to interact with an instance of this contract without having to regenerate this information from source.

Stand Alone Package with a Deployed Contract

Now that we've seen what a package looks like which includes a fully functional contract that is ready to be deployed, lets explore a package that also includes a deployed instance of that contract.

Solidity Libraries are an excellend example of this type of package, so for this example we are going to write a library for safe math operations called safe-math-lib. This library will implement functions to allow addition and subtraction without needing to check for underflow or overflow conditions. Our package will have a single solidity source file ./contracts/SafeMathLib.sol

pragma solidity ^0.4.0;


/// @title Safe Math Library
/// @author Piper Merriam <[email protected]>
library SafeMathLib {
    /// @dev Subtracts b from a, throwing an error if the operation would cause an underflow.
    /// @param a The number to be subtracted from
    /// @param b The amount that should be subtracted
    function safeAdd(uint a, uint b) returns (uint) {
        if (a + b > a) {
            return a + b;
        } else {
            throw;
        }
    }

    /// @dev Adds a and b, throwing an error if the operation would cause an overflow.
    /// @param a The first number to add
    /// @param b The second number to add
    function safeSub(uint a, uint b) returns (uint) {
        if (b <= a) {
            return a - b;
        } else {
            throw;
        }
    }
}

This will be our first package which includes the deployments section which is the location in the Release Lockfile where information about deployed contract instances is found. Lets look at the Release Lockfile for this package. Some parts have been truncated for readability but the full file can be found here

{
  "lockfile_version": "1",
  "version": "1.0.0",
  "package_name": "safe-math-lib",
  "sources": {
    "./contracts/SafeMathLib.sol": "ipfs://QmVN1p6MmMLYcSq1VTmaSDLC3xWuAUwEFBFtinfzpmtzQG"
  },
  "contract_types": {
    "SafeMathLib": {
      "bytecode": "0x606060405234610000575b60a9806100176000396000f36504062dabbdf050606060405260e060020a6000350463a293d1e88114602e578063e6cb901314604c575b6000565b603a600435602435606a565b60408051918252519081900360200190f35b603a6004356024356088565b60408051918252519081900360200190f35b6000828211602a57508082036081566081565b6000565b5b92915050565b6000828284011115602a57508181016081566081565b6000565b5b9291505056",
      "runtime_bytecode": "0x6504062dabbdf050606060405260e060020a6000350463a293d1e88114602e578063e6cb901314604c575b6000565b603a600435602435606a565b60408051918252519081900360200190f35b603a6004356024356088565b60408051918252519081900360200190f35b6000828211602a57508082036081566081565b6000565b5b92915050565b6000828284011115602a57508181016081566081565b6000565b5b9291505056",
      "abi": [
        ...
      ],
      "compiler": {
        "type": "solc",
        "version": "0.4.6+commit.2dabbdf0.Darwin.appleclang",
        "settings": {
            "optimize": true
        }
      },
      "natspec": {
        "title": "Safe Math Library",
        "author": "Piper Merriam <[email protected]>",
        ...
      }
    }
  },
  "deployments": {
    "blockchain://41941023680923e0fe4d74a34bdac8141f2540e3ae90623718e47d66d1ca4a2d/block/1e96de11320c83cca02e8b9caf3e489497e8e432befe5379f2f08599f8aecede": {
      "SafeMathLib": {
        "contract_type": "SafeMathLib",
        "address": "0x8d2c532d7d211816a2807a411f947b211569b68c",
        "transaction": "0xaceef751507a79c2dee6aa0e9d8f759aa24aab081f6dcf6835d792770541cb2b",
        "block": "0x420cb2b2bd634ef42f9082e1ee87a8d4aeeaf506ea5cdeddaa8ff7cbf911810c"
      }
    }
  }
}

The first thing to point out is that unlike our standard-token contract, we've included the bytecode, runtime_bytecode and compiler information in the SafeMathLib section of the contract_type definition. This is because we are also including a deployed instance of this contract and need to require adequate information for package managers to verify that the contract sound at the deployed address is in fact from the source code included in this package.

The next thing to look at is the deployments section. The first thing you should see is the BIP122 URI.

blockchain://41941023680923e0fe4d74a34bdac8141f2540e3ae90623718e47d66d1ca4a2d/block/1e96de11320c83cca02e8b9caf3e489497e8e432befe5379f2f08599f8aecede

This URI defines the chain on which the SafeMathLib library was deployed. The first hash you see, 41941023680923e0fe4d74a34bdac8141f2540e3ae90623718e47d66d1ca4a2d is the genesisi block hash for the Ropsten test network. The later hash 1e96de11320c83cca02e8b9caf3e489497e8e432befe5379f2f08599f8aecede is the block hash for block numbr 168,238 from the Ropsten chain.

Under that URI there is a single contract instance. It specifies that it's contract type is SafeMathLib, the address that the contract instance can be found at, the transaction hash for the transaction that deployed the contract, and the block hash in which the deploying transaction was mined.

Package which uses a Reusable Contract from a depenency

For our next example we'll be creating a package includes a deployed instance of a contract type from that comes from a package dependency. This differs from our previous safe-math-lib example where our deployment is referencing a local contract from the local contract_types. In this package's Release Lockfile we will be referencing a contract_type from one of the build_dependencies

We are going to use the standard-token package we created earlier and include a deployed version of the StandardToken contract.

Our package will be called piper-coin and will not contain any source files since it merely makes use of the contracts from the standard-token package. The Release Lockfile is listed below with some sections truncated for improved readability. The full Release Lockfile can be found at ./examples/piper-coin/1.0.0.json

{
  "lockfile_version": "1",
  "version": "1.0.0",
  "package_name": "piper-coin",
  "deployments": {
    "blockchain://41941023680923e0fe4d74a34bdac8141f2540e3ae90623718e47d66d1ca4a2d/block/cff59cd4bc7077ae557eb39f84f869a1ea7955d52071bad439f0458383a78780": {
      "PiperCoin": {
        "contract_type": "standard-token:StandardToken",
        "address": "0x11cbb0604e47e0f8501b8f56c1c05f92088dc1b0",
        "transaction": "0x1f8206683e4b1dea1fd2e7299b7606ff27440f33cb994b42b4ecc4b0f83a210f",
        "block": "0xe94a700ef9aa2d7a1b07321838251ea4ade8d4d682121f67899f401433a0d910",
        "bytecode": "...",
        "runtime_bytecode": "...",
        "compiler": {
          "type": "solc",
          "version": "0.4.6+commit.2dabbdf0.Darwin.appleclang"
        }
      }
    }
  },
  "build_dependencies": {
    "standard-token": "ipfs://QmegJYswSDXUJbKWBuTj7AGBY15XceKxnF1o1Vo2VvVPLQ"
  }
}

Most of this should be familiar but it's worth pointing out how we reference contract types from dependencies. Under the PiperCoin entry within the deployments you should see that the contract_type key is set to standard-token:StandardToken. The first portion represents the name of the package dependency within the build_dependencies that should be used. The later portion indicates the contract type that should be used from that dependencies contract types.

Stand Alone package with a deployed Library and a contract which Links against that Library

In the previous safe-math-lib package we demonstrated what a package with a deployed instance of one of it's local contracts looks like. In this example we will build on that concept with a package which includes a library and a contract which uses that library as well as deployed instances of both.

The package will be called escrow and will implementing a simple escrow contract. The escrow contract will make use of a library to safely send ether. Both the contract and library will be part of the package found in the following two solidity source files.

The full source for these files can be found here: ./examples/escrow/.

The Release Lockfile is listed below with some sections truncated for improved readability. The full Release Lockfile can be found at ./examples/escrow/1.0.0.json

{
  "lockfile_version": "1",
  "version": "1.0.0",
  "package_name": "escrow",
  "sources": {
    "./contracts/SafeSendLib.sol": "ipfs://QmcnzhWjaV71qzKntv4burxyix9W2yBA2LrJB4k99tGqkZ",
    "./contracts/Escrow.sol": "ipfs://QmSwmFLT5B5aag485ZWvHmfdC1cU5EFdcqs1oqE5KsxGMw"
  },
  "contract_types": {
    "SafeSendLib": {
      ...
    },
    "Escrow": {
      ...
    }
  },
  "deployments": {
    "blockchain://41941023680923e0fe4d74a34bdac8141f2540e3ae90623718e47d66d1ca4a2d/block/e76cf1f29a4689f836d941d7ffbad4e4b32035a441a509dc53150c2165f8e90d": {
      "SafeMathLib": {
        "contract_type": "SafeSendLib",
        "address": "0x80d7f7a33e551455a909e1b914c4fd4e6d0074cc",
        "transaction": "0x74561167f360eaa20ea67bd4b4bf99164aabb36b2287061e86137bfa0d35d5fb",
        "block": "0x46554e3cf7b768b1cc1990ad4e2d3a137fe9373c0dda765f4db450cd5fa64102"
      },
      "Escrow": {
        "contract_type": "Escrow",
        "address": "0x35b6b723786fd8bd955b70db794a1f1df56e852f",
        "transaction": "0x905fbbeb6069d8b3c8067d233f58b0196b43da7a20b839f3da41f69c87da2037",
        "block": "0x9b39dcab3d665a51755dedef56e7c858702f5817ce926a0cd8ff3081c5159b7f",
        "link_dependencies": [
          {"offset": 524, "value": "SafeSendLib"},
          {"offset": 824, "value": "SafeSendLib"}
        ]
      }
    }
  }
}

This Release Lockfile is the first one we've seen thus far that include the link_dependencies section within one of the contract instances. The runtime_bytecode value for the Escrow contract has been excluded from the example above for readability, but the full value is as follows (wrapped to 80 characters).

0x606060405260e060020a600035046366d003ac811461003457806367e404ce1461005d57806369
d8957514610086575b610000565b3461000057610041610095565b60408051600160a060020a0390
92168252519081900360200190f35b34610000576100416100a4565b60408051600160a060020a03
9092168252519081900360200190f35b34610000576100936100b3565b005b600154600160a06002
0a031681565b600054600160a060020a031681565b60005433600160a060020a0390811691161415
61014857600154604080516000602091820152815160e260020a6324d048c7028152600160a06002
0a03938416600482015230909316316024840152905173__SafeSendLib_____________________
______92639341231c926044808301939192829003018186803b156100005760325a03f415610000
57506101e2915050565b60015433600160a060020a039081169116141561002f5760008054604080
51602090810193909352805160e260020a6324d048c7028152600160a060020a0392831660048201
52309092163160248301525173__SafeSendLib___________________________92639341231c92
60448082019391829003018186803b156100005760325a03f41561000057506101e2915050565b61
0000565b5b5b56

You can see that the placeholder __SafeSendLib___________________________ is present in two locations within this bytecode. This is referred to as a link reference. The entries in the link_dependencies section of a contract instance describe how these link references should be filled in.

The offset value specifies the number of characters into the unprefixed bytecode where the replacement should begin. The value defines what address should be used to replace the link reference. In this case, the value is referencing the SafeSendLib contract instance from this release lockfile.

Package with a contract with link dependencies on a contract from a package dependency

Now that we've seen how we can handle linking dependencies that rely on deployed contract instances from the local package we'll explore an example with link dependencies that rely on contracts from the package dependencies.

In this example we'll be writing the wallet package which includes a wallet contract which makes use of the previous safe-math-lib package. We will also make use of the owned package from our very first example to handle authorization. Our package will contain a single solidity source file ./contracts/Wallet.sol. The version below has been trimmed for readability.

import {SafeMathLib} from "safe-math-lib/contracts/SafeMathLib.sol";
import {owned} from "owned/contracts/owned.sol";

contract Wallet is owned {
    using SafeMathLib for uint;

    mapping (address => uint) allowances;

    function() {
    }

    function send(address recipient, uint value) public onlyowner {
        recipient.send(value);
    }

    function approve(address recipient, uint value) public onlyowner {
        allowances[recipient] = value;
    }

    function withdraw(uint value) public {
        allowances[msg.sender] = allowances[msg.sender].safeSub(value);
        if (!msg.sender.send(value)) throw;
    }
}

The Release Lockfile for our wallet package can been seen below. It has been trimmed to improve readability. The full Release Lockfile can be found at ./examples/wallet/1.0.0.json

{
  "lockfile_version": "1",
  "version": "1.0.0",
  "package_name": "wallet",
  "sources": {
    "./contracts/Wallet.sol": "ipfs://QmYKibsXPSTR5UjywQHX8SM4za1K3QHadtFGWmZqGA4uE9"
  },
  "contract_types": {
    "Wallet": {
      "bytecode": "...",
      "runtime_bytecode": "...",
      ...
    }
  },
  "deployments": {
    "blockchain://41941023680923e0fe4d74a34bdac8141f2540e3ae90623718e47d66d1ca4a2d/block/3ececfa0e03bce2d348279316100913c42ca2dcd51b8bc8d2d87ef2dc6a479ff": {
      "Wallet": {
        "contract_type": "Wallet",
        "address": "0xcd0f8d7dab6c682d3726693ef3c7aaacc6431d1c",
        "transaction": "0x5c113857925ae0d866341513bb0732cd799ebc1c18fcec253bbc41d2a029acd4",
        "block": "0xccd130623ad3b25a357ead2ecfd22d38756b2e6ac09b77a37bd0ecdf16249765",
        "link_dependencies": [
          {"offset": 678, "value": "safe-math-lib:SafeMathLib"}
        ]
      }
    }
  },
  "build_dependencies": {
    "owned": "ipfs://QmXDf2GP67otcF2gjWUxFt4AzFkfwGiuzfexhGuotGTLJH",
    "safe-math-lib": "ipfs://QmfUwis9K2SLwnUh62PDb929JzU5J2aFKd4kS1YErYajdq"
  }
}

Just like our previous example, the runtime_bytecode has been omitted for improved readability, but the full value is as follows (wrapped to 80 characters).

0x606060405236156100355760e060020a6000350463095ea7b381146100435780632e1a7d4d1461
006a578063d0679d341461008e575b34610000576100415b5b565b005b3461000057610056600435
6024356100b5565b604080519115158252519081900360200190f35b346100005761005660043561
00f8565b604080519115158252519081900360200190f35b34610000576100566004356024356101
da565b604080519115158252519081900360200190f35b6000805433600160a060020a0390811691
16146100d157610000565b50600160a060020a038216600090815260016020819052604090912082
90555b5b92915050565b600160a060020a0333166000908152600160209081526040808320548151
830184905281517fa293d1e800000000000000000000000000000000000000000000000000000000
8152600481019190915260248101859052905173__SafeMathLib___________________________
9263a293d1e89260448082019391829003018186803b156100005760325a03f41561000057505060
4080518051600160a060020a0333166000818152600160205293842091909155925084156108fc02
91859190818181858888f1935050505015156101d157610000565b5060015b919050565b60008054
33600160a060020a039081169116146101f657610000565b604051600160a060020a038416908315
6108fc029084906000818181858888f19450505050505b5b9291505056

As you can see, this bytecode contains a link reference to the SafeMathLib library from the safe-math-lib package dependency. If you look in the link_dependencies section of our Wallet contract you'll see it's items are similar to the ones from our previous example.

"link_dependencies": [
  {"offset": 678, "value": "safe-math-lib:SafeMathLib"}
]

However, unlike the previous example which linked against a local contract type, value portion is prefixed with the name of the package which contains the address of the contract instance that this should be linked against.