Contract Metadata
- Encoding of the Metadata Hash in the Bytecode
- Usage of automatic interface generation and natspac
- Usage for Source Code Verification
The Solidity compiler automatically generates a JSON file, the contract metadata, that contains information about the current contract. You can use this file to query the compiler version, the sources used, the ABI and NatSpec documentation to more safely interact with the contract and verify its source code.
The compiler appends a Swarm hash of the metadata file to the end of the bytecode (for details, see below) of each contract, so that you can retrieve the file in an authenticated way without having to resort to a centralized data provider.
You have to publish the metadata file to Swarm (or another service) so that others can access it. You create the file by using the solc --metadata command that generates a file called ContractName_meta.json. It contains Swarm references to the source code, so you have to upload all source files and the metadata file.
The metadata file has the following format. The example below is presented in a human-readable way. Properly formatted metadata should use quotes correctly, reduce whitespace to a minimum and sort the keys of all objects to arrive at a unique formatting. Comments are not permitted and used here only for explanatory purposes.
{
// Required: The version of the metadata format
version: "1",
// Required: Source code language, basically selects a "sub-version"
// of the specification
language: "Solidity",
// Required: Details about the compiler, contents are specific
// to the language.
compiler: {
// Required for Solidity: Version of the compiler
version: "0.4.6+commit.2dabbdf0.Emscripten.clang",
// Optional: Hash of the compiler binary which produced this output
keccak256: "0x123..."
},
// Required: Compilation source files/source units, keys are file names
sources:
{
"myFile.sol": {
// Required: keccak256 hash of the source file
"keccak256": "0x123...",
// Required (unless "content" is used, see below): Sorted URL(s)
// to the source file, protocol is more or less arbitrary, but a
// Swarm URL is recommended
"urls": [ "bzzr://56ab..." ]
},
"mortal": {
// Required: keccak256 hash of the source file
"keccak256": "0x234...",
// Required (unless "url" is used): literal contents of the source file
"content": "contract mortal is owned { function kill() { if (msg.sender == owner) selfdestruct(owner); } }"
}
},
// Required: Compiler settings
settings:
{
// Required for Solidity: Sorted list of remappings
remappings: [ ":g=/dir" ],
// Optional: Optimizer settings. The fields "enabled" and "runs" are deprecated
// and are only given for backwards-compatibility.
optimizer: {
enabled: true,
runs: 500,
details: {
// peephole defaults to "true"
peephole: true,
// jumpdestRemover defaults to "true"
jumpdestRemover: true,
orderLiterals: false,
deduplicate: false,
cse: false,
constantOptimizer: false,
yul: false,
yulDetails: {}
}
},
metadata: {
// Reflects the setting used in the input json, defaults to false
useLiteralContent: true
}
// Required for Solidity: File and name of the contract or library this
// metadata is created for.
compilationTarget: {
"myFile.sol": "MyContract"
},
// Required for Solidity: Addresses for libraries used
libraries: {
"MyLib": "0x123123..."
}
},
// Required: Generated information about the contract.
output:
{
// Required: ABI definition of the contract
abi: [ ... ],
// Required: NatSpec user documentation of the contract
userdoc: [ ... ],
// Required: NatSpec developer documentation of the contract
devdoc: [ ... ],
}
}
Warning
Since the bytecode of the resulting contract contains the metadata hash, any change to the metadata results in a change of the bytecode. This includes changes to a filename or path, and since the metadata includes a hash of all the sources used, a single whitespace change results in different metadata, and different bytecode.
Note
Note the ABI definition above has no fixed order. It can change with compiler versions.
Encoding of the Metadata Hash in the Bytecode
Because we might support other ways to retrieve the metadata file in the future, the mapping {"bzzr1": <Swarm hash>, "solc": <compiler version>} is stored CBOR-encoded. Since the mapping might contain more keys (see below) and the beginning of that encoding is not easy to find, its length is added in a two-byte big-endian encoding. The current version of the Solidity compiler usually adds the following to the end of the deployed bytecode:
0xa2 0x65 'b' 'z' 'z' 'r' '1' 0x58 0x20 <32 bytes swarm hash> 0x64 's' 'o' 'l' 'c' 0x43 <3 byte version encoding> 0x00 0x32
So in order to retrieve the data, the end of the deployed bytecode can be checked to match that pattern and use the Swarm hash to retrieve the file.
Whereas release builds of solc use a 3 byte encoding of the version as shown above (one byte each for major, minor and patch version number), prerelease builds will instead use a complete version string including commit hash and build date.
Note
The CBOR mapping can also contain other keys, so it is better to fully decode the data instead of relying on it starting with 0xa265. For example, if any experimental features that affect code generation are used, the mapping will also contain "experimental": true.
Note
The compiler currently uses the “swarm version 1” hash of the metadata, but this might change in the future, so do not rely on this sequence to start with 0xa2 0x65 'b' 'z' 'z' 'r' '1'. We might also add additional data to this CBOR structure, so the best option is to use a proper CBOR parser.
Usage for Automatic Interface Generation and NatSpec
The metadata is used in the following way: A component that wants to interact with a contract (e.g. Mist or any wallet) retrieves the code of the contract, from that the Swarm hash of a file which is then retrieved. That file is JSON-decoded into a structure like above.
The component can then use the ABI to automatically generate a rudimentary user interface for the contract.
Furthermore, the wallet can use the NatSpec user documentation to display a confirmation message to the user whenever they interact with the contract, together with requesting authorization for the transaction signature.
For additional information, read Ethereum Natural Language Specification (NatSpec) format.
Usage for Source Code Verification
In order to verify the compilation, sources can be retrieved from Swarm via the link in the metadata file. The compiler of the correct version (which is checked to be part of the “official” compilers) is invoked on that input with the specified settings. The resulting bytecode is compared to the data of the creation transaction or CREATE opcode data. This automatically verifies the metadata since its hash is part of the bytecode. Excess data corresponds to the constructor input data, which should be decoded according to the interface and presented to the user.
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