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authorAlex Beregszaszi <alex@rtfs.hu>2017-09-05 21:20:53 +0800
committerAlex Beregszaszi <alex@rtfs.hu>2017-09-11 22:05:46 +0800
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Promote metadata to its own chapter (split from miscellaneous)
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+#################
+Contract Metadata
+#################
+
+.. index:: metadata, contract verification
+
+The Solidity compiler automatically generates a JSON file, the
+contract metadata, that contains information about the current contract.
+It can be used to query the compiler version, the sources used, the ABI
+and NatSpec documentation in order to more safely interact with the contract
+and to 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.
+
+Of course, you have to publish the metadata file to Swarm (or some other service)
+so that others can access it. The file can be output by using ``solc --metadata``
+and the file will be called ``ContractName_meta.json``.
+It will contain 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 of course also not permitted and used here only for explanatory purposes.
+
+.. code-block:: none
+
+ {
+ // 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 (enabled defaults to false)
+ optimizer: {
+ enabled: true,
+ runs: 500
+ },
+ // 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: [ ... ],
+ }
+ }
+
+.. note::
+ Note the ABI definition above has no fixed order. It can change with compiler versions.
+
+.. note::
+ Since the bytecode of the resulting contract contains the metadata hash, any change to
+ the metadata will result in a change of the bytecode. Furthermore, since the metadata
+ includes a hash of all the sources used, a single whitespace change in any of the source
+ codes will result in a different metadata, and subsequently a different bytecode.
+
+Encoding of the Metadata Hash in the Bytecode
+=============================================
+
+Because we might support other ways to retrieve the metadata file in the future,
+the mapping ``{"bzzr0": <Swarm hash>}`` is stored
+`CBOR <https://tools.ietf.org/html/rfc7049>`_-encoded. Since 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 thus adds the following
+to the end of the deployed bytecode::
+
+ 0xa1 0x65 'b' 'z' 'z' 'r' '0' 0x58 0x20 <32 bytes swarm hash> 0x00 0x29
+
+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.
+
+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) 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, Mist can use the userdoc to display a confirmation message to the user
+whenever they interact with the contract.
+
+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.