79 lines
3.4 KiB
Markdown
79 lines
3.4 KiB
Markdown
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---
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eip: 1052
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title: EXTCODEHASH opcode
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author: Nick Johnson <arachnid@notdot.net>, Paweł Bylica <pawel@ethereum.org>
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discussions-to: https://ethereum-magicians.org/t/extcodehash-opcode/262
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status: Final
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type: Standards Track
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category: Core
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created: 2018-05-02
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requires: 161
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---
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## Abstract
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This EIP specifies a new opcode, which returns the keccak256 hash of a contract's code.
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## Motivation
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Many contracts need to perform checks on a contract's bytecode, but do not necessarily need the bytecode itself. For instance, a contract may want to check if another contract's bytecode is one of a set of permitted implementations, or it may perform analyses on code and whitelist any contract with matching bytecode if the analysis passes.
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Contracts can presently do this using the `EXTCODECOPY` (`0x3c`) opcode, but this is expensive, especially for large contracts, in cases where only the hash is required. As a result, we propose a new opcode, `EXTCODEHASH`, which returns the keccak256 hash of a contract's bytecode.
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## Specification
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A new opcode, `EXTCODEHASH`, is introduced, with number `0x3f`. The `EXTCODEHASH`
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takes one argument from the stack, zeros the first 96 bits
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and pushes to the stack the keccak256 hash of the code of the account
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at the address being the remaining 160 bits.
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In case the account does not exist or is empty (as defined by [EIP-161](./eip-161.md)) `0` is pushed to the stack.
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In case the account does not have code the keccak256 hash of empty data
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(i.e. `c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470`)
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is pushed to the stack.
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The gas cost of the `EXTCODEHASH` is 400.
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## Rationale
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As described in the motivation section, this opcode is widely useful, and saves
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on wasted gas in many cases.
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The gas cost is the same as the gas cost for the `BALANCE` opcode because the
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execution of the `EXTCODEHASH` requires the same account lookup as in `BALANCE`.
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Only the 20 last bytes of the argument are significant (the first 12 bytes are
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ignored) similarly to the semantics of the `BALANCE` (`0x31`), `EXTCODESIZE` (`0x3b`) and
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`EXTCODECOPY` (`0x3c`).
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The `EXTCODEHASH` distincts accounts without code and non-existing accounts.
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This is consistent with the way accounts are represented in the state trie.
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This also allows smart contracts to check whenever an account exists.
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## Backwards Compatibility
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There are no backwards compatibility concerns.
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## Test Cases
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1. The `EXTCODEHASH` of the account without code is `c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470`
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what is the keccack256 hash of empty data.
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2. The `EXTCODEHASH` of non-existent account is `0`.
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3. The `EXTCODEHASH` of an precompiled contract is either `c5d246...` or `0`.
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4. If `EXTCODEHASH` of `A` is `X`, then `EXTCODEHASH` of `A + 2**160` is `X`.
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5. The `EXTCODEHASH` of an account that selfdestructed in the current transaction.
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6. The `EXTCODEHASH` of an account that selfdestructed and later the selfdestruct has been reverted.
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7. The `EXTCODEHASH` of an account created in the current transaction.
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8. The `EXTCODEHASH` of an account that has been newly created and later the creation has been reverted.
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9. The `EXTCODEHASH` of an account that firstly does not exist and later is empty.
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10. The `EXTCODEHASH` of an empty account that is going to be cleared by the state clearing rule.
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## Implementation
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TBD
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## Copyright
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Copyright and related rights waived via [CC0](../LICENSE.md).
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