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Update EIP-1052: fix eip-1052.md #9176

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94 changes: 59 additions & 35 deletions EIPS/eip-1052.md
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---
eip: 1052
title: EXTCODEHASH opcode
author: Nick Johnson <[email protected]>, Paweł Bylica <[email protected]>
description: Returns the keccak256 hash of a contract's code
author: Nick Johnson (@arachnid), Paweł Bylica (@chfast)
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This is a final proposal, so it isn't important to update the author line. No one has automatic merge permissions anyway.

Suggested change
author: Nick Johnson (@arachnid), Paweł Bylica (@chfast)
author: Nick Johnson <arachnid@notdot.net>, Paweł Bylica <[email protected]>

discussions-to: https://ethereum-magicians.org/t/extcodehash-opcode/262
status: Final
type: Standards Track
Expand All @@ -11,68 +12,91 @@ requires: 161
---

## Abstract
This EIP specifies a new opcode, which returns the keccak256 hash of a contract's code.

This EIP specifies a new opcode, which returns the keccak256 hash of a contract's
code.

## Motivation
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.

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.
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.

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.

## Specification

A new opcode, `EXTCODEHASH`, is introduced, with number `0x3f`. The `EXTCODEHASH`
takes one argument from the stack, zeros the first 96 bits
and pushes to the stack the keccak256 hash of the code of the account
at the address being the remaining 160 bits.
A new opcode, `EXTCODEHASH`, is introduced, with number `0x3f`. The `EXTCODEHASH`
takes one argument from the stack, zeros the first 96 bits, and pushes to the
stack the keccak256 hash of the code of the account at the address being the
remaining 160 bits.

In case the account does not exist or is empty (as defined by [EIP-161](./eip-161.md)) `0` is pushed to the stack.
In case the account does not exist or is empty (as defined by
[EIP-161](./eip-161.md)), `0` is pushed to the stack.

In case the account does not have code the keccak256 hash of empty data
(i.e. `c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470`)
is pushed to the stack.
In case the account does not have code, the keccak256 hash of empty data (i.e.
`c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470`) is pushed to
the stack.

The gas cost of the `EXTCODEHASH` is 400.


## Rationale

As described in the motivation section, this opcode is widely useful, and saves
on wasted gas in many cases.
As described in the motivation section, this opcode is widely useful and saves on
wasted gas in many cases.

The gas cost is the same as the gas cost for the `BALANCE` opcode because the
The gas cost is the same as the gas cost for the `BALANCE` opcode because the
execution of the `EXTCODEHASH` requires the same account lookup as in `BALANCE`.

Only the 20 last bytes of the argument are significant (the first 12 bytes are
ignored) similarly to the semantics of the `BALANCE` (`0x31`), `EXTCODESIZE` (`0x3b`) and
`EXTCODECOPY` (`0x3c`).

The `EXTCODEHASH` distincts accounts without code and non-existing accounts.
This is consistent with the way accounts are represented in the state trie.
This also allows smart contracts to check whenever an account exists.
Only the last 20 bytes of the argument are significant (the first 12 bytes are
ignored), similarly to the semantics of the `BALANCE` (`0x31`), `EXTCODESIZE`
(`0x3b`), and `EXTCODECOPY` (`0x3c`).

The `EXTCODEHASH` distinguishes accounts without code and non-existing accounts.
This is consistent with the way accounts are represented in the state trie. This
also allows smart contracts to check when an account exists.

## Backwards Compatibility

There are no backwards compatibility concerns.


## Test Cases

1. The `EXTCODEHASH` of the account without code is `c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470`
what is the keccack256 hash of empty data.
2. The `EXTCODEHASH` of non-existent account is `0`.
1. The `EXTCODEHASH` of the account without code is
`c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470`,
which is the keccak256 hash of empty data.
2. The `EXTCODEHASH` of a non-existent account is `0`.
3. The `EXTCODEHASH` of a precompiled contract is either `c5d246...` or `0`.
4. If `EXTCODEHASH` of `A` is `X`, then `EXTCODEHASH` of `A + 2**160` is `X`.
5. The `EXTCODEHASH` of an account that selfdestructed in the current transaction.
6. The `EXTCODEHASH` of an account that selfdestructed and later the selfdestruct has been reverted.
5. The `EXTCODEHASH` of an account that selfdestructed in the current
transaction.
6. The `EXTCODEHASH` of an account that selfdestructed and later the
selfdestruct has been reverted.
7. The `EXTCODEHASH` of an account created in the current transaction.
8. The `EXTCODEHASH` of an account that has been newly created and later the creation has been reverted.
9. The `EXTCODEHASH` of an account that firstly does not exist and later is empty.
10. The `EXTCODEHASH` of an empty account that is going to be cleared by the state clearing rule.
8. The `EXTCODEHASH` of an account that has been newly created and later
the creation has been reverted.
9. The `EXTCODEHASH` of an account that did not exist at first and then
became empty.
10. The `EXTCODEHASH` of an empty account that will be cleared by
the state clearing rule.

## Reference Implementation

_TBD_

## Implementation
TBD
## Security Considerations

This opcode does not introduce new security risks beyond those already
associated with the usage of code-hashing. However, it allows for more efficient
code checks, so developers should be aware that this may enable cheaper
verification of contracts, potentially influencing design decisions.

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

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