ARC: 0032: Adding BlockHash and BlockHeight

[evanmarshall]

---

arc: 0032
title: Add block_hash and block_height keywords
authors: [email protected] [email protected]
discussion: #23
topic: Application
status: Draft
created: 12/2/2022

Abstract

Currently, it's not possible to get the block_height or blockhash within an Aleo program.
These opcodes would enable Aleo Dapps to take advantage of time without a Time Oracle.

For example, if you have a game like Battleship, a Time Oracle is required to handle the case where one of the players fails to respond. Using block_height or blockhash, a game could introduce timeouts to handle such edge cases.

Specification

program example.aleo;

record some_record:
    owner as address.private;
    gates as u64.private;
    hash as blockhash.private;
    height as u64.private;

function create_record:
    cast self.caller 0u64 blockhash block_height into r0 as some_record.record;
    output r0 as some_record.record;

We should also considering mirroring Solidity global variables:

blockhash[block_height]: hash of the given block when blocknumber is one of the 256 most recent blocks; otherwise returns zero
block.basefee: current block’s base fee (EIP-3198 and EIP-1559)
block.chainid: current chain id
block.timestamp: current block timestamp as seconds since unix epoch

Dependencies

This impacts snarkVM, snarkOS, aleo, & leo repositories. As a new opcode determined from consensus, it will impact all parts of the Aleo ecosystem.

Backwards Compatibility

Adding new opcodes seems to require a regeneration of the genesis block.
Existing programs should not be impacted.

Security & Compliance

There should not be any security or compliance considerations.

DApp developers should not use these variables as a source of randomness as they could be influenced by validators.
Depending on the usecase, validators may have trouble influencing results if the programs are sufficiently private.

References

Global variables in Solidity: https://docs.soliditylang.org/en/v0.8.17/units-and-global-variables.html

[d0cd]

Thanks for the proposal! This would be a useful addition but there are some technical challenges.
The main issue is providing on-chain metadata to a transition before it becomes invalid.
Note that reading on-chain metadata during a finalize block is fairly straightforward.