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Contract Access To Subjective Data

Simple Summary

Give contracts access to subjective data. This includes CPU charges for the current transaction, wall-clock time, and a random number generator.

Abstract

Contracts don't have access to subjective data. e.g. they can't examine CPU costs. They don't have access to wall-clock time. They don't have access to an unpredictable entropy source. They could, if producers recorded all subjective data they passed to the contracts.

This proposal defines a mechanism for producers to record the subjective data they provide to contracts. It also defines a set of intrinsics for accessing this data.

Overview

There is 1 consensus upgrade in this proposal, which adds the following:

  • A block extension which records subjective data provided to contracts
  • A new intrinsic get_trx_cpu_bill that returns the current CPU charges for the transaction
  • A new intrinsic get_wall_time that returns the wall-clock time with microsecond accuracy
  • A new intrinsic get_random that returns a random number, assuming you trust the producers

There are some constraints on these functions (e.g. get_wall_time is non-decreasing during a single transaction), but this doesn't propose a way to prevent producers from manipulating this data to their advantage (e.g. get_random).

Specification

Block extension

When it's producing a block, nodeos will record subjective data that it provides to contracts into a block extension. This allows other nodes to provide this data to the contracts while applying blocks.

Primitives

All of these primitives abort the transaction when used inside a deferred transaction. They charge NET since they increase block size.

uint32_t get_trx_cpu_bill();
uint32_t get_wall_time();
void get_random(char* dest, size_t size);

get_trx_cpu_bill returns the current CPU charges for the transaction. During validation, nodeos verifies the following:

  • The result is non-decreasing across a transaction.
  • The result is <= the final billed amount.

get_wall_time returns the wall-clock time with microsecond accuracy. During validation, nodeos verifies the following:

  • The result is non-decreasing across a transaction.
  • The result is >= (the block time of the current block - 0.5s)
  • The result is <= (the block time of the current block + 0.5s)

get_random fills a buffer with random data. Even though the reference implementation of nodeos fills this with random data, there are no guarantees that individual producers can not change this. Get Producer proposes a potential counter-measure against producers manipulating subjective data.