EIP4399 - Supplant DIFFICULTY opcode with RANDOM

# Abstract

This EIP supplants the semantics of the return value of existing DIFFICULTY opcode and renames the opcode to RANDOM.

The return value of the DIFFICULTY opcode after this change is the output of the randomness beacon provided by the beacon chain.

# Motivation

Applications may benefit from using the randomness accumulated by the beacon chain. Thus, randomness outputs produced by the beacon chain should be accessible in the EVM.

At the point of TRANSITION_BLOCK of the Proof-of-Stake (PoS) upgrade described in EIP-3675, the difficulty block field MUST be 0 thereafter because there is no longer any Proof-of-Work (PoW) seal on the block. This means that the DIFFICULTY opcode no longer has it's previous semantic meaning, nor a clear "correct" value to return.

Given prior analysis on the usage of DIFFICULTY, the value returned by the opcode mixed with other values is a common pattern used by smart contracts to obtain randomness. The instruction with the same number as the DIFFICULTY opcode returning the output of the beacon chain randomness makes the upgrade to PoS backwards compatible for existing smart contracts obtaining randomness from the DIFFICULTY opcode.

Additionally, changes proposed by this EIP allow for smart contracts to determine whether the upgrade to the PoS has already happened. This can be done by analyzing the return value of the DIFFICULTY opcode. A value greater than 2**64 indicates that the transaction is being executed in the PoS block.

# Specification

# Definitions

TRANSITION_BLOCK The definition of this block can be found in the Definitions section of EIP-3675.

# Block structure

Beginning with TRANSITION_BLOCK, client software MUST set the value of the mixHash, i.e. the field with the number 13 (0-indexed) in a block header, to the output of the randomness beacon provided by the beacon chain.

# EVM

Beginning with TRANSITION_BLOCK, the DIFFICULTY opcode, i.e. the EVM instruction with the number 0x44, MUST return the value of the mixHash field.

Note: The gas cost of the DIFFICULTY opcode remains unchanged.

# Renaming

The mixHash field SHOULD further be renamed to random.

The DIFFICULTY opcode SHOULD further be renamed to RANDOM.

# Rationale

# Including randomness output in the block header

Including the randomness output in the block header provides a straightforward method of accessing it from inside of the EVM as block header data is already available in the EVM context.

Additionally, this ensures that the execution layer can be fully executed with the block alone rather than requiring extra inputs from the PoS consensus layer.

Mixing the randomness into a block header may contribute to uniqueness of the block hash in the case when values of other fields of the block header match the corresponding values of the header of another block.

# Using `mixHash` field instead of `difficulty`

The mixHash header field is used instead of difficulty to avoid a class of hidden forkchoice bugs after the PoS upgrade.

Client software implementing pre-EIP-3675 logic heavily depends on the difficulty value as total difficulty computation is the basis of the PoW fork choice rule. Setting the difficulty field to 0 at the PoS upgrade aims to reduce the surface of bugs related to the total difficulty value growing after the upgrade.

Additionally, any latent total difficulty computation after the PoS upgrade would become overflow prone if the randomness output supplanted the value of the difficulty field.

# Reusing existing field instead of appending a new one

The mixHash field is deprecated at the PoS upgrade and set to zero bytes array thereafter. Reusing an existing field as a place for the randomness output saves 32 bytes per block and effectively removes the deprecation of one of the fields induced by the upgrade.

# Reusing the `DIFFICULTY` opcode instead of introducing a new one

See the Motivation.

# Renaming the field and the opcode

The renaming should be done to make the field and the opcode names semantically sound.

# Using `TRANSITION_BLOCK` rather than a block or slot number

By utilizing TRANSITION_BLOCK to trigger the change in logic defined in this EIP rather than a block or slot number, this EIP is tightly coupled to the PoS upgrade defined by EIP-3675.

By tightly coupling to the PoS upgrade, we ensure that there is no discontinuity for the usecase of this opcode for randomness -- the primary motivation for re-using DIFFICULTY rather than creating a new opcode.

# Using `2**64` threshold to determine PoS blocks

The probability of randomness output to fall into the range between 0 and 2**64 and, thus, to be mixed with PoW difficulty values, is drastically low. Though, proposed threshold might seem to have insufficient distance from difficulty values on Ethereum Mainnet (they are currently around 2**54), it requires a thousand times increase of the hashrate to make this threshold insecure. Such an increase is considered impossible to occur before the upcoming consensus upgrade.

# Backwards Compatibility

This EIP introduces backward incompatible changes to the execution and validation of EVM state transitions. As written, this EIP utilizes TRANSITION_BLOCK and is thus tightly coupled with the PoS upgrade introduced in EIP-3675. If this EIP is to be adopted, it MUST be scheduled at the same time as EIP-3675.

Additionally, the changes proposed might be backward incompatible for the following categories of applications:

Applications that use the value returned by the DIFFICULTY opcode as the PoW difficulty parameter
Applications with logic that depends on the DIFFICULTY opcode returning a relatively small number with respect to the full 256-bit size of the field.

The first category is already affected by switching the consensus mechanism to PoS and no additional breaking changes are introduced by this specification.

The second category is comprised of applications that use the return value of the DIFFICULTY opcode in operations that might cause either overflow or underflow errors. While it is theoretically possible to author an application where a change in the range of possible values this opcode may return could lead to a security vulnerability, the chances of that are negligible.

# Test Cases

Testing this functionality goes beyond this document and should be done in a form of integration testing.

# Reference Implementation

TBD

# Security Considerations

The RANDOM opcode in the PoS network should be considered as a source of randomness output of a higher strength than the output provided by either the BLOCKHASH or the DIFFICULTY opcode in the PoW network.

However, the randomness output returned by the RANDOM opcode is revealed to the network within the parent block.