EVM Opcode: DIFFICULTY / PREVRANDAO

The DIFFICULTY opcode (0x44) pushes the current block's difficulty value onto the stack. In Solidity, this is exposed as block.difficulty.

After Ethereum's Merge in 2022, the same opcode was repurposed and renamed to PREVRANDAO (EIP-4399). In Solidity it became block.prevrandao. The opcode number 0x44 is the same; only what it returns changed.

On Cronos, this opcode returns 0.

Background

On proof-of-work Ethereum, DIFFICULTY returned how hard it was to mine the current block, a large number that adjusted over time. Smart contracts occasionally read this value, though it was not safe to use as a random number source since miners could influence it.

After the Merge, Ethereum replaced proof-of-work with proof-of-stake and repurposed the opcode. It now returns the RANDAO mix value from the previous block, which is a pseudo-random value produced by combining validator signatures over many blocks. This made block.prevrandao a better (though still imperfect) source of on-chain randomness.

Cronos uses Cosmos SDK BFT consensus and has neither proof-of-work difficulty nor Ethereum's RANDAO mechanism. There is no meaningful value to return, so the opcode returns 0.

Behaviour Comparison

• Ethereum (pre-Merge): DIFFICULTY returns the current block's mining difficulty.

• Ethereum (post-Merge): PREVRANDAO returns the previous block's RANDAO mix, a pseudo-random value.

• Cronos: Returns 0. There is no proof-of-work and no RANDAO.

What This Means for Your Contract

• Do not use block.difficulty or block.prevrandao as a source of randomness on Cronos. On Cronos, both always return 0. Any contract logic that depends on this value being random or variable will not work correctly.

// This does NOT produce randomness on Cronos — always returns 0
uint256 rand = uint256(block.prevrandao);

• Porting contracts from Ethereum that use block.difficulty for randomness needs a fix. A common pattern in older Ethereum contracts is using block.difficulty combined with other values to generate a pseudo-random number. This pattern breaks on Cronos since the input is always 0.

// Common Ethereum pattern, broken on Cronos
uint256 rand = uint256(keccak256(abi.encodePacked(block.difficulty, block.timestamp, msg.sender)));

Audit any Ethereum contract that reads block.difficulty or block.prevrandao before deploying on Cronos. If the contract uses it for randomness or any variable logic, it needs to be updated.

Recommended Alternative

Use a decentralised oracle for randomness on Cronos. The following are available on Cronos mainnet:

• Witnet: https://docs.cronos.com/for-dapp-developers/dev-tools-and-integrations/witnet#randomness-oracle-in-smart-contracts

• Pyth Entropy: on-chain random number generation by Pyth Network.

Checking the Value

Via JSON-RPC

curl https://evm.cronos.com/ \
  -X POST \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","method":"eth_getBlockByNumber","params":["latest",false],"id":1}'

The difficulty and mixHash fields in the response correspond to this opcode. On Cronos, difficulty is 0:

{
   "id": 1,
   "result": {
       ...
       "difficulty": "0x0",
       "mixHash": "0x0000000000000000000000000000000000000000000000000000000000000000",
       ...
   },
   "jsonrpc": "2.0"
}