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Randomness

How unpredictable pseudo-randomness is achieved on the Celo blockchain and offered as a service for dapp developers.


Producing Pseudo-randomness

Producing unpredictable pseudo-randomness without a trusted third party is not trivial. Several solutions for this problem exist or are being currently researched. They include Verifiable Random Functions (for example, based on BLS threshold signatures), Verifiable Delay Functions, and commit-reveal schemes.

Currently, Celo implements a simple RANDAO commit-reveal scheme which is secure enough for many uses, offering validators only 1 bit of influence: a validator can affect randomness only by choosing not to propose a block, which results in the next validator revealing their pre-commited randomness. A more sophisticated solution might be implemented as the network evolves, especially if randomness becomes necessary for other purposes that require stronger assumptions about the randomness’s security (for example if it was decided that a randomized leader election algorithm should replace the current round robin).

In a proposed block, the proposer attaches two values related to the randomness scheme - randomness corresponding to their previous commitment, and a new commitment to freshly generated random bytes that will be revealed in the future. The revealed randomness is added to an entropy pool accessible on-chain from the Random smart contract.

Randomness Equation

More formally, the nthn*{th} block proposed by a given validator contains values (rn,sn)(r_n, s_n) such that keccack256(rn)=sn1\text{keccack256}(r_n) = s*{n-1}. The one exception to this is the validator’s first block, the case where n=1n = 1, since they have not previously committed to randomness yet. Here, the protocol instead requires that r1=1r_1 = 1.

Using Onchain Randomness

This randomness can be used by any smart contracts deployed to a Celo network using the Random core contract, e.g.:

import "celo-monorepo/packages/protocol/identity/interfaces/IRandom.sol";
import "celo-monorepo/packages/protocol/common/interfaces/IRegistry.sol";

contract Example {
function test() external view returns (bytes32 randomness) {
randomness = IRandom(
IRegistry(0x000000000000000000000000000000000000ce10)
.getAddressFor(keccak256(abi.encodePacked("Random")))
).random();
}
}

Alternatively, through inheritance of UsingRegistry.

import "celo-monorepo/packages/protocol/common/UsingRegistryV2.sol";

contract Example is UsingRegistryV2 {
function test() external view returns (bytes32 randomness) {
randomness = getRandom().random();
}
}