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On-Chain Randomness with NEAR

Generating random numbers on a blockchain is fundamentally different from traditional programming. While you might use Math.random() in JavaScript or /dev/urandom in Linux, blockchains require every node to agree on the same "random" value - making true randomness impossible. This tutorial will teach you how to implement secure, verifiable randomness in NEAR smart contracts.

The Challenge of Blockchain Randomness​

In a blockchain environment, randomness faces unique constraints:

  • Determinism: All validators must compute identical results
  • Unpredictability: Users shouldn't be able to predict outcomes before transactions
  • Manipulation Resistance: Miners or validators shouldn't influence results

NEAR solves this through a Verifiable Random Function (VRF) that provides cryptographically secure randomness available directly in your smart contracts.

How NEAR's Randomness Works​

NEAR provides randomness through:

  • Rust: env::random_seed()
  • JavaScript: near.randomSeed()

Both return a 32-byte array derived from:

  • Block producer's cryptographic signature
  • Previous epoch's random value
  • Block height and timestamp
  • Network-specific constants

This ensures randomness that is unpredictable for users but deterministic for validators.

info

The complete source code for this tutorial is available in the near-examples/coin-flip-examples repository.

A live version is deployed on testnet at coinflip.near-examples.testnet for testing.

What You Will Build​

You'll create a coin flip game that demonstrates:

Let's start by understanding the fundamentals of on-chain randomness.