Zk-SNARKs in Substrate

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zk-SNARKs in Substrate: Bringing Privacy to Custom Blockchains

Zero-knowledge proofs are no longer just theory. They are becoming one of the most powerful tools for building privacy-preserving blockchain applications. Among them, zk-SNARKs (Zero-Knowledge Succinct Non-Interactive Arguments of Knowledge) are especially popular because they allow efficient verification of proofs without revealing sensitive data.

On the other hand, Substrate, the blockchain framework that powers Polkadot and many other chains, was designed to be modular and extensible. This makes it a natural place to experiment with zk-SNARKs. In this post, we’ll explore what zk-SNARKs bring to Substrate and outline the first steps for developers who want to integrate them.

Why zk-SNARKs in Substrate?

Blockchains are transparent by design. That’s great for trust, but not for privacy. Suppose you want:

  • To prove you’re over 18 without showing your exact birthdate.

  • To prove you have enough funds without revealing your account balance.

  • To let users vote without exposing their choices.

Normally, you would have to reveal the underlying data. With zk-SNARKs, you only reveal a proof, and the blockchain can still verify it.

Substrate gives us a perfect testing ground, because its runtime can be extended with pallets that add zk-SNARK verification as a native feature.

How It Fits Together

The zk-SNARK workflow in a Substrate chain looks like this:

  1. Circuit design (off-chain):
    Define the mathematical constraints of your proof in a tool like Circom or ZoKrates.

  2. Proof generation (off-chain):
    A user compiles the circuit, generates a trusted setup (if needed), and produces a zk-SNARK proof using private inputs.

  3. Transaction submission (on-chain):
    The user submits the proof and any required public inputs to the Substrate chain via an extrinsic.

  4. Proof verification (on-chain):
    A custom Substrate pallet, using Rust cryptography libraries like arkworks, verifies the proof inside the runtime.

  5. State update (on-chain):
    If valid, the blockchain accepts the transaction, executes logic, and updates state—without ever knowing the private data.

A Minimal Rust Example

Inside a custom pallet, verification might look like this:

 
use ark_groth16::{verify_proof, PreparedVerifyingKey, Proof};
use ark_bn254::Bn254;
pub fn verify_snark(
pvk: &PreparedVerifyingKey<Bn254>,
proof: &Proof<Bn254>,
inputs: &[ark_ff::Field],
) -> bool {
verify_proof(pvk, proof, inputs).unwrap_or(false)
}

This function checks whether a given zk-SNARK proof is valid. If it returns true, the extrinsic can continue; otherwise, it fails.

Starting Steps for Developers

If you’re curious to try this yourself, here’s a simple roadmap:

1. Set Up a Substrate Node

2. Install Cryptography Libraries

  • Add arkworks dependencies in your Cargo.toml:

     
    ark-ff = "0.4"
    ark-groth16 = "0.4"
    ark-bn254 = "0.4"

3. Build a zk-SNARK Circuit

  • Use ZoKrates or Circom to design a simple circuit.

  • Example: prove that a number x is in a certain range without revealing x.

4. Generate Proofs Off-Chain

  • Run the circuit compiler, create proving and verification keys.

  • Generate a proof with private inputs.

5. Create a Verification Pallet

  • Write a pallet function (extrinsic) that takes (proof, public_inputs) as arguments.

  • Call verify_snark() from inside the pallet.

  • Update blockchain state only if the proof verifies.

6. Test Locally

  • Submit your proof via Polkadot.js Apps or CLI.

  • Confirm the runtime verifies it correctly.

 Use Cases

  • Confidential transactions (hide amounts but prove validity).

  • Private voting (votes counted without revealing identities).

  • Selective disclosure (prove identity attributes like age, citizenship).

With Substrate’s modular design, each of these can be implemented as a pallet.

Final Thoughts

zk-SNARKs solve the tension between transparency and privacy in blockchains. Substrate provides the flexibility to integrate them directly at the runtime level.

The heavy cryptography happens off-chain, but the on-chain verification is fast and practical. That’s the magic: you get privacy, efficiency, and trust—all at once.

If you’re building a new blockchain or experimenting with privacy features, zk-SNARKs on Substrate are worth exploring. Start small—prove something simple, verify it in a pallet, and build from there.

https://brightinventions.pl/blog/zk-snarks-in-substrate-part-1/https://brightinventions.pl/blog/zk-snarks-in-substrate-part-2-using-groth16-and-running-circom-proof/ 

 

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