PoseidonPolkaVM

Poseidon hashing for Polkavm with solidity compatibility, making it more gas effective to do Poseidon hashing in dotsama

This is a port of Light-Poseidon that is compatible with circom and PolkaVM, allowing you to do poseidon hashing on polkavm.

Poseidon paper

Poseidon Hash Function

The hash(uint256[2]) function implements the Poseidon hash algorithm with:

• Field: BN254 (alt_bn128)
• Arity: 2 (takes two inputs)
• Rounds: 8 full rounds + 57 partial rounds
• S-box: x^5

Usage

After deploying this PolkaVM program, you can call it using this solidity interface:

pragma solidity ^0.8.30;

interface IPoseidonT3 {
    function hash(uint256[2] memory input) external pure returns (uint256);
}

# Call the hash function with two uint256 inputs
cast call <CONTRACT_ADDRESS> "hash(uint256[2]):(uint256)" "[123,456]" \
  --rpc-url <RPC_URL> --private-key <PRIVATE_KEY>

Example Results

Input	Output
[0, 0]	5520371747610818048552497760483731695918538905235353263918705622436011791040
[1, 2]	3240460917331939313458239639914504962640333851982787431747809795119847651274
[123, 456]	10422317022970317265083564129867363010880980031113186224756990573079674352133

Architecture

┌─────────────────┐    ┌──────────────────┐    ┌─────────────────┐
│   Rust Source   │ -> │   PolkaVM Build  │ -> │  Smart Contract │
│   (src/erc20.rs)│    │   (build.sh)     │    │   (.polkavm)    │
└─────────────────┘    └──────────────────┘    └─────────────────┘

┌─────────────────┐    ┌──────────────────┐
│  TypeScript     │ -> │   Contract ABI   │
│  (ts/erc20.ts)  │    │   (JSON)         │
└─────────────────┘    └──────────────────┘

Installation

Rust

curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

Polkatool

cargo install polkatool

Node, Typescript and Yarn

It is recommended to install via nvm.

# Download and install nvm:
curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.40.3/install.sh | bash

# in lieu of restarting the shell
\. "$HOME/.nvm/nvm.sh"

# Download and install Node.js:
nvm install 22

# Verify the Node.js version:
node -v # Should print "v22.15.0".
nvm current # Should print "v22.15.0".

# Verify npm version:
npm -v # Should print "10.9.2".

npm install -g typescript
npm install -g yarn

Build

Run ./build.sh, you will get an erc20.polkavm file for deployment and test

Build Process

# Build the PolkaVM contract
./build.sh

# The output is erc20.polkavm - ready for deployment

Deployment & Testing

1. Go to ts folder, mv env.example .env. Input a key with some token in Westend Asset Hub.
2. Run yarn to install dependency
3. Run yarn erc20 to deploy ERC20 contract and interact with it.

user@user-X870-EAGLE-WIFI7:~/github/papermoon/polkavm-erc20-in-rust/ts$ yarn erc20
yarn run v1.22.22
$ ts-node main.ts
contract address is:  0xD670137a3CfAaF08B5395CDaaEaFf617672896D2
recipientBalance is:  1000000000000000000n
Name:  name
Symbol:  symbol
Decimals:  18n
Total Supply:  1234000000000000000000n
Balance:  1234000000000000000000n
Allowance:  0n
My balance after transfer:  1233n
Random wallet balance after transfer:  1n
Approve allowance:  2n
Approve allowance after transferFrom:  1n
Done in 38.25s.

Testing Poseidon Hash

Use cast to test the Poseidon hash function:

# Test Poseidon hash with different inputs
cast call CONTRACT_ADDRESS "hash(uint256[2]):(uint256)" "[123,456]" \
  --rpc-url http://eth-pas-hub.laissez-faire.trade:8545 --private-key <PRIVATE_KEY>

cast call CONTRACT_ADDRESS "hash(uint256[2]):(uint256)" "[1,2]" \
  --rpc-url http://eth-pas-hub.laissez-faire.trade:8545 --private-key <PRIVATE_KEY>

Implementation Details

Rust Contract (src/erc20.rs)

• No-std environment for PolkaVM compatibility
• Custom field arithmetic for BN254 operations
• ABI decoding for Ethereum-compatible function calls
• Memory management with SimpleAlloc

Poseidon Implementation (src/zk.rs)

• Field element operations (Fr struct with 4x u64 limbs)
• Round constants for BN254 Poseidon
• MDS matrix for linear layer
• S-box implementation (x^5 exponentiation)

Function Selector

• Hash function: hash(uint256[2])
• Selector: 0x561558fe
• Input format: ABI-encoded array of two uint256 values

Contract Address

Latest deployment: 0xD670137a3CfAaF08B5395CDaaEaFf617672896D2 (Polkadot Asset Hub Testnet)

Contributing

When modifying the Poseidon implementation:

1. Ensure field arithmetic remains correct
2. Verify round constants match reference implementation
3. Test with known input/output pairs
4. Build and test on PolkaVM before deployment

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