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Examples

An extensive collection of examples is available in the GitHub repository. Below we discuss the HodlVault example from the repository in full detail. The focus of this page is on the use of the SDK, while the Examples page in the language section focuses on the CashScript syntax.

HodlVault​

We will break up the development of the smart contract application in 4 manageable steps:

  1. Creating the keypairs
  2. Generating a Contract
  3. Building the Oracle
  4. Sending a Transaction

Creating the keypairs​

To put the HodlVault.cash contract to use in a TypeScript application, we have to use the CashScript SDK in combination with a BCH library such as Libauth, Mainnetjs or BCHJS. These libraries are used to generate public/private keys for the contract participants. In this example we'll use Libauth to generate the keys alicePriv, alicePub, oracle & oraclePub. Then we can use these keys to create the smart contract.

caution

These 'private keys' are public just for testing, in other contexts you want to very carefully treat private keys as environment variables. You would also make sure to use a secure seed phrase and not 'CashScript Examples'...

common.ts
import { hash160 } from '@cashscript/utils';
import {
  deriveHdPrivateNodeFromSeed,
  deriveHdPath,
  deriveSeedFromBip39Mnemonic,
  secp256k1,
  encodeCashAddress,
} from '@bitauth/libauth';
import { PriceOracle } from './PriceOracle.js';

// Generate entropy from BIP39 mnemonic phrase and initialise a root HD-wallet node
const seed = deriveSeedFromBip39Mnemonic('CashScript Examples');
const rootNode = deriveHdPrivateNodeFromSeed(seed, true);
const baseDerivationPath = "m/44'/145'/0'/0";

// Derive Alice's private key, public key, public key hash and address
const aliceNode = deriveHdPath(rootNode, `${baseDerivationPath}/0`);
if (typeof aliceNode === 'string') throw new Error();
export const alicePub = secp256k1.derivePublicKeyCompressed(aliceNode.privateKey) as Uint8Array;
export const alicePriv = aliceNode.privateKey;
export const alicePkh = hash160(alicePub);
export const aliceAddress = encodeCashAddress('bchtest', 'p2pkhWithTokens', alicePkh);

// Initialise a price oracle with a private key
const oracleNode = deriveHdPath(rootNode, `${baseDerivationPath}/2`);
if (typeof oracleNode === 'string') throw new Error();
export const oraclePub = secp256k1.derivePublicKeyCompressed(oracleNode.privateKey) as Uint8Array;
export const oraclePriv = oracleNode.privateKey;
export const oracle = new PriceOracle(oracleNode.privateKey);
export const oraclePkh = hash160(oraclePub);
export const oracleAddress = encodeCashAddress('bchtest', 'p2pkhWithTokens', oraclePkh);

Generating a Contract​

For the networkprovider, we'll use the ElectrumNetworkProvider from the SDK and for Simple Transaction Builder for this example. Once you have a smart contract address you can send funds to it. To spend the Bitcoin cash locked in the contract you will have to satisfy the spending conditions on the contract.

hodl_vault.ts
import { stringify } from '@bitauth/libauth';
import { Contract, SignatureTemplate, ElectrumNetworkProvider } from 'cashscript';
import { compileFile } from 'cashc';
import { URL } from 'url';

// Import keys and price oracle from common.ts
import {
  alicePub,
  oraclePub,
} from './common.ts';

// Compile the HodlVault contract to an artifact object
const artifact = compileFile(new URL('hodl_vault.cash', import.meta.url));

// Initialise a network provider for network operations on CHIPNET
const provider = new ElectrumNetworkProvider('chipnet');

// Instantiate a new contract using the compiled artifact and network provider
// AND providing the constructor parameters
const parameters = [alicePub, oraclePub, 100000n, 30000n];
const contract = new Contract(artifact, parameters, { provider });

// Get contract balance & output address + balance
console.log('contract address:', contract.address);
console.log('contract balance:', await contract.getBalance());

Building the Oracle​

We need the create the functionality for generating and signing the oracle message to use in the HodlVault contract:

PriceOracle.ts
import { padMinimallyEncodedVmNumber, flattenBinArray, secp256k1 } from '@bitauth/libauth';
import { encodeInt, sha256 } from '@cashscript/utils';

export class PriceOracle {
  constructor(public privateKey: Uint8Array) {}

  // Encode a blockHeight and bchUsdPrice into a byte sequence of 8 bytes (4 bytes per value)
  createMessage(blockHeight: bigint, bchUsdPrice: bigint): Uint8Array {
    const encodedBlockHeight = padMinimallyEncodedVmNumber(encodeInt(blockHeight), 4);
    const encodedBchUsdPrice = padMinimallyEncodedVmNumber(encodeInt(bchUsdPrice), 4);

    return flattenBinArray([encodedBlockHeight, encodedBchUsdPrice]);
  }

  signMessage(message: Uint8Array): Uint8Array {
    const signature = secp256k1.signMessageHashSchnorr(this.privateKey, sha256(message));
    if (typeof signature === 'string') throw new Error();
    return signature;
  }
}

Sending a Transaction​

Finally, we can put all of this together to create a working smart contract application. We use the generated keys as a contract arguments directly or in a SignatureTemplate to create a transaction signature.

hodl_vault.ts
import { stringify } from '@bitauth/libauth';
import { Contract, SignatureTemplate, ElectrumNetworkProvider } from 'cashscript';
import { compileFile } from 'cashc';
import { URL } from 'url';

// Import keys and price oracle from common.ts
import {
  alicePriv,
  alicePub,
  oracle,
  oraclePub,
} from './common.ts';

// Compile the HodlVault contract to an artifact object
const artifact = compileFile(new URL('hodl_vault.cash', import.meta.url));

// Initialise a network provider for network operations on CHIPNET
const provider = new ElectrumNetworkProvider('chipnet');

// Instantiate a new contract using the compiled artifact and network provider
// AND providing the constructor parameters
const parameters = [alicePub, oraclePub, 100000n, 30000n];
const contract = new Contract(artifact, parameters, { provider });

// Get contract balance & output address + balance
console.log('contract address:', contract.address);
console.log('contract balance:', await contract.getBalance());

// Produce new oracle message and signature
const oracleMessage = oracle.createMessage(100000n, 30000n);
const oracleSignature = oracle.signMessage(oracleMessage);

// Spend from the vault
const tx = await contract.functions
  .spend(new SignatureTemplate(alicePriv), oracleSignature, oracleMessage)
  .to(contract.address, 1000n)
  .send();

console.log(stringify(tx));