Using metadata hash

Transactions are encoded and readed using the runtime metadata, for offline wallets this implies that they need to have access to a trustable source to provide its metadata.

With the introduction of the frame_metadata V15 and the RFC-78

It was introduced the merkleization of the metadata as an extra measure of security to prevent offline devices from signing transactions from fake metadatas.

Polkadart through the substrate_metadata package provides a way to generate the merkleized metadata hash and use it to sign transactions.

Make a transfer using metadata hash

import 'package:demo/generated/polkadot/polkadot.dart';
import 'package:demo/generated/polkadot/types/sp_runtime/multiaddress/multi_address.dart';
import 'package:polkadart/polkadart.dart';
import 'package:polkadart/scale_codec.dart';
import 'package:polkadart_keyring/polkadart_keyring.dart';
import 'package:substrate_metadata/metadata/merkleize.dart';

Future<void> main(List<String> arguments) async {
  final provider = Provider.fromUri(Uri.parse('wss://rpc.polkadot.io'));
  final polkadot = Polkadot(provider);

  final wallet = await KeyPair.sr25519.fromUri("//Alice");
  print('Alice\' wallet: ${wallet.address}');

  // Get information necessary to build a proper extrinsic
  final runtimeVersion = await polkadot.rpc.state.getRuntimeVersion();
  final currentBlockNumber = (await polkadot.query.system.number()) - 1;
  final currentBlockHash = await polkadot.query.system.blockHash(currentBlockNumber);
  final genesisHash = await polkadot.query.system.blockHash(0);
  final nonce = await polkadot.rpc.system.accountNextIndex(wallet.address);

  // Make the encoded call
  final multiAddress = $MultiAddress().id(wallet.publicKey.bytes);
  final transferCall = polkadot.tx.balances.transferKeepAlive(dest: multiAddress, value: BigInt.one);
  final encodedCall = transferCall.encode();

  // Get the MetadataHash
  final typedMetadata = await state.getTypedMetadata();
  final merkleizer = MetadataMerkleizer.fromMetadata(typedMetadata.metadata,
      decimals: 10, tokenSymbol: 'DOT');
  final metadataHash = hex.encode(merkleizer.digest());

  // Make the payload
  final payload = SigningPayload(
      method: encodedCall,
      specVersion: runtimeVersion.specVersion,
      transactionVersion: runtimeVersion.transactionVersion,
      genesisHash: encodeHex(genesisHash),
      blockHash: encodeHex(currentBlockHash),
      blockNumber: currentBlockNumber,
      eraPeriod: 64,
      nonce: nonce,
      metadataHash: metadataHash,
      tip: 0).encode(polkadot.registry);

  // Sign the payload and build the final extrinsic
  final signature = wallet.sign(payload);
  final extrinsic = ExtrinsicPayload(
    signer: wallet.bytes(),
    method: encodedCall,
    signature: signature,
    eraPeriod: 64,
    blockNumber: currentBlockNumber,
    nonce: nonce,
    tip: 0,
    metadataHash: metadataHash,
  ).encode(polkadot.registry, SignatureType.sr25519);

  // Send the extrinsic to the blockchain
  final author = AuthorApi(provider);
  await author.submitAndWatchExtrinsic(extrinsic, (data) {
    print(data);
  });
}

// Output:
// Alice' wallet: 5GrwvaEF5zXb26Fz9rcQpDWS57CtERHpNehXCPcNoHGKutQY
// ready: null
// broadcast: [12D3KooWH6tanw7GDdvspfEpt2gEbn82Na84deJDnMNJptJCfAKU, 12D3KooWN8pzDyuAAZ126uR6WcwYThbMWSPnXf7cimKHCZVn34iB, 12D3KooWDUjbGEP1fExf7LDze8iScY6ycsB8B7K3Gfnb2wwffhxK, 12D3KooWHTuZXi5jfAztC68Wf9wbHk8g3pFs9uUVpw9tib13zQWf, 12D3KooWGGBCu1AwCLt5LWE63RTPorR3PjX7Q3cDZrRku59NPkB5, 12D3KooWB6iwHFUoLpSxp6jNCkExzukVJF1hKWNRBvZWtt34gVXh, 12D3KooWEtK675m8iU8Hpc1E5SLnBMPZgzZfdp4uUGDmyh7LEgR3, 12D3KooW9zVuJRTqsPJ5KoA7QSPzCxesYT8FhAKRDwdLdYXAkim3, 12D3KooWPgHhBcG2zD6uND3M6XWdd8MuXCU2Z6xjZs5typTVqUVd, 12D3KooWGQudnpbyypxqUmQdsmpGCfHDu9ip4oqeMM4M4pPxcSmJ, 12D3KooWSS1FFy4VgJmf5kioR5MotCS3LFNHWM3Z1GxQfmUcKqiy, 12D3KooWQZkoqjez2aJ8GsTf1K5vCY2PV3XxwVf9B3iuuYYxViEE, 12D3KooWHHyDiQXNvgtW2pxNsgGntpz6AuSni3Dgem6EYY7PeMZM, 12D3KooWLJ5DEEcifVAXnJZAyznxd6xWsEvwS6U43nD5JmUxQGPb, 12D3KooWADSbUQbTLHf4dzp8NtWK3DJFj3TSyWxv3v1wooCU77YC, 12D3KooWCmUArt3vWrmNTCtvXf7RCWWhgSgU5jUcqD1cgPiL7w1z, 12D3KooWPe66mj1D4T4KcwsJikeJScxSVNS2iae6oJHGnF99DTLc, 12D3KooWH3toosYujUH3jvZDrVsudJp9CEUc8ajyNGAUeMvRYqYd, 12D3KooWAuKeacpNPdFq7sLjXmgS23SyGu4Us7Z4fRiJoVK3aky2, 12D3KooWFUUj8yJ3sJzXjtPZAvhuK1bAhbn9JPa7KAYCoDN1MLmZ, 12D3KooWCx3GoZeH2MZ4eFmqeYa9RPppC4eZopW8GbEgyYcrrhQA, 12D3KooWPWHUu17HfaPuK7fbszNd6chFDzwHL3HCVw5miNfAKjLN, 12D3KooWPWRX8XUxV3pYeYb1fZpK2RSycTFhrVZ4Su65HxuYzRSh, 12D3KooWH57Fx8xR8FJhkDuvZDg2Jav3ZEJohjMtyLC1CGXVZ1vK, 12D3KooWLc4ieE6EGZ3B8yZ7JnaQqJKU7ocGXRVNufkE9Nh8E3XQ, 12D3KooWCRcoJqxeKVDTcKxqv4kdeUGkoJkWggZJyhrfhh4jAHY4, 12D3KooWAwWmTPHPeUruUrAm53KjUrAGVqdwSKPmRa7RC65yE7aL, 12D3KooWRuAURXtSTkbiz6tccf6QhABKuYmQw2w3wy16ZjhAQEsA, 12D3KooWAgv6KKySREnXRy6V136VM7orMtLTdXifcLkofUTeN76o, 12D3KooWLnf7gQkZVjDcKkoxfxAYuoJZcFukkV3zvkNMvYwvZLV4, 12D3KooWDjsafUnHj8SfCuG8pYBzNFMbq8GuTWQ9AT1SbSa4X3qy, 12D3KooWQj2P4jDanJtktEJnTFqbtv8rqo8s3iqaTi58yE5GMoPa, 12D3KooWJhMu1NSHC2xJgSi73GznghPXBc6oq2VZdUYvJJd8MLU7, 12D3KooWSz85vPmCcQ9fwaHpSrPyaAXRP2fB5phetz6txD444Jmn, 12D3KooWKz8dMRwWnw7UVZrrWegYBPBufBhpxBYSjk4JELyxKrTS, 12D3KooWLcFJj4mvUBD38FckCqafTYavJxZBeQknxVAvkhMFATiB, 12D3KooWNgzmWzP2cdDoinwsCva1vrXdN1bKHbnrzvkz66hQhmtc, 12D3KooWAbPXnoRSng2B5W9rXwz2A3tVrPMy8brnSvmDcyvnJs7X, 12D3KooWRmVgYuBa54Xo225zBAhfesdSPM8TL5JruNmYFUa5m9dP, 12D3KooW9xysa5Q6KuANonoMBp8AgYpCAcpyUk6Z2wxpEbN4ru2N]
// inBlock: 0xc3c341788f41a28046b7e0cac16629a2205acf1259ebfb5b52da0f2ae9e09959
// finalized: 0xc3c341788f41a28046b7e0cac16629a2205acf1259ebfb5b52da0f2ae9e09959

You will notice that the example above is very similar to the one in Make a balance transfer. The difference here are the following:

import 'package:substrate_metadata/metadata/merkleize.dart';

final typedMetadata = await state.getTypedMetadata();
final merkleizer = MetadataMerkleizer.fromMetadata(typedMetadata.metadata, decimals: 10, tokenSymbol: 'DOT');
final metadataHash = hex.encode(merkleizer.digest());

1. We use state.getTypedMetadata() to get the metadata v15 from the blockchain.
2. We use the MetadataMerkleizer to merkleize the metadata.
3. We use the digest() method to generate the metadata hash.

// Make the payload
  final payload = SigningPayload(
      method: encodedCall,
      specVersion: runtimeVersion.specVersion,
      transactionVersion: runtimeVersion.transactionVersion,
      genesisHash: encodeHex(genesisHash),
      blockHash: encodeHex(currentBlockHash),
      blockNumber: currentBlockNumber,
      eraPeriod: 64,
      nonce: nonce,
      metadataHash: metadataHash,
      tip: 0).encode(polkadot.registry);

// Sign the payload and build the final extrinsic
  final signature = wallet.sign(payload);
  final extrinsic = ExtrinsicPayload(
    signer: wallet.bytes(),
    method: encodedCall,
    signature: signature,
    eraPeriod: 64,
    blockNumber: currentBlockNumber,
    nonce: nonce,
    tip: 0,
    metadataHash: metadataHash,
  ).encode(polkadot.registry, SignatureType.sr25519);

Now, notice that we are providing the metadataHash in the SigningPayload and ExtrinsicPayload objects. This will ensure that your transaction was signed with the correct metadata and if not it will be rejected by the blockchain.