Transaction Construction and Signing

This page will discuss the transaction format in Polkadot and how to create, sign, and broadcast transactions. Like the other pages in this guide, this page demonstrates some of the available tools. Always refer to each tool's documentation when integrating.

Transaction Format

Polkadot has some basic transaction information that is common to all transactions.

• Address: The SS58-encoded address of the sending account.
• Block Hash: The hash of the checkpoint block.
• Block Number: The number of the checkpoint block.
• Genesis Hash: The genesis hash of the chain.
• Metadata: The SCALE-encoded metadata for the runtime when submitted.
• Nonce: The nonce for this transaction.*
• Spec Version: The current spec version for the runtime.
• Transaction Version: The current version for transaction format.
• Tip: Optional, the tip to increase transaction priority.
• Mode: The flag indicating whether to verify the metadata hash or not.
• Era Period: Optional, the number of blocks after the checkpoint for which a transaction is valid. If zero, the transaction is immortal
• MetadataHash: Optional, the metadata hash which should match the RUNTIME_METADATA_HASH environment variable.

caution

There are risks to making a transaction immortal. If an account is reaped and a user re-funds the account, then they could replay an immortal transaction. Always default to using a mortal extrinsic.

*The nonce queried from the System module does not account for pending transactions. You must track and increment the nonce manually if you want to submit multiple valid transactions at the same time.

Each transaction will have its own (or no) parameters to add. For example, the transferKeepAlive function from the Balances pallet will take:

• dest: Destination address
• #[compact] value: Number of tokens (compact encoding)

Refer to the protocol specifications, for the concrete specifications and types to build a transaction.

Mode and MetadataHash

The mode and metadataHash fields were introduced in transaction construction to support the optional CheckMetadataHash Signed Extension. This enables trustless metadata verification by allowing the chain to verify the correctness of the metadata used without the need of a trusted party. This functionality was included in v1.2.5 runtime release by the Fellowship. A user may up out of this functionality by setting the mode to 0. When the mode is 00, the metadataHash field is empty/None.

Serialized transactions and metadata

Before being submitted, transactions are serialized. Serialized transactions are hex encoded SCALE-encoded bytes. The Polkadot runtimes are upgradable and therefore any interfaces are subject to change, the metadata allows developers to structure any extrinsics or storage entries accordingly. The metadata provides you with all of the information required to know how to construct the serialized call data specific to your transaction. You can read more about the metadata, its format and how to get it in the Substrate documentation.

Summary

The typical transaction workflow for Polkadot is as follows:

1. Construct an unsigned transaction.
2. Create a signing payload.
3. Sign the payload.
4. Serialize the signed payload into a transaction.
5. Submit the serialized transaction.

Parity provides the following tools to help perform these steps.

Polkadot-JS Tools

Polkadot-JS Tools contains a set of command line tools for interacting with a Substrate client, including one called "Signer CLI" to create, sign, and broadcast transactions.

This example will use the signer submit command, which will create and submit the transaction. The signer sendOffline command has the exact same API, but will not broadcast the transaction. submit and sendOffline must be connected to a node to fetch the current metadata and construct a valid transaction. Their API has the format:

yarn run:signer <submit|sendOffline> --account <from-account-ss58> --ws <endpoint> <module.method> [param1] [...] [paramX]

Signing:

yarn run:signer sign --account <from-account-ss58> --seed <seed> --type <sr25519|ed25519> <payload>

For example, let's send 0.5 DOT from 121X5bEgTZcGQx5NZjwuTjqqKoiG8B2wEAvrUFjuw24ZGZf2 to 15vrtLsCQFG3qRYUcaEeeEih4JwepocNJHkpsrqojqnZPc2y.

yarn run:signer submit --account 121X5bEgTZcGQx5NZjwuTjqqKoiG8B2wEAvrUFjuw24ZGZf2 --ws ws://127.0.0.1:9944 balances.transferKeepAlive 15vrtLsCQFG3qRYUcaEeeEih4JwepocNJHkpsrqojqnZPc2y 5000000000

This will return a payload to sign and an input waiting for a signature. Take this payload and use your normal signing environment (e.g. air gapped machine, VM, etc.). Sign the payload:

yarn run:signer sign --account 121X5bEgTZcGQx5NZjwuTjqqKoiG8B2wEAvrUFjuw24ZGZf2 --seed "pulp gaze fuel ... mercy inherit equal" --type sr25519 0x040300ff4a83f1...a8239139ff3ff7c3f6

Save the output and bring it to the machine that you will broadcast from, enter it into submit's signature field, and send the transaction (or just return the serialized transaction if using sendOffline).

Tx Wrapper

If you do not want to use the CLI for signing operations, Parity provides an SDK called TxWrapper Core to generate and sign transactions offline. For Polkadot, Kusama, and select parachains, use the txwrapper-polkadot package. Other Substrate-based chains will have their own txwrapper-{chain} implementations. See the examples for a guide.

Import a private key

import { importPrivateKey } from '@substrate/txwrapper-polkadot';

const keypair = importPrivateKey(“pulp gaze fuel ... mercy inherit equal”);

Derive an address from a public key

import { deriveAddress } from '@substrate/txwrapper-polkadot';

// Public key, can be either hex string, or Uint8Array
const publicKey = “0x2ca17d26ca376087dc30ed52deb74bf0f64aca96fe78b05ec3e720a72adb1235”;
const address = deriveAddress(publicKey);

Construct a transaction offline

import { methods } from "@substrate/txwrapper-polkadot";

const unsigned = methods.balances.transferKeepAlive(
  {
    dest: "15vrtLsCQFG3qRYUcaEeeEih4JwepocNJHkpsrqojqnZPc2y",
    value: 5000000000,
  },
  {
    address: "121X5bEgTZcGQx5NZjwuTjqqKoiG8B2wEAvrUFjuw24ZGZf2",
    blockHash: "0x1fc7493f3c1e9ac758a183839906475f8363aafb1b1d3e910fe16fab4ae1b582",
    blockNumber: 4302222,
    genesisHash: "0xe3777fa922cafbff200cadeaea1a76bd7898ad5b89f7848999058b50e715f636",
    metadataRpc, // must import from client RPC call state_getMetadata
    nonce: 2,
    specVersion: 1019,
    tip: 0,
    eraPeriod: 64, // number of blocks from checkpoint that transaction is valid
    transactionVersion: 1,
  },
  {
    metadataRpc,
    registry, // Type registry
  }
);

Construct a signing payload

import { methods, createSigningPayload } from '@substrate/txwrapper-polkadot';

// See "Construct a transaction offline" for "{...}"
const unsigned = methods.balances.transferKeepAlive({...}, {...}, {...});
const signingPayload = createSigningPayload(unsigned, { registry });

Serialize a signed transaction

import { createSignedTx } from "@substrate/txwrapper-polkadot";

// Example code, replace `signWithAlice` with actual remote signer.
// An example is given here:
// https://github.com/paritytech/txwrapper-core/blob/b213cabf50f18f0fe710817072a81596e1a53cae/packages/txwrapper-core/src/test-helpers/signWithAlice.ts
const signature = await signWithAlice(signingPayload);
const signedTx = createSignedTx(unsigned, signature, { metadataRpc, registry });

Decode payload types

You may want to decode payloads to verify their contents prior to submission.

import { decode } from "@substrate/txwrapper-polkadot";

// Decode an unsigned tx
const txInfo = decode(unsigned, { metadataRpc, registry });

// Decode a signing payload
const txInfo = decode(signingPayload, { metadataRpc, registry });

// Decode a signed tx
const txInfo = decode(signedTx, { metadataRpc, registry });

Check a transaction's hash

import { getTxHash } from ‘@substrate/txwrapper-polkadot’;
const txHash = getTxHash(signedTx);

Submitting a Signed Payload

There are several ways to submit a signed payload:

1. Signer CLI (yarn run:signer submit --tx <signed-transaction> --ws <endpoint>)
2. Substrate API Sidecar
3. RPC with author_submitExtrinsic or author_submitAndWatchExtrinsic, the latter of which will subscribe you to events to be notified as a transaction gets validated and included in the chain.

Notes

Some addresses to use in the examples. See Subkey documentation.

$ subkey --network polkadot generate
Secret phrase `pulp gaze fuel ... mercy inherit equal` is account:
  Secret seed:      0x57450b3e09ba4598 ... ... ... ... ... ... ... .. 219756eeba80bb16
  Public key (hex): 0x2ca17d26ca376087dc30ed52deb74bf0f64aca96fe78b05ec3e720a72adb1235
  Account ID:       0x2ca17d26ca376087dc30ed52deb74bf0f64aca96fe78b05ec3e720a72adb1235
  SS58 Address:     121X5bEgTZcGQx5NZjwuTjqqKoiG8B2wEAvrUFjuw24ZGZf2

$ subkey --network polkadot generate
Secret phrase `exercise auction soft ... obey control easily` is account:
  Secret seed:      0x5f4bbb9fbb69261a ... ... ... ... ... ... ... .. 4691ed7d1130fbbd
  Public key (hex): 0xda04de6cd781c98acf0693dfb97c11011938ad22fcc476ed0089ac5aec3fe243
  Account ID:       0xda04de6cd781c98acf0693dfb97c11011938ad22fcc476ed0089ac5aec3fe243
  SS58 Address:     15vrtLsCQFG3qRYUcaEeeEih4JwepocNJHkpsrqojqnZPc2y