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Implementing Chain Signatures

Chain signatures enable NEAR accounts, including smart contracts, to sign and execute transactions across many blockchain protocols.

This unlocks the next level of blockchain interoperability by giving ownership of diverse assets, cross-chain accounts, and data to a single NEAR account.

Create a Chain Signature​

There are five steps to create a Chain Signature:

  1. Deriving the Foreign Address - Derive the address that will be controlled on the target blockchain.
  2. Creating a Transaction - Create the transaction or message to be signed.
  3. Requesting a Signature - Call the NEAR MPC contract requesting it to sign the transaction.
  4. Formatting the Signature - Format the signature from the MPC contract and add it to the transaction.
  5. Relaying the Signed Transaction - Send the signed transaction to the destination chain for execution.

chain-signatures Diagram of a chain signature in NEAR

The chainsig.js library provides a convenient interface for completing each of these steps.

tip

For building transactions inside of NEAR smart contracts written in Rust, you can use the Omni Transaction library to easily build transactions for different blockchains (like Bitcoin and Ethereum).

MPC Contracts

There is an MPC contract available on both mainnet and testnet:

  • Mainnet: v1.signer
  • Testnet: v1.signer-prod.testnet

The MPC network is made up of 8 nodes.

Chain Adapters​

Before being able to use the methods the chainsig.js library provides, we need to instantiate the relevant chain adapter object.

The EVM chain adapter takes the MPC contract address as an argument as well as publicClient which is constructed from an EVM RPC URL.

tip

To use different EVM networks just specify an RPC URL for the EVM network you require. A list of different EVM RPC URLs can be found here.

1. Deriving the Foreign Address​

Chain Signatures use derivation paths to represent accounts on the target blockchain. The foreign address to be controlled can be deterministically derived from:

  • The NEAR account calling the MPC contract (e.g., example.near, example.testnet, etc.)
  • A derivation path (a string such as ethereum-1, ethereum-2, etc.)
  • The MPC service's master public key (we don't need to worry about this as it is defined in the library we're using).

To derive the address call the deriveAddressAndPublicKey method passing the near account Id from which the address is being derived and the derivation path.

info

The same NEAR account and path will always produce the same address on the target blockchain.

  • example.near + ethereum-1 = 0x1b48b83a308ea4beb845db088180dc3389f8aa3b
  • example.near + ethereum-2 = 0x99c5d3025dc736541f2d97c3ef3c90de4d221315

2. Creating the Transaction​

To construct the transaction to be signed use the method prepareTransactionForSigning.

Constructing a transaction to transfer ETH is very simple. The value is the amount of ETH in Wei as type BigInt (1 ETH = 1018 Wei).

This method returns the unsigned transaction and the transaction hash(es) (also known as the payload).

3. Requesting the Signature​

Once the transaction is created and ready to be signed, a signature request is made by calling sign on the MPC smart contract.

The method requires three parameters:

  1. The payload (or hash) to be signed for the target blockchain
  2. The derivation path for the account we want to use to sign the transaction
  3. The key_version, 0 for Secp256k1 signatures and 1 for Ed25519 signatures.
info

The contract will take some time to respond, as the sign method yields execution, waiting for the MPC service to sign the transaction.

4. Formatting the Signature​

Once the signature is returned from the MPC it needs to be formatted and added to the transaction to produce a signed transaction.

5. Relaying the Signed Transaction​

Now that we have a signed transaction, we can relay it to the target network using broadcastTx.

The method returns a transaction hash which can be used to locate the transaction on an explorer.

info

⭐️ For a deep dive into the concepts of Chain Signatures, see What are Chain Signatures?

⭐️ For a complete example of a NEAR account using chain signatures in a frontend, see our web app example.

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