1. Using Rust project (or some JS/TS in the future) a client generates an Ethereum public key, address and sig v value (for ECDSA recovery).
2. The client builds an Ethereum TX client side (Chain ID matters).
3. The client calls sign with their near account.
4. An indexer picks up the response transaction. The response contains the big_r and s values for the Ethereum TX.
5. The client can build a signed Ethereum TX with v, r, s, and broadcast it to an Ethereum network.

Steps 1 and 2 are already done for the current testnet contract, but this contract and your NEAR network may differ

1. How to get the MPC Public Key: NEAR_ENV=[testnet/mainnet] near view [MPC_CONTRACT] public_key (for testnet: NEAR_ENV=testnet near view multichain-testnet-2.testnet public_key)
2. Example response: secp256k1:5Kwe7Ho7gicqBeTUGQLjKeRo87A3xyXjw1MbJVFe6GSiGzL4rK6i6Ycx8ksXJsFBPuxHv97U481HbM96KRYvbkX6
3. Put into lib.rs
4. Choose your Chain ID: https://chainlist.org/
5. Add your NEAR AccountId
6. Add your path based on naming conventions: https://github.com/near/near-fastauth-wallet/blob/dmd/chain_sig_docs/docs/chain_signature_api.org
7. cargo run
8. Note all values including v sig value you will need it later

Reference: https://cypherpunks-core.github.io/ethereumbook/06transactions.html (section: Transaction Signing in Practice)

To sign a transaction in Ethereum, the originator must:

1. Create a transaction data structure, containing nine fields: nonce, gasPrice, gasLimit, to, value, data, chainID, 0, 0.

Example:

[
	nonce,
	gasPrice,
	gasLimit,
	to,
	value,
	data,
	chainId,
	0,
	0
]

2. Produce an RLP-encoded serialized message of the transaction data structure.

Example: ethers.utils.RLP.encode([payload from step 1])

3. Compute the Keccak-256 hash of this serialized message.

Example:

4. CALL NEAR MPC CONTRACT METHOD sign: Compute the ECDSA signature, signing the hash with the originating EOA’s private key.

NEAR_ENV=testnet near call multichain-testnet-2.testnet sign '{"path":",ethereum,1","payload":[BYTES OF: Keccak-256 hash of this serialized message]} --accountId=[YOUR_ACCOUNT_ID]'

5. GET MPC CONTRACT RESPONSE

Example:

{
  "big_r": "029CA55598B0F280EE5D9023B74B62FFCC68D9CDCF0D33636715F0AE72CA7CE213",
  "receipt_id": [...some bytes...],
  "s": "3392F41127631F364AF9068C10F8F4B524489C9A0506943FEBC4FB867AECAE5E"
}

6. Create a new RLP encoded transaction like in step 1 & 2 (ethers.utils.RLP.encode([...])) but remove the last 3 values with:

• v (from rust project output) ECDSA signature parts from MPC Contract Response
• big_r
• s

Some notes about final transaction, every piece is prefixed with a length in RLP encoding: https://ethereum.stackexchange.com/a/38726

7. Broadcast this tx to your ethereum network (same chain_id from rust project!)

Example: ethers.provider.sendTransaction(YOUR_FULL_RLP_ENCODED_TX_IN_HEX) (should have prefix 0x)

• Right now the focus is on Ethereum

[] Add cli args [] Generalize to other chains