How to sign a transaction using secretKey with polkadot/api?

Question

I generated the secretKey and publicKey using the steps in https://polkadot.js.org/docs/keyring/start/create/.

After I get the secretKey and publicKey, I want to sign a tx using only the secretKey. How do I do that? I have tried to use keyring, but what is the addFrom... method that I use for secretKey? I have tried using the addFromSeed, but it produces error Bad seed size

This is the gist of the code I use

import { Keyring } from '@polkadot/api'
import {
  mnemonicGenerate,
  mnemonicToMiniSecret,
  ed25519PairFromSeed,
} from '@polkadot/util-crypto'

async function main() {
  // Create mnemonic string for Alice using BIP39
  const mnemonicAlice = mnemonicGenerate()
  // Create valid Substrate-compatible seed from mnemonic
  const seedAlice = mnemonicToMiniSecret(mnemonicAlice)
  // Generate new public/secret keypair for Alice from the supplied seed
  const { publicKey, secretKey } = ed25519PairFromSeed(seedAlice)
  const keyring = new Keyring()
  const pair = keyring.addFromSeed(secretKey, { name: 'first pair' }, 'ed25519') // this results in error
}

main()

Answer 1

I found out the problem, so the problem is, I thought secretKey generated from ed25519PairFromSeed is equivalent to the private key that can be exported from for example metamask.

But actually, the seedAlice I think is the equivalent one, so I can just use:

const secretKey = Buffer.from(seedAlice).toString('hex')

and use keyring to sign transactions

const keyPair = keyring.addFromSeed(seedAlice, { name: 'first pair' }, 'ed25519')
// and you can use this keyPair to send tx
api.tx.balance.transfer(...).signAndSend(keyPair)