Example of safely save some secret info (polkadot mnemonic) in the local storage

box_substrate_secret.ts

import {Sr25519Account} from '@unique-nft/sr25519'
import {algorithms} from '@unique-nft/utils/address'
import {getRandomValues} from 'node:crypto' // remove for browser code
import {secretbox, secretbox_open} from 'tweetnacl-ts'

const textEncoder = new TextEncoder()
const textDecoder = new TextDecoder()

const mnemonic = Sr25519Account.generateMnemonic()
const mnemonicByteArray = textEncoder.encode(mnemonic)

const password = 'Pa$$w0rd'
const passwordHash = algorithms.keccak_256(textEncoder.encode(password))

const nonce = getRandomValues(new Uint8Array(24))

const boxed = secretbox(mnemonicByteArray, nonce, passwordHash)

const SAVE_TO_LOCALSTORAGE = {
  address: Sr25519Account.fromUri(mnemonic).address,
  nonce: nonce,
  boxed: boxed,
}

console.dir(SAVE_TO_LOCALSTORAGE)
console.log(`source mnemonic: ${mnemonic}`)

const unboxed = secretbox_open(boxed, nonce, passwordHash)
if (!unboxed) {
  throw new Error('Cannot unbox')
}

const decodedMnemonic = textDecoder.decode(unboxed)
console.log(`decoded mnemonic: ${decodedMnemonic}`)