Function to convert entropy to seed? about bip39

[junderw]

The seed is just random bytes. This library is a way to create / store entropy in the form of mnemonic words so that it can later be converted into a seed.

If you don't want to use a mnemonic, just use the entropy directly as the seed.

If you are creating a seed for use with BIP32 HD keys, there is a restriction on length.

Generate a seed byte sequence S of a chosen length (between 128 and 512 bits; 256 bits is advised) from a (P)RNG.

So in terms of bytes, 16 - 64 bytes, 32 bytes recommended.

BIP39, on the other hand, requires the entropy be 128 ~ 256 bits (16-32 bytes), and hashes the mnemonic into a 64 byte hash value for use in the BIP32 seed.

This code is verbose, but the short answer is "just use the entropy directly in bip32" with the caveat that "you can only use between 16 and 64 bytes for bip32 seed, so be careful".

const bip39 = require('bip39')
const bip32 = require('bip32')
const crypto = require('crypto')

const MAX_BITS_BIP39 = 256 // 32 bytes
const MAX_BITS_BIP32 = 512 // 64 bytes

// generate x bits of entropy (rounded up to nearest byte)
const genEntropy = x => crypto.randomBytes(Math.ceil(x / 8))

// generate seed using BIP39, returns a 64 byte Buffer
const genSeedBIP39 = () =>
  bip39.mnemonicToSeedSync(
    // generates a 24 word mnemonic string
    bip39.entropyToMnemonic(
      genEntropy(MAX_BITS_BIP39)
    )
  )

// generate seed without using BIP39, returns a 64 byte Buffer
const genSeedNoBIP39 = () => genEntropy(MAX_BITS_BIP32)

// bip32 library accepts both
console.log(bip32.fromSeed(genSeedBIP39()).toBase58())
console.log(bip32.fromSeed(genSeedNoBIP39()).toBase58())

[kenorb]

I think it's not possible, as I've checked nativePBKDF2Sync which is used in mnemonicToSeedSync() and it's using mnemonic/password words directly for hashing.

[yoonaiu]

good work!