How to convert mnemonic to 32 byte seed?

Question

I have a 24-word mnemonic, and I want to convert it to public and private keys.

This is how I did it:

  const hex = HexCoder.instance;

  final seed = bip39.mnemonicToSeedHex(_mnmonic);

  final algorithm = Ed25519();

  // The hex.decode(seed) have 64 bytes lengths.
  final keyPair = await algorithm.newKeyPairFromSeed(hex.decode(seed));

  final newPublicKey = await keyPair.extractPublicKey();

But I get this error:

ArgumentError (Invalid argument(s): Seed must have 32 bytes)

What am I missing?

Answer 1

Update

Use sha256:

import 'package:crypto/crypto.dart' show sha256;

final seed = bip39.mnemonicToSeed(mnemonic);
final digest = sha256.convert(seed); // 32 bytes

Old answer

Here are some examples how people generate keys from mnemonics:

final seed = bip39.mnemonicToSeed(mnemonic);
final root = bip32.BIP32.fromSeed(seed);
getAddress(root.derivePath("m/0'/0/0"));

Here is another example:

Uint8List seed = await compute(bip39.mnemonicToSeed, mnemonic);
bip32.BIP32 masterNode = bip32.BIP32.fromSeed(seed);
String hdPath = "m/44'/12586'/$accountIndex'/0/0";
bip32.BIP32 child0 = masterNode.derivePath(hdPath);

Uint8List rawPrivateKey = child0.privateKey!;
rawPrivateKey[0] &= 0x3f;
final privateKeyHex = HEX.encode(rawPrivateKey);