(Android™) BIP39 Compute Binary Seed from Mnemonic

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Creates a binary seed from a mnemonic. Uses the PBKDF2 function with a mnemonic sentence (in UTF-8 NFKD) used as the password and the string "mnemonic" + passphrase (again in UTF-8 NFKD) used as the salt. The iteration count is set to 2048 and HMAC-SHA512 is used as the pseudo-random function. The length of the derived key is 512 bits (= 64 bytes).

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// Important: Don't forget to include the call to System.loadLibrary
// as shown at the bottom of this code sample.
package com.test;

import android.app.Activity;
import com.chilkatsoft.*;

import android.widget.TextView;
import android.os.Bundle;

public class SimpleActivity extends Activity {

  private static final String TAG = "Chilkat";

  // Called when the activity is first created.
  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    // This example assumes the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    CkCrypt2 crypt = new CkCrypt2();

    // Test with the test vectors at https://github.com/trezor/python-mnemonic/blob/master/vectors.json

    // This is the 2nd test vector..
    String mnemonic = "legal winner thank year wave sausage worth useful legal winner thank yellow";
    String passphrase = "TREZOR";
    String expectedSeed = "2e8905819b8723fe2c1d161860e5ee1830318dbf49a83bd451cfb8440c28bd6fa457fe1296106559a3c80937a1c1069be3a3a5bd381ee6260e8d9739fce1f607";
    String expectedMasterKey = "xprv9s21ZrQH143K2gA81bYFHqU68xz1cX2APaSq5tt6MFSLeXnCKV1RVUJt9FWNTbrrryem4ZckN8k4Ls1H6nwdvDTvnV7zEXs2HgPezuVccsq";

    // The mnemonic sentence (in UTF-8 NFKD) used as the password.
    // The string "mnemonic" + passphrase (again in UTF-8 NFKD) used as the salt.
    // The iteration count is set to 2048 and HMAC-SHA512 is used as the pseudo-random function.
    // The length of the derived key is 512 bits (= 64 bytes). 

    // We want the computed seed to be lowercase hex, therefore our salt must also be hex.
    // The seed is the keyword "mnemonic" + passphrase (in this case is "TREZOR") converted to hex.
    CkBinData bdSalt = new CkBinData();
    bdSalt.AppendString("mnemonic","utf-8");
    bdSalt.AppendString(passphrase,"utf-8");

    String computedSeed = crypt.pbkdf2(mnemonic,"utf-8","sha512",bdSalt.getEncoded("hex_lower"),2048,512,"hex_lower");

    Log.i(TAG, "Expected: " + expectedSeed);
    Log.i(TAG, "Computed: " + computedSeed);

    // To compute the hd_master_key, duplicate this Python code:

    //     def to_hd_master_key(seed: bytes, testnet: bool = False) -> str:
    //         if len(seed) != 64:
    //             raise ValueError("Provided seed should have length of 64")
    // 
    //         # Compute HMAC-SHA512 of seed
    //         seed = hmac.new(b"Bitcoin seed", seed, digestmod=hashlib.sha512).digest()
    // 
    //         # Serialization format can be found at: https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki#Serialization_format
    //         xprv = b"\x04\x88\xad\xe4"  # Version for private mainnet
    //         if testnet:
    //             xprv = b"\x04\x35\x83\x94"  # Version for private testnet
    //         xprv += b"\x00" * 9  # Depth, parent fingerprint, and child number
    //         xprv += seed[32:]  # Chain code
    //         xprv += b"\x00" + seed[:32]  # Master key
    // 
    //         # Double hash using SHA256
    //         hashed_xprv = hashlib.sha256(xprv).digest()
    //         hashed_xprv = hashlib.sha256(hashed_xprv).digest()
    // 
    //         # Append 4 bytes of checksum
    //         xprv += hashed_xprv[:4]
    // 
    //         # Return base58
    //         return b58encode(xprv)

    // First compute the HMAC-SHA512 of the computedSeed
    CkBinData bdSeed = new CkBinData();
    bdSeed.AppendEncoded(computedSeed,"hex_lower");
    crypt.put_EncodingMode("hex_lower");
    crypt.put_HashAlgorithm("sha512");
    crypt.SetMacKeyString("Bitcoin seed");
    String hmacSha512_hex = crypt.macBdENC(bdSeed);

    CkBinData bdHmac = new CkBinData();
    bdHmac.AppendEncoded(hmacSha512_hex,"hex_lower");

    CkBinData bdXprv = new CkBinData();
    bdXprv.AppendEncoded("0488ade4","hex_lower");
    bdXprv.AppendEncoded("000000000000000000","hex_lower");
    bdXprv.AppendEncoded(bdHmac.getEncodedChunk(32,32,"hex_lower"),"hex_lower");
    bdXprv.AppendByte(0);
    bdXprv.AppendEncoded(bdHmac.getEncodedChunk(0,32,"hex_lower"),"hex_lower");

    // Double hash using SHA256
    crypt.put_EncodingMode("hex_lower");
    crypt.put_HashAlgorithm("sha256");

    CkBinData bdHash = new CkBinData();
    bdHash.AppendEncoded(crypt.hashBdENC(bdXprv),"hex_lower");
    String secondHash = crypt.hashBdENC(bdHash);
    bdHash.Clear();
    bdHash.AppendEncoded(secondHash,"hex_lower");

    // Append the 1st 4 bytes of the bdHash to bdXprv.
    bdXprv.AppendEncoded(bdHash.getEncodedChunk(0,4,"hex_lower"),"hex_lower");

    // Base58 encode bdXprv
    String computedMasterKey = bdXprv.getEncoded("base58");

    Log.i(TAG, "Expected Master Key: " + expectedMasterKey);
    Log.i(TAG, "Computed Master Key: " + computedMasterKey);

  }

  static {
      System.loadLibrary("chilkat");

      // Note: If the incorrect library name is passed to System.loadLibrary,
      // then you will see the following error message at application startup:
      //"The application <your-application-name> has stopped unexpectedly. Please try again."
  }
}