X25519 calculated shared key is different in two programs

Question

I have written two programs which will exchange X25519 public keys and then calculate shared secret. This is just a test, so public keys are exchanged trough text files (in PEM format), program is until you tell it to continue in order to generate keypairs and exchange them.

First one is in Dart:

import 'dart:convert';
import 'dart:io';
import 'package:cryptography/cryptography.dart';
import 'package:pem/pem.dart';

Future<void> main(List<String> arguments) async {
  //https://pub.dev/documentation/cryptography/latest/cryptography/X25519-class.html

  final algorithm = Cryptography.instance.x25519();
  final keyPair = await algorithm.newKeyPair();
  var file = File('/home/razj/dartPublic.txt');
  file.writeAsStringSync(PemCodec(PemLabel.publicKey)
      .encode(await keyPair.extractPublicKeyBytes()));

  print('PRESS AFTER C# PROGRAM GENERATES KEYPAIR');
  stdin.readLineSync();

  String remotePem = File('/home/razj/sharpPublic.txt').readAsStringSync();
  SimplePublicKey remotePublicKey = SimplePublicKey(
      PemCodec(PemLabel.publicKey).decode(remotePem),
      type: KeyPairType.x25519);
  final sharedSecretKey = await algorithm.sharedSecretKey(
    keyPair: keyPair,
    remotePublicKey: remotePublicKey,
  );
  List<int> sharedKeyBytes = await sharedSecretKey.extractBytes();
  print(base64.encode(sharedKeyBytes));
}

extension SimpleKeyPairExtension on SimpleKeyPair {
  Future<List<int>> extractPublicKeyBytes() {
    return extract().then((value) => value.bytes);
  }
}

Second is in .NET CORE:

using System.Security.Cryptography;
using X25519;

internal class Program
{
    private static void Main(string[] args)
    {
        //https://github.com/HirbodBehnam/X25519-CSharp

        var keyPair = X25519KeyAgreement.GenerateKeyPair();
        File.WriteAllText("/home/razj/sharpPublic.txt", new string(PemEncoding.Write("PUBLIC KEY", keyPair.PublicKey)));

        Console.WriteLine("PRESS AFTER DART PROGRAM GENERATES KEYPAIR");
        Console.ReadKey(true);

        string remotePem = File.ReadAllText("/home/razj/dartPublic.txt").Replace("-----BEGIN PUBLIC KEY-----\n", "").Replace("\n-----END PUBLIC KEY-----\n", "");
        byte[] sharedKeyBytes = X25519KeyAgreement.Agreement(keyPair.PrivateKey, Convert.FromBase64String(remotePem));
    
        Console.WriteLine(Convert.ToBase64String(sharedKeyBytes));
    }
}

In the end of each program I print base64 encoded byte array which represents the shared key. Unfortunately outputs doesn't match. Any idea how to fix this or what might be wrong? Thanks for answering.

Answer 1

As @Topaco said,

However, you simply apply the raw 32 bytes key instead of the DER encoded key, so the PEM key is invalid. This has no consequence, since the reverse direction returns the raw keys that both libraries use in the end. It is unnecessary though, just apply the (Base64 encoded) raw keys directly.

Just exchanged keys by encoding raw key bytes to Base64 string:

Dart:

import 'dart:io';
import 'dart:convert';
import 'package:cryptography/cryptography.dart';

Future<void> main(List<String> arguments) async {
  //https://pub.dev/documentation/cryptography/latest/cryptography/X25519-class.html

  final algorithm = Cryptography.instance.x25519();
  final keyPair = await algorithm.newKeyPair();
  var file = File('/home/razj/dartPublic.txt');
  SimplePublicKey publicKey = await keyPair.extractPublicKey();
  //saving key bytes as base64 string
  file.writeAsStringSync(base64.encode(publicKey.bytes));

  print('PRESS AFTER C# PROGRAM GENERATES KEYPAIR');
  stdin.readLineSync();

  String remoteKeyBase64 =
      File('/home/razj/sharpPublic.txt').readAsStringSync();
  SimplePublicKey remotePublicKey =
      SimplePublicKey(base64.decode(remoteKeyBase64), type: KeyPairType.x25519);
  final sharedSecretKey = await algorithm.sharedSecretKey(
    keyPair: keyPair,
    remotePublicKey: remotePublicKey,
  );
  List<int> sharedKeyBytes = await sharedSecretKey.extractBytes();
  print(base64.encode(sharedKeyBytes));
}

.NET CORE

using X25519;

internal class Program
{
    private static void Main(string[] args)
    {
        //https://github.com/HirbodBehnam/X25519-CSharp

        var keyPair = X25519KeyAgreement.GenerateKeyPair();
        //saving key bytes as base64 string
        File.WriteAllText("/home/razj/sharpPublic.txt", Convert.ToBase64String(keyPair.PublicKey));

        Console.WriteLine("PRESS AFTER DART PROGRAM GENERATES KEYPAIR");
        Console.ReadKey(true);

        string remoteKeyBase64 = File.ReadAllText("/home/razj/dartPublic.txt");
        byte[] sharedKeyBytes = X25519KeyAgreement.Agreement(keyPair.PrivateKey, Convert.FromBase64String(remoteKeyBase64));
    
        Console.WriteLine(Convert.ToBase64String(sharedKeyBytes));
    }
}

Dart output:

Hz2Rf2nUFbwmg4wgaXOl3qAi8ha5h61fHcMOXpNQ23o=

.NET-CORE output

Hz2Rf2nUFbwmg4wgaXOl3qAi8ha5h61fHcMOXpNQ23o=