how to convert AsymmetricKeyPair to base64 encoding string in dart

I‘m a rookie in PointyCastle,how can I generate ecc base64 key pairs with PointyCastle in dart

AsymmetricKeyPair<PublicKey, PrivateKey> generateKeyPair({curve = 'secp256r1'}) {
    var param = curve == 'secp256r1' ? ECCurve_secp256r1() : ECCurve_secp256k1();
    var keyParams = ECKeyGeneratorParameters(param);

    var random = FortunaRandom();
    random.seed(KeyParameter(_seed()));

    var generator = ECKeyGenerator();
    generator.init(ParametersWithRandom(keyParams, random));

    return generator.generateKeyPair();
}
Uint8List _seed() {
    var random = Random.secure();
    var seed = List<int>.generate(32, (_) => random.nextInt(256));
    return Uint8List.fromList(seed);
}

above I can generate a AsymmetricKeyPair object,how to get public base64 and private base64 like my already done js bundle

{"priKey":"CVK3r/UxdGCwQBjtn5vN/orUMxKf9E/1TlJzLkMz9t4=","pubKey":"BJH/mWJqgchGxXGA5/E79SsWRwVo3rpduBmD8FOs7UlKiK8PIvwrkCDvUcwhKdysW35OByPjoVcwFqg1NyumLKM="}

besides, at first I want to use my js bundle file in flutter android, but that would be very tough as I know

how to use js bundle in flutter android

I also need to make sign and verify function with ecdsa in sha256

Signer signer = new Signer('SHA-256/ECDSA');
// 签名，参数：私钥，明文
ECSignature sign(String privateKeyStr, String txt) {
    SecureRandom random = _setRadom();
    // how to convert a plain txt to kTestBytes
    final Uint8List kTestBytes = new Uint8List.fromList([1, 2, 3]);

    // if I pass in base64 privateKey str, is the radix 64?
    ECPrivateKey privateKey = new ECPrivateKey(BigInt.parse(privateKeyStr, radix: 10), ecDomain);

    PrivateKeyParameter signParams = new PrivateKeyParameter(privateKey);
    signer.init(true, new ParametersWithRandom(signParams, random));
    ECSignature signature = signer.generateSignature(kTestBytes) as ECSignature;
    return signature;
}
// 验签
//  参数： 明文，签名，公钥
bool verify(txt, signature, publicKey) {
    // how to make the txt to kTestBytes
    var kTestBytes = txt
    signer.init(false, new PublicKeyParameter(publicKey));
    bool verify = signer.verifySignature(kTestBytes, signature);
    // print(verify);
    return verify;
}
// I don't know what this function actually do,as I know in other language ecdsa don;t need a random number.
SecureRandom _setRadom() {
    List<int> key = [67, 3, 241, 75, 143, 78, 115, 99, 21, 242, 180, 43, 26, 7, 194, 20];
    List<int> iv = [87, 117, 137, 182, 2, 199, 132, 230, 120, 12, 109, 177, 34, 197, 186, 206];
    KeyParameter keyParam = new KeyParameter(new Uint8List.fromList(key));
    ParametersWithIV<KeyParameter> paramsIV = new ParametersWithIV(keyParam, new Uint8List.fromList(iv));
    SecureRandom random = new SecureRandom('AES/CTR/AUTO-SEED-PRNG')..seed(paramsIV);
    return random;
}

Any place can I find sample codes, these functions were changed by some copies on the internet。and I got many confuses list in the comment

The EC keys are to be exported/imported as a Base64 encoded raw private key and as a Base64 encoded raw uncompressed public key.

First of all it can be stated that signing and verifying with the posted code works when a fresh key pair is generated:

AsymmetricKeyPair<PublicKey, PrivateKey> kp = generateKeyPair();
ECSignature signature = sign(kp.privateKey as ECPrivateKey, "The quick brown fox jumps over the lazy dog");
bool verified = verify("The quick brown fox jumps over the lazy dog", signature, kp.publicKey as ECPublicKey);
print(verified); // true

I.e. in the following I will focus on the import/export of the keys. The raw keys are encapsulated in ECPrivateKey and ECPublicKey and can be exported and imported as follows:

String p256 = 'secp256r1';
AsymmetricKeyPair<PublicKey, PrivateKey> kp = generateKeyPair(curve: p256);

// Export
BigInt d = (kp.privateKey as ECPrivateKey).d!;
BigInt x = (kp.publicKey as ECPublicKey).Q!.x!.toBigInteger()!;
BigInt y = (kp.publicKey as ECPublicKey).Q!.y!.toBigInteger()!;

// Import
ECDomainParameters domain = ECDomainParameters(p256);
ECPrivateKey ecPrivateKey = ECPrivateKey(d, domain);
ECPublicKey ecPublicKey = ECPublicKey(domain.curve.createPoint(x, y), domain);

Here d is the raw private key and x and y are the x and y coordinates of the raw public key.

---

On the JavaScript side, the Base64 encoded raw private key is used and the Base64 encoded raw uncompressed key, where the uncompressed key is the concatenation 0x04 + <x> + <y>. So, for export and import, the following applies:

• Regarding the export, the Base64 encoded raw private and public key can be derived from d, x, and y as follows:

  String rawPrivateB64 = exportPrivate(d);
  String rawUncompressedPublicB64 = exportPublic(x, y, 32);
  

  with

  import 'dart:convert';
  import 'package:nanodart/nanodart.dart';
  
  String exportPrivate(BigInt d){
    return base64Encode(NanoHelpers.bigIntToBytes(d));
  }
  
  String exportPublic(BigInt x, BigInt y, int size){
    return base64Encode(NanoHelpers.concat([Uint8List.fromList([4]), pad(NanoHelpers.bigIntToBytes(x), size), pad(NanoHelpers.bigIntToBytes(y), size)]));
  }
  
  Uint8List pad(Uint8List list, int size){
    Uint8List padded = list;
    int currSize = list.length;
    if (currSize < size){
      Uint8List pad = Uint8List(size - currSize);
      padded = NanoHelpers.concat([pad, list]);
    }
    return padded;
  }
  

  The size passed in exportPublic() as 3rd parameter is the size of the order of the generator point (32 bytes for secp256r1). If x or y are smaller, they are each padded from the front with 0x00 values until the required length is reached.

  For the conversion from BigInt to Uint8List and the concatenation I used the NanoHelpers class from the NanoDart package for simplicity. Of course, other implementations can be used here as well.

• Regarding the import, d, x, and y can be derived from the Base64 encoded raw private and public key as follows:

  BigInt d = importPrivate(rawPrivateB64);
  List xy = importPublic(rawUncompressedPublicB64);
  BigInt x = xy.elementAt(0);
  BigInt y = xy.elementAt(1);
  

  with

  BigInt importPrivate(String d){
    return NanoHelpers.byteToBigInt(base64Decode(d));
  }
  
  List importPublic(String xy){
    Uint8List xyBytes = base64Decode(xy);
    xyBytes = xyBytes.sublist(1, xyBytes.length);
    int size = xyBytes.length ~/ 2;
    Uint8List x = xyBytes.sublist(0, size);
    Uint8List y = xyBytes.sublist(size);
    return [NanoHelpers.byteToBigInt(x), NanoHelpers.byteToBigInt(y)];
  }
  

---

Test

The posted key pair

{"priKey":"CVK3r/UxdGCwQBjtn5vN/orUMxKf9E/1TlJzLkMz9t4=","pubKey":"BJH/mWJqgchGxXGA5/E79SsWRwVo3rpduBmD8FOs7UlKiK8PIvwrkCDvUcwhKdysW35OByPjoVcwFqg1NyumLKM="}

can be imported and used for signing and verification as follows:

String rawPrivateB64 = "CVK3r/UxdGCwQBjtn5vN/orUMxKf9E/1TlJzLkMz9t4=";
String rawUncompressedPublicB64 = "BJH/mWJqgchGxXGA5/E79SsWRwVo3rpduBmD8FOs7UlKiK8PIvwrkCDvUcwhKdysW35OByPjoVcwFqg1NyumLKM=";

BigInt d = importPrivate(rawPrivateB64);
List xy = importPublic(rawUncompressedPublicB64);
BigInt x = xy.elementAt(0);
BigInt y = xy.elementAt(1);

ECDomainParameters domain = ECDomainParameters('secp256r1');
ECPrivateKey private = ECPrivateKey(d, domain);
ECPublicKey public = ECPublicKey(domain.curve.createPoint(x, y), domain);

ECSignature signature = sign(private, "The quick brown fox jumps over the lazy dog");
bool verified = verify("The quick brown fox jumps over the lazy dog", signature, public);
print(verified); // true

---

Edit: Regarding your comment: sign() and verify() are the methods you posted, but according to the changes, the keys are now passed directly (instead of strings) and the actual message is applied (instead of [1,2,3]) using utf8.encode() for UTF-8 encoding:

import 'dart:convert';

ECSignature sign(ECPrivateKey privateKey, String txt) {
  SecureRandom random = _setRandom();
  Uint8List txtBytes = Uint8List.fromList(utf8.encode(txt));
  PrivateKeyParameter signParams = PrivateKeyParameter(privateKey);
  signer.init(true, ParametersWithRandom(signParams, random));
  ECSignature signature = signer.generateSignature(txtBytes) as ECSignature;
  return signature;
}

bool verify(String txt, ECSignature signature, ECPublicKey publicKey) {
  signer.init(false, PublicKeyParameter(publicKey));
  bool verify = signer.verifySignature(Uint8List.fromList(utf8.encode(txt)), signature);
  return verify;
}

As _setRandom() I applied the implementation from generateKeyPair() instead of your implementation (i.e. a FortunaRandom() based CSPRNG).