How to generate RSA private/public key pair in Dart with Pointy Castle

rsa.dart

import 'dart:html';

import 'dart:math';
import 'dart:typed_data';
import "package:bignum/bignum.dart";
import "package:pointycastle/export.dart";

void main() {
  var keyParams = new RSAKeyGeneratorParameters(new BigInteger("65537"), 2048, 5);

  var secureRandom = new FortunaRandom();
  var random = new Random.secure();
  var seeds = [];
  for (int i = 0; i < 32; i++) {
    seeds.add(random.nextInt(255));
  }
  secureRandom.seed(new KeyParameter(new Uint8List.fromList(seeds)));

  var rngParams = new ParametersWithRandom(keyParams, secureRandom);
  var k = new RSAKeyGenerator();
  k.init(rngParams);

  var keyPair = k.generateKeyPair();
  var cipher = new RSAEngine()
    ..init( true, new PublicKeyParameter(keyPair.publicKey) )
  ;
  var cipherText = cipher.process(new Uint8List.fromList("Hello World".codeUnits));

  print("Encrypted: ${new String.fromCharCodes(cipherText)}");

  cipher.init( false, new PrivateKeyParameter(keyPair.privateKey) )
  ;
  var decrypted = cipher.process(cipherText);
  print("Decrypted: ${new String.fromCharCodes(decrypted)}");
}

What if my public key is a dot cer file? How to encrypt？
javapublic PublicKey getPublicKey(InputStream in) { try { try { CertificateFactory certificateFactory = CertificateFactory .getInstance("X.509"); Certificate certificate = certificateFactory.generateCertificate(in); return certificate.getPublicKey(); } finally { if (in != null) in.close(); } } catch (Exception e) { throw new RuntimeException("init exception", e); } }