Encryption/Decryption in flutter by adapting java code

I used this document as my reference: https://developers.emsigner.com/signer-gateway/api-reference/signing-documents.html

You need 2 packages: pointycastle and x509, and to import them as follows:

import 'package:pointycastle/export.dart';
import 'package:x509/x509.dart';

Then you need these helper functions:

Uint8List generateSessionKey() {
  final r = Random();
  return Uint8List.fromList(List<int>.generate(32, (_) => r.nextInt(256)));
}

RSAPublicKey parseCert(String pemData) {
  final cert = parsePem(pemData).first as X509Certificate;
  final pub = cert.publicKey as RsaPublicKey;
  return RSAPublicKey(pub.modulus, pub.exponent);
}

Uint8List encryptUsingPublicKey(RSAPublicKey key, Uint8List data) {
  final cipher = PKCS1Encoding(RSAEngine())
    ..init(true, PublicKeyParameter<RSAPublicKey>(key));
  return cipher.process(data);
}

Uint8List encryptUsingSessionKey(Uint8List key, Uint8List data) {
  final cipher = PaddedBlockCipher('AES/ECB/PKCS7')
    ..init(true, PaddedBlockCipherParameters(KeyParameter(key), null));
  return cipher.process(data);
}

Uint8List sha256Digest(Uint8List data) {
  return SHA256Digest().process(data);
}

And you'd build your 3 parameters like this:

  final pem = File('cert2.pem').readAsStringSync();
  final publicKey = parseCert(pem);

  final sessionKey = generateSessionKey();

  final encryptedSessionKey = encryptUsingPublicKey(publicKey, sessionKey);

  final jsonString = json.encode(<String, dynamic>{
    'FileType': 'PDF',
    'SignaturePosition': 'Top-Left',
    'AuthToken': 'some token',
    'File': '',
    'SUrl': 'http://localhost:3000/Success',
    'FUrl': 'http://localhost:3000/Error',
    'CUrl': 'http://localhost:3000/Cancel',
    'ReferenceNumber': 'generate unique reference number',
  });

  final jsonBytes = utf8.encode(jsonString) as Uint8List;

  final encryptedJson = encryptUsingSessionKey(sessionKey, jsonBytes);

  final hash = sha256Digest(jsonBytes);
  final encryptedHash = encryptUsingSessionKey(sessionKey, hash);

  final p1 = base64.encode(encryptedSessionKey);
  final p2 = base64.encode(encryptedJson);
  final p3 = base64.encode(encryptedHash);

BUT, the big problem I see is how you then do the POST, because you want to be in a web page, right? And the normal flutter web view doesn't support an initial post. It does look like there's another package. Just search for flutter webview post.

By the way, if you don't want to use the pointycastle registry, you can rewrite encryptUsingSessionKey without as:

  final cipher = PaddedBlockCipherImpl(
    PKCS7Padding(),
    ECBBlockCipher(AESEngine()),
  )..init(true, PaddedBlockCipherParameters(KeyParameter(key), null));
  return cipher.process(data);

Finally, at least until you get the web view issue understood, you can just use http to do the post. But, let it do the work of encoding the parameters and setting the content type, as follows:

  final response = await http.post(
    Uri.parse('https://somewhere/V3_0/Index'),
    body: <String, String>{
      'Parameter1': p1,
      'Parameter2': p2,
      'Parameter3': p3,
    },
  );
  print(response.statusCode);
  print(response.body);