How to implement RSA in flutter(decryption) and Node(encryption)?

Question

I need to implement RSA. I am generating public and private keys from nodejs i am sending private key to client. I am able to encrypt data using public key from nodejs but i am unable to decrypt from flutter i had tried various libraries in flutter(simple_rsa, encrypt etc…) but those are not working i am getting padding errors, invalid private key errors. Can any one suggest me how to implement this ?

Here is my code

Nodejs

    crypto.generateKeyPair('rsa', {
      modulusLength: 4096,
      publicKeyEncoding: {
        type: 'pkcs1',
        format: 'pem'
      },
      privateKeyEncoding: {
        type: 'pkcs1',
        format: 'pem',
        cipher: 'aes-256-cbc',
        passphrase: '',
      }
    }, (err, publicKey, privateKey) => {
      // Handle errors and use the generated key pair.
      if(err)
        throw err
        //Publickey, PrivateKey
    });

encrypt = function(data, publicKey) {
    var buffer = Buffer.from(data);
    var encrypted = crypto.publicEncrypt(publicKey, buffer);
    return encrypted.toString("base64");
};

i am sending encrypt function return value to client which is msg in flutter
Flutter Code (Tried with many libraries this is one among them using simple_rsa)

    final decpKey = "-----BEGIN RSA PRIVATE KEY-----\nProc-Type:.........";
    final msg = "fH2EBmBS4fRHG1............."; 

    final decryptedText = await decryptString(msg, decpKey); //Error: Invalid private key
    print(decryptedText);

It would be great if you could share your solution or some useful links — Karalina Lavishak
– Karalina Lavishak, Commented Sep 2, 2020 at 17:24

Karalina Lavishak · Accepted Answer · 2020-09-07 09:46:09Z

Keygen (NodeJS)

Docs: https://nodejs.org/api/crypto.html#crypto_crypto_generatekeypair_type_options_callback

const { generateKeyPair } = require('crypto');

generateKeyPair('rsa', {
    modulusLength: 4096,    // key size in bits
    publicKeyEncoding: {
        type: 'spki',
        format: 'pem',
    },
    privateKeyEncoding: {   
        type: 'pkcs8',      // !!! pkcs1 doesn't work for me
        format: 'pem',
    },
}, (err, publicKey, privateKey) => {
    // Handle errors and use the generated key pair.
});

NodeJS encryption via JSEncrypt library

node-jsencrypt: https://www.npmjs.com/package/node-jsencrypt
JSEncrypt: https://travistidwell.com/jsencrypt/#

const JSEncrypt = require('node-jsencrypt');  

function encrypt(text, key) {
    const crypt = new JSEncrypt();
    crypt.setKey(key);
    return crypt.encrypt(text);
}

function decrypt(encrypted, privateKey) {
    const crypt = new JSEncrypt();
    crypt.setPrivateKey(privateKey);
    return crypt.decrypt(encrypted);
}

Dart encryption via crypton

GitHub: https://github.com/konstantinullrich/crypton

import 'package:crypton/crypton.dart';

import 'encryption.dart';

class AsymmetricCrypt implements Encryption {
  final String _key;
  RSAPublicKey _publicKey;
  RSAPrivateKey _privateKey;

  AsymmetricCrypt._(this._key);

  @override
  String encrypt(String plain) {
    _publicKey ??= RSAPublicKey.fromPEM(_key);
    return _publicKey.encrypt(plain);
  }

  @override
  String decrypt(String data) {
    _privateKey ??= RSAPrivateKey.fromPEM(_key);
    return _privateKey.decrypt(data);
  }
}

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