RSA Encrypting in c# and Decrypting data in java

I'm trying to encrypt data in my C# client with public RSA key from the Java server. To do this I generated KeyPair in Java

KeyPairGenerator.java

KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA"); kpg.initialize(2048); KeyPair kp = kpg.generateKeyPair(); Key pub = kp.getPublic(); Key pvt = kp.getPrivate(); String outFile = "rsa_key"; FileOutputStream outPvt = new FileOutputStream(outFile + ".key"); outPvt.write(pvt.getEncoded()); outPvt.close(); FileOutputStream outPub = new FileOutputStream(outFile + ".pub"); outPub.write(pub.getEncoded()); outPub.close(); 

It gave me two files (rsa_key.key and rsa_key.pub) and then I encoded public key with Base64 so C# could read it:

PublicKeyBase64.java

String keyFile2 = "rsa_key.pub"; Path path2 = Paths.get(keyFile2); byte[] bytes2 = Files.readAllBytes(path2); X509EncodedKeySpec ks2 = new X509EncodedKeySpec(bytes2); KeyFactory kf2 = KeyFactory.getInstance("RSA"); PublicKey pub = kf2.generatePublic(ks2); Base64.Encoder encoder = Base64.getEncoder(); String outFile = "en_rsa_key"; Writer out = new FileWriter(outFile + ".pub"); out.write(encoder.encodeToString(pub.getEncoded())); out.close(); 

Then I created my C# class to encrypt data

Encrypter.cs

class Encrypter { private const string PATH = "..\\..\\key\\en_rsa_key.pub"; private string PublicKey; public Encrypter() { PublicKey = File.ReadAllText(PATH); Console.WriteLine(PublicKey.Length); } public string encryptData(string dataToEncrypt) { Asn1Object obj = Asn1Object.FromByteArray(Convert.FromBase64String(PublicKey)); DerSequence publicKeySequence = (DerSequence)obj; DerBitString encodedPublicKey = (DerBitString)publicKeySequence[1]; DerSequence publicKey = (DerSequence)Asn1Object.FromByteArray(encodedPublicKey.GetBytes()); DerInteger modulus = (DerInteger)publicKey[0]; DerInteger exponent = (DerInteger)publicKey[1]; RsaKeyParameters keyParameters = new RsaKeyParameters(false, modulus.PositiveValue, exponent.PositiveValue); RSAParameters parameters = DotNetUtilities.ToRSAParameters(keyParameters); RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(); rsa.ImportParameters(parameters); //Console.WriteLine(dataToEncrypt); byte[] encryptedData = rsa.Encrypt(Encoding.UTF8.GetBytes(dataToEncrypt), true); //Console.WriteLine(Convert.ToBase64String(encryptedData)); return Convert.ToBase64String(encryptedData); } } 

And at last my Decrypter class

public class Decrypter { private static final String PATH = "I:\\rsa_key.key"; private File privateKeyFile = new File(PATH); private RSAPrivateKey privateKey; public Decrypter() throws Exception { DataInputStream dis = new DataInputStream(new FileInputStream(privateKeyFile)); byte[] privateKeyBytes = new byte[(int)privateKeyFile.length()]; dis.read(privateKeyBytes); dis.close(); KeyFactory keyFactory = KeyFactory.getInstance("RSA"); PKCS8EncodedKeySpec privSpec = new PKCS8EncodedKeySpec(privateKeyBytes); privateKey = (RSAPrivateKey) keyFactory.generatePrivate(privSpec); } public String decrypt(String data) throws Exception{ Cipher cipher = Cipher.getInstance("RSA"); cipher.init(Cipher.ENCRYPT_MODE, privateKey); return new String(cipher.doFinal(data.getBytes())); } } 

I send encrypted data in C# via WebSocket and receive it without any problem because data while sending and receiving is the same. The problem is that data length is 344 characters (bytes) so when I decrypt my data it shows me and error: javax.crypto.IllegalBlockSizeException: Data must not be longer than 245 bytes I'm using 2048 bit key so it's 256B - 11B for padding that's why it's 245B. But the problem is that it always generates 344B and it doesn't depend on message to encode length. For example encoding "A" gives 344B and "Hello world" also 344B.

In this topic people say to use symmetric key, Encrypt the data with the symmetric key and then Encrypt the symmetric key with rsa. But it won't help because any encrypted string in my code has 344B size, so I won't be able to decrypt encrypted symmetric key.

What's wrong with my code?