Sending RC4 encrypted data over UDP causes changes in decrypted plaintext

I'm trying to send some data across a UDP connection using Java.

At the Client, I encrypt the data with an RC4 algorithm which returns a string. This string, I convert to a byte array and send it as a Datagram Packet.

However, at the Server, the decrypted plaintext gives a result similar to the original but with some differences.

The RC4 encryption program works fine when I run it independently and both Client and Server use the same source code so I really don't understand why this is happening.

Here's the code for the Client side:

import java.net.*; import javax.crypto.Cipher; import java.io.*; import java.security.*; import java.security.spec.*; import java.util.*; import java.util.Base64.*;  
 
class EchoClient 
{
	public static void main( String args[] ) throws Exception 
	{
		Scanner sc = new Scanner (System.in);
		
		DatagramSocket socket = new DatagramSocket();
		socket.setSoTimeout(120000);
		
		//Encrypting and sending data
		System.out.print("Enter data: ");
		String password = sc.nextLine();
		String credentials = password;
		String encryptedCredentials = RC4.encryptRC4(credentials, "637443");
		byte encCred[] = encryptedCredentials.getBytes();
		DatagramPacket packetCred = new DatagramPacket(encCred,encCred.length,InetAddress.getByName(args[0]),1500);
		socket.send(packetCred);
		System.out.println("\nEncrypted Credentials sent");
		System.out.println("Plain: " + credentials);
		System.out.println("Encrypted data: " + encryptedCredentials);
	
	}
}

And here's the Server:

import java.net.*; import javax.crypto.Cipher; import java.io.*; import java.security.*; import java.security.spec.*; import java.util.*; import java.util.Base64.*;
 
class EchoServer 
{
	public static void main( String args[] ) throws Exception 
	{
		System.out.println("Server running. Awaiting Connection Request...\n");

		Scanner sc = new Scanner (System.in);
		
		DatagramSocket socket = new DatagramSocket(1500);
		
		
		//Recieving Credentials
		DatagramPacket packetCred = new DatagramPacket(new byte[512],512);
		socket.receive(packetCred);
		String stringCred = new String(packetCred.getData(),0,packetCred.getLength());
		String decryptedCred = RC4.decryptRC4(stringCred, "637443");
		System.out.println("\nReceived Authentication Data");
		System.out.println("Bob at: "+new Date()+" "+packetCred.getAddress()+":"+packetCred.getPort()+"\nData: "+new String(packetCred.getData(),0,packetCred.getLength()));
		System.out.println("Decrypted Credentials: " + decryptedCred);
}

Here's the Client terminal output:

> Enter data: Hello there. How's it going? I'm doing fine; thanks.
>
> Setting up RC4...
>
> Starting RC4 encryption...
>
> Encryption Complete:
>
> Encrypted Credentials sent
>
> Plain: Hello there. How's it going? I'm doing fine; thanks.
>
> Encrypted data: ?d¥érSÿ?dNé?=P°?Whéá=Nù?H!)îërC??f@?ï<B¬?i
?í!

And here's the Server terminal output:

> Server running. Awaiting Connection Request...
>
> Setting up RC4...
>
> Starting RC4 decryption...
>
> Decryption Complete:
>
> Received Authentication Data
>
> Bob at: Wed Jul 29 05:01:51 IST 2020 /127.0.0.1:54835
>
> Data: ?d¥érSÿ?dNé?=P°?Whéá=Nù?H!)îërC??f@?ï<B¬?i
?í!
>
> Decrypted Credentials: Hello thÆre. How'Æ it goinÆ? I'm doÆng fine;
> thanks.

I think it's a problem with string encoding. Never use String.getBytes() without specifying an encoding unless you're absolutely sure that both ends use the same platform encoding.

Everywhere (RC4.decryptRC4()/RC4.encryptRC4(), new String(), String.getBytes()) where you need to convert byte[]/String do this:

From String to byte[]: .getBytes(StandardCharsets.UTF_8)

From byte[] to String: new String(byte[], int, int, StandardCharsets.UTF_8)

It doesn't really matter if it's UTF-8 or ISO-8859-1, as long as it is the same in both ends.

Oh, as @MarquisofLorne said, please don't use RC4 unless its for educational purposes only.