Java Stream Cipher (With input and output txt files) outputting random characters not complete decrypted string

Question

Don't know where I went wrong necessarily but my program is supposed to be a stream cipher that takes an input.txt file of chars and encrypts it into numbers and then decrypts it back to chars.

My issue is that I type in:

java Program4 -e 71 < inp.txt > out.txt

(to encrypt txt to output file and it works fine,) input file looks like:

guess what? Chicken butt

output file looks like:

222 204 220 202 202 153 206 209 216 205 134 153 250 209 208 218 210 220 215 153 219 204 205 205

Then when I decrypt the file..

java Program4 -d 71 < out.txt

it comes out like this:

g  E ?  ? 8 º ? Ä ì  ß ê ( ? ½ ^ ~ ? ? X  ?

I don't know what I'm doing wrong but I'm guessing it's something with my decryption method or how my encryption gives the same number on some values? I truly appreciate any help!

import java.util.Scanner;
import java.util.Random;
public class Program4
{
public static void main(String[] args)
{
    if(args.length < 2)
    {
        usage();
    }
    else if(args[0].equals("-e"))
    {
        encrypt(args);
    }
    else if(args[0].equals("-d"))
    {
        decrypt(args);
    }

}   
        //Intro (Usage Method)
    public static void usage()
    {
        System.out.println("Stream Encryption program by my name");
        System.out.println("usage: java Encrypt [-e, -d] < inputFile > outputFile" );
    }
        //Encrypt Method
        public static void encrypt(String[] args)
    {   Scanner scan = new Scanner(System.in);
        String key1 = args[1];
        long key = Long.parseLong(key1);
        Random rng = new Random(key);
        int randomNum = rng.nextInt(256);
        while (scan.hasNextLine())
        {
            String s = scan.nextLine();
            for (int i = 0; i < s.length(); i++)
            {
                char allChars = s.charAt(i);
                int cipherNums = allChars ^ randomNum;
                System.out.print(cipherNums + " ");
            }

        }
    }   

        //Decrypt Method
        public static void decrypt(String[] args)
    {   String key1 = args[1];
        long key = Long.parseLong(key1);
        Random rng = new Random(key);
        Scanner scan = new Scanner(System.in);

            while (scan.hasNextInt())
        {
            int next = scan.nextInt();
            int randomNum = rng.nextInt(256);
            int decipher = next ^ randomNum;
            System.out.print((char)decipher + " ");

        }

    }

}

Answer 1

You're using your random number generator differently in the two cases. In your encryption code, you generate one random number, and use it for all characters:

Random rng = new Random(key);
int randomNum = rng.nextInt(256);
while (scan.hasNextLine())
{
    String s = scan.nextLine();
    for (int i = 0; i < s.length(); i++)
    {
        char allChars = s.charAt(i);
        int cipherNums = allChars ^ randomNum;
        System.out.print(cipherNums + " ");
    }
}

In your decryption code, you generate a new random number per character:

while (scan.hasNextInt())
{
    int next = scan.nextInt();
    int randomNum = rng.nextInt(256);
    int decipher = next ^ randomNum;
    System.out.print((char)decipher + " ");
}

The best way to fix this (to avoid each 'e' always encrypting to the same number, for example) would be to use a new random number for each character when encrypting:

Random rng = new Random(key);
while (scan.hasNextLine())
{
    String s = scan.nextLine();
    for (int i = 0; i < s.length(); i++)
    {
        char allChars = s.charAt(i);
        int randomNum = rng.nextInt(256);
        int cipherNums = allChars ^ randomNum;
        System.out.print(cipherNums + " ");
    }
}

(Of course, this code shouldn't be used for real encryption - I assume it's only for the purposes of education.)