Thread: ArrayIndexOutOfBoundsException problem in decryption algorithm

I don't need a lesson in what the exception is, I know what it is. What I don't know is why I'm getting it. The encryption algorithm uses a similar line, but the call from the array is exactly the same. The encryption uses a simple-substitution with a pattern buster variable. The decryption algorithm worked until I after I implemented that variable. The array being returned by the toArray() method is correct and full and so is the array in the call, it should have a length of 11 and does until that point for some reason.

Code for testing class:

public class tester {	
	public static void main(String args[]){
 
 
 
			long start = System.currentTimeMillis();
 
			String enc = Encryptor.hardEncrypt("Mississippi",7,1);
			System.out.println(enc);
			System.out.println("Decrypted: " + Encryptor.keyDecrypt(enc, 7, 5));
 
			long end = System.currentTimeMillis();
			System.out.println("   Elapsed time: " + (end-start) + "ms.");	
	}
}

Code for Encryption class:

package algorithms;
 
import javax.swing.JOptionPane;
 
public class Encryptor {
 
	private static String[] vals = {" ","!", "\"","#","$","%","&","'","(",")","*","+",",","-",".","/","0","1","2","3","4","5","6","7","8","9"
		,":",";","<","=",">","?","@","A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V"
		,"W","X","Y","Z","a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z","{","}","|"
		,"}","~"};
	private static int max = vals.length;
 
	public Encryptor(){}
	//takes a String a makes it into an array containing every character
	private static String[] toArray(String enc)
	{
		String[] arr = new String[enc.length()];
		for(int i = 0; i < enc.length(); i++)
		{
			arr[i]=enc.substring(i,i+1);
		}
		return arr;
	}
	//assigns number values to each character
	private static int[] numValues(String[] enc)
	{
		int[] numvals = new int[enc.length];
 
		for(int i = 0; i < enc.length;i++)
		{
			for(int j = 0; j < max; j++)
			{
				if(enc[i].equals(vals[j]))
				{
					numvals[i] = j;
					System.out.println("numValues(): " + i+": " + j);
					break;
				}
			}
		}
		System.out.println("numValues(): " + numvals.length);
		return numvals;
	}
 
	//shifts each character by the specified increment
	public static String hardEncrypt(String enc, int inc, int times)
	{
		String fin = "";//variable for final encrypted String
		int chng = enc.length()*enc.length();//pattern breaker and makes the substitution much less obvious
		int[] nums = numValues(toArray(enc));//integer array for the characters in the String to be encrypted
		for(int amount = 0; amount < times; amount++)//encryption and re-encryption loop
		{	
			fin = "";//resets fin
			chng = enc.length()*enc.length();//resets the breaker
			for(int i = 0; i < enc.length(); i++)//encryption algorithm loop
			{
				int ind = (nums[i] + inc)*chng;//the index in vals[] containing the correct character
				if(ind < 0)//makes sure the number is positive
				{
					ind *= -1;
				}
				if(ind >= max)//makes sure the index is in bounds
				{
					while(ind >= max)
						ind -= max;
				}
				fin += vals[ind];//adds the character at vals[ind] to fin
				chng = Math.round(chng*1/2);//algorithmically changes the breaker integer
				if(chng == 1)//resets the breaker if it equals one to enc.length^2*i
				{
					chng = enc.length()*enc.length() * i;
				}
			}
			nums = numValues(toArray(fin));//resets the nums array
		}
 
		return fin;//returns the encrypted String
	}
	//decryption method based on hardEncrypt()
	/*
	 * Problem Code!! vvvvvvvvv
	 */
	public static String keyDecrypt(String enc, int key, int times)
	{
		String fin = "";//String for final decrypted String
		int chng = enc.length()*enc.length();//copies the algorithm and attempts to reverse it
		System.out.println("keyDecrypt(): " + enc);
		System.out.println("keyDecrypt(): " + chng);
		int[] nums = numValues(toArray(enc));//array getting the exception
		for(int amount = 0; amount < times; amount++)
		{	
			fin = "";
			chng = enc.length()*enc.length();
			for(int i = 0; i < enc.length(); i++)
			{
				int ind = (nums[i] - key)*chng;//getting an ArrayIndexOutOfBoundsException and I can't figure out the cause
				if(ind < 0)
				{
					ind += max;
				}
				if(ind >= max)
				{
					while(ind >= max)
						ind -= max;
				}
				fin += vals[ind];
				nums = numValues(toArray(fin));
				chng = Math.round(chng*1/2);
				if(chng == 1)
				{
					chng = enc.length()*enc.length() * i;
				}
			}
		}
 
		return fin;
	}
	/*
	 * Problem Code!! ^^^^^^^^^
	 */
 
	public static String easyEncrypt(String enc)
	{
		return hardEncrypt(enc, 3,1);
	}
 
	public static String decryptEasyEncrypt(String dec)
	{
		return hardEncrypt(dec, -3,1);
	}
}

I found errors in you algorithm. After you encrypt the word mississippi, the numeric cypher text would be 82, 30, 0, 0 each at nums[0], nums[1] and so on. When the turns comes of the third value that is 0. you see that the ind value remains negative. that is (0 - 7) * 121 = -847 which is negative.
And even after adding the max value to it, it remains negative still. The ind value does not change at all. And when the program reach at fin += vals[ind] then the ind value is still negative. So this might have cause the problem. Whats your encryption algorithm. I wanted to check if the algorithm itself is correct or not. Hope this will help you

i found the error now but your algorithm is wrong.
In you keydecrypt method nums = numValues(toArray(fin)); should be out of the loop statement. This was causing the problem.
overall it should be

 
				if(ind < 0)
				{
					ind *= -1;
				}
 
 
				if(ind >= max)
				{
					while(ind >= max)
						ind -= max;
				}
 
				fin += vals[ind];
 
				chng = Math.round(chng*1/2);
				if(chng == 1)
				{
					chng = enc.length()*enc.length() * i;
				}
			}
			nums = numValues(toArray(fin));
		}
 
		return fin;
	}

It works but the decryption fails. I will work on your algorithm and find out the problem. Hope it helps buddy

Thanks, what about this as a solution:

if(ind < 0)
{
        while(ind < 0)
                 ind+=max;
}

That is what I did, however, I still am getting the exception at that line. If you look at the same line in the encryption algorithm, where it calls the value at

int ind = (nums[i] - key)*chng;

It's the same in both spots, however, I only get the exception in the decryption algorithm. Also, I was working on the decryption algorithm, it won't be right yet, but I can't make it right with the exception popping up everytime I run it.

the ind could be negative even you did ind *= -1;. Try with this:

				int ind = (nums[i] + inc)*chng;//the index in vals[] containing the correct character
				if(ind < 0) ind = math.abs(ind);
				if(ind >= max)//makes sure the index is in bounds
				{
					while(ind >= max)
						ind -= max;
				}
				fin += vals[ind];//adds the character at vals[ind] to fin

In that case, it couldn't be negative.

if(ind < 0) //ind has to be negative to return true
      ind *= -1; //will only use this line if the if statement returns true meaning ind has to be negative.

int ind = (nums[i] - key)*chng;//getting an ArrayIndexOutOfBoundsException and I can't figure out the cause

That is the line I'm getting the exception from. the only variable working with nums[] is "i". nums[] length is correct before the loop and should be correct the whole time, however, it only goes through one time meaning there's only one value in nums[]. It should s

Originally Posted by aesguitar

In that case, it couldn't be negative.

if(ind < 0) //ind has to be negative to return true
      ind *= -1; //will only use this line if the if statement returns true meaning ind has to be negative.

int ind = (nums[i] - key)*chng;//getting an ArrayIndexOutOfBoundsException and I can't figure out the cause

That is the line I'm getting the exception from. the only variable working with nums[] is "i". nums[] length is correct before the loop and should be correct the whole time, however, it only goes through one time meaning there's only one value in nums[]. It should s

Uh..oh I've copied your Encryptor and surveyed it...have a question

int[] nums = numValues(toArray(enc));//array getting the exception
...
for(int i = 0; i < enc.length(); i++)

What happens if you change to:

int[] nums = numValues(toArray(enc));//array getting the exception
...
for(int i = 0; i < nums.length; i++)

...I've copied and tested...it worked fine.

Alright, but it's supposed to decrypt the encrypted phrase. Basically, in this case, change "x> jnb<wMF" back into Mississippi. It printed "Decrypted: 1". It should say "Decrypted: Mississippi".

can you tell the name of encryption algorithm you are using. I actually wanted to check your algorithm correctness.