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Thread: Passing an array to the constructor!

  1. #1
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    Unhappy Passing an array to the constructor!

    I implemented RC4 cryptography algorithm using Java but I still don't know why it doesn't work when I try to pass an array from main method to the constructor! How can I make it works?
    Thank you in advance

     
    package rc4.generator;
     
     
    public class RC4_Generator {
        private int[] S;
        private int[] T;
        private int keylen;    
     
        public RC4_Generator(byte[] key) throws Exception 
        {
                if (key.length < 1 || key.length > 256) 
                	throw new Exception("The key should be between 1 to 256 Byte!"); 
                else 
                	// The initialization of both S[] and T[].
                {
                        keylen = key.length;
                        for (int i = 0; i < 256; i++) {
                                S[i] = i;
                                T[i] = key[i % keylen];
                }
     
                    // The S permutation.
                        int j = 0;
                        for (int i = 0; i < 256; i++) 
                        {
                                j = (j + S[i] + T[i]) % 256;
                                S[i] = S[i] ^ S[j];
                                S[j] = S[j] ^ S[i];
                                S[i] = S[i] ^ S[j];
                        }
                }
        }
     
     
        public int[] encryption(int[] plaintext) 
        {
                int[] ciphertext = new int[plaintext.length];
                int i = 0, j = 0, k, t;
                for (int c = 0; c < plaintext.length; c++) {
                        i = (i + 1) % 256;
                        j = (j + S[i]) % 256;
                        S[i] = S[i] ^ S[j];
                        S[j] = S[j] ^ S[i];
                        S[i] = S[i] ^ S[j];
                        t = (S[i] + S[j]) % 256;
                        k = S[t];
                        //ciphertext[c] = plaintext[c] ^ k;
                }
                return ciphertext;
        }
     
     
        public static void main(String[] args){
     
        	byte[] k ={0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30};
     
        	RC4_Generator rc4= new RC4_Generator(k);
        }
     
    }


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: Passing an array to the constructor!

    it doesn't work
    Please explain what "doesn't work" means.
    Copy the full text of the error messages and paste it here.
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
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    Default Re: Passing an array to the constructor!

    You've commented to initialize S[] and T[], but do you actually initialize them anywhere? An array needs to be initialized with a length.
    Also, when asking a question, make sure to tell us what is going wrong. "It doesn't work" is not a helpful statement for us. Is the output different than you expect, and how is it different? Are you getting exceptions or errors, and what type of exceptions and errors?
    NOTE TO NEW PEOPLE LOOKING FOR HELP ON FORUM:

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    Default Re: Passing an array to the constructor!

    Oh! I'm sorry for that! I'm a new user here! I will be clear

    I got this error:
    -----------------------------------------------------------------
    Exception in thread "main" java.lang.NullPointerException
    at rc4.generator.RC4_Generator.<init>(RC4_Generator.j ava:18)
    at rc4.generator.RC4_Generator.main(RC4_Generator.jav a:62)
    ----------------------------------------------------------------
    Do you see guys why? I try to understand it but I couldn't!

  5. #5
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    Default Re: Passing an array to the constructor!

    I would guess because S[] and T[] have not been initialized, but you attempt to add values to them.
    You need to initialize them beforehand in your constructor like this:
    S = new int[256]; // 256 is the size of the array, you can change this to whatever length seems appropriate
    T = new int[256];
    NOTE TO NEW PEOPLE LOOKING FOR HELP ON FORUM:

    When asking for help, please follow these guidelines to receive better and more prompt help:
    1. Put your code in Java Tags. To do this, put [highlight=java] before your code and [/highlight] after your code.
    2. Give full details of errors and provide us with as much information about the situation as possible.
    3. Give us an example of what the output should look like when done correctly.

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  6. #6
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    Default Re: Passing an array to the constructor!

    It worked!
    Thank you so much (aussiemcgr ) for your help! Really Appreciate that

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