Thread: Need help fixing error to get correct output.

  1. #1
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    Need help fixing error to get correct output.

    Code:
    #include <stdio.h>
     char K[6]={'k','c','u','n','n','i'};
     int i, j, T;
    void reset( int S[])
    {   for(i=0; i<=255; i++)
         S[i]=i;
    }
    void init( int S[])
    {
       for(i=0; i<=255; i++)
          j=0;  
          j=(j + S[i] + K[i%6])%256;
          T=S[i];
          S[i]=S[j];
          S[j]=T;    
    }
    int main(void)
    {  int c, S[256], d, i=0, j=0, reset S[], init S[]; 
    reset(S); 
    init(S);
      while(1)
      {
        c = getchar(); if( c < 0) break;
        i=(i+1)%256;
        j=(j+S[i])%256;
        T=S[i];
        S[i]=S[j];
        S[j]=T;
        d=S[(S[i] + S[j])%256];
        printf( "%c", c^d);   
      }
      return 0;
    }
    Error is with reset/init in main. I also can't get my output to match, if i could get any help thanks.

  2. #2
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    Code:
    {  int c, S[256], d, i=0, j=0, reset S[], init S[];
    this wont even compile, what do you mean by reset S[] and init S[]? This isn't valid C.

  3. #3
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    Assignment is RC4 Decryption (General Idea)

    It says im suppose to call reset and init to initialize the array in main.

  4. #4
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    Quote Originally Posted by kctrk4
    Assignment is RC4 Decryption (General Idea)

    It says im suppose to call reset and init to initialize the array in main.
    Give us a link to the description. But this call of S looks like a method call.

  5. #5
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    Use global variables for the key and keylength; the key will be your VU LDAP UserID, all lowercase. For example:
    char key[] = "frodo";
    int keylength = 5;

    reset the RC4 permutation array:
    for i from 0 to 255
    S[i] := i

    initialize the permutation array:
    j := 0
    for i from 0 to 255
    j := (j + S[i] + key[i mod keylength]) mod 256
    swap(S[i],S[j])

    Declare the array and other parameters in your main program:
    int S[256], i = 0, j = 0;

    and call reset and init to initialize the array.

    Then for each byte read from stdin using getchar() until end-of-file, compute the next RC4 "output" byte
    i := (i + 1) mod 256
    j := (j + S[i]) mod 256
    swap(S[i],S[j])
    output S[(S[i] + S[j]) mod 256]

    and exclusive-OR that byte with the input byte to decrypt it.

  6. #6
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    well i got it to compile without errors but my output is still not matching, can anyone help me?

    Should be:
    Money is better

    Mine:
    M-yM-t&^@M-2I{dM-^BPnzM

  7. #7
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    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #define SSIZE 4
    
    void initArray(int *array, int arraysize){
     int i;
     
     for (i = 0; i < arraysize; i++) {
         array[i] = 0;
     }
     
    }
    
    void resetArray(int *array, int arraysize){
     int i = 0;
     //array[SSIZE] = '\0';
     for (i = 0; i < arraysize; i++) {
         array[i] = i;
     }
    }
    
    int main(void){
    
     int s[SSIZE];
     int i, j;
        initArray(s,SSIZE);
        for (i = 0; i < SSIZE; i++ ){
            printf("%d\n",s[i]); 
        }
        resetArray(s,SSIZE);
        
        for (j = 0; j < SSIZE; j++){
            printf("%d\n",s[j]);
        }
     return 0;
    }

  8. #8
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    Thanks, although your code is quite different, can you explain it?

  9. #9
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    Quote Originally Posted by kctrk4
    Thanks, although your code is quite different, can you explain it?
    Well Im passing an array and initializing it, I pass the array size so I know when Ive filled all positions, you must of course adapt it to fill it properly.

  10. #10
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    I don't understand C all that well. I already thanked you but is there anyway you can tell me the problem in my program, which is causing me to print a random mess.

  11. #11
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    Give us your last source version.

  12. #12
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    Not sure what you by last source.
    Implement RC4 decryption in C as follows using the algorithm description from Wikipedia: (Wikipedia RC4 file)
    Format taken from Wikipedia

    I just don't understand what i have wrong, i already posted my full code and orginal information given. My ouput is junk and does not match the "fortune". I have been trying to work with it since before my first post.

  13. #13
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    Quote Originally Posted by kctrk4
    Not sure what you by last source.
    Implement RC4 decryption in C as follows using the algorithm description from Wikipedia: (Wikipedia RC4 file)
    Format taken from Wikipedia

    I just don't understand what i have wrong, i already posted my full code and orginal information given. My ouput is junk and does not match the "fortune". I have been trying to work with it since before my first post.
    The code you posted doesn't compiles give us a compilling version.

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