Thread: Help needed in C Programming

Hi guys, I am new to this forum and C programming language.
Currently, I am doing an application and faced with some problems.
Hope you guys here can help!!!

I am doing an application on cryptography.
Basically what the program does is to read in ciphertexts(encrypted texts).
The texts will then be decrypt with a key to generate out the plain english texts.

My program should decrypt letters(a,b,c,...,z) of the ciphertext leaving the others characters(1234567890.,!@#$~%^&<>"space") unaltered. Unaltered characters will not advance the shift to its
next key.

E.g. ciphertext: "sgdty &hy, GHYTR?" decrypt with the key "boy" should decrypt using this method

"boybo &yb, OYBOY?" and
NOT to "boybo &oy, YBOYB?"

Currently, my program is able to leave the unaltered characters unchanged. The nummbers, !@#$%^ even "space" seems to work fine. However, all my alphabets(a-z) after going through the algorithmn display out texts that is not plain english texts.

Below is a portion of my source code that i used to decrypt the ciphertext to english text.

Code:

char *vig_dec_str(char *str, char *key)
{
  int ii, key_len = strlen(key);
  if(key_len < 1) { vig_error(VIG_ELENGTH); }


#define VIG_DEC_CHAR(C, K)  (K == '-') ? ' ' : \
                                      ('A' + (26 + (C - 'A') - (K - 'A')) % 26)
  for(ii = 0; str[ii]; ii++)
  {
         if(isalpha(str[ii]))
         {
          str[ii] = VIG_DEC_CHAR(str[ii], key[ii % key_len]);
         }
         else if(isdigit(str[ii]))
         {
          str[ii] = str[ii];
         }
         else if(ispunct(str[ii]))
         {
          str[ii] = str[ii];
         }
         else if(iscntrl(str[ii]))
         {
          str[ii] = str[ii];
         }
         putchar(str[ii]);
  }
  return str;
}

I believe the problem is because my source code decrypt the texts using this method "boybo &oy, YBOYB?" which is wrong. I would like my program to decrypt the texts using this method "boybo &yb, OYBOY?".

One of the methods that i try is removing all numbers, !@#$%^, and "space" from the ciphertexts combine the whole string texts and do the decryption. This method is not good because I am able to
decrypt the texts correctly but i lose all my spacing, numbers and !@#$%.

E.g.

Results using this method: IneedsomehelpfromMrcanyouhelpmePlease
Actual text message: I need some help from Mr.123 can you help me? Please!

Hope you guys can help me out!

Thanks in advance.

Thanks Quzah! Yah I try out the codes in orange is actually redunctant.

Originally Posted by ProgC

Hope you guys can help me out!

Thanks in advance.

I'd suggest something like this:

Code:

key[] = "boy\0"

for (i = 0; i < stringlength of ciphertext ; i++)  {  /* work thru the ciphertext with i */
    /* if char is a letter, replace it with the next key letter */ 

    if (isalpha(ciphertext[i]))
        ciphertext[i] = key[j++] 

     if (j >= stringlenth of key - 1)
        j = 0

You'll need to adjust the "stringlength" references to your own ciphertext char array. Shorten by one if you have a '\0' (end of string char), at the end of your char array.

Right off hand, I don't remember whether isalpha('&') returns a 1 or not. Pop back with a post if that's a problem you can't surmount.

I wouldn't use your compression method, because at the end of the program, you want to print out the encoded ciphertext, not a compressed version.


Adak

Hi Adak,

I tried your method but i am still not able to assign the key accordingly to each characters that I want.

Below are the updated codes that i added.
Please see if there is anything wrong with the logic.

Code:

char *vig_dec_str(char *str, char *key)
{
   int ii, key_len = strlen(key);
   int jj = 0;
   if(key_len < 1) 
  { 
    vig_error(VIG_ELENGTH); 
  }
 /* The check for a key letter being '-' is used in vig_disp_text */
#define VIG_DEC_CHAR(C, K)  (K == '-') ? ' ' : \
                                      ('A' + (26 + (C - 'A') - (K - 'A')) % 26)
  for(ii = 0; str[ii]; ii++)
  {
         if(isalpha(str[ii]))
         {
          str[ii] = VIG_DEC_CHAR(str[ii], key[ii % key_len]);
          
          if(key[jj] >= key_len - 1)
          {
           key[jj] = 0;
          }
         }
         putchar(str[ii]);
  }
  return str;
}

Yes, that logic is wrong. You're actually modifying the value of the key instead of the key index.

Code:

for(ii = 0; str[ii]; ii++)
  {
         if(isalpha(str[ii]))
         {
              str[ii] = VIG_DEC_CHAR(str[ii], key[jj % key_len]);
              jj++;
         }
         putchar(str[ii]);
  }

something along the lines of the above should work.

Thanks KONI!
I am able to decipher most of the texts out correctly.

Million Thanks!!!