Question

encrypt decrypt in C problem

Asked by: jazzIIIlove

Hi there;

I am trying to enc/dec a string but I have a problem that the enc/dec. string are not matching.

What to do?

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
 
 
char * encipher(char * input, char * key)
{
     unsigned int i,j;
	 char final_enc [12];
    
	 for(i=0,j=0;i<strlen(input);i++,j++)
     {
          //repeat the key if you are at end of it.
          if(j>=strlen(key))
          {
               j=0;
          }
          //actual logic -> character from input + character from key % 26 is encrypted charater
          final_enc [i] = 65+(toupper(input[i])-65)+(toupper(key[j])-65)%26;
     }
	 return final_enc;
 
}
 
char * decipher(char * input, char * key)
{
     unsigned int i,j;
	 char final_dec[12];   
     int value;
	
     for(i=0,j=0;i<strlen(input);i++,j++)
     {
          //repeat the key if you are at end of it.
          if(j>=strlen(key))
          {
               j=0;
          }
          //similar to encipher only difference is you need to subtract
          value = (toupper(input[i])-64)-(toupper(key[j])-64);
          //make positive if value is negative.
          if( value < 0)
          {
               value = value * -1;
          }          
          final_dec [i] = 65 + (value % 26);
     }
	 return final_dec;
 
}
 
int main(void){
 
	char * str = "192.168.2.1"; //12 chars
	
	char *k ;
	char *l;
 
	k = encipher(str,"123");
 
	
	l = decipher(k,"123");
 
	
 
	return 0;
}

                                  
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Asked On
2009-10-22 at 19:25:58ID24836816
Topic

C Programming Language

Participating Experts
3
Points
500
Comments
6

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Answers

 

by: mrjoltcolaPosted on 2009-10-22 at 19:47:35ID: 25641020

Why in your encipher do you use 65 to add, when in your decipher you use 64 to subtract. Seems it should be symmetrical to me. Isn't this a Caesar cipher?

 

by: Subrat2009Posted on 2009-10-22 at 21:27:11ID: 25641360

+1

needs to add '\0' at the end of string before returning form function.
Your code will show asserts bcz in a unsafe manner u are storing the strings.don't use char* only in main to point it. May use static allocation like array or better use dynamic allocation inside functions and after use in main u can free the memory.
Include ctype.h to make toupper() work.
Needs to check ur logic... of incript and decript.Try to debug and find it ursef why it's not giving expected result.

 

by: Infinity08Posted on 2009-10-23 at 01:09:02ID: 25642138

A few problems have already been mentioned.

There is also the problem that you're returning a pointer to a local array (final_enc and final_dec resp.) ... This array no longer exists once the function ends, so in main you end up with a pointer to a block of memory that doesn't necessarily hold the data you expect any more.

Now in the encryption function, you have (65+(toupper(input[i])-65)), which adds 65, and then subtracts it again - that doesn't achieve anything ;)

Further, ((toupper(key[j])-65)%26) ... The example key "123" you used has no alphabetic characters, so toupper will have no effect. You'll end up with a negative value though due to the - 65. Is that what you intended ?

In the decryption function, you have :

>> value = (toupper(input[i])-64)-(toupper(key[j])-64);

which does not correspond to your encryption function (where you used 65 instead of 64, but more importantly you used the % 26, which you don't use here).

You can't also just do this :

>> //make positive if value is negative.

because that changes the value ...

 

by: jazzIIIlovePosted on 2009-10-23 at 01:21:09ID: 25642206

hmm...my code seems a mess...Let me fix it according to your suggestions...

 

by: Infinity08Posted on 2009-10-23 at 01:25:48ID: 25642232

I suggest that you make the encryption function do something simple first, like simply add the key. Then the decryption function can subtract the key.

Once that works, you can make it more complicated, as you wish ...

 

by: jazzIIIlovePosted on 2009-10-23 at 03:57:36ID: 25643099

Yep, that's what I am gone try...

Best regards...

20120131-EE-VQP-002

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