Thread: XOR encryption

I need some help with a XOR program I'm trying to write.

I need to know if I'm on the right track here, and how I could decrypt it all. Reverse the process, so to speak. Please have a look at this code, but ignore all the unnecessary printf's, I'm just trying to follow the code through step by step.

Are the for loops correct? I found some pseudo code in the forum which I decided to write the code from..
If the encryption is in order, which I believe it is, then there's the small matter of decrypting it.. The process has to be reversed, that much I know, but it seems it will only decrypt as many characters as the key is based on. Somwhere, somehow, this needs to increment..

itsme86, you've been of great help explaining the modulo and the caesar program. You mind taking a peek at this?

Code:

#include <stdio.h>
#include <string.h>
main()
{
      char        plain[51];
      char        encry[51];
      char        key[5] = "ABCD";
      int           i, keylen = 0, keypos = 0;
      
      printf("Enter text: ");
      scanf("%[^\n]", plain);
      printf("You entered: %s\n", plain);
      keylen = strlen(key);
      printf("Encyption key is: %s\n", key);
      printf("Length of key is: %d\n", keylen);
      
      for ( i = 0; plain[i] != '\0'; i++ ) 
      {
          encry[i] = plain[i] ^ key[keypos];
          keypos++;
                if ( keypos > keylen )
                   keypos = 0;
          
      }    
      printf("\nEncrypted text: %s\n", encry);        

      for ( i = 0; encry[i] != '\0'; i++ ) {
          plain[i] = encry[i] ^ key[i]; /*???*/   
          /* ??? */
          }
          
      printf("Decrypted: %s", plain);
      
      fflush(stdin);
      getchar();
}

Originally Posted by Salem

> printf("\nEncrypted text: %s\n", encry);
'A' ^ 'A' is 0, which will not print very well with %s
Similarly, other pairs of letters will produce similar non-printable results.

> fflush(stdin);
Ugh, read the FAQ.

Yes, I know about the fflush(stdin); issue, but I thought I would leave it as it is and concentrate on the encryption and decryption instead.

I've changed both my "Encrypted" and "Decrypted" part of the code to

Code:

puts("Encrypted text: ");
puts(encry);

Same for the decrypted text, but it's not decrypting the way it should.. Actually I don't even know if it's encrypting as it should.. Would you mind giving it a go?

Originally Posted by Salem

Well xor is so simple that decrypt is the same as encrypt

So given something like
void encrypt( char *message, const char *key, size_t messageLen, size_t keyLen );

Calls of
len = strlen(message);
encrypt( message, key, len, strlen(key) );
encrypt( message, key, len, strlen(key) );

Will get you back to where you started.
Just don't do any str... functions on the intermediate result.

Yes, well.. this will most certainly be embarrasing.. Functions is in the next chapter of our book.. :-) But I'll do my best and read up on what that "void encrypt" part of the code does. I'm guessing the *'s are pointers and those are not discussed for a loooong time in our book.
I'll read up on functions, as best I can, and maybe I'll be able to break that sucker.

Originally Posted by dwks

As Salem said, encrypting is the same as decrypting.

Code:

      for ( i = 0; plain[i] != '\0'; i++ ) 
      {
          encry[i] = plain[i] ^ key[keypos];
          keypos++;
                if ( keypos > keylen )
                   keypos = 0;
          
      }    
      printf("\nEncrypted text: %s\n", encry);        

      for ( i = 0; encry[i] != '\0'; i++ ) {
          plain[i] = encry[i] ^ key[i]; /*???*/   
          /* ??? */
          }

So you need to use key[keypos] for the decrypting too.

I've tried different combinations, but I can't seem to get it right.. Something, somewhere, needs to increment, in order for it to allow more characters to be decrypted, and that's where I fail.. I'll try some more though..

Originally Posted by dwks

It's quite simple.
Use the code in the first loop in the second one.

Code:

/* decrypt loop */
      for ( i = 0; encry[i] != '\0'; i++ ) 
      {
          plain[i] = encry[i] ^ key[keypos];
          keypos++;
                if ( keypos > keylen )
                   keypos = 0;
          
      }

Well, that's actually the first thing I tried, but I get garbage for output. I thought it would be the logical choice.. Here's the code, and whenever I try to decrypt I get garbage.. There has to be something wrong with the loops.. Saturday evening and I'm trying to get a XOR program to work.. Well, at least I'm committed :-)

Code:

#include <stdio.h>
#include <string.h>
main()
{
      char        plain[51];
      char        encry[51];
      char        key[5] = "ABCD";
      int         i, keylen = 0, keypos = 0;
      
      printf("Enter text: ");
      scanf("%[^\n]", plain);
      printf("You entered: %s\n", plain);
      keylen = strlen(key);
      printf("Encryption key is: %s\n", key);
      printf("Length of key is: %d\n", keylen);
      
      for ( i = 0; plain[i] != '\0'; i++ ) 
      {
          encry[i] = plain[i] ^ key[keypos];
          keypos++;
                if ( keypos > keylen )
                   keypos = 0;
                         
      }                 
      puts("Encrypted text: ");
      puts(encry);   
                        
      for ( i = 0; encry[i] != '\0'; i++ ) {
          plain[i] = encry[i] ^ key[keypos];
          keypos++;
                if ( keypos > keylen )
                   keypos = 0;
          }
          
      puts("Decrypted: ");
      puts(plain);
      
      fflush(stdin);
      getchar();
}

Originally Posted by dwks

You have to set keypos to zero before the second loop. Otherwise it starts whereever the first loop left off.

Your indentation is better.

Code:

fflush(stdin);

But this is not.

• FAQ > Explanations of... > Why fflush(stdin) is wrong
• FAQ > How do I... (Level 2) > Flush the input buffer

[edit] BTW, puts() appends a newline to the end of the string, so

Code:

      puts("Decrypted: ");
      puts(plain);

will print

Code:

Decrypted: 
plaintext

[/edit]

Haha, everyone keeps bugging me about the stdin :-) I promise, I'll do my best to get rid of it asap. Thanks for helping me with the program. It works quite nicely now.. I also changed my puts to fputs instead and got rid of the new line. Looks much better. I'm learning as I go..

Code:

      fputs("Decrypted: ", stdout);
      puts(plain);

There is one more thing, when it has decrypted the text, it spits out several of these garbage characters after the decrypted text. Is there something I could throw at that?

If I encrypt and decrypt "testtest", for example - 39 characters get printed to the screen. 8 of them being the testtest and the rest is just arbitrary characters. Sample

Code:

Enter text: testtest
You entered: testtest
Encryption key is: ABCD
Length of key is: 4
Encrypted text: 5'00t$17↑¯É|p♣æ|    m♣æ|Ì┬┬w
Keypos set to: 0
Decrypt ch count: 39
Decrypted: testtest\¯Ð>3Aæ=¢╝╗ ,GÊ8ÌâÇ4Ò┬┬w¶©├w`$=

Originally Posted by dwks

Code:

      for ( i = 0; encry[i] != '\0'; i++ ) {

You never add a terminating NULL to encry[] so you'll never find one. (At least where you expect to.) Add this

Code:

encry[i] = 0;

just after the first loop.

[edit] You could use the isprint() function to only print characters that are printable. [/edit]

How can you possibly keep track of all those "small" details? That small program took me the better part of the day to put together. Oh, it's wonderful being a noob :-) I really appreciate your help!

I'll rewrite the program for a little cleaner output and try to make the whole en-/decryption thingie into a function. Should be fun.

Thanks again!