Thread: xor Encryption

I am trying to work out the encryption function to the following decryption function.

Code:

char* buf;
	unsigned int nKey = buf[3] << 24;
	nKey |= buf[2] << 16;
	nKey |= buf[1] << 8;
	nKey |= buf[0];
	unsigned int nIndex = 4;

	for(unsigned int nBitNr = 0; nBitNr < 32; ++nBitNr, nIndex++)
	{
		int nDecryptBit = (nKey & (1 << nBitNr)) >> nBitNr;
		int chEncryptedByte = buf[nIndex];
		int chDecryptedByte = chEncryptedByte;

		chDecryptedByte = chDecryptedByte ^ nDecryptBit;
		chDecryptedByte = chDecryptedByte & 1;
		chDecryptedByte = chDecryptedByte ^ chEncryptedByte;
		buf[nIndex] = chDecryptedByte;
		if(nBitNr == 31)
			break;
	}
	printf("File Decrypted is %s\n",buf);

I see that the decrpt key is the first for bytes of Buf and that only 32 bytes are decrypted.

From searching these forums I now know that
a = b ^ c goes to c = b ^ a

would a = b & 1 go to a = b & 0?

also what is this doing?

Code:

int nDecryptBit = (nKey & (1 << nBitNr)) >> nBitNr;

This is a fairly badly coded function that performs a fairly bad encryption scheme.
To answer your last question first:

Code:

 
int nDecryptBit = (nKey & (1 << nBitNr)) >> nBitNr;
/* can be written */
int nDecryptBit = (nKey >> nBitNr) & 1;

gets the value of a single bit in nKey which is bit number nBitNr counting from the least significant bit where the least significant bit is number 0.

a=b & 1 clears all but the least significant bit of a. the lsb of a = lsb of b. There is no inverse.

If you work out the logic of what the rest of the code does - it just replaces the least significant bit of the encrypted bit with the corresponding bit in the key. So your encoding algorithm should create the key out of the least significant bits of the plain text and, I would suggest, randomly toggle the lsb of the rest of the message:

Code:

key[i/8] = (key[i/8] << 1) | (plaintextByte[i] & 1);
encryptedByte[4+i] = plaintextByte[i] ^ (rand() % 2);

or something similar...

::edit:: ah, no, the key calculation gets the bits the wrong way round - but I'll leave that as an exercise for the reader (as they say in all the best text books!)...

The way to see if you are close is to try it out. I used this short program to test out your encoder:

Code:

#include <stdlib.h>
#include <stdio.h>

void decrypt (unsigned char buf[37])
{
  unsigned int nBitNr;
  unsigned int nIndex = 4;
  unsigned int nKey = (unsigned int)buf[3] << 24;
  nKey |= (unsigned int)buf[2] << 16;
  nKey |= (unsigned int)buf[1] << 8;
  nKey |= (unsigned int)buf[0];

  for(nBitNr = 0; nBitNr < 32; ++nBitNr, nIndex++)
  {
    int nDecryptBit = (nKey & (1 << nBitNr)) >> nBitNr;
    int chEncryptedByte = buf[nIndex];
    int chDecryptedByte = chEncryptedByte;

    chDecryptedByte = chDecryptedByte ^ nDecryptBit;
    chDecryptedByte = chDecryptedByte & 1;
    chDecryptedByte = chDecryptedByte ^ chEncryptedByte;
    buf[nIndex] = chDecryptedByte;
    if(nBitNr == 31)
      break;
  }
  buf[36] = '\0';  /* added to make sure the string is properly terminated */
  printf("File Decrypted is %s\n",buf);
}

void cruxisEncrypt (char buf[37])
{
  unsigned int nBitNr;
  unsigned int nIndex = 4;

  for(nBitNr = 0; nBitNr < 32; ++nBitNr, nIndex++)
  {
    // make key ???
    unsigned int nKey = buf[nIndex] << 1 | (buf[nIndex] & 1);

    int nEncryptBit = (nKey >> nBitNr) & 1;

    int	chEncryptedByte = nEncryptBit ^ buf[4+nIndex]; 

    buf[4+nIndex] = chEncryptedByte;

    if(nBitNr == 31)
	    break;
  }
  printf("File Encrypted is %s\n",buf);
}

void davtEncrypt (char buf[37])
{
  unsigned int i;

  for (i = 35; i > 3; i--) {
    buf[(i-4)/8] = (buf[(i-4)/8] << 1) | (buf[i] & 1);
    buf[i] ^= rand() % 2;
  }
  buf[36] = '\0'; /* shouldn't need this line - but just in case... */
  printf("File Encrypted is %s\n", buf);
}

int main (void)
{
  char buf[37] = "key*This is my plaintext. How dull!!";

//  davtEncrypt(buf);
  cruxisEncrypt(buf);
  decrypt(buf);

  return 0;
}

Notice that I have had to make some changes to the code to get it to compile - some because my compiler is pre-C99 and some because it just won't work. Here is what I get as output:

Code:

File Encrypted is key*This is my!plaintext. How dull!!
File Decrypted is key*Uihs is mx!plainudxu/!Inv!dulm

Which is clearly not right. Using my function gives:

Code:

File Encrypted is lce¬Tihs hs ly ql`ioudyu/!How dtll  
File Decrypted is lce¬This is my plaintext. How dull!!

Which shows that it works. (note that the first 4 symbols above are not right - I couldn't type the actual values produced by the program). All my code does is store the least significant bit of each character in the key and randomly mess up the least significant bit of each character in the message. The decrypt algorithm can be simplified to:

Code:

void decrypt (char buf[37])
{
  unsigned int i;

  for (i = 0; i < 32; i++) {
    unsigned int keyBit = ((buf[i/8] >> (i%8)) & 1);
    buf[i+4] &= (~1); /* clear lsb */
    buf[i+4] |= keyBit; /* set lsb = keyBit */
  }
  buf[36] = '\0';
  printf("File Decrypted is %s\n", buf);
}