Thread: somthing funky

If you start to think in terms of functions AND pointers this could be proven easier:

Code:

#include <stddef.h>
#include <stdint.h>

// Both dest and src points to buffers with the same size.
void xorblk( void *dest, const void *src, size_t blksize,
             const void *key, size_t keysize )
{
  uint8_t *d;
  const uint8_t *s, *de, *k, *ke;

  d = dest;
  s = src;
  de = dest + blksize;
  k = key;
  ke = key + keysize;

  while ( d < de )
  {
    *d++ = *s++ ^ *k++;
    if ( k == ke ) k = key;
  }  
}

// test.
#include <stdio.h>
#include <string.h>

static void display( const char *, const void *, size_t );

int main( void )
{
  char dest[32];    // with enough space to hold "encoded" data.
  char src[] = "TOMORROW NEVER DIES";
  const char key[] = "hel";
  size_t size, ksize;

  size = strlen( src );   // or (sizeof src - 1)
  ksize = strlen( key );  // or (sizeof key - 1)

  xorblk( dest, src, size, key, ksize );
  display( "original :", src, size );
  display( "encoded :", dest, size );

  xorblk( src, dest, size, key, ksize );
  display( "decoded :", src, size );
}

void display( const char *prompt, const void *p, size_t size )
{
  const uint8_t *q, *qe;
  ptrdiff_t offset;

  fputs( prompt, stdout );

  q = p;
  qe = p + size;

  while ( q < qe )
  {
    offset = q - (const uint8_t *)p;
    if ( offset % 16 == 0 )
      printf( "\n%08tx:", offset );

    printf( " %02x", *q );
    q++;
  }
  putchar( '\n' );    
}

Code:

$ ./xorenc
original:
00000000: 54 4f 4d 4f 52 52 4f 57 20 4e 45 56 45 52 20 44
00000010: 49 45 53
encoded :
00000000: 3c 2a 21 27 37 3e 27 32 4c 26 20 3a 2d 37 4c 2c
00000010: 2c 29 3b
decoded :
00000000: 54 4f 4d 4f 52 52 4f 57 20 4e 45 56 45 52 20 44
00000010: 49 45 53