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#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <assert.h>
#include <unistd.h>
#define flip_block(TMP_VAR, TYPE, BUF, POS) \
TMP_VAR = *(TYPE *)&BUF[POS]; \
TMP_VAR &= ~mask(step*8); \
TMP_VAR = ~TMP_VAR; \
*(TYPE *)&BUF[POS] = TMP_VAR; \
static char *
read_file(const char *fname, size_t *size_out)
{
FILE *fp;
char *buf;
size_t bufsize;
if (access(fname, F_OK) != 0) {
fprintf(stderr, "%s does not exist!\n");
return NULL;
}
fp = fopen(fname, "r");
/* Get file size */
fseek(fp, 0, SEEK_END);
bufsize = ftell(fp);
fseek(fp, 0, SEEK_SET);
buf = malloc(bufsize);
fread(buf, sizeof(char), bufsize, fp);
fclose(fp);
*size_out = bufsize;
return buf;
}
static void
writeback_file(const char *fname, const char *buf, size_t buf_size)
{
FILE *fp;
fp = fopen(fname, "w");
fwrite(buf, buf_size, sizeof(char), fp);
fclose(fp);
}
static inline uint64_t
mask(size_t n)
{
return (1 << n) - 1;
}
static char *
encrypt(char *buf, size_t buf_size)
{
size_t current_pos;
size_t step;
uint64_t tmp;
current_pos = 0;
step = 8; /* Start at 8 bytes (64 bits) */
while (current_pos < buf_size) {
/* Ensure we aren't over 8 bytes and a power of two */
if (step != 1) {
assert((step & 1) == 0 && step <= 8);
}
switch (step) {
case 8:
flip_block(tmp, uint64_t, buf, current_pos);
break;
case 4:
flip_block(tmp, uint32_t, buf, current_pos);
break;
case 2:
flip_block(tmp, uint16_t, buf, current_pos);
break;
case 1:
flip_block(tmp, uint8_t, buf, current_pos);
break;
}
current_pos += step;
/* Ensure we don't cause any overflows */
while (((current_pos + step) >= buf_size) && step > 1)
/* Essentially divide the step by 2, just faster */
step >>= 1;
}
}
int
main(int argc, const char **argv)
{
size_t buf_size;
char *buf;
if (argc < 2) {
fprintf(stderr, "Usage: %s <file>\n", argv[0]);
return 1;
}
buf = read_file(argv[1], &buf_size);
encrypt(buf, buf_size);
writeback_file(argv[1], buf, buf_size);
free(buf);
return 0;
}
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