descramble kindle firmware

descramble.c

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

enum {
        BITS_PER_BYTE = 8,
        TYPE_AUTO = 1,
        TYPE_MANUAL = 2,
};

int swap_byte(int byte)
{
        int low, high;

        low = (0x0F & byte) << 4;
        high = (0xF0 & byte) >> 4;

        return (high | low);
}

int main()
{
        char sig[5];
        int byte, type;
        long offset;

        /* check signature (first 4 bytes) */
        fread(sig, 4, 1, stdin);
        sig[4] = '\0';

        if (strncmp("FB01", sig, 4) == 0)
                type = TYPE_MANUAL;
        else if (strncmp("FC02", sig, 4) == 0
                || strncmp("FD03", sig, 4) == 0)
                type = TYPE_AUTO;
        else {
                fprintf(stderr, "unknown signature: %s\n", sig);
                exit(EXIT_FAILURE);
        }

        /* seek */
        offset = (type == TYPE_MANUAL) ? 0x20000: 0x40;
        fseek(stdin, offset, SEEK_SET);

        /* read gzipped tar */
        while ((byte = fgetc(stdin)) != EOF) {
                byte = swap_byte(byte ^ 0x7A);
                fwrite(&byte, 1, 1, stdout);
        }
}