hex string to byte array, C

gistfile1.c

unsigned char* hexstr_to_char(const char* hexstr)
{
    size_t len = strlen(hexstr);
    IF_ASSERT(len % 2 != 0)
        return NULL;
    size_t final_len = len / 2;
    unsigned char* chrs = (unsigned char*)malloc((final_len+1) * sizeof(*chrs));
    for (size_t i=0, j=0; j<final_len; i+=2, j++)
        chrs[j] = (hexstr[i] % 32 + 9) % 25 * 16 + (hexstr[i+1] % 32 + 9) % 25;
    chrs[final_len] = '\0';
    return chrs;
}