# the usual crc32 polynomial
my $crc_poly = 0xedb88320;
my $crc_size = 32;

my $initial_str = "foo bar";
my $final_str   = "baz blat";
my $desired_crc = 0x12345678;

my $magic = forward_crc($initial_str, 0xffffffff)
    ^ reverse_crc("\0\0\0\0".$final_str,
        $desired_crc^0xffffffff);
my $forged_str = $initial_str . pack("V",$magic) .
    $final_str;

use String::CRC32;
printf "%x\n", crc32($forged_str);

# calculate an ordinary crc of $string,
# given the initial register contents
sub forward_crc {
  my ($str, $reg) = @_;
  foreach (split //, $str) {
    $reg ^= ord;
    for (1..8) {
      $reg = ($reg & 1) ?
          ($reg >> 1) ^ $crc_poly : $reg >> 1;
    }
  }
  return $reg;
} # forward_crc

# given the final register contents, calculate
# what the register state was before $str
sub reverse_crc {
  my ($str, $reg) = @_;
  my $poly = ($crc_poly << 1) ^ 1;
  my $highbit = 1 << ($crc_size - 1);
  foreach (reverse split //, $str) {
    for (1..8) {
      $reg = ($reg & $highbit) ?
          ($reg << 1) ^ $poly : $reg << 1;
    }
    $reg ^= ord;
  }
  return $reg;
} # reverse_crc