[ Prev ][ Table of Contents ][ Front Page ][ Talkback ][ FAQ ][ Next ]

"Linux Gazette...making Linux just a little more fun!"

Securely Erasing a Hard Drive with Perl

By Mark Nielsen


  1. Introduction
  2. What problems I have.
  3. The Perl Script
  4. Conclusion
  5. References


When moving from Ohio to California, had some hard drives (along with other hardware and software) which were to be donated to COLUG (Central Ohio Linux Users Group). They needed to be deleted before they were donated. 2 out of the 3 hard drives had bad sectors on them, and the third I ended up using as a hard drive for testing purposes, like creating this article, so I ended up not giving any away. Still, I will need to wipe a hard drive in the future, so I created this Perl script (which I will later convert to Python and make it have more options).

The goal of this Perl script is to just delete the hard drive at /dev/hdb (the slave drive on the Primary IDE controller) since I have a hard drive removeable kit there. I want it to delete all partitions, create one partition that takes up the whole hard drive, and then fill up the hard drive with garbage data (including some random encrypted data just to ruin a hacker's day trying to find out what the data is).

The Problems

Here is a list of problems I had and how I solved them:
  1. How do I get it to delete all the partitions?

    I remember researching many different options to alter partitions on a hard drive, and doing it manually yielded the best results. I had used a Perl Expect script to automate the fdisk program (fdisk partitions hard drives in Linux) in the past, and I decided to continue to do it that way. I believe there are better alternatives for the simple task of deleting all the partitions, like sfdisk and others, but if one solution covers all possibilities with 100% power and flexibility, I usually just stick to one way of doing things so that I don't have to remember too many things and if it ever gets more complicated, I don't have to learn anything new.

    Thus, I used Expect code to simulate a user typing in the commands for fdisk. The Expect code deleted all the partitions and then it created one big partition.

  2. How do I fill up the hard drive with garbage data?

    Just deleting partitions isn't enough to delete the data. I want to overwrite all old data with garbage to make sure the previous data was deleted. I used sfdisk to get the size of the partition that fdisk created. Then, I created a loop which would continuously print garbage data until the amount printed was equal or greater than the size of the partition.

  3. How do I put binary data on the hard drive to confuse a hacker?

    I created random binary data using the random function and "chr" function in Perl. Then, I encrypted the random data using the Perl Blowfish module. If someone manages to decrypt the data, it will still look like garbage and confuse them. I wanted to encrypt the data so that it didn't look purely random in a mathematical sense.

  4. How do I reformat the big partition?

    This was easy. I just used a simple "mkfs" command.

The Perl Script

The version of Perl I was using for this Perl script was out of date. I was using Perl 5.005_03 and I believe Perl is up to 5.6 as of 1/2001.

There are a lot of things I need to enhance to make this script more user friendly. There should be a lot more error checking, considering how dangerous this script is, and prompts to ask a user if they really want to do so stuff. I am waiting until I restart my MILAS project (which will be written in Python) before I make this script better. It was only to get me through moving from Columbus to the Bay Area.

I have commented a lot of the code, so hopefully a novice Perl programmer can understand most of what I am trying to do. (Text version of this listing)


##### Things to do
# 1. Make sure we create a brand new directory for temporary mounting
#     in order to avoid security risks in case someone is logged in.
# 2. Use perl functions to handle a lot of the system calls. 
# 3. Let it autodetect hard drives, and floppy drives, and only perform
#    actions on unmounted hard drives and floppy drives.

use strict;
use Expect;
use Crypt::Blowfish;

my $Junk;
  ### Set the drive to the slave drive on the Primary IDE controller.
my $Drive = "hdb";

  ### Let us do a lot of random stuff, and get the last line from the
  ### /etc/passwd file to make it really random, assuming one person
  ### has been added to the computer. 
my $time = time();
my $Ran = rand($time);
my $Ran = rand(10000000000000);
my $LastLine = `tail -n 1 /etc/passwd`; chomp $LastLine;
$LastLine = substr ($LastLine,0,30);
my $Blowfish_Key = $LastLine . $Ran . $time;
$Blowfish_Key = substr ($Blowfish_Key,0,20);
while (length ($Blowfish_Key) < 56) 
  $Blowfish_Key .= $Ran = rand($time);
$Blowfish_Key = substr ($Blowfish_Key,0,56);

  ### Done making up random key, now create Blowfish Encryption object.
my $Blowfish_Cipher = new Crypt::Blowfish $Blowfish_Key;

system "clear";
print "This will wipe out the hard drive on Drive /dev/$Drive\n";
print "Press enter to continue\n";
my $R = <STDIN>;

  ### Get the list of mounted partitions on the drive we want to wipe out
my @Mounted = `df`;
@Mounted = grep($_ =~ /\/dev\/hdb/, @Mounted);
  ### Foreach mounted partition, umount it
foreach my $Mount (@Mounted)
  my ($Partition,$Junk) = split(/\s+/, $Mount,2);
  print "Unmounting $Partition\n";
  my $Result = system ("umount $Partition");
  if ($Result > 0) 
    print "ERROR, unable to umount $Partition, aborting Script, Error = $Result\n";

  ### Start the expect script, which will simulate someone doing this
  ### commands manually.
my $Fdisk = Expect->spawn("/sbin/fdisk /dev/$Drive");

  ### Get a list of mounted partitions by printing the partition table 
print $Fdisk "p\n";
my $match=$Fdisk->expect(30,"Device Boot    Start");

my $Temp = $Fdisk->exp_after();
my @Temp = split(/\n/, $Temp);
  ## Get the lines that tell us about the partitions
my @Partitions = grep($_ =~ /^\/dev\//, @Temp);

  ## Foreach line, delete the partition
foreach my $Line (reverse @Partitions)
    ## Get the /dev/hdb part, and its number
  my ($Part,$Junk) = split(/[\t ]/, $Line,2);
  my $No = $Part;
  $No =~ s/^\/dev\/$Drive//;

  print "Deleting no $Drive $No\n";     

    ## Delete command
  print $Fdisk "d\n";    
  $match=$Fdisk->expect(30,"Partition number");
    ## Which partition number to delete
  print $Fdisk "$No\n";
  $match=$Fdisk->expect(30,"Command (m for help):");


  ### If we had partitions, write changes, or otherwise, just end it
if (@Partitions < 1) {print $Fdisk "q\n"; $Fdisk->expect(2,":");}
  print $Fdisk "w\n";
  $Fdisk->expect(30,"Command (m for help):");

  ## Get the geometry of the hard drive
my $Geometry = `/sbin/sfdisk -g /dev/$Drive`;
my ($Junk, $Cyl, $Junk2, $Head, $Junk3, $Sector,@Junk) = split(/\s+/,$Geometry);
if ($Cyl < 1) 
   {print "ERROR: Unable to figure out cylinders for drive. aborting\n"; exit;}

  ### Create a new expect script to simulate a person using fdisk
my $Fdisk = Expect->spawn("/sbin/fdisk /dev/$Drive");

   #### Tell fdisk to create new partition
print $Fdisk "n\n";

  ### Tell it the new partition should be a primary partition
print $Fdisk "p\n";

  ### Which partition, number 1
print $Fdisk "1\n";

  ### Start at cylinder 1
print $Fdisk "1\n";

  ### Go to the end
print $Fdisk "$Cyl\n";

  ### Write and save
print $Fdisk "w\n"; 
$Fdisk->expect(30,"Command (m for help):");

### Format the partition and mount it

my $Partition = "/dev/$Drive" . "1";
my $Result = system ("mkfs -t ext2 $Partition");
if ($Result > 0) {print "Error making partition, aborting.\n"; exit;}

   ### There should be better error checking here
system "umount /tmp/WIPE_IT";
system "rm -rf /tmp/WIPE_IT";
system "mkdir -p /tmp/WIPE_IT";
system "chmod 700 /tmp/WIPE_IT";

  ## See if we can mount the new partition.
my $Result = system ("mount $Partition /tmp/WIPE_IT");
if ($Result > 0) {print "Error mounting drive, aborting.\n"; exit;}
system "chmod 700 /tmp/WIPE_IT";

### Now create the file and stop when we hit the size.

my $Count = 0;
my $Written_Size = 0;  

  ### Open up a new file.
   ### If someone actually wants to screw around with your hard drive, 
   ### let us play with them and waste their time by adding a teaser. 
my $Ran = rand 259200000;   # between now and ten years ago (approx)
($Ran, $Junk) = split(/\./, $Ran, 2);
   ## New date minus random number of seconds
my $Date = `date --date '-$Ran seconds'`;

print FILE "DATE CREATED $Date\n";
my $Ran = rand 50;
($Ran, $Junk) = split(/\./, $Ran, 2);
$Ran = $Ran + 10;
print FILE "This document is extremely secure. It is a violation to let 
any unauthorized persons read it. Known password holders need to 
apply Method $Ran in order to decrypt binary data.\n"; 

  ### Create random number plus 25000
my $Ran = rand 25000;
($Ran, $Junk) = split(/\./, $Ran, 2);
$Ran = $Ran + 25000;

  ### Create an array of numbers which we will use most of the time.
my @Blank =  (1..$Ran);
  ### Take the array and make into a string.
my $Blank = "@Blank";
  ### Empty the array to free up memory.
@Blank = ();
my $B_Length = length $Blank;

  ### Let us get the amount of real space we have for the partition
my @Temp = `df`;
@Temp = grep($_ =~ /^$Partition/, @Temp);
my $Line = $Temp[0];
my ($Junk,$Blocks,@Junk) = split(/\s+/, $Line,4);
  ### We are assuming 1k blocks. 
my $Size = $Blocks*1000;

  ## While the file we have written is less than the size of the
  ## partition, print some more data. 
while ($Written_Size < $Size)

        ### 9 out of ten times, we just want to print blank spaces to hurry
        ### up printing. One out of ten times, print garbage binary.
     my $Ran = rand (10);
     if ($Ran > 1) 
       print FILE $Blank; 
       $Written_Size = $Written_Size + $B_Length; 
         ## This part makes a long string (upto 10000 bytes) of random data. 
       my $Garbage = "";
       my $Length = rand(10000);
       ($Length, $Junk) = split(/\./, $Length, 2);
       for (my $i = 0; $i < $Length; $i++)
         my $Ran = rand 256;
         ($Ran, $Junk) = split(/\./, $Ran, 2);
         $Garbage .= chr $Ran;
         ## This parts encrypts the random data 8 bytes at a time. 
       my $Temp = $Garbage;
       my $Encrypted = "";
       while (length $Temp > 0)  
         while (length $Temp < 8) {$Temp .= "\t";}
         my $Temp2 = $Blowfish_Cipher->encrypt(substr($Temp,0,8));
         $Encrypted .= $Temp2; 
         if (length $Temp > 8) {$Temp = substr($Temp,8);} else {$Temp = "";}

         ### Print the encrypted random data to file. 
       print FILE $Encrypted;
       $Length = length $Encrypted;

       $Written_Size = $Written_Size + $Length;
       my $Rest = $Size - $Written_Size;
       print "$Size - $Written_Size = $Rest to go\n";

   ### At every 500 prints, start saving to a new file.  
  if ($Count =~ /500$/) 
    close FILE;

close FILE;

my $Result = system ("umount $Partition");
if ($Result > 0) {print "Error unmounting partition $Partition, aborting.\n"; exit; }

  ### Let us reformat the partition. Doesn't delete data, just removes it
  ### from the directory. 
my $Result = system ("mkfs -t ext2 $Partition");
if ($Result > 0) {print "Error making partition, aborting.\n"; exit;}


Using Expect was not necessary (other programs could have solved the simple problems I had). Using Blowfish was not necessary. As a matter of fact, the whole darn script was way too long if you just wanted to wipe a hard drive and fill it with blanks. However, I wanted to use fdisk because I always want to use fdisk, Expect is such a powerful tool, it is good to let people see how it works, and putting random garbage encrypted binary data in to confuse a hacker is just an extra touch.

I don't understand the complete complexity of hard drives, so I am not sure if there are residual data left on the hard drive. For my purposes, and my level of security, it does exactly what I need. As I develop MILAS more, I am sure there will be tighter checks and enhancements to delete all data off of a hard drive.

I tend to look forward in time trying to anticipate things which might be needed in the future, which always causes a programmer to work more than is required for the project at hand. However, the mood struck me, and I like the direction the script is going, and so, it doesn't bother me to write up this article on an airplane flight. Making something cool doesn't wear me out, unlike having to do work for someone else, which is real work.


  1. website.
  2. Expect Perl Module
  3. Blowfish Perl Module
  4. Original site for this article. - (any updates will be placed here)
[You can also use /dev/random or /dev/urandom to overwrite a disk. See
The Answer Gang: "Classified Disk - Low-level Format" in issue 60. But it doesn't do encryption. -Mike.]

Copyright © 2001, Mark Nielsen.
Copying license
Published in Issue 63 of Linux Gazette, Mid-February (EXTRA) 2001

[ Prev ][ Table of Contents ][ Front Page ][ Talkback ][ FAQ ][ Next ]