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468 lines
18 KiB
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<H4 ALIGN="center">
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"Linux Gazette...<I>making Linux just a little more fun!</I>"
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<P> <HR> <P>
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<center>
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<H1><font color="maroon">Securely Erasing a Hard Drive with Perl</font></H1>
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<H4>By <a href="mailto:articles@gnujobs.com">Mark Nielsen</a></H4>
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<P> <HR> <P>
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<H3>Contents</H3>
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<ol>
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<li>
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<a href="#Introduction">Introduction</a></li>
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<li>
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<a href="#problems">What problems I have.</a></li>
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<li>
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<a href="#perl">The Perl Script</a></li>
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<li>
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<a href="#Conclusion">Conclusion</a></li>
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<li>
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<a href="#REF">References</a></li>
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</ol>
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<h3>
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<a NAME="Introduction"></a>Introduction</h3>
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When moving from Ohio to California, GNUJobs.com had some hard drives
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(along with other hardware and software)
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which were to be donated to
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<a href="http://www.colug.net">COLUG</a> (Central Ohio Linux Users Group).
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They needed to be deleted
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before they were donated.
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2 out of the 3 hard drives had bad sectors on them, and the third
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I ended up using as a hard drive for testing purposes, like creating
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this article, so I ended up not giving any away. Still, I will need to
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wipe a hard drive in the future, so I created this Perl script (which I
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will later convert to Python and make it have more options).
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<p>
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The goal of this Perl script is to just delete the hard drive at /dev/hdb
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(the slave drive on the Primary IDE controller) since I have a hard drive
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removeable kit there. I want it to delete all partitions, create one
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partition that takes up the whole hard drive, and then fill up the
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hard drive with garbage data (including some random encrypted data
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just to ruin a hacker's day trying to find out what the data is).
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<h3>
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<a NAME="problems"></a>The Problems</h3>
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Here is a list of problems I had and how I solved them:
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<ol>
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<li> <font size= +1>How do I get it to delete all the partitions?</font>
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<p>I remember researching many different options to alter partitions on
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a hard drive, and doing it manually yielded the best results. I had used
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a Perl Expect script to automate the fdisk program (fdisk partitions hard
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drives in Linux) in the past, and I decided to continue to do it
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that way. I believe
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there are better alternatives for the simple task of deleting all the
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partitions, like sfdisk and others, but if one solution covers all
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possibilities with 100% power and flexibility, I usually just stick to
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one way of doing things so that I don't have to remember too many things
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and if it ever gets more complicated, I don't have to learn anything new.
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<p>Thus, I used Expect code to simulate a user typing in the commands
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for fdisk. The Expect code deleted all the partitions and then
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it created one big partition.<p></li>
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<li><font size=+1>How do I fill up the hard drive with garbage data?</font>
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<p> Just deleting partitions isn't enough to delete the data. I want
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to overwrite all old data
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with garbage to make sure the previous data was deleted.
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I used sfdisk to get the size of the partition that fdisk created.
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Then, I created a loop which would continuously print garbage data
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until the amount printed was equal or greater than the size of the partition.
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<p></li>
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<li>
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<font size=+1> How do I put binary data on the hard
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drive to confuse a hacker?</font>
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<P> I created random binary data using the
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random function and "chr" function in Perl. Then, I encrypted the random
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data using the Perl Blowfish module. If someone manages to decrypt the
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data, it will still look like garbage and confuse them. I wanted to encrypt
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the data so that it didn't look purely random in a mathematical sense.
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<p> </li>
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<li> <font size=+1> How do I reformat the big partition?</font>
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<P> This was easy. I just used a simple "mkfs" command.
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</ol>
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<h3>
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<a NAME="perl"></a>The Perl Script</h3>
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The version of Perl I was using for this Perl script was out of date.
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I was using Perl 5.005_03 and I believe Perl is up to 5.6 as of 1/2001.
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<p>
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There are a lot of things I need to enhance to make this
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script more user friendly. There should be a lot more error checking,
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considering how dangerous this script is, and prompts to ask a user
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if they really want to do so stuff. I am waiting until I restart my MILAS
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project (which will be written in Python) before I make this script better.
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It was only to get me through moving from Columbus to the Bay Area.
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<p> I have commented a lot of the code, so hopefully a novice Perl
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programmer can understand most of what I am trying to do.
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<A HREF="misc/nielsen2/Wipe_It.pl.txt">(Text version of this listing)</A>
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<pre>
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#!/usr/bin/perl
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##### Things to do
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# 1. Make sure we create a brand new directory for temporary mounting
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# in order to avoid security risks in case someone is logged in.
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# 2. Use perl functions to handle a lot of the system calls.
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# 3. Let it autodetect hard drives, and floppy drives, and only perform
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# actions on unmounted hard drives and floppy drives.
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#####
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use strict;
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use Expect;
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use Crypt::Blowfish;
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#-----------------------------------------------
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my $Junk;
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### Set the drive to the slave drive on the Primary IDE controller.
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my $Drive = "hdb";
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### Let us do a lot of random stuff, and get the last line from the
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### /etc/passwd file to make it really random, assuming one person
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### has been added to the computer.
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my $time = time();
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my $Ran = rand($time);
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my $Ran = rand(10000000000000);
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my $LastLine = `tail -n 1 /etc/passwd`; chomp $LastLine;
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$LastLine = substr ($LastLine,0,30);
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my $Blowfish_Key = $LastLine . $Ran . $time;
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$Blowfish_Key = substr ($Blowfish_Key,0,20);
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while (length ($Blowfish_Key) < 56)
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{
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$Blowfish_Key .= $Ran = rand($time);
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}
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$Blowfish_Key = substr ($Blowfish_Key,0,56);
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### Done making up random key, now create Blowfish Encryption object.
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my $Blowfish_Cipher = new Crypt::Blowfish $Blowfish_Key;
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#------------------------------------
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system "clear";
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print "This will wipe out the hard drive on Drive /dev/$Drive\n";
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print "Press enter to continue\n";
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my $R = <STDIN>;
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### Get the list of mounted partitions on the drive we want to wipe out
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my @Mounted = `df`;
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@Mounted = grep($_ =~ /\/dev\/hdb/, @Mounted);
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### Foreach mounted partition, umount it
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foreach my $Mount (@Mounted)
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{
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my ($Partition,$Junk) = split(/\s+/, $Mount,2);
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print "Unmounting $Partition\n";
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my $Result = system ("umount $Partition");
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if ($Result > 0)
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{
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print "ERROR, unable to umount $Partition, aborting Script, Error = $Result\n";
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exit;
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}
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}
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### Start the expect script, which will simulate someone doing this
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### commands manually.
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my $Fdisk = Expect->spawn("/sbin/fdisk /dev/$Drive");
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### Get a list of mounted partitions by printing the partition table
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print $Fdisk "p\n";
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my $match=$Fdisk->expect(30,"Device Boot Start");
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my $Temp = $Fdisk->exp_after();
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my @Temp = split(/\n/, $Temp);
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## Get the lines that tell us about the partitions
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my @Partitions = grep($_ =~ /^\/dev\//, @Temp);
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## Foreach line, delete the partition
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foreach my $Line (reverse @Partitions)
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{
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## Get the /dev/hdb part, and its number
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my ($Part,$Junk) = split(/[\t ]/, $Line,2);
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my $No = $Part;
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$No =~ s/^\/dev\/$Drive//;
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print "Deleting no $Drive $No\n";
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## Delete command
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print $Fdisk "d\n";
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$match=$Fdisk->expect(30,"Partition number");
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## Which partition number to delete
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print $Fdisk "$No\n";
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$match=$Fdisk->expect(30,"Command (m for help):");
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}
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$Fdisk->clear_accum();
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### If we had partitions, write changes, or otherwise, just end it
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if (@Partitions < 1) {print $Fdisk "q\n"; $Fdisk->expect(2,":");}
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else
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{
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print $Fdisk "w\n";
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$Fdisk->expect(30,"Command (m for help):");
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}
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#-------------------------------
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## Get the geometry of the hard drive
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my $Geometry = `/sbin/sfdisk -g /dev/$Drive`;
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my ($Junk, $Cyl, $Junk2, $Head, $Junk3, $Sector,@Junk) = split(/\s+/,$Geometry);
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if ($Cyl < 1)
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{print "ERROR: Unable to figure out cylinders for drive. aborting\n"; exit;}
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### Create a new expect script to simulate a person using fdisk
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my $Fdisk = Expect->spawn("/sbin/fdisk /dev/$Drive");
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#### Tell fdisk to create new partition
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print $Fdisk "n\n";
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$Fdisk->expect(5,"primary");
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### Tell it the new partition should be a primary partition
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print $Fdisk "p\n";
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$Fdisk->expect(5,":");
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### Which partition, number 1
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print $Fdisk "1\n";
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$Fdisk->expect(5,":");
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### Start at cylinder 1
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print $Fdisk "1\n";
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$Fdisk->expect(5,":");
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### Go to the end
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print $Fdisk "$Cyl\n";
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$Fdisk->expect(5,":");
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### Write and save
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print $Fdisk "w\n";
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$Fdisk->expect(30,"Command (m for help):");
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#------------------------------------------
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### Format the partition and mount it
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my $Partition = "/dev/$Drive" . "1";
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my $Result = system ("mkfs -t ext2 $Partition");
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if ($Result > 0) {print "Error making partition, aborting.\n"; exit;}
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### There should be better error checking here
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system "umount /tmp/WIPE_IT";
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system "rm -rf /tmp/WIPE_IT";
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system "mkdir -p /tmp/WIPE_IT";
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system "chmod 700 /tmp/WIPE_IT";
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## See if we can mount the new partition.
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my $Result = system ("mount $Partition /tmp/WIPE_IT");
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if ($Result > 0) {print "Error mounting drive, aborting.\n"; exit;}
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system "chmod 700 /tmp/WIPE_IT";
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#--------------------------------
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### Now create the file and stop when we hit the size.
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my $Count = 0;
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my $Written_Size = 0;
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### Open up a new file.
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open(FILE,">>/tmp/WIPE_IT/Message.txt");
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### If someone actually wants to screw around with your hard drive,
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### let us play with them and waste their time by adding a teaser.
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my $Ran = rand 259200000; # between now and ten years ago (approx)
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($Ran, $Junk) = split(/\./, $Ran, 2);
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## New date minus random number of seconds
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my $Date = `date --date '-$Ran seconds'`;
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print FILE "DATE CREATED $Date\n";
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my $Ran = rand 50;
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($Ran, $Junk) = split(/\./, $Ran, 2);
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$Ran = $Ran + 10;
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print FILE "This document is extremely secure. It is a violation to let
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any unauthorized persons read it. Known password holders need to
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apply Method $Ran in order to decrypt binary data.\n";
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### Create random number plus 25000
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my $Ran = rand 25000;
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($Ran, $Junk) = split(/\./, $Ran, 2);
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$Ran = $Ran + 25000;
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### Create an array of numbers which we will use most of the time.
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my @Blank = (1..$Ran);
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### Take the array and make into a string.
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my $Blank = "@Blank";
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### Empty the array to free up memory.
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@Blank = ();
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my $B_Length = length $Blank;
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### Let us get the amount of real space we have for the partition
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my @Temp = `df`;
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@Temp = grep($_ =~ /^$Partition/, @Temp);
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my $Line = $Temp[0];
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my ($Junk,$Blocks,@Junk) = split(/\s+/, $Line,4);
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### We are assuming 1k blocks.
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my $Size = $Blocks*1000;
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## While the file we have written is less than the size of the
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## partition, print some more data.
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while ($Written_Size < $Size)
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{
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$Count++;
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### 9 out of ten times, we just want to print blank spaces to hurry
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### up printing. One out of ten times, print garbage binary.
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my $Ran = rand (10);
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if ($Ran > 1)
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{
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print FILE $Blank;
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$Written_Size = $Written_Size + $B_Length;
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}
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else
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{
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## This part makes a long string (upto 10000 bytes) of random data.
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my $Garbage = "";
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my $Length = rand(10000);
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($Length, $Junk) = split(/\./, $Length, 2);
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for (my $i = 0; $i < $Length; $i++)
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{
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my $Ran = rand 256;
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($Ran, $Junk) = split(/\./, $Ran, 2);
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$Garbage .= chr $Ran;
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}
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## This parts encrypts the random data 8 bytes at a time.
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my $Temp = $Garbage;
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my $Encrypted = "";
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while (length $Temp > 0)
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{
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while (length $Temp < 8) {$Temp .= "\t";}
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my $Temp2 = $Blowfish_Cipher->encrypt(substr($Temp,0,8));
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$Encrypted .= $Temp2;
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if (length $Temp > 8) {$Temp = substr($Temp,8);} else {$Temp = "";}
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}
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### Print the encrypted random data to file.
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print FILE $Encrypted;
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$Length = length $Encrypted;
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$Written_Size = $Written_Size + $Length;
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my $Rest = $Size - $Written_Size;
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print "$Size - $Written_Size = $Rest to go\n";
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}
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### At every 500 prints, start saving to a new file.
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if ($Count =~ /500$/)
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{
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close FILE;
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open(FILE,">>/tmp/WIPE_IT/$Count");
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}
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}
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close FILE;
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#----------------------------------------------------
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my $Result = system ("umount $Partition");
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if ($Result > 0) {print "Error unmounting partition $Partition, aborting.\n"; exit; }
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### Let us reformat the partition. Doesn't delete data, just removes it
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### from the directory.
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my $Result = system ("mkfs -t ext2 $Partition");
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if ($Result > 0) {print "Error making partition, aborting.\n"; exit;}
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</pre>
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<h3>
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<a NAME="Conclusion"></a>Conclusion</h3>
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Using Expect was not necessary (other programs could have solved
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the simple problems I had). Using Blowfish was not necessary. As a matter
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of fact, the whole darn script was way too long if you just wanted to
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wipe a hard drive and fill it with blanks. However, I wanted to use
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fdisk because I always want to use fdisk, Expect is such a powerful tool,
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it is good to let people see how it works, and putting random garbage
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encrypted binary data in to confuse a hacker is just an extra touch.
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<p>
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I don't understand the complete complexity of hard drives, so I am not
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sure if there are residual data left on the hard drive.
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For my purposes, and my level of security, it does exactly what
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I need. As I develop MILAS more, I am sure there will be tighter checks
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and enhancements to delete all data off of a hard drive.
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<p>
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I tend to look forward in time trying to anticipate things which might be
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needed in the future, which always causes a programmer to work more than is
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required for the project at hand. However, the mood struck me, and I like
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the direction the script is going, and so, it doesn't bother me to write
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up this article on an airplane flight. Making something cool doesn't
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wear me out, unlike having to do work for someone else, which is real work.
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<h3>
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<a NAME="REF"></a>References</h3>
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<ol>
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<li><a href="http://www.perl.com">Perl.com</a> website.
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<li><a href="http://www.perl.com/CPAN-local/modules/by-category/21_File_Handle_Input_Output/Expect/Expect-1.10.readme">Expect Perl Module</a></li>
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<li><a href="http://www.perl.com/CPAN-local/modules/by-category/14_Security_and_Encryption/Crypt/Crypt-Blowfish-2.06.readme">Blowfish Perl Module</a></li>
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<li><a href="http://www.gnujobs.com/Articles/14/Wipe_It.html">Original site for this article. - http://www.gnujobs.com/Articles/14/Wipe_It.html</a>
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(any updates will be placed here)</li>
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</ol>
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<BLOCKQUOTE><EM>
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[You can also use /dev/random or /dev/urandom to overwrite
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a disk. See<BR>
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<A HREF="../issue60/lg_answer60.html#tag/9">The Answer Gang: "Classified Disk - Low-level Format"</A>
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in issue 60. But it doesn't do encryption. -Mike.]
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</EM></BLOCKQUOTE>
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<!-- *** BEGIN copyright *** -->
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Copyright © 2001, Mark Nielsen.<BR>
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Copying license <A HREF="../copying.html">http://www.linuxgazette.com/copying.html</A><BR>
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Published in Issue 63 of <i>Linux Gazette</i>, Mid-February (EXTRA) 2001</H5>
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