Patrick’s development blog

Delete data from hard drive securely

Posted in Computer Science, Security by Patrick on May 10, 2011

It’s common for people to say that it’s necessary to overwrite the hard drive many times before the data becomes unrecoverable. This is however just a myth. The probability of recovering only a single byte is less than 1%, so it’s very unlikely that someone can restore something much bigger like 100 MB.

In other words, overwriting the hard drive many times is just a waste of time. Writing 0:s on the whole hard drive is sufficient and can be done with tools like dd.

More information:


Encrypt chat conversations in Pidgin using pidgin-otr

Posted in Articles, Security by Patrick on March 17, 2009

Pidgin is an excellent “chat client” or instant messaging client. I’ve even replaced the MSN client on my Windows system with Pidgin. It’s open source and has support for many different chat networks like MSN, ICQ…

There’s a plugin called pidgin-otr (Off-the-Record messaging) which allows you to encrypt your conversations (assuming the other part also has pidgin-otr installed). Regretfully, people never seems to care about encryption even if they seem to get close to crazy if someone invades their privacy, quite the paradox… Well that’s just another story which i’m not going to post here, as i’m just spreading the word about everything that’s good : )

The Off-the-Record messaging plugin uses public and private keys. It’s very easy to use. Just download the plugin, activate it in the Pidgin add-on menu and generate a key. In the conversation window, a button will appear that makes it easy to toggle encryption on/off.

Download Pidgin from:
Download Pidgin-otr from:

Encrypting mail in Thunderbird using GnuPG and Enigmail

Posted in Articles, Security by Patrick on March 17, 2009

Thunderbird is a mail user agent developed by Mozilla. GnuPG is an encryption program (free software) that uses the standard OpenPGP. This standard is based on encryption using a private and public key. The private key is used to decrypt the data while the public key is used to encrypt the data.

The Thunderbird add-on Enigmail, provides an “back-end” interface to GnuPG so the user can use Thunderbird to encrypt/decrypt mail. After installing Enigmail, generate a keypair. This will create a public and private key for the current account. The public key is meant to be distributed so other people can send mail encrypted to you. The private key however, is important NOT to distribute. Since it is used to encrypt the messages sent to you with your public key. The public key is usually uploaded to a keyserver.

It’s possible to search for public keys on the keyservers and add public keys into a local list and configure Thunderbird to encrypt all messages by default (supposing the public key to the person in question is added into your key list). Both Thunderbird, GnuPG and Enigmail, are very useful indeed : )

For more information about GnuPG and Enigmail:

become anonymous on the internet using Tor

Posted in Articles, Security by Patrick on July 23, 2008

Tor is a platform independent program that protects you from traffic analysis. Traffic analysis is a form of surveillance of your network traffic which is a threat to your personal integrity. Tor is usually used to surf anonymously, but can also be used with for example instant-messaging applications.

How Tor works
When using Tor, your communication with the internet is protected by distributing it trough a network of different relays trough the world which are run by volunteers. The communication is also encrypted so no one can see what you’re doing or learn your location.

I recommend using the Firefox add-on Torbutton which provides an easy way to disable/enable Tor in Firefox. This way, you don’t have to configure your browser either.

Pitfalls with Tor
Watch out for cookies, flash files, java applets and similiar web applications. They can reveal your IP-address even if you use Tor. Using Add-ons for Mozilla Firefox like No Script and Flash Block can prevent this. Be sure that you don’t fall for things like this.

Even if Tor encrypts your traffic inside the Tor-network and makes you anonymous. The last relay you are connected to, which is directly communication with the webserver can still see your traffic. Don’t use Tor if you do things that can expose your identity, for example logging in to your webmail (if you truly want to be anonymous that is). Using an encrypted protocol like HTTPS prevents this though.

Read more about potential pitfalls here:

To sum it up
If you want to surf anonymously, this is a very useful program as it also hides your IP-address. But remember that web apps like Flash can still expose your IP. A lot of people believes that installing Tor will automagically make them anonymous. That’s wrong however. You have to configure the application correctly you want to use with Tor. If you’re using Firefox, the Torbutton add-on makes this automatically.

A good idea might be to install a portable browser which you use only when you want to be anonymous. Configure this browser so it doesn’t accept cookies and install a flash blocker, block java and other “media” that could expose your IP-address. It’s recommended to only use services like webmail if the site uses SSL or another secure connection.

There is a portable version of Firefox called Firefoxportable.
Tor’s official website:

Useful free software to optimize and secure your system

Posted in Security by Patrick on June 28, 2008

Here’s a list of useful programs in different categories. Most of these programs are for Windows, but some of them are also open source and works for Linux. I only list programs that are free. I might update this post over time.


Mozilla Firefox
The web-browser Firefox, has optional add-ons that you can install which makes it possible to extends the browsers functionality. I’ve picked out three add-ons that I find very useful. They speed up the browser and adds more security.

It blocks all Javascript, Java and other malicious code from sites you don’t trust. Ever since I installed this, i’ve never had any spywares on my XP computer at all. A most have for people who wants a secure browser. My favorite add-on.

AdBlock Plus
Get rid of all those annoying banners and ads on websites.

Flash Block
Blocks all flash animations on all websites. If you want to play a certain flash animation though, just click on it, that’s what I like most about this add-on.


Zonealarm Firewall – A free version of the popular firewall Zonealarm. This software also has program control and port stealthing for example.

AVG Anti-Virus Free Edition – Free anti-virus program, that is easy to use, provides high level of detection capability and doesn’t use so much system resources.

Ad-Aware 2008 – Scan for spywares, data-mining, advertising, and tracking components in general.

Spybot Search & Destroy – Spyware cleaner that also has real time protection.

TrueCrypt – Open Source encryption software. Can encrypt partitions, make virtual drives and encrypt harddrives completely.


Optimize and cleaning software
JkDefrag – Free disk defragmenter program. Optimizes your harddrive.

CCleaner – System optimization tool which cleans registry and removes temporary files. It’s very fast.

secure hashes in PHP using salt

Posted in Articles, Security by Patrick on February 12, 2008

This tutorial shows the principle of using a salt in order to secure your password hashes. It’s written with my scripting-language of choice which is PHP, but the principle is the same with whatever server-language you might be using. Before going on, i’ll explain some facts that might be good to know before reading the article.

Brute force
Brute force is a comparison technique which goes trough all possible characters and in this case runs trough an algorithm to compare with the hashed password. To put it simply, it compares all different characters until it finds the right password.

One way encryption
The process of encrypting a string so it cannot be decrypted. These are also called hashing algorithms. The most used ones are md5 and sha1. The general difference between these two is that sha1 is generally stronger since it returns a 160-bit fingerprint while md5 is 128-bit. You might ask how we can make use of something that cannot be decrypted. It’s quite easy, we compare different hashes and do not decrypt them.

Whenever you store information about a user in your database. You most likely store the passwords using a one way encryption like md5 or sha1. If not, you really have to think over your security, big time. Still, even if you encrypt the passwords, they can still be cracked using brute force, rainbow tables or dictionary attacks. If we assume the passwords are very sloppy and insecure while the hashes have leaked to the cracker. Unfortunately it’s very uncommon that the users use a smart password, so it’s no use relying on the users.

This is where salt comes in. It simply adds a certain string into the password before hashing it. It’s not any harder to do than regular hashing.

The principle of using salt

// this is what it normally looks like when hashing a string using md5
$password = md5($password);

// using a salt, you instead concentate a string to the password before hashing it
$password = md5($salt.$password);

If we assume $password contains a very weak password, for example “abcdef”. Which would be very easy to brute force. The salt can make this password practically as strong as you want it. There are many methods out there and people constantly invent new ways to generate a salt. In my option, the salt doesn’t need to be that complicated though. The only way someone can brute force the password in this state, is by knowing which salt you use. If they’ve come that far, it wouldn’t matter in any case.

A salt can be as complicated or simple as you want it. Here is an example.

Creating a simple salt

$password = "banana";
$salt = "aB1cD2eF3G";
$password = md5($salt.$password);

This might not be the best and most used salt method out there, but it works. Suppose you use the password “banana” from the example. Then the generated hash would actually become “aB1cD2eF3Gbanana” which is a considerably stronger password than “banana”. This takes care of the user’s bad habit of using easy passwords with only lowercase characters. Just remember that you don’t have to use the salt in the beginning of the string, the principle is the same if you type md5($password.$salt). You can do however you want.

I have seen it’s very common to combine different encryption methods as well. This is in my option very effective. If we create a salt based on the password that uses sha1 and md5 together and then use md5 to concentate these strings like a normal salt. We would get a salt that contains a double encryption of two different encryption methods. The hash we get from this is then added to the password before hashing it once again.

$password = "banana"
$salt = sha1(md5($password));
$password = md5($password.$salt);

This would be equal to a password that is a hash itself + it’s regular password.


Brute forcing the password hash in this case would mean to brute force the sha1 generated string with the password”f8bbff024e7d04d98a349ccb0984ad85d8ba86fabanana”. In other words close to impossible unless you have over million years and a supercomputer at hand. Well, even if you had, you would only get another hash out of the current one, if you want it to be that way.

How to crack salted hashes
The only known way to crack salted hashes is to know the salt algorithm. If the cracker knows this, it’s possible to brute force it by adding the necessary data before starting the brute force process. It’s hard to explain in words so i’ll give another example.

// the regular salt process, this is the “algorithm”
$salt = md5($password);
$password = md5($salt.$password);

// we suppose $test_string is the string it tests
// since this is brute force it’ll be various different strings
// “a”, “b”, “c”, “d” .. “aa”, “ab” … etc

// if md5(md5(“abc”). “abc”) is equal to
// md5(md5($password). $password) then
// “abc” is the password we are searching for

if (md5(md5($test_string). $test_string) == md5($salt.$password))
echo $test_string; // this is the password

To put it simply, if the cracker knows what salt algorithm is used. We will be back to step one and the salted hash is just as vulnerable as the regular md5 hash that isn’t salted. It’s very unlikely that the cracker do know though, i’m just pointing out the flaws.

This is how webmasters can protect their users from evil crackers or annoying skiddies that have come across a cracking program. Not many webmasters secure their website and use salted passwords though, some even leave them in clear text in the database. So i’ll let my next article be about how to create strong passwords so the users themselves can protect their password/account better. Strong passwords should be an obvious thing, but it sadly isn’t.