Category Archives: Computing

Color blindness and accessibility

color blindness testIn an effort to meet accessibility requirements, I was looking for tools to check whether users with a variety of color blindness conditions can actually use our websites. Posters and brochures can already be difficult to read for color blind people. On web sites another dimension is added to this problem because certain features may be rendered useless by choosing a bad color palette. Colored links for example, if not underlined, may not be seen as links because they appear to be of the same color as all other text. Continue reading

Email Address Validation Tool

I have been doing do a lot of email marketing lately. Our email marketing service does a pretty good job at removing records with an invalid email address from the imported data set. However, some invalid addresses still slip through and cause unnecessary bounces and license costs. We therefore need to correct or remove all invalid addresses before we import them. Some can be easily corrected without involving the subscriber, like

someone @

Others can be removed from the subscriber list because they originate from fake subscriptions, like test@justtesting.

I was looking for an online tool to clean up an email address list but could only find tools that process one address at a time. So I read RFC 3696 to learn more about valid email address formats and wrote my own. The tool does email format checks for most of the RFC requirements. It also queries the email domain’s DNS server to look for MX and A records.

Enter email addresses into this box. Unless you have the admin password, no more than 100 will be processed at a time (20 if DNS checks are requested) and your IP address will be recorded, so spammers must not use this to validate their address lists :-). If you abuse the tool, your IP will be blocked automatically.

A future version of this tool will work with the name@[IP address] address format and also telnet into port 25 of the email server to see whether there is a live SMTP service running.

DNS tests on domains?

Admin password
(only needed if more than 100 addresses, or more than 20 with DNS test):

Continue reading

Free encryption software (3)
Hard disc encryption

Truecrypt[Update Nov. 11, 2014: The Truecrypt developers no longer support this product]

Since version 5, released on February 8, 2008, Truecrypt can encrypt an entire drive or partition, including the one that contains the operating system installation. Truecrypt’s implementation of strong drive encryption is particularly impressive. Listen to Security Now episode 133 for more information. Finally high quality free open source software is available for encrypting an entire hard disk. I immediately encrypted my Laptop’s hard disk. Continue reading

Free encryption software (2)
File encryption on USB flash drives

Free encryption softwareI am a PGP user since 1996 and I still use Ståle Schumacher’s international DOS version 2.63i to encrypt files on my USB flash drives. All I need is PGP’s small exectutable file (pgp.exe, 237.737 kb). On first use, PGP will create a second small file that contains some random seed data for the encryption. With this minimal setup, PGP will warn you about a missing configuration file, but this does not affect the strength of encryption. The USB drive on my key ring contains an encrypted passwords file and pgp.exe, providing easy access to the many cryptic passwords I use. All I need is one strong master pass phrase to decrypt the file and get instant access to more passwords than I could ever remember.

I insert the USB drive and open a command prompt window (Start > Run > cmd).

PGP file encryption:

pgp -c filename

PGP file decryption:

pgp filename.pgp

PGP 2.36i in action

In conventional -c encryption mode PGP uses a symmetric block encryption algorithm (IDEA) with a key size of 128 bits. Together with a good pass phrase this is really strong encryption.

Don’t forget to securely delete the plaintext file afterwards:

pgp -w filename

Since this version of PGP was developed for DOS, it only supports 8 character file names (8.3).

PGP 2.63i is still available from Ståle’s site  [download].

Back in 1996 I contributed the “self-sign FAQ” to the PGP community.

Other Flash drive encrytion options for Windows, Mac and Linux

Many different free and commercial PGP versions and clones are available. See,, and I still use PGP 2.63i because it is an easy-to-use lightweight program that provides strong encryption and can be put on any device by simply copying one small file.

Truecrypt in “Traveller Mode” can be used to create an encrypted file container on Flash memory. Truecrypt is free and provides super strong encryption, too. However, you need to have administrator privileges on all machines that you decrypt the container on. I’m not an admin on my machine at work but need access to the passwords on my keyring there, too.

Some USB drives (SanDisk, Kingston, IronKey) have hardware encryption built in, but when it comes to encryption, I prefer to stay away from proprietary implementations.

Free encryption software (1): Introduction

Free encryption software (3): Hard disc encryption

Free encryption software (4): GNU Privacy Guard

My first Laptop

My first PCYou’ve probably met some wise computer veterans who constantly tell you that they started computing decades ago. And usually they talk about their first Atari or Commmodore 64, and how they wrote cool Assembler code to get it to do what they wanted it to do. I can’t impress anyone with my first 80286 desktop PC, but take a look at my first Laptop… Continue reading