MavEtJu's Distorted View of the World - 2004-05

Posted on 2004-05-22 11:53:25, modified on 2006-01-09 16:29:21
Tags: Rant

Today somebody tried to convince me that the Imperial system for measuring sizes was much better "because you could easily divide distances by 3". His example was cutting a wooden plank in three equal size pieces. Read on to see where his ideas are flawed.

Dividing objects in equal parts only works if the objects are properly pre-measured. For example I can split 100 marbles into 10 groups, because 100 divided by 10 doesn't have any leftovers.

For the same reason I can split 2 wooden planks into 2 groups, each of one plank.

But then... splitting one object is more difficult. For example, cutting an apple (pretty solid material) in half leaves you with two more-or-less half-apple pieces. And some leftover apple-juice on your knife. So you actually splitted it in three pieces: two nearly-half-apple-pieces and an some-juice-piece.

Now back to the plank. You measure it up, it's one yard. Since one yard is 36 inches, dividing it into three pieces would make every piece 12 inches. Take a measure tape, mark the plank at 12 inch and at 24 inch. And start sawing, trying to stick to the marks you've set as good as possible.

At the end, you end up with: one piece a little bit smaller than 12 inch, one piece two little bits smaller than 12 inch, another piece a little bit smaller than 12 inch and some saw dust.

In the metric system, it's the same idea. You get a plank of one meter, mark it somewhere between 32.25 and 33.5 centimeters and somewhere between 66.5 and 66.75 centimeters and start sawing, also here trying to stick to the marks you've set as good as possible.

At the end, you end up with: one piece about 33.3 centimeters, one piece just not yet 33.3 centimeters, another piece about 33.3 centimeters and some saw dust.

Moral of the story: either with the Imperial or the Metric system, you don't end up with three equal pieces because a. the mark you've set is always too wide to be exactly on 1/3rd of the size and b. you're losing material due to the sawing.

In areas where you do need to be as precise as you can be, people don't use normal rulers anymore but use the more precise caliper (dutch: schuifmaat) and they don't use a plank of one meter or yard to start with but one which is just a little bit bigger so they take the loss of sawing into account.

And remember, if a plank is sawn and is just a little bit too big, you can easily shave it a little bit off with a plane (dutch: houtschaaf).

Posted on 2004-05-14 09:54:30, modified on 2006-01-09 16:29:22
Tags: Happiness

What if everybody had an RFID payment card implemented in their right hand palm. Imagine sitting at outside at a pub, the waiter puts down your drinks and says "that's is 2 dollars". All you need to do is wave your hand in front of his machine and say with your best Jedi voice "These drinks have been paid for.". Waiter will check his machine and go "You are right, these drinks have been paid for!".

Now where is my lightsaber and my Jedi cloak?

Posted on 2004-05-02 00:03:20, modified on 2006-01-09 16:29:22
Tags: Networking, SSL

We at BarNet purchased a wildcard certificate from FreeSSL to secure our web transactions, to authenticate our IMAP and POP servers and to support TLS for the SMTP sessions.

TLS (Transport Layer Security) for SMTP consists of two parts

• The server side for receiving email, where the identity of your server can be verified:
  Verified: subject_CN=*.barnet.com.au, issuer=ChainedSSL CA
  
• A client side for sending of email, where your server identifies itself at the other MTA:
  Verified: subject_CN=edwin.adsl.barnet.com.au, issuer=BarNet Root Certificate Authority
  

The above example works because the certificate my MTA presents is an internal client certificate. But the official certificate which we purchased is only valid for server usage, not for client usage.

   Issuer: C=US, O=FreeSSL, CN=ChainedSSL CA
    Subject: C=AU, O=*.barnet.com.au,
        OU=https://services.choicepoint.net/get.jsp?430367485,
        OU=See www.freessl.com/cps (c)04,
        OU=Domain Control Validated,
        CN=*.barnet.com.au
    X509v3 extensions:
       Netscape Cert Type: 
            SSL Server

Note there that it only says "SSL Server" and not "SSL Client".

With the result that we see this in our logfiles:

May  1 00:01:03 mag postfix-dbmail/smtpd[54595]: verify error:num=26:unsupported certificate purpose
May  1 00:01:03 mag postfix-dbmail/smtpd[54595]: Unverified: subject_CN=*.barnet.com.au, issuer=ChainedSSL CA

Oh well, this just means that we for outgoing sessions are using our self-signed certficates again. Moral of the story: make sure you know what you get.