Skip to main content


Showing posts from June, 2011

Density of expanded polystyrene and expanded polyurethane foam

I recently posted about an experiment with expanded polyurethane foam and I was asked in a forum about the relative density of it and the expanded polystyrene I had previously used.

1. GAGA-1 Expanded Polystyrene Container

Weight: 213g
Outside dimensions: 245mm cube
Inside dimensions: 150mm cube

Thus, volume is 0.01471 m^3 - 0.003375 m^3 = 0.011335 m^3

Which gives a density of 18,791.4 g/m^3

2. My experimental capsule using expanding polyurethane foam

Weight: 25g
Diameter: 90mm
Height: 200mm
Internal space: 100 mm x 65 mm x 35 mm

Thus, volume is 0.001272 m^3 - 2.275×10^-4 m^3 = 0.001045 m^3

Which gives a density of 23,923 g/m^3.

Thus the expanding polyurethane foam is about 25% heavier than expanded polystyrene for the same volume.

Various sources give the R-value give an approximate 2x increase in R-value of polyurethane over polystyrene. Thus for the same thickness it would be expected that the polyurethane is a better insulator at a slightly greater weight. And the insulation i…

Experiments with polyurethane foam for injection molding a HAB capsule

Back when I worked on GAGA-1 I used a polystyrene container for the capsule. This worked well but it was quite large and polystyrene is annoying to cut.

So, I've been experimenting with expanding polyurethane foam to injection mold a HAB capsule.

My first experiment was to work with a big block of foam that I sprayed onto a sheet of baking parchment. The foam doesn't stick to the parchment and it can be easily peeled off. The set foam can be easily cut with both a saw (a bit messy since small parts break off) or with a Stanley knife. The Stanley knife gave clean cuts. It was also possible to hollow out a space inside the foam where the electronics could be stored.

One test I did was to see how strong the cured foam is by hitting it with a hammer. Here you can see the hollowed out capsule as I hit it with a geological hammer. It seemed strong enough to me for a HAB landing.

I then tested various glues and paints on the surface. Here I've painted it with spray pa…

Making a 'Ponyo' or putt-putt boat

If you've seen the film Ponyo (or if you grew up in Asia) you may have seen a sort of toy boat powered by a very simple steam engine. This sort of boat is variously called a putt-putt or pop-pop.

They work by having a very simple boiler powered by a candle in which water is turned to steam. The steam forces water out of a pair of tubes creating a partial vacuum which sucks water back in. The distinctive pop sound comes from the movement of the thin metal boiler's walls.

I came across lovely plans for building such a boat from a drink can and a milk carton from Science Toy Maker. The explanation and videos are really clear and easy to follow and the boat worked first time. Here's a short video showing it under test in my sink:

The first step in the instructions is making the engine. It uses a drink can, two straws and some epoxy glue. If you follow the video instructions it's not hard to make. Here's the can with the lid cut off using scissors:

Then it's…

Mean and standard deviation of the distances between letters

The following table shows is computed by calculating the distance between occurrences of letters in Jane Austen's Pride and Prejudice (for example, the distance between e's in the sentence "Hello, my name is Jeremy" is 9 and 2; all punctuation and spaces are ignored).

LetterMeanStandard Deviatione7.76.3t11.510.0a12.911.1o13.312.0i14.212.3n14.212.5h16.014.2s16.315.6r16.514.4d24.122.2l25.025.9u35.634.5m36.535.1c39.239.2y42.342.3w43.941.9f44.643.9g52.850.4b58.955.6p63.568.3v94.492.0k165.1172.8j569.2638.9z582.8834.4x635.0669.6q859.4873.9

Importing an existing SSL key/certificate pair into a Java keystore

I'm writing this blog post in case anyone else has to Google that. In Java 6 keytool has been improved so that it now becomes possible to import an existing key and certificate (say one you generated outside of the Java world) into a keystore.

You need: Java 6 and openssl.

1. Suppose you have a certificate and key in PEM format. The key is named host.key and the certificate host.crt.

2. The first step is to convert them into a single PKCS12 file using the command: openssl pkcs12 -export -in host.crt -inkey host.key > host.p12. You will be asked for various passwords (the password to access the key (if set) and then the password for the PKCS12 file being created).

3. Then import the PKCS12 file into a keystore using the command: keytool -importkeystore -srckeystore host.p12 -destkeystore host.jks -srcstoretype pkcs12. You now have a keystore named host.jks containing the certificate/key you need.

For the sake of completeness here's the output of a full session I performe…

Identification of homophone sequences in the Zodiac 408 cipher

In my previous post I mentioned an algorithm that I thought would identify homophone sequences used in the Zodiac 408 cipher. I wrote a small test program. Here's the output (note that here I have not used Unicode characters but the encoding scheme from this site).

The output shows the cipher being split on successive symbols and then shows the subsequences found (and within which of the splits it was found). It's clear that lots of subsequences are found (validating that the Zodiac 408 uses simple sequential homophone sequences) and it finds the longer sequences used.

This technique looks pretty viable for this type of cipher. However, tests on the Zodiac 340 show that it isn't working there. Either The Zodiac switched from a sequential pattern for the homophones, or there's some scrambling (in the form of transposition).

I've marked in bold a few very likely sequences (long ones, or those that occur frequently). These are all actual sequences used by The Zo…

How The Zodiac enciphered the Zodiac 408 cipher

I was looking through the Zodiac Killer ciphers the other day and woke in the middle of the night wondering how The Zodiac actually enciphered the first message (the one that was decoded).

The message consists of 408 symbols; there are 54 different symbols used for the alphabet. So there are multiple symbols for each letter of the alphabet. The Zodiac used a homophonic cipher to disguise the most common letters of the alphabet by using multiple symbols for common letters. For example, for the letter E The Zodiac used seven different symbols.

I started to wonder how when The Zodiac was writing out the message he picked which symbol to use. And it occurred to me that he might have used a really simple system: cycling through the symbols for each letter in the same order.

A quick look at the cipher showed that it was likely that the simple scheme was used. Here I looked at the letter E and the letter N and discovered that a simple pattern was used for each.

Using a small program I wr…

My Email Canary

Despite the fact that I use really long passwords and two-factor authentication wherever it's available I still worry that someone might break into my online accounts. And, my greatest worry is my Google Mail account.

In fact, everyone should be worrying about their online email accounts because they are the Achilles' Heel of your online identity. So much information passes through your personal email that it's a gold-mine for a hacker. Just imagine what could be done with the information on your online email account. Think of all the password reminders and password reset messages: access to your email means that an attacker could likely access many other accounts you own.

So, for my email I built in a canary: a tempting looking email that's sitting in my inbox that's entirely fake and designed to tempt an attacker into clicking on it. Here's a shot of my inbox:

That starred email from "Barclays Private Banking" is entirely fake. If you click it…

Plan 28: June 2011 Update

It's been a while since I gave an update on Plan 28. I apologize for the long silence. The wheels of British bureaucracy turn slower than the gears of the Difference Engine No.2 being driven by a field mouse.

The current status is:

1. Doron and I continue to work on the project. We have assembled a team of trustees and the paperwork for the creation of an official "Plan 28" charity is in progress.

2. We have been working closely with The Science Museum and the Computer Conservation Society to ensure that the project runs smoothly and has the maximum support (in terms of expertise) once it kicks off.

I hope that by July (at the latest) I will be able to make concrete announcements about the project's progress and timeline, and to be able to start to receive donations.

In case you missed it, Wired Magazine published an article about the project called A £400,000 PC downgrade: Rebooting Babbage’s Analytical Engine.

How to break the New Scientist cipher puzzle (part 3)

New Scientist had a little cipher challenge with four ciphers that needed breaking. The hardest of which (at least for me) was part 3. It's a form of transposition cipher where the letters are simply juggled around.

Here's the cipher text:
E U I N R E C S T N C I H R L Y Y E O R Y N D P U P T A I P T S N I O N O U E I O D M N T H I B T N T H E T E S S R E E C W O E E C L F O H N R P F R E S R L I Q D I H C Y O N O U E F C U O F L I C S E R P D T H N L Y V O P E O E K T S T L V O T B E Y E…