Regarding books and ciphers

Nov 8, 2011 - 01 a.m.

I've been trying to write about something (anything really) at least once a week. Last week I failed at this for a variety of reasons, and the week before I wrote a draft of a recipe post for my (and my girlfriend's) other blog, Usversusdinner.com, which is about our cooking adventures. I'll post that once I get it edited and put some pictures into it. This week, I'm getting right back on the horse with this post.

I'm a big fan of Penny Arcade, and about a month ago, the news post was written by a game developer who also happened to be an expert in cryptography. In the post, she included two blocks of numbers: "28-1 10-2 10-3 6-4 18-4 43-1 36-3 9-2 43-4 32-9 41-2 1-1 2-1 45-1 42-3 33-2 3-4" and "42-4 14-2 26-1 32-10 35-3 54-3 56-1 3-3".

I was curious about these, so I did a bit of poking around and discovered that they were the cipher text produced by a type of Book Cipher. You, much like myself before this exercise, may not be familiar with a book cipher. Simply put, a book cipher relies on an existing piece of text, usually a book (go figure), as a key for decoding a message. The coding of the message often isn't complex - usually numbers refer to words in the key text. The obscurity of the cipher comes more from knowing what the key text is, less from the complexity of the code.

Every news post on Penny Arcade accompanies a given comic, so it isn't terribly difficult to guess what the key text for this particular cipher text is (it is this comic).

In this instance, the way the message is coded is "word number-word letter". I was curious what the message was, so I started to counting through the words and then counting through the letters. As it turns out, this is an incredibly dull, slow, and completely unrewarding process. In the past, this was no doubt was a "security feature" of this type of cipher.

Fortunately for me, we live in a world filled with computers, and computers are great at this type of thing. I whipped up a little program in python that de-crypted the text in almost no time. The longest part of the coding session may have been transcribing the text of the comic.

base_cipher = "28-1 10-2 10-3 6-4 18-4 43-1 36-3 9-2 43-4 32-9 41-2 1-1 2-1 45-1 42-3 33-2 3-4 42-4 14-2 26-1 32-10 35-3 54-3 56-1 3-3"
key_text = "Sir I need you to turn off your my what you want me to turn off my book oh you'd like that I bet you'd love to turn off all books wouldn'tcha yeah just \
like hitler uh oh too far you're gonna shock my genitals huh the genital region I think we might start there yeah"

code = base_cipher.split(' ')
key = key_text.split(' ')

for i in code:
   parts = i.split('-')
   index = int(parts[0]) - 1
   print key[index][int(parts[1]) - 1]

It's quite simple thanks to the way that python keys strings just like arrays. I've considered trying to re-write it as a one liner, but it didn't seem like a good use of time.

I split both the cipher text and the key up by spaces, and then cycle through each word-letter pair of the cipher. I split those by the dash, and then use the word part of the pair to grab the right word, and the letter part of the pair to grab the write letter from that word. There is also the subtract by one to compensate for the cipher using an index that starts with one and python using an index that starts with zero. Easy stuff.

You can grab the code if you like from my bitbucket account. I won't say what the message is, but it actually isn't very interesting. No worry though, as I had a great time finding it. I've found that with most worthwhile pursuits, the fun isn't in the destination as much as it is in the journey. Enjoy!