Kids Code & Computer Science

Hide information in plain sight using a clever code and a good book.

Are you a bit of a bookworm? Do you have any friends who also enjoy reading? If you noticed that you both own the same books, you’ve already got the basics for making a book cipher! Book ciphers are special codes which are created by referencing the words and/or letters in it to make your own sentences. People reading your code will only see numbers, but you and your code recipient will know the truth!

There are a few different ways to make a book cipher, but in order for it to work, you and your recipient should have the exact same book. Make sure that both books are the same edition, as different editions may have different page numbers and word placement, which is very important for the book cipher to work!

Word Ciphers

One of the simpler variants of the book cipher is a code that points to individual words within the book. Let’s say you want to send the word “cat” You take the book you’re using for your cipher, and find an instance of the word “cat” in it. Let’s say you find the word “cat” on page 64, line 5, the 8th word on the line. To represent the word “cat” you write it as (64, 5, 8), which tells your recipient to go to that page, line, and word.

Letter Ciphers

Another way to do a book cipher is to use the starting letter of words in the book to represent each letter in your secret message. To start, you and your recipient decide on a page to use as reference; perhaps you can write the page number at the top of your code so your reader doesn’t get lost.

Then, you find a word that starts with the letter you want to use, and write down what number word it is on the page. For example, if you want the letter “E”, and the word “Elephant” is the 12th letter on the page, you write the number 12. You may want to separate words with a comma to tell your recipient where to put spaces in their letters.

If you want to make your code extra strong, you can use different words that begin with the same letter in your code. For example, while “Elephant” is the 12th word, you may also find “Eagle” as the 26th letter, and can use either to represent an E. The benefit of this is that it makes it harder to crack; if you simply use the same number for each letter every time, it becomes a simple substitution code (which we’ve covered before!) and is very easy to crack.

Time for you to try it! Find out what the message is in this small passage using the numbers provided.

1 9, 7 12 6 20 29

Harold could not believe his eyes. The man in front of him, who Harold thought was the villain, was really his own father all along. He could not even imagine why he would join the evil side.

Book ciphers are a lot of fun, but they’ve played an important role in history as well! Book ciphers were used during the American Revolution to help encrypt messages. Two spies called André and Arnold used a copy of William Blackstone’s “Commentaries on the Laws of England” to perform the word cipher and send each other secret messages. When you try the book cipher for yourself, you can feel good knowing you’re using a code that even spies once used in history!

Answer

Answer to puzzle: HI THERE

Learn More

Beale Cipher

https://en.wikibooks.org/wiki/Cryptography/Beale_cipher

Ciphers from the American Revolution

http://www.ouramericanrevolution.org/index.cfm/page/view/prq0003

The Book Cipher Algorithm

http://www.drdobbs.com/security/the-book-cipher-algorithm/210603676