Boy thinking with coloured light and code background

Operation Decode – Lockdown Edition

Mission Five: Book Ciphers

Like the grille cipher, book ciphers are a way of passing on a secret message using a book or other text. Instead of using a grille, book ciphers work by using a series of numbers to refer to the location of specific words of characters in a book. When combined, these words/characters form the text of the secret message.

Although referred to as book ciphers, theoretically any body of text can be used as source material for encrypting a message in this way. As long as the source material contains text, a consistent layout and is available to both the sender and receiver; almost anything will work!

Activity 5

Book Cipher 1 – Words

In this activity, the aim is to find the secret code by using the number coordinates as show below using the format (line number, word number). Start by finding the line number of the first word and then count each along the line separating the words by the spaces in the line, ignore any numbers or symbols, you only want to count words.


(5,1) (5,6) (5,8) (1,4) (4,3)

Check your answer below, make sure to separate each word with a space.

Book Cipher 2 – Letters

This one is a little bit more difficult this time, as you are now looking for letters instead of words. To do this we will use the coordinate format as follows: (line, word number, letter number). Hint: each letter will create a single code word.


(7,1,1) (2,6,5) (13,5,1) (8,8,3) (1,1,5) (11,5,6)

Check your answer below.

Strengths and Weaknesses


  • Key can be sent separately to the source text, and reveals nothing about the message if intercepted
  • Can be used with any type of text, from books, to web pages, and even advertisements
  • Using coordinates means that letters and words from the source text can be reused at different points in the message. This means that there is no limit to the length of message that can be sent, even if the source text is very short.


  • Reliant on sender and recipient having access to exactly the same source text. If the source texts are even slightly different, the coordinates may not match.
  • Unless it is disguised in some way, a book cipher key is easily recognisable. Even though it may not reveal the content of the message, it can still draw unwanted attention if intercepted