How to Solve the Book Cipher

No key is given. Detective days hand you the ciphertext and nothing else, so the work is finding the key — or proving which cipher it is — before you can read a word of it.

Free tool Have one to decode right now? Use the Book Cipher tool →

How to recognise it

If the card says "key unknown" or names no method at all, it's a detective day. Look at what the text keeps: word lengths, letter frequencies, which characters appear.

Book Cipher Every number is a word in the opening line of Treasure Island, counting from 1. Take the first letter of each word.

The key text

Every daily book cipher counts through Treasure Island, the opening line of Robert Louis Stevenson’s novel, printed in full on the puzzle page:

Squire Trelawney, Dr. Livesey, and the rest of these gentlemen having asked me to write down the whole particulars about Treasure Island, from the beginning to the end, keeping nothing back but the bearings of the island, and that only because there is still treasure not yet lifted, I take up my pen in the year of grace 17— and go back to the time when my father kept the Admiral Benbow inn and the brown old seaman with the sabre cut first took up his lodging under our roof.

A worked example

No key was given, so the first job was finding one. Word lengths and letter patterns were the evidence. This one is Book Cipher. The key: Treasure Island, opening line. Every number is a word in the opening line of Treasure Island, counting from 1. Take the first letter of each word. Count to word 2 in the passage: it starts with T. Do that for every number. The answer was TREASURE MAP.

Working one out from scratch

Narrow it down before you decode. Count the letters, look for the one-letter words, and test the likeliest cipher first. Short words and common letters will give you the first foothold, and one correct letter usually leads to the next.