Book cipher

Article summary

, a highly available publication suitable for the book cipher. A book cipher is a cipher in which each word or letter in the plaintext of a message is replaced by some code that locates it in another text, the key. A simple version of such a cipher would use a specific book as the key, and would replace each word of the plaintext by a number that gives the position where that word occurs in that book. For example, if the chosen key is H. G. Wells's novel The War of the Worlds, the plaintext "all plans failed, coming back tomorrow" could be encoded as "335 219 881, 5600 853 9315" — since the 335th word of the novel is "all", the 219th is "plans", etc. Instead of the position of the word, sender can also use for each word a triplet indicating page number, line number in the page and word number in the line, avoiding error-prone counting of words from the start of the book. With the Bible, triplet can be chapter number, verse number, word number. This method requires that the sender and receiver use exactly the same edition of the key book. This simple version fails if the message uses a word that does not appear in the text. A variant that avoids this problem works with individual letters rather than words. Namely each letter of the plaintext message would be replaced by a number that specifies where that letter occurs in the key book.…

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Discussion questions

1. Summarise the article in your own words. Which single claim would you verify first, and why?
2. Which claim in this article is most likely to be revised as the sources update? How would you check its current status?
3. If two sources disagreed about one of these facts, what would you need in order to trust one over the other?

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