New answers tagged substitution-cipher
If the book cipher refers to words as opposed to letters then the book cipher could be made more secure by the different ciphers refering to the same word. Therefore where a word appears more than once in the book likely use a different reference to it. Another way of making it more secure is not to use a book in your native language for example an ...
The only way for this cipher to be secure these days is to use a truly random running key that is the same size or larger then the plaintext. Then to never ever use that same key again. This makes the cipher a One-Time Pad which is the one cipher that is truly unbreakable. But key generation, management, and storage would be extremely difficult.
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