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Don't bet your life on it. Codes of this type have been broken without even knowing the book.

There are several pitfalls. A part of the book cannot be reused in other messages else the overlap allows unfolding of the code. It's not easy to select a book for two people to share and that is not obvious to the adversary; Bible, Quotations of Mao, Shakespeare, Book of Common Prayer, Gone With the Wind, Dictionary, etc., are all rather obvious. It's easy to check the code against many known books. Some books just do not have enough words (even The Dictionary) and using the book to select letters rather than words leaks even more information. The adversaries may also be suspicious that a suspected spy has a copy of a particular book.

'Zackly. And along with the shortcomings of the method itself, there's still the problems inherent in any secure communications - how do you know your key exchange isn't compromised? How do you know the black hats aren't watching you and observing that you always pull out a copy of "Little House on the Prairie" when you get e-mail from "John"?

One-time pads are uncrackable, given two conditions. One, they must be random. Two, they must be uncompromised. The Venona stuff failed the first test - the Soviets got lazy and started re-using pads when they should have tossed them. John Walker was possibly the most dangerous spy in the history of the United States. Why? He was was giving the Soviets the pads and the "word of the day" to tell them what pad to use - there was a ten-year period during which the Soviet Union read every single piece of flash traffic that the US Navy generated.

"Don't bet your life on it. Codes of this type have been broken without even knowing the book."

You say yes, I say no so we will have to disagree! Noodle this around. If raghead #1 wants to tell raghead #2 that tomorrow is the day he does not send out a 100 page document. He encrypts "tomorrow" using the selected book. Using only the bare minimum of code, out of a book with 200 pages, 40 lines per page, 80 characters per line. Any of the 5 characters can come from any page, any line with a source of 640,000 possible characters. To decrypt you need some pattern or some type of mathmatical repetition. Raghead 1 can use any "t", any "o", any "m", any R, and any "w" in an entire book.

Calling all number crunching type people what are the odds of finding the sequence of just 5 characters in 640,000 in the correct order?

The page,line,character sequence must be know. P,L,C or C,L,P or C,P,L etc then the book title must be known. You have to also know that this is the type of code being used.