Especially in the modern era of computers, it would not be hard to write a program to try to decrypt a message repeatedly using each of a collection of digitized books as keys, and to apply a statistical test to the output to see how random it looks. the number of distinct usable books, is not as large as one might think. by comparing ciphertext with known plaintext) which pages are which, and thus be able to tell with fairly high accuracy which ciphertext indices denote numbers.Īlso worth noting is that the keyspace, i.e. A clever attacker might then be able to deduce (again e.g. a collection of statistical tables, in which some pages consist mostly of text and others mostly of numbers. It's then not much of a leap of deduction to guess that index 123:7:47 probably denotes a T.Ī more realistic case might be one where the book used as a key happens to be e.g. from known plaintext) that the indices 123:7:41 to 123:7:46 spell out E-N-C-R-Y-P. Thus, in principle, if two indices are chosen too close to each other, an adversary could deduce some statistical information about how the corresponding plaintext letters may be correlated.Īs a toy example, suppose that an adversary has already deduced (e.g.
So you are at the mercy of Google (and of whoever acquires Google's services).Īn obstacle to proving that a book cipher is secure is that the letters in (most) books are not chosen independently at random. The book then ceases to be an inconspicuous object: instead of a brain-only key ("use this book"), you have a physical key ("use this, I mean that specific printed instance, book").Īnd, of course, anybody who has scanned millions of book and extracted the text into searchable computer structures, can easily try all existing books in a few minutes, and report a hit when the decryption result appears to look like meaningful words. by marking the already used letters in the book. That's tedious.Īlso, in order to avoid reusing a given index, you must keep a list of the index places you have used, e.g. An unbiased process would have you use dice to select at random the page, line and column and if that does not correspond to your target letter, try again. So the encryption process is important, and not completely described. But non-random choices can exhibit biases. Human beings are very bad at making random choices in their head. When you want to encrypt a letter, you must choose one of the index values in the book which correspond to that letter. This reduces the lifetime of a given book as a cipher key.Īnother problem is in the "random choice". Therefore, merely not reusing exact index values is not sufficient you should refrain from using two index values from the same word. But that's true only to some extent: a book consists of words, with meaning, which implies that characters which may appear at position 321:42:35 are not uncorrelated with characters which appear at positions 321:42:34 and 321:42:36. Using the book as a key is relatively similar to one-time pad, insofar as the book can be considered as a random stream of characters.
BOOK CIPHER CRACK
N.B: I'm not trying to get you to crack this, just adding it because it nicely demonstrates the way this works :) The book is Charles Darwin's On The Origin of Species, 6th edition. I'm using a page:line:character format, ignoring titles, spaces and punctuation. Without knowing what the book is, I would imagine this is provably secure as long as the person encrypting never re-used an index. We might write 238:4:64 to specify that we're on the 238th page, 4th line, 64th character. Imagine a book that has 500 pages, with approximately 60 lines per page and 80 characters per line. Essentially it's a keyed substitution cipher, where the key is the name and exact edition number of the book. I've seen ciphers (usually in spy drama shows) that involve taking a book and writing down an index to individual characters.
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