Cracking the BookPad cipher

BookPad is a paper and pencil “one-time pad” cipher, described in this other post. Real cryptographers are leery of ciphers claiming to be incarnations of the unbreakable one-time pad for good reasons, the best of them being that true one-time pads must contain perfectly random numbers, which not even a computer can produce. In this post, therefore, I put on my cryptanalyst’s hat and attempt to break a longish message encrypted with BookPad.

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A new look at one-time pads

In another article, I describe how text taken from a book in your library can possibly be used to serve as a one-time pad of sorts, since normal text also contains some unpredictability. The trick is to use a piece of text from an agreed-upon book that is five times the length of the plaintext. That method uses a computer-based hash function, but in this article I tell you how to obtain good security from simple paper and pencil calculations, actually using a key text out of the book that is only three times the length of the plaintext.

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BookPad, a paper and pencil “one time pad” cipher

In another article, I describe how text taken from a book in your library can possibly be used to serve as a one-time pad of sorts, since normal text also contains some unpredictability. The trick is to use a piece of text from an agreed-upon book that is five times the length of the plaintext. That method uses a computer-based hash function, but in this article I tell you how to obtain good security from simple paper and pencil calculations, actually using a key text out of the book that is only three times the length of the plaintext.

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Absolute forward secrecy

swat-team-e1302113424146 (1)Case scenario: Alice and Bob are emailing messages back and forth between them. They know their email is not secure, so they use encryption to preserve their privacy. Suddenly, SWAT teams break simultaneously into Alice’s and Bob’s apartments. Their respective computers are seized and they are asked at gunpoint for their encryption keys. Can their prior conversation, which has been duly recorded before the break-in, remain private?

Answer: it can, but it requires a very stringent form of secrecy, which I will call Absolute Forward Secrecy (AFS). This is one step beyond Perfect Forward Secrecy (PFS), which is touted a lot these days. In this article, I discuss the different kinds of forward secrecy, and how to obtain the absolute kind with a minimum of hassle. Read More