Book Ciphers

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Book Ciphers

The kind of cipher used in The Beale Papers. See Monday’s notes for a description of how these ciphers work.

Practice

In groups of 1 or 2, pick a document to use as a key for a book cipher. Then come up with some message and encrypt it using your key. Write the ciphertext in a shared Google doc

When you have posted ciphertext to this document, find some way to securely exchange keys with another group. Verify that you can decrypt that team’s ciphertext.

Finally, time permitting, see if you can decrypt the ciphertext from some team that did not deliberately share their key with you. As the “...did not deliberately share their key…” wording suggests, this will probably entail guessing or surreptitiously acquiring their key.

Discussion

For example, how did people come up with and devise their cipher? How easy are book ciphers to use? Are they secure? What can go wrong with them? Other reactions?

• It’s not easy to find a key document that can encrypt every letter; for instance, not many English words begin with “X.”
• One of the surprisingly frustrating things about book ciphers is needing to count the words in the key accurately. If preparing a numbered copy of the key is considered part of key preparation, then book cipher keys are hard to create; if counting words is part of encrypting and decrypting messages, then those steps are hard but key generation is easy.
• If you get sloppy with a book cipher and always use the same word to encrypt each letter, then all you really have is a simple (monoalphabetic) substitution cipher.

Next

Modern (since 1976) cryptography.

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