Questions?
Book Ciphers
How they Work
The plaintext is some sort of document.
There’s also a key document (e.g., the Declaration of Independence)
The ciphertext is a series of numbers, namely the positions of words in the key.
Ciphertext number n denotes the 1st letter of the nth word in the key.
Decrypt the first few words of “document 2” for yourself (but beware that “Beale” didn’t count very well, see the note at the end of the the “unmuseum” reprint, and what Beale apparently took to be word 680 is actually 691):
Add 10 to everything beyond 490, add 11 to everything beyond 691
| Cipher Text | 115 | 73 | 24 | 807 (818) | 37 | 52 | 49 | 17 | 31 | 62 | 647 (657) | 22 | 7 | 15 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Plain Text | I | H | A | V | E | D | E | P | O | S | I | T | E | D |
Do It Yourself
Make your own book cipher and try it with others in the room.
See if you can break one of the other book ciphers from this exercise.
Observations
Book ciphers are cumbersome, and it’s easy to make mistakes.
If you don’t know another person’s key document, it’s extremely hard to break a book cipher -- so they’re secure. But on the other hand, you need to worry about whether the other party to a communication has exactly the same key as you do.
If you want to break a book cipher, knowing what key documents are going to seem “natural” to the cryptographer is helpful.
Next
Peer reviews of Turing drafts.
Bring 2 printed copies or be prepared to share with reviewer(s).
Also share the draft with me or email me a copy by Tuesday’s class.