Cryptology - Unofficial St. Mary's College of California Web Site

8.8. TERMS AND TOPICS 155
multiple of the lengths of the individual keys. While this method may keep
short texts secure, the amount of keyword needed is directly related to the
amount of text one wishes to send (approximately 1 page of keyword must be
used to safely encipher 20 pages of plaintext). Producing and exchanging the
huge of amount keys needed by a modern business or government prevents any
practical use of a repeated Vigenère system.
Vigenère did not invent the Vigenère Cipher, but, rather that Auto Key
Cipher. This cipher uses a priming key following by the plaintext as its key,
thus producing a key without repetitions. Its security is much better than that
of the Vigenère. Nonetheless, the patterns in English provide enough of an entry
that Friedman was eventually able to break this system.
The only unbreakable cipher system is the One-Time Pad. A random series
of letters (or numbers) is used as the key. As long as each key is used exactly
twice, once for enciphering and once for deciphering, and is kept secret, the
system cannot be broken. But the key is as long as the message, so the problem
of transmitting the message is simply replaced by that of transmitting the key.
So the immense quantity of keys that must be distributed to use this system
prevent it from being used except in very special circumstances.
8.8 Terms and Topics
1. What is a “coincidence” What is the connection between coincidences
and Kasiski’s test
2. What is the Index of Coincidence What does it measure
3. What is the formula for Φ
4. How do the frequency counts of a monoalphabetic cipher and a polyalphabetic
cipher with long keylength differ
5. What does M.R. stand for What does it measure
6. Under what circumstances do we expect M.R. to be a larger number A
smaller number
7. What is the Friedman test
8. How can we use Φ to estimate the keylength of a polyalphabetic cipher
9. What is the Double Vigenère How does it differ from the Vigenère cipher
10. Performing multiple encipherments with different keywords is equivalent
to enciphering once with a keyword of what length
11. Who invented the Auto Key cipher
12. What is the Auto Key cipher Explain how to use it.