Codebusters Test Codebusters Polyalphabetic Ciphers 3

Question 1

The Porta cipher uses a tableau where each key letter selects one of how many alphabets, each covering a pair of key letters?

Accepted Answer

13 alphabets, one per pair of adjacent letters (AA/BB … YY/ZZ)

Answer

26 alphabets

Answer

5 alphabets

Answer

6 alphabets

Question 2

What key property makes the Vigenère cipher significantly harder to break than a simple Caesar cipher?

Accepted Answer

The same plaintext letter can be enciphered as different ciphertext letters depending on its position

Answer

It uses a 52-letter alphabet

Answer

It reverses the plaintext before enciphering

Answer

It requires two passes of encryption

Question 3

In the context of Codebusters, what does 'key period' refer to in a polyalphabetic cipher?

Accepted Answer

The length of the repeating keyword before it cycles back to the start

Answer

The time allowed to solve the cipher

Answer

The number of plaintext letters enciphered per key letter

Answer

The total number of cipher alphabets available

Question 4

Which manipulation of a polyalphabetic ciphertext would NOT help a cryptanalyst break a Vigenère cipher?

Accepted Answer

Rearranging the ciphertext letters alphabetically

Answer

Finding repeated trigrams

Answer

Computing the Index of Coincidence

Answer

Testing assumed key lengths and running frequency analysis

Question 5

The Quagmire ciphers (I–IV) used in advanced Codebusters are all polyalphabetic. What distinguishes Quagmire IV from the others?

Accepted Answer

Both the plaintext and key alphabets are mixed (scrambled), not in standard A–Z order

Answer

It uses only 13 alphabets

Answer

It applies a double transposition

Answer

The key is numeric rather than alphabetic

Question 6

When the IC of a full ciphertext is calculated to be approximately 0.045—between the English value (~0.065) and random (~0.038)—what does this suggest about the key length?

Accepted Answer

A medium-length key of perhaps 3–7 letters, partially flattening the frequency distribution

Answer

The cipher is a simple Caesar shift

Answer

The key length equals the ciphertext length (one-time pad)

Answer

The text is not in English