Codebusters Test Codebusters Logical Puzzles and Riddles 4

Question 1

A Vigenère cipher uses keyword 'KEY'. What is the shift applied to the 4th plaintext letter?

Accepted Answer

K (shift 10)

Answer

E (shift 4)

Answer

Y (shift 24)

Answer

K (shift 10) again

Question 2

You have two hourglasses: one measures 7 minutes, one measures 4 minutes. What is the minimum time needed to measure exactly 9 minutes?

Accepted Answer

9 minutes

Answer

11 minutes

Answer

7 minutes

Answer

4 minutes

Question 3

In index-of-coincidence analysis, a high IoC value (close to English's ~0.065) suggests what about an intercepted cipher?

Accepted Answer

The cipher is likely a simple monoalphabetic substitution

Answer

The cipher uses a very long keyword

Answer

The cipher is a transposition cipher

Answer

The text is random noise

Question 4

A weighing puzzle: you have 9 balls, one heavier. What is the minimum number of balance scale weighings needed to identify the heavy ball?

Accepted Answer

2

Answer

3

Answer

4

Answer

1

Question 5

If the plaintext frequency of letter E is ~12.7% and you notice ciphertext letter X appears ~12.5% of the time, what is the most likely shift in a Caesar cipher?

Accepted Answer

The shift is E→X = 19

Answer

The shift is 23

Answer

The shift is 3

Answer

The shift is X→E = 19 reversed

Question 6

A classic river-crossing puzzle: a farmer has a fox, a chicken, and a bag of grain. He can only take one item at a time. Which item must he take first to keep everything safe?

Accepted Answer

The chicken

Answer

The fox

Answer

The grain

Answer

Any of them

Question 7

In a Playfair cipher, if two plaintext letters are in the same row of the 5×5 key square, each is replaced by the letter to its:

Accepted Answer

Right

Answer

Left

Answer

Above

Answer

Below