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

The keyword cycles: positions 1=K, 2=E, 3=Y, 4=K; so the 4th letter uses shift K (10).

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

Start both; when 4-min ends (t=4), flip it; when 7-min ends (t=7), the 4-min has 1 min left; flip 7-min; let 4-min run out (t=8) then run 7-min for 1 min to total 9.

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

A high IoC matching natural language indicates letter frequencies are preserved, typical of monoalphabetic substitution.

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

Divide into three groups of three; first weighing identifies which group, second weighing identifies the ball — 2 weighings total.

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

E is position 4, X is position 23; shift = 23 − 4 = 19, meaning each plaintext letter was shifted forward by 19.

Codebusters Test Practice Test

Codebusters Test Practice Test

Codebusters Test Codebusters Logical Puzzles and Riddles 4