Codebusters Test Codebusters Pattern Recognition 4

Question 1

What does the index of coincidence primarily help determine in cipher analysis?

Accepted Answer

Whether a cipher is monoalphabetic or polyalphabetic

Answer

The index of coincidence measures letter frequency regularity; ~0.065 indicates monoalphabetic substitution, while lower values suggest polyalphabetic ciphers.

Question 2

Using A=1, B=2...Z=26, what does the number sequence '8-5-12-12-15' decode to?

Accepted Answer

HELLO

Answer

8=H, 5=E, 12=L, 12=L, 15=O spells HELLO using the standard A=1 through Z=26 mapping.

Question 3

A ciphertext has nearly flat letter frequency (all 26 letters appear almost equally often). This most strongly suggests which cipher type?

Accepted Answer

Vigenère or polyalphabetic substitution

Answer

Polyalphabetic ciphers like Vigenère spread letter frequencies evenly by using multiple cipher alphabets, flattening the distribution.

Question 4

In a Baconian cipher, each letter is encoded as a 5-character sequence of A's and B's. How many unique letters can this system encode?

Accepted Answer

32

Answer

Five binary positions (A or B) give 2^5 = 32 possible values, more than enough to cover the 26-letter alphabet.

Question 5

A ciphertext word has pattern 'ABBA' (e.g., 'NOON'). Which 4-letter English word also fits this pattern?

Accepted Answer

DEED

Answer

DEED (D-E-E-D) fits ABBA: first and last letters match (D), middle two letters match (E), just like NOON.

Codebusters Test Practice Test

Codebusters Test Practice Test

Codebusters Test Codebusters Pattern Recognition 4