Codebusters Test Codebusters Cipher Decryption 5

Question 1

The Hill cipher encrypts plaintext in blocks by multiplying them with a key matrix. For a 2×2 key matrix, how many plaintext letters are encrypted per block?

Accepted Answer

2

Answer

1

Answer

4

Answer

3

Question 2

In an affine cipher with encryption E(x) = (ax + b) mod 26, which condition must the multiplier 'a' satisfy for the cipher to be invertible?

Accepted Answer

gcd(a, 26) = 1

Answer

a must be a prime number

Answer

a must be an even number

Answer

a must be greater than 13

Question 3

In RSA encryption with public key (e, n), how is a plaintext number m encrypted to produce ciphertext c?

Accepted Answer

c = m^e mod n

Answer

c = m × e mod n

Answer

c = e^m mod n

Answer

c = m + e mod n

Question 4

In RSA, the private key exponent d is computed so that ed ≡ 1 (mod φ(n)). Here φ(n) represents:

Accepted Answer

Euler's totient function of n

Answer

The product p × q

Answer

The sum p + q

Answer

The GCD of p and q

Question 5

The Baconian cipher encodes each letter as a five-character sequence using only two symbols (A and B). How many distinct letters or symbols can a five-character binary sequence represent?

Accepted Answer

32

Answer

26

Answer

10

Answer

64

Question 6

To decrypt a Hill cipher, you must find the inverse of the key matrix. This inverse is computed in which arithmetic system?

Accepted Answer

Modular arithmetic mod 26

Answer

Standard real-number linear algebra

Answer

Binary (base-2) arithmetic

Answer

Complex number arithmetic

Question 7

The Porta cipher uses a keyword to select from 13 reciprocal alphabets, meaning the same operation both encrypts and decrypts. How is the Porta cipher classified?

Accepted Answer

Polyalphabetic substitution

Answer

Monoalphabetic substitution

Answer

Columnar transposition

Answer

Product cipher