Symmetric Encryption II

In the classification of cryptography, what are the two main sub-branches of cryptology?

Cryptography and Cryptanalysis.

What two categories is modern symmetric key cryptography divided into?

Block Ciphers and Stream Ciphers.

Which specific property of identity is achieved when both non-forgeability and non-mutability are present?

Nonrepudiation

What is the fixed block length b typically used in modern block ciphers like AES?

128 bits

Why is padding necessary for block ciphers?

Block ciphers require the plaintext length n to be a multiple of the block size b.

Under the PKCS5 standard, what is the assumed fixed block size?

8 bytes

How does the PKCS7 padding method indicate the length of the padding added to a block?

It uses a sequence of identical bytes where each byte's value equals the number of padding bytes.

If a 16-byte block cipher is used to encrypt the 7-character string "Roberto", how many padding bytes are required?

9 bytes

In PKCS7 padding, what is the value of each padding byte if 4 bytes of padding are needed to complete a block?

4

Which classical block cipher treats English letters as numbers modulo 26 and uses an n x n matrix as a key?

The Hill Cipher

What mathematical property must the key matrix K possess to be valid for a Hill Cipher?

The matrix must be invertible modulo 26.

What is the encryption formula for a Hill Cipher given key matrix K and message vector M?

C = K * M mod 26

How is the decryption matrix D calculated for a Hill Cipher if the key matrix K is known?

D = K^-1 mod 26

Which cipher type functions by shuffling the positions of plaintext characters according to a specific permutation?

Transposition Cipher

In a transposition cipher, what serves as the decryption key relative to the encryption permutation p_i?

The inverse permutation p_i^-1.

How can a transposition cipher be mathematically represented using the Hill Cipher framework?

It can be represented using a permutation matrix as the key matrix K.

To what type of attack are Hill Ciphers particularly vulnerable due to their linear nature?

Known plaintext attacks.

What was the official encryption standard used by the U.S. from 1976 until 2005?

Data Encryption Standard (DES).

What is the effective key length of DES after dropping the parity bits from the initial 64-bit key?

56 bits

Which variant of DES uses three separate 56-bit keys to enhance security?

Triple DES

What is the main security weakness of the original DES algorithm in modern computing?

The 56-bit key size is small enough for a computer to brute force all possible keys.

The current standard that replaced DES and supports 128-bit blocks is called _____.

Advanced Encryption Standard (AES)

What are the three supported key lengths for the AES algorithm?

128, 192, and 256 bits.

How many rounds are performed in the 128-bit version of the AES encryption algorithm?

10 rounds

In AES, what is the name of the 128-bit array that undergoes transformations in each round?

State

What operation is performed to create the initial state X₀ in AES given plaintext P and key K?

X₀ = P XOR K

The AES step involving an S-box substitution to transform bytes is called _____.

SubBytes

Which AES round step performs a permutation by cyclically shifting the rows of the state matrix?

ShiftRows

Which AES round step involves matrix multiplication to provide diffusion across the columns?

MixColumns

The AES step where the state is combined with a subkey using the XOR operation is called _____.

AddRoundKey

In the AES S-box lookup, which part of the 8-bit value determines the row index?

The leftmost 4 bits.

In the AES S-box lookup, which part of the 8-bit value determines the column index?

The rightmost 4 bits.

What is the block size of the AES algorithm?

128 bits

Which organization selected the final algorithm for the Advanced Encryption Standard in 1997?

National Institute for Standards and Technology (NIST).

If the determinant of a Hill Cipher key matrix K is d, what is the formula for the decryption matrix D?

D = [(d^-1 mod 26)(d(K))] mod 26

How does an attacker determine the key for a transposition cipher given sufficient plaintext and ciphertext?

By examining the destination of each character position from the plaintext to the ciphertext.

What is the peak performance of the El Capitan supercomputer mentioned as a context for brute force capabilities?

1.8 Exa Flop/s

Which term describes the number of characters needed to uniquely determine a key through brute force?

Unicity distance

What was the intended lifespan of DES when it was first adopted in 1976?

10 to 15 years

In AES-128, which state is designated as the final ciphertext C?

X₁₀

The sub-branch of cryptology focused on breaking codes is called _____.

Cryptanalysis

In the context of the Hill Cipher, how are the letters of the English alphabet represented numerically?

A through Z are mapped to the numbers 0 through 25.

When encrypting with AES, how many rounds are used for a 256-bit key?

14 rounds

Which symmetric cipher uses a key matrix K to transform a column vector of plaintext into a column vector of ciphertext?

The Hill Cipher

Why is 'padding with just zeroes' considered ambiguous in block ciphers?

It is impossible to distinguish between added padding zeroes and legitimate data zeroes at the end of a message.

What is the block size of the Data Encryption Standard (DES)?

64 bits

In modern cryptography, a cipher that processes input one block at a time is called a _____.

Block Cipher

What is the primary difference between PKCS5 and PKCS7 padding?

PKCS5 is defined specifically for 8-byte blocks, while PKCS7 allows for arbitrary block sizes.

How is a plaintext message of length n bits partitioned for encryption in a block cipher with block size b?

It is divided into a sequence of m blocks M[0], …, M[m-1].

In a Hill Cipher example where det(K) = 433, what is the modular inverse of the determinant modulo 26?

23