Cryptography and Encryption Principles Quiz

Symmetric Encryption

•  Symmetric encryption involves using the same secret key for both encryption and decryption.
•  Sender and receiver must exchange a common secret key.
•  The sender encrypts data using this key, and the receiver decrypts data using the same key.
•  Advanced Encryption Standard (AES) is a common example of symmetric encryption.

Symmetric Encryption Attacks

•  Cryptanalytic attacks rely on the algorithm's nature, knowledge of general plaintext characteristics, and example plaintext-ciphertext pairs.
•  Brute-force attacks attempt every possible key until a match is found.

Attack Models

•  Ciphertext-only attacks: only the ciphertext is known.
•  Known-plaintext attacks: some plaintext-ciphertext pairs are known.
•  Chosen-plaintext attacks: the attacker can choose plaintext to be encrypted.
•  Chosen-ciphertext attacks: the attacker can choose ciphertext to be decrypted.

Chosen Plaintext Attack

•  An adversary selects two messages, m₀ and m₁.
•  Oracle chooses a random bit b and encrypts m_b.
•  Adversary must not guess b with non-negligible probability.

Chosen Plaintext Attack Indistinguishability

•  A private-key encryption scheme (Gen, Enc, Dec) is CPA-secure if for all probabilistic polynomial-time adversaries A, there's a negligible function negl such that Pr(PrivKсpa А,П (n) = 1) ≤ 1/2 + negl(n).

Block and Stream Ciphers

•  Block ciphers encrypt data in fixed-size blocks.
•  Stream ciphers encrypt data bit by bit or in small segments.

Substitution and Permutation

•  Substitution: For each of 2^k possible input values, the input bit string is replaced with its corresponding output bit string.
•  Permutation: Specifies the output position for each input bit (e.g., 1st bit → 13th bit of output).

Block Cipher Example

•  The process involves substitution functions derived from a key, followed by permutation.

Stream cipher operation

•  Uses a key stream generator to produce a stream of key bits.
•  Keystream bits are combined with plaintext bits using XOR.

Stream Cipher Design Considerations

•  Encryption sequence should have a large period to avoid repetition.
•  Use true random number generation to ensure randomness of keystream bits.
•  Use long keys for enhanced security.

Substitution Techniques

•  Caesar cipher: Each letter is replaced by the letter k places away. - C = E(k, p) = (p + k) mod 26 - p = D(k, C) = (C - k) mod 26

Monoalphabetic Ciphers

•  A finite set of elements with each element appearing exactly once.
•  e.g., {a, b, c}: 3! = 6 permutations (abc, acb, bac, bca, cab, cba).

Monoalphabetic Ciphers: Cryptanalysis

•  The frequency of letters (e.g., E, T, A) used in messages can reveal the substitutions.

Multiple-Letter Ciphers: Playfair

•  Uses a 5x5 matrix with a keyword to encrypt pairs of letters.

Polyalphabetic ciphers

•  Uses multiple substitution rules to encrypt.
•  Vigenere cipher uses a keyword for multiple Caesar ciphers, each with a different shift.

Relative occurrence of letters

•  Analysis of the relative frequency of letters (e.g., E, T, A) or digrams (e.g., TH) can reveal how plaintext letters are substituted.

One-Time Pads

•  Use a random key as long as the message.
•  Provides perfect security.
•  Key distribution is a significant challenge.

Advanced Encryption Standard (AES)

•  Overview of AES encryption and decryption process. Includes details like rounds, subkeys, expansions, and various steps.

Shift rows

•  AES encryption involves shifting rows of the state matrix.
•  Row 1: no shift.
•  Row 2: 1 byte circular shift.
•  Row 3: 2 byte circular shift.
•  Row 4: 3 byte circular shift.

MixColumns

•  Matrix-based calculation to combine columns of the state matrix during encryption
•  Uses a mathematical matrix multiplication to thoroughly mix the input

AddRoundKey

•  XOR operation between the state block and a specific round key.

Key Expansion

•  Generating sub-keys needed for each AES round. - A procedure to generate all necessary round sub-keys.

Block Ciphers

•  Design Criteria (Overhead, Error recovery and propagation, Diffusion, Security).

•  Modes of operation (ECB (Electronic Code Book), CBC (Cipher Block Chaining), CFB (Cipher Feedback), OFB (Output Feedback), CTR (Counter)). - ECB (Electronic Code Book) - CBC (Cipher Block Chaining) - CFB (Cipher FeedBack) - OFB (Output FeedBack)

  - CTR (Counter)

Electronic Code Book (ECB) Encryption

•  Simple approach, encrypting each block independently.
•  Block repetition in the message may lead to repetition in the ciphertext

Electronic Code Book (ECB) Decryption

•  Reverses the process, decrypting each block based on its individual representation in the ciphertext

Electronic Code Book (ECB) Problems

•  Identical message blocks lead to identical ciphertext blocks. This can allow attackers to identify patterns in messages.

Randomized Electronic Code Book Encryption

•  Combines randomness to prevent identical message blocks leading to identical ciphertext blocks.

Cipher Block Chaining (CBC) Encryption

•  Uses a previous ciphertext block to encrypt the current message block, ensuring each block encryption depends on others.

Cipher Block Chaining (CBC) Problems

•  Reception of all preceding blocks is essential for decryption of a block.
•  Error in one block impacts subsequent blocks