Public Key Cryptosystems and Block Cipher Modes

Vocabulary flashcards covering key concepts from cryptography lecture notes including block cipher structures, DES, AES, Public Key Cryptosystems, Euclidean Algorithm, and Modes of Operation.

Block Cipher

An encryption scheme where a plaintext message of $b$ bits is encrypted using an encryption algorithm and key to produce $b$ bits of ciphertext, which can then be decrypted back to the message using a decryption algorithm.

Feistel Cipher

A symmetric block cipher structure where plaintext bits are divided into left and right halves $(L_0, R_0)$ and processed through $m$ rounds using round function $F(R_i, K_i)$ and subkeys $K_i$.

Data Encryption Standard (DES)

An encryption standard adopted by NIST in 1977 that uses a Feistel cipher structure with $m = 16$ rounds and a 56-bit key length; it was broken in 1999.

DES Round Function (F Function)

A four-step round function taking a 32-bit input $R_{i-1}$: (1) Expansion permutation to 48 bits, (2) Adding/XORing the key $K_{i-1}$, (3) Substitution using S-boxes to 32 bits, and (4) Permutation on the output.

DES Key Schedule

The procedure of generating 16 subkeys $K_{i-1}$ from an 8-byte key (where the last bit of each byte is a parity bit, giving 56 effective key bits) by dividing into left and right halves and circularly shifting left by 1 or 2 bits per round.

Diffusion

A cipher design goal where a change in a single plaintext bit propagates to as many ciphertext bits as possible; in DES, changing 1 bit of 64-bit plaintext affects 34 bits of ciphertext.

Confusion

A cipher design goal meant to make the relationship between the secret key and ciphertext as complex as possible; in DES, a single bit change in the secret key changes 35 bits of ciphertext.

Advanced Encryption Standard (AES)

A block cipher announced by NIST in 1997 and approved in 2003 as a successor to DES, featuring a block length of 128 bits, 10 rounds of encryption, and a key length of 128 bits.

AES Encryption Round Steps

The four steps executed in a standard AES encryption round: (1) Substitution (SubBytes via lookup table), (2) ShiftRows (shifting rows left in a $4 \times 4$ byte matrix), (3) MixColumns, and (4) AddRoundKey.

Avalanche Effect

A property in block ciphers like AES where a small change in the secret key or plaintext produces a large change in the resulting ciphertext.

Public Key Cryptosystem

A cryptosystem where encryption and decryption use different keys: sender Bob encrypts plaintext using recipient Alice's public key, and Alice decrypts the ciphertext using her private key.

Requirements of Public Key Cryptosystem

The security criteria specifying that Alice's private key is known only to Alice, Alice's public key and algorithms are public, and recovering plaintext from ciphertext and the public key is computationally infeasible.

Euclidean Algorithm

An algorithm to compute the greatest common divisor $\gcd(a, b)$ for $a \ge b$ by repeatedly dividing $a = b q_1 + r_1$ and setting $\gcd(a, b) = \gcd(b, r_1)$ until remainder 0 is obtained.

Electronic Codebook (ECB) Mode

A block cipher mode of operation where plaintext is divided into blocks of $b$ bits and each block is encrypted independently with the same key, leaving repeating patterns visible in the ciphertext.

Cipher Block Chaining (CBC) Mode

A block cipher mode of operation where each plaintext block is XORed with the preceding ciphertext block (or an Initialization Vector $IV$ for the first block) before encryption to obscure repetitive structure.

Cipher Feedback (CFB) Mode

A block cipher mode of operation that uses a shift register to encrypt plaintext segments of size $s$ bits (smaller than block size $b$) by XORing them with encrypted shift register output.

Output Feedback (OFB) Mode

A block cipher mode of operation where the cipher's output is fed back into the shift register to generate keystream bits independently of plaintext, allowing precomputation and preventing error propagation in ciphertext transmission.

Counter (CTR) Mode

A block cipher mode of operation where a counter (initialized at random and incremented by 1 per block) is encrypted to produce a keystream that is XORed with plaintext blocks, enabling parallelization and precomputation without error propagation.

Counter Mode Decryption

Counter Mode Encryption

Output Feedback Mode Decryption

Output Feedback Mode Encryption

Cipher Feedback Mode Decryption

Cipher Feedback Mode Encryption

Cipher Block Chaining mode Decryption

Cipher Block Chaining mode Encryption

Electronic Code Book Encryption/Decryption