gen-define-block-versus-stream-ciphers-and-give-one-short-worked-5b98

Define block versus stream ciphers and give one short worked example.

# Block vs Stream Ciphers ## **Block Cipher** Encrypts data in **fixed-size blocks** (typically 64, 128, or 256 bits). Each block is processed independently using the same key. **Characteristics:** - Processes fixed-length groups of bits - Same plaintext block → same ciphertext block (in ECB mode) - Examples: AES, DES, Blowfish ## **Stream Cipher** Encrypts data **one bit or byte at a time** by generating a continuous keystream that's XORed with plaintext. **Characteristics:** - Processes data as a continuous stream - Same plaintext → different ciphertext (if keystream doesn't repeat) - Examples: RC4, ChaCha20, Salsa20 --- ## **Worked Example: Simple Stream Cipher (XOR)** **Setup:** - Plaintext: `HELLO` (in binary, using ASCII) - Keystream: `10110...` (pseudorandom bits) **Encryption:** ``` Plaintext: H E L L O ASCII: 72 69 76 76 79 Binary: 01001000 01000101 01001100 01001100 01001111 Keystream: 10110011 11010110 00111001 10101010 11110000 ⊕ ⊕ ⊕ ⊕ ⊕ Ciphertext: 11111011 10010011 01110101 11100110 10111111 (251) (147) (117) (230) (191) ``` **Decryption:** XOR ciphertext with same keystream to recover plaintext.