Which statement best characterizes stream ciphers in a cryptographic system?

• A. Encrypts fixed-size blocks independently
• B. Generates a keystream to XOR with the plaintext
• C. Uses public-key pairs for each block
• D. Requires a block cipher mode

Answer: (B)

Stream ciphers are defined by generating a pseudorandom keystream and combining it with the plaintext using XOR to produce ciphertext. Decryption uses the same keystream to recover the plaintext, since P = C XOR KS. The keystream is produced from a secret key (and often a nonce or IV), and it must be unpredictable and never reused with the same key and nonce. This direct keystream XOR behavior is what sets stream ciphers apart from other types.

The other statements describe concepts that belong to block ciphers or public-key cryptography. Encrypting fixed-size blocks independently matches how many block ciphers operate, not stream ciphers. Using public-key pairs for each block describes asymmetric encryption, which is a different paradigm. Requiring a block cipher mode pertains to how block ciphers handle longer messages, not the core operation of a stream cipher.