What defines a stream cipher?

• A. It encrypts data one bit or byte at a time using a keystream
• B. It uses block processing with padding
• C. It requires a public key
• D. It provides integrity by default

Answer: (A)

A stream cipher encrypts data by generating a keystream and combining it with the plaintext one bit or one byte at a time. The keystream comes from a secret key (and often a nonce or IV) and is typically mixed with the message using a simple operation like XOR. This per-bit or per-byte process makes stream ciphers well suited for streaming data or situations where data arrives continuously.

This contrasts with block ciphers, which operate on fixed-size blocks of data and often require padding to fill a block. Block processing with padding is how many traditional ciphers handle larger messages in chunks.

Public key usage refers to asymmetric cryptography, where separate keys are used for encryption and decryption, whereas stream ciphers are symmetric and rely on a shared secret key. Finally, encryption alone does not guarantee integrity or authenticity; you’d need a MAC or an AEAD scheme to provide that, since encryption by itself focuses on confidentiality.