Which of the following is a stream cipher used for confidentiality among the listed symmetric algorithms?

• A. IDEA
• B. RC4
• C. 3DES-EDE
• D. AES

Answer: (B)

A stream cipher encrypts data one byte (or bit) at a time by generating a continuous keystream that is XORed with the plaintext. This allows fast, byte-wise encryption without needing to pad to a fixed block size. RC4 fits this model because it takes a secret key to initialize a keystream generator and then outputs a stream of bytes that are XORed with the message to produce ciphertext. The other options are block ciphers, which operate on fixed-size blocks (for example, 64-bit blocks for IDEA and DES-based 3DES, and 128-bit blocks for AES). They encrypt data in blocks and often require modes of operation to handle longer messages. Thus RC4 is the stream cipher among the listed symmetric algorithms. Note that RC4 is no longer recommended for modern protocols due to known weaknesses.