Which of the following statements accurately describes RC4?

• A. RC4 is extremely fast.
• B. RC4 always uses a 40-bit key.
• C. Both A and B
• D. Neither A nor B

Answer: (A)

RC4 is a fast stream cipher because it relies on a simple key-scheduling process and byte-wise operations to generate a keystream that is XORed with the plaintext. This straightforward approach means very little per-byte computation, leading to high speed in software and hardware, which is why RC4 earned a reputation for being extremely fast.

Key lengths for RC4 are not fixed at 40 bits. RC4 supports key sizes from 1 to 256 bytes (8 to 2048 bits). The 40-bit key you may have heard about comes from specific historical uses like WEP, where a 40-bit key was chosen by the protocol, not a limitation of RC4 itself. So the statement that RC4 always uses a 40-bit key is incorrect.

Therefore, the best description is that RC4 is extremely fast. (Note: despite its speed, RC4 has known vulnerabilities and is not recommended for modern secure use.)