What does a stream cipher do?

What does a stream cipher do?

• A. Encrypts continuous streams of data
• B. Encrypts data in fixed-size blocks
• C. It uses modular arithmetic
• D. Is always slower than block ciphers

Answer: (A)

A stream cipher encrypts data as a continuous stream by generating a keystream from a secret key (and usually an IV or nonce) and then XORing that keystream with the plaintext as the data arrives. This allows encryption bit-by-bit or byte-by-byte, so you can secure data in real time as it streams in. In contrast, block ciphers operate on fixed-size blocks (like 64 or 128 bits) and require modes of operation to handle longer messages; those modes can mimic streaming but the underlying cipher type is still a block cipher, not a true stream cipher. While some designs may use simple operations that feel fast, the defining idea is the continuous, byte- or bit-wise processing with a keystream, which is why this option best captures what a stream cipher does.

A stream cipher encrypts data as a continuous stream by generating a keystream from a secret key (and usually an IV or nonce) and then XORing that keystream with the plaintext as the data arrives. This allows encryption bit-by-bit or byte-by-byte, so you can secure data in real time as it streams in. In contrast, block ciphers operate on fixed-size blocks (like 64 or 128 bits) and require modes of operation to handle longer messages; those modes can mimic streaming but the underlying cipher type is still a block cipher, not a true stream cipher. While some designs may use simple operations that feel fast, the defining idea is the continuous, byte- or bit-wise processing with a keystream, which is why this option best captures what a stream cipher does.