In the context of encryption ciphers, what is a Steam Cipher?

A stream cipher is a method of encryption that processes data one bit or byte at a time and encodes plaintext with a keystream, which is generated from a key. This characteristic allows stream ciphers to be particularly efficient for applications where data is being transmitted continuously or in varying lengths. The keystream is a sequence of bits that is combined with the plaintext to produce ciphertext, typically using operations such as XOR. Because each bit of plaintext is paired with a bit of keystream for encryption, this method ensures that even small changes in the plaintext result in significant changes in the ciphertext.

The option relating to fixed-size chunks refers to block ciphers, which operate differently by encrypting data in blocks of fixed length. The mention of cloud storage is irrelevant to the fundamental nature of stream ciphers, as they can be used in various contexts beyond cloud-based solutions. Lastly, key control is essential to the function of a stream cipher, as the security of the encryption relies heavily on the effective management of the key and the generated keystream. Thus, the defining aspect of a stream cipher is indeed that it encodes plaintext data using a keystream controlled by a key.