In stream ciphers, what is typically combined with plaintext to produce ciphertext?

In stream ciphers, pseudorandom characters are combined with the plaintext to produce the ciphertext through a process called the keystream. The keystream is generated using a cryptographic algorithm along with a secret key, ensuring that the output appears random and unpredictable. Each bit or byte of the plaintext is then XORed with the corresponding bit or byte of the keystream to create the ciphertext.

The use of a pseudorandom sequence is crucial in stream ciphers because it provides security by varying the result significantly even if the same plaintext is encrypted multiple times, assuming the keystream (derived from the key) is unique for each encryption session. This prevents patterns in the plaintext from being easily discernible in the ciphertext.

In contrast, while options like random keys, initial vectors, and the reuse of keys are relevant in other contexts of cryptography, they do not specifically represent what is combined directly with plaintext in the operational mechanism of creating ciphertext in stream ciphers. Random keys refer to the secret keys used for generating the pseudorandom characters, and while initial vectors can play roles in other cipher modes, they are not the main component in the workings of stream ciphers. Reusing keys can lead to vulnerabilities, which is contrary to